COLLECTION FOR MYANMAR GEOLOGY STUDENTS AND LEARNERS-1

COLLECTION FOR MYANMAR GEOLOGY STUDENTS AND LEARNERS-1
Original Articlerge_145 1..29 Geology of the High Sulfidation Copper Deposits, Monywa Mine, Myanmar Andrew H. G. Mitchell,1 Win Myint,2 Kyi Lynn,2 Myint Thein Htay,2 Maw Oo2 and Thein Zaw2 1 IMHL, U Wisara Road, Kamayut, Yangon, Myanmar and 2 Myanmar Ivanhoe Copper Company Limited, Mine Site, Monywa, Myanmar Abstract The 50 km2 Monywa copper district lies near the Chindwin River within the northward continuation of the Sunda-Andaman magmatic arc through western Myanmar. There are four deposits; Sabetaung, Sabetaung South, Kyisintaung, and the much larger Letpadaung 7 km to the southeast. Following exploration drilling which began in 1959, production of copper concentrates from a small open pit started at Sabetaung in 1983. Since 1997, when resources totaled 7 million tonnes contained copper in 2 billion tonnes ore, a heap leach– electro-winning operation has produced over 400,000 t copper cathode from Sabetaung and Sabetaung South. Ore is hosted by mid-Miocene andesite or dacite porphyry intrusions, and by early mid-Miocene sandstone and overlying volcaniclastics including eruptive diatreme facies which the porphyries intrude. District-wide rhyolite dykes and domes with marginal breccias probably post-date andesite porphyries in the mine area and lack ore-grade copper. Host rocks to mineralization are altered to phyllic and advanced argillic hydrothermal assemblages within an outer chlorite zone; hypogene alunite is most abundant at Letpadaung and Kyisintaung. Most mineralization is structurally-controlled with digenite-chalcocite in breccia dykes, in steeply dipping NE-trending sheeted veins, and in stockwork and low-angle sulfide veins. A high-grade pipe at Sabetaung grades up to 30% Cu, and much of the ore at Sabetaung South is in a NE-trending zone of mega-breccia and stockworked sandstone. The hydrothermal alteration, together with replacement quartz, alunite and barite in breccia dykes and veins, the virtual absence of vein quartz, and the presence of chalcopyrite and bornite only as rare veins and as inclusions within the abundant pyrite, indicate that the deposits are high sulfidation. Regional uplift, resistance to erosion and leaching of the altered and mineralized rocks have resulted in porous limonite-stained leached caps over 200 m thick forming the Letpadaung and Kyisintaung hills. The barren caps pass abruptly downwards at the water table into the highest grade ore at the top of the supergene enrichment zone, within which copper grade, supergene kaolinite and cubic alunite decrease, and pyrite increases with depth; in contrast, marcasite is mostly shallow. Much of the copper to depths exceeding 200 m below the water table occurs as supergene digenite-chalcocite and minor covellite. Disseminated chalcocite is mostly near- surface and hence almost certainly supergene. We infer that during prolonged uplift at all four deposits, oxidation of residual pyrite at the water table generated enough acid to leach all the copper from earlier supergene-enriched ore; below the water table the resulting acid sulfate solutions partly replaced enargite, covellite, chalcopyrite, bornite and pyrite with supergene chalcocite. Undeformed upward-fining cross- bedded conglomerates and sands of the ancestral Chindwin River floodplain overlie the margins of the Received 25 January 2010. Accepted for publication 17 August 2010. Correspondening author: A. H. G. MITCHELL, IMHL, 234/A-1, U Wisara Road, Kamayut Township, Yangon, Myanmar. Email: imhle@myanmar.com.mm Present address: 20 Dale Close, Oxford, OX1 1TU, UK. doi: 10.1111/j.1751-3928.2010.00145.x Resource Geology Vol. 61, No. 1: 1–29 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 1
Sabetaung deposits, form a major aquifer up to 40 m thick, and are a potential host for exotic copper mineralization. A mid-Miocene pluton is inferred to underlie the Monywa deposits, but the possibility of porphyry-type mineralization within the district is at best highly speculative. Keywords: heap leachable ore, high sulfidation, Miocene breccia dykes, Monywa copper mine, Myanmar, supergene enrichment. 1. Introduction The Monywa copper district is 115 km west- northwest of Mandalay in an elevated flood plain west of the Chindwin River in western Myanmar (Fig. 1). The district is on the northern margin of Myanmar’s dry zone; annual rainfall is about 800 mm confined to the May–October period. Maximum daily tempera- tures from March to June can exceed 45°C. Four major high sulfidation deposits of Miocene age define the district: the almost contiguous Sabetaung, Sabetaung South and Kyisintaung deposits and the much larger Letpadaung 7 km to the southeast (Fig. 2). Production is from Sabetaung and the adjacent smaller Sabetaung South, now within a single open pit. Kyis- intaung lies beneath a prominent hill immediately west of Sabetaung, and Letpadaung underlies an isolated 6 km2 hill bisected by a northeast-trending valley (Figs 2, 3). Together these deposits comprise the northernmost major copper resource in the 7000 km long Banda-Sunda-Myanmar arc; with pre-mining combined resources totaling 2 billion tonnes ore with over 7 million tonnes contained copper, they constitute a giant deposit. In Southeast Asia only the undeveloped high sulfidation Tampakan deposit in the Philippines has a larger copper resource. The Monywa deposits differ from most high sulfidation systems (e.g. Heald et al., 1987; Hedenquist et al., 2000) in lacking economically significant enargite and gold mineralization, and their classification as high sulfidation epithermal became widely accepted only in the late 1990s. This followed a quarter of a century during which they were widely but not unanimously interpreted as supergene-enriched porphyry-type deposits. Despite their size the geology of deposits is not widely known, and since mining began the only published accounts are those by Kyaw Win and Kirwin (1998) drawing on D.J. Kirwin (1995, unpublished data), and by Mitchell et al. (2008). Here we describe the geology based largely on recent systematic mapping of levels in the Sabetaung–Sabetaung South pit, on mid- 1990s feasibility study reports, on logging and selective re-logging of core drilled between 1973 and 2008, and on results of regional mineral exploration and geological mapping. Elevations in the mine area are reported as sea level plus 500 m, or m RL (meters relative level). The Chind- win flood plain (Fig. 2) is around 590 m RL, Let- padaung and Kyisintaung hills rise to 832 m RL and 775 m RL, respectively, Sabetaung was a 655 m RL hill prior to mining, and Sabetaung South a low rise. 2. Regional geological setting 2.1 Wuntho-Popa magmatic arc and Central belt The Monywa deposits lie on the eastern flank of the 460 km long Wuntho-Popa or western Myanmar Fig. 1 Location map for Monywa copper district, showing position within Myanmar and within Burma plate (shaded), plate boundary from Steckler et al. (2008). A. H. G. Mitchell et al. 2 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
magmatic arc, within the regional Central belt or Central basin (Fig. 4) of Myanmar. The arc, the northern continuation of the Sunda-Andaman arc, is a northerly-trending geanticlinal uplift which exposes Mesozoic intrusions and their host rocks in four inliers. The 160 km long Wuntho-Banmauk inlier is in the north, the much smaller Okkan inlier is west of the Monywa deposits (Fig. 2), and Salingyi (Barber, 1936; United Nations, 1979a) and Shinmataung are to the south. The inliers consist largely of dioritic to grano- dioritic plutons of early Upper Cretaceous age, and Tertiary dykes and stocks, all intrusive into pre-Albian basalts and a locally exposed greenstone, amphibolitic and gneissic basement. The Wuntho-Banmauk inlier includes the Kanzachaung batholith (United Nations, 1978a). There are three extinct andesitic strato- volcanoes (Fig. 4) of which the southernmost is Mt. Popa where late Cenozoic volcanic rocks (Stephenson & Marshall, 1984) include potash-rich andesites with mid-Miocene Ar-Ar ages (Yang, 2008). In the Okkan inlier west of Monywa, Mesozoic rocks consist of basalt breccias and pillow lavas (Fig. 5a) intruded by Cretaceous diorites and biotite granodior- ites. These are overlain unconformably to the east by a latest Oligocene and Miocene succession equivalent to the regional Upper Pegu Group. The succession consists of a basal conglomerate with local limestone overlain by the Powintaung Sandstones forming a west-facing scarp. Above are shales, cross-bedded sandstone, and local basalt breccia, with minor interbedded andesitic tuff in the upper part, comprising the Magyigon Formation, which includes debris flow deposits with rhyolite blocks west of Letpadaung. In and within 5 km of the copper deposits rhyolite dykes and domes intrude the Magyigon Formation and older rocks. Gently folded Lower Irrawaddian sandstone and overlying Upper Irrawaddian gravel beds, of Pliocene age, occupy a sinuous anticline northwest of the Okkan inlier. Immediately north of Sabetaung in the Lower Chindwin District there are spectacular low-aspect Fig. 2 Geological map, Monywa copper deposits and surroundings, simplified from Myint Naing Win and Myint Thein Htay (2006, unpublished data). Ticked line is approximate western limit of Kangon Sands, black line is motor road. Eastern part of Okkan inlier is Jurassic and Cretaceous at western edge of map. Location on Figures 1 and 4. Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 3
basaltic craters with volcanic aprons which include boulders of garnet-hornblende rock described by Chhibber (1934a). The basalts are shoshonitic and include absarokite (Yang, 2008). 2.2 Chindwin-Minbu basins and Indo-Myanmar (Indo-Burman) Ranges West of the Wuntho-Popa magmatic arc the Chindwin and Minbu basins (Fig. 4) contain 15-km thick syncli- norial unconformity-bound sedimentary successions of Upper Albian to Cenomanian, Senonian to Danian, mid-Palaeocene to Miocene, and Pliocene age. The mid-Palaeocene and younger rocks comprise the Ter- tiary fore-arc basin (Fig. 6) of the magmatic arc (Win Swe et al., 1972, unpublished data); the late Oligocene to Pliocene part of the succession extends eastwards across the arc into the Shwebo basin. The Kabaw Fault is a zone with local W-directed thrusts near the western margin of the Chindwin-Minbu basins. In the oil and gas fields of the Minbu basin (Fig. 4) 100 km southwest of Monywa, there are Oligocene to early Miocene extensional faults and ENE-directed thrusts which offset Pliocene strata. The thrusts have been related to stresses associated with dextral strike- slip movement on the Sagaing Fault to the east (Pivnik et al., 1998). The Indo-Myanmar Ranges (Figs 4, 6), west of the Chindwin-Minbu basins, record the early subduction history of the Wuntho-Popa arc. They consist of two belts (United Nations, 1979b, Mitchell et al., 2010) sepa- rated by an east-dipping thrust. The Eastern belt consists of Upper Triassic flysch and associated ophiolitic rocks underlain by mica schists. In the Western belt, highly-deformed Senonian pelagic limestones, mudstones and turbidite sandstones are overlain by Pale- ocene and Eocene marine clastics with upright open folds. The Tertiary rocks pass westwards into the sedimentary sequence of the Chittagong–Tripura fold belt (CTFB) and the Bengal (Alam et al., 2003) or Ganges- Brahmaputra (Steckler et al., 2008) delta basin in Bang- ladesh and India. 2.3 Myanmar plate The Central belt and Indo-Burman Ranges form part of the small Myanmar (formerly Burma) plate (Fig. 1) of Curray et al. (1979). The western boundary of the plate (Figs 1, 6), the northern continuation of the Sunda Trench, is an eastward-dipping seismic zone (Steckler et al., 2008), which lies about 100 km beneath the Monywa segment of the Wuntho-Popa magmatic arc; present day convergence is oblique. The subducting plate beneath the Bay of Bengal and probably that beneath the Monywa deposits (Fig. 6) is oceanic, but the crust beneath the CTFB and Bengal delta to the north and west could be either oceanic or thinned continental crust of the Indian margin. Fig. 3 View of mine looking southeast from hill above Kyisintaung deposit, across Sabetaung (left foreground) and Sabe- taung South (centre) pits to leached cap above Letpadaung deposit (hills in right background). Leach pads are behind Sabetaung South pit. A. H. G. Mitchell et al. 4 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
The eastern boundary of the Myanmar plate is the Sagaing dextral fault (Figs 4, 6). South of latitude 23°N, the fault forms the western boundary of the Shan-Thai block, part of the Asian plate. The block margin is intruded by Cretaceous and Tertiary granites and volcanic rocks, possibly the northern continuation of the Wuntho-Popa arc offset on the Sagaing Fault. Total northward movement of the Myanmar plate relative to Asia has been variously estimated at 350 to 1100 km. The Naga Thrust (Fig. 4) can be related to collision of the Myanmar plate with the southeastern passive margin of India southeast of the Foreland Spur. 2.4 Mineralization in the magmatic arc Beyond the Monywa district, mineralization in the magmatic arc is largely confined to the large Banmauk- Wuntho inlier (Fig. 4). Here there are artisanal workings on mesothermal quartz-gold veins in grano- diorites (Chhibber, 1934b) and in the basaltic and greenstone host rocks to the intrusions (United Nations, 1978a; Mitchell et al., 1999); porphyry copper- gold prospects of which the most promising is at Shan- galon near Wuntho; quartz-molybdenite veins also at Shangalon (United Nations, 1978b); and some high sulfidation gold prospects. Low sulfidation epithermal quartz-gold veins occur within Neogene sedimentary rocks south of Shangalon, and there are traces of epithermal gold in quartz-alunite-replaced Neogene vol- canics near Mt. Popa. Scattered artisanal workings of placer gold with platinum group metals extend southwards from the Hukawng Valley in northernmost Myanmar to immediately north of the Monywa copper deposits. Near the Monywa deposits, regional exploration in 1995–97 (O. Radislao et al., 1996, 1977, unpublished data) identified gold-silver mineralization within Miocene sedimentary rocks intruded by rhyolitic sills and domes with peperite margins. The rhyolites with associated mineralization are found at Kyaukmyet, first described by Kelterborn (1925, in Chhibber, 1934a); the small Shwebontha-Nache Taung hills; and Taungzone (Fig. 2). At Kyaukmyet colloform-banded chalcedonic quartz veins and spectacular stockworks of comb- textured open-centre veinlets occur within silicified rocks (Figs 2, 5b). Maximum gold values in 6000 m of diamond drilling were around 3 g t-1 with Ag : Au ratios of 300, accompanied by high zinc; antimony (up to 569 ppm) and arsenic (up to 6370 ppm) do not cor- relate with gold. Higher gold values from trenches suggest significant supergene enrichment. Vein tex- tures at Kyaukmyet (D.J. Kirwin, 1994, unpublished data) and minor adularia reported by M. Simpson and L. Zhang (1996, unpublished data) imply low sulfidation epithermal systems. Quartz veins with Pb Zn Cu sulfides but no gold occur in an ENE-trending discontinuous zone extending for a kilometer through andesite porphyry west of Kyisintaung. Fig. 4 Sketch map of northern and central Myanmar showing location of Monywa copper deposits relative to main structural features. CTFB, Chittagong-Tripura fold belt. Indian continent: FS, Foreland Spur. Indo- Burman Ranges: EB, Eastern belt; J, Jade mines uplift; WB, Western belt. Strato-volcanoes: L, Mt. Loimye; P, Popa; T, Taungthonlon. Structures: NT, Naga Thrust; SF, Sagaing Fault; SZ, subduction zone; TTU, 22°N uplift. Basins: CB, Chindwin; HB, Hukawng; PB, Pathein; SB, Shwebo; SMB, Salin (Minbu). Mesozoic inliers with magmatic arc rocks (dark grey): M, Mabein granodiorite; SA, Salingyi; SH, Shinmataung; W, B, Wuntho-Banmauk. Ma, Mandalay, Ya, Yangon. Modi- fied from Mitchell et al. (2008). Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 5
3. Exploration history and resource Copper at Myanmar was extracted from shallow underground workings and smelted long before the British occupation in 1885. Barber (1936) noted veins of malachite and chalcanthite in rhyolites at Letpadaung, and Chhibber (1934a) reported similar occurrences at Kyisintaung, his Hill 937. Until 2008, adits and shafts from c.1905 (D.J. Kirwin, 1994, unpublished data) could be seen at the base of slope at Letpadaung. The first exploration drilling was undertaken at Letpadaung, Sabetaung and Kyisintaung in 1959–60 by predecessors of the Geological Survey and Mineral Exploration Department (DGSE) and totaled 5830 m; a further 54 holes were drilled by 1972. The first ground and air- borne geophysical surveys in the area were carried out in 1969, and in 1971 a UK-Myanmar government Colombo Plan survey discovered a major induced polarization anomaly south of Sabetaung (T. Green- wood and J. H. Tooms, 1973, unpublished data). Drill- ing of this anomaly by DGSE in 1972 led to discovery of the Sabetaung South deposit. From 1972 to 1976, a Japan-Myanmar (Colombo Plan) project drilled a total of 134 wireline holes, most of them vertical and at Kyisintaung, demonstrated an economic resource at Sabetaung South, and completed a 92 m adit in Sabetaung (MMAJ-OTCA 1973, 1974a, b, Mincorp and Minproc, 1996, unpublished data). MMAJ-OTCA applied stratigraphic concepts to the pre-intrusion rocks, recognized that Kyisintaung and Fig. 5 Photographs of representa- tive rock types near and in the copper district (a) basalt pillow lava, Jurassic (?), Okkan inlier; (b) low-temperature quartz-gold veins in sandstone and rhyolite, Kyaukmyet; (c) Sabetaung South Sandstone with laminated mudstone block and mudstone rip-up clasts; (d) Mine Pyroclastics drill core, top to left, reverse-graded bed with clasts of quartz-pyrite replaced rock overlain by silty laminated tuff with spherical concretions. Fig. 6 Schematic west-east regional cross-section, Indian Shield through Indo-Burman (Indo– Myanmar) Ranges (IBR) and Wuntho-Popa arc (WPA) with Monywa copper deposits. GB, Ganges-Brahmaputra. Partly re-labelled from Steckler et al. (2008), location on Figure 1. A. H. G. Mitchell et al. 6 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
Sabetaung were epithermal deposits with mineralization in NE-trending breccia dykes, and inferred the presence of mushroom-shaped bodies of zoned quartz, alunite, and kaolinite alteration analogous to those at the Kasuga gold mine in Japan. Exploration at Letpadaung resumed from 1974–78 when a United Nations-executed project with the DGSE undertook a resistivity IP and Turam survey, identifying deep strong anomalies in the northeastern third of the hill, east of the central valley. The project drilled 66 vertical wireline holes, encountering chalcocite-covellite ore in steep breccia dykes beneath a leached cap up to 200 m thick (United Nations, 1978c) and giving a resource of 152 million tonnes at 0.70% Cu (Watts et al., 1995, unpublished data). By 1986 a further 52 vertical core holes drilled largely by DGSE brought the total holes at Letpadaung to 147, or 31,286 m. From 1978 to 1983 the Bor Copper Institute of Yugo- slavia undertook a feasibility study on Sabetaung and Kyisintaung, constructed a small commercial-scale plant, and drilled 68 holes. The government-owned No. (1) Mining Enterprise (ME-1) then developed an open pit at Sabetaung and extracted about 14.4 million tonnes of ore between 1983 and 1996, containing 138,000 tonnes copper-in-concentrate. Exploration at Sabetaung–Kyisintaung was resumed in 1994, and by 1996 Ivanhoe Myanmar Holdings had completed 143 diamond holes, 81 at Kyisintaung, 44 at Sabetaung, and 18 at Sabetaung South, bringing the total holes drilled at Sabetaung-Kyisintaung to over 400. At this time resources for Kyisintaung were 391 million tonnes at 0.31% Cu with a 0.15% cut-off. In 1998 an ME-1–Ivanhoe joint venture, the Myanmar Ivanhoe Copper Company Limited (MICCL), began open-pit mining at Sabetaung with annual production of 25,000 tonnes copper cathode from a heap leach–electro- winning process. Later a second pit was opened at Sabetaung South and in 2008 the two pits became one. In 2005, published resources at Sabetaung-Sabetaung South were 213 million tonnes @ 0.26% Cu with a 0.14% cut-off. At Letpadaung, a feasibility study was completed early in 1997 by Minproc and MRDI (1997, unpublished data) for Ivanhoe. This included diamond core drilling of 91,871 m in 304 angled exploration holes, initially on a 70 m ¥ 140 m grid with later infill drilling to give a 70 m ¥ 70 m grid in the northeastern part of Letpadaung hill. The drilling indicated a published measured, indicated and inferred resource at Let- padaung of 1478 million tonnes averaging 0.37% Cu at a 0.10% cut-off. An air-borne radiometric and magnetic survey over the Monywa district in 1997 showed that the deposits had no detectable magnetic signature. The deposits were regarded as transitional between porphyry-type and Kuroko-type by P. H. Krisl (1975, unpublished data), Goossens (1978), Bender (1983) and ESCAP (1996), and as supergene-enriched porphyry- type by United Nations (1978c) and R. H. Sillitoe (in United Nations, 1978c). Following D. J. Kirwin’s (1995, unpublished data) identification of the Monywa deposits as a deep-level example of high sulfidation epithermal mineralization, detailed petrographic work by M. Simpson et al. (1996, unpublished data), T. M. Leach (1996, unpublished data) and others confirmed the acid sulfate nature of the hydrothermal systems. P. H. Krisl’s (1975, unpublished data) interpretation may have resulted from a misunderstanding of the Japanese term “massive sulfide” for the Monywa (MMAJ- OTCA, 1974a, b) and other pyrite-rich high sulfidation epithermal deposits, also known as acid sulfate, high sulfide, pyritic, quartz-alunite, enargite-gold, and quartzite deposits. Since 2000, exploration and evaluation have included preliminary geological mapping of Letpadaung, the Sabetaung and Sabetaung South pits and Kyisintaung (Marjoribanks, R., 2004, unpublished data); drilling in 2004, 2005 and 2008 of 156 exploration and condemna- tion holes at Sabetaung and Sabetaung South for a total meterage of 17,775 m; some mineralization and alteration studies on all four deposits; and bench mapping carried out as mining advanced. Of the 1997 resource with over 7 million tonnes contained copper, MICCL have produced more than 400,000 t from ore averaging 0.56% Cu. 4. Host rocks to the copper mineralization Mineralization at Monywa is hosted (Fig. 7) by dykes and sills of porphyritic biotite andesite, quartz andesite and minor dacite, and by the folded Magyigon Formation which the porphyritic rocks intrude; rhyolite dykes are only weakly mineralized. The Magyigon in the mine area consists of the Mine Pyroclastics and the Sabetaung South Sandstone. The Sabetaung South pit is mostly in sandstone, and Sabetaung in Mine Pyroclastics and andesite porphyry intrusions; the Kyisintaung deposit is in andesite porphyry (Fig. 8) and Letpadaung in quartz andesite porphyry and Mine Pyroclastics. The Sabetaung South Sandstone is more than 250 m thick and can be correlated with the Mid-Miocene Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 7
Obogon Formation of the Minbu basin oil fields to the southwest. The Sandstone consists of felspathic quartz arenite (D. R. Mason, 2004, unpublished data) with interbedded mudstone and includes high-angle cross- bedded sandstones, sandstones with flaser bedding and ripple marks, and abundant mud-pebble conglomerates with rip-up clasts (Fig. 5c), implying an estua- rine depositional environment (Dr Thura Oo, oral commun., 2009); minor conglomerates with volcanic clasts also occur. Within the Sandstone a well-bedded basalt breccia or hyaloclastite up to 60 m thick consists of aphyric green-grey chloritised amygdaloidal pebbles and grains with a calcareous matrix and calcite veinlets. On its northwestern side the sandstone is cut by a mineralized breccia zone which separates the sandstone from Mine Pyroclastics and rhyolites to the northeast. The Mine Pyroclastics at Sabetaung–Sabetaung South (Fig. 9a) consist of a lower structureless unit, overlain by about 60 m of mostly fine-grained well-bedded tuffs with distinctive concentrically zoned spherical concretions (Fig. 5d), the pisolites of MMAJ-OTCA (1974a, b). The tuffs dip NW at up to 60° and pass up into layered but poorly-sorted coarse andesitic volcaniclastics with high-angle cross-bedding locally forming large-scale channel-fill deposits. Disrupted mudstone dykes occur and tuffisite water escape structures are common. Pebble and cobble clasts of andesite porphyry, totally replaced by fine-grained quartz with minor sericite and up to 30% pyrite, are concentrated in layers and help to define bedding in the coarser beds (Fig. 5d). The clasts, mostly angular but including some well- rounded pebbles, were termed “mineralization of the first period” by MMA-OTCAJ (1973, 1974a). At Fig. 7 Schematic stratigraphic column, Monywa copper deposits, succession under Sabetaung South Sandstone is inferred from regional geology west and south of copper district. A. H. G. Mitchell et al. 8 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
Letpadaung the Mine Pyroclastics are most abundant in the northeast and are locally intersected down to 130 m sub-surface where R. Majoribanks (2004, unpublished data) described arkosic grits of pyroclastic origin. Thin concretion-bearing pyroclastic beds within sandstones and mudstones up to 10 km from the mine resemble those in the Mine Pyroclastics. Andesite and quartz-andesite porphyries are the predominant intrusive rocks, occupying much of the Sabetaung pit (Figs 8, 9b), and Kyisintaung hill; dacites predominate at Letpadaung (M. Sheehan, 2007, unpublished data). Most intrusions are coarsely plagiophyric with biotite, lesser hornblende and minor pyroxene phenocrysts, and rod-like rutile crystals; plagioclase phenocrysts are zoned, often crowded, poorly sorted and up to 12 mm long. At Sabetaung xenoliths of microdiorite are widespread; local sandstone, aphyric andesite and rare diorite xenoliths also occur. Rafts or screens of sandstone within andesite porphyry are present in drill core from Letpadaung (United Nations, Fig. 8 Simplified bedrock geology map of the Sabetaung-Kyisintaung area, location on Figure 2. From outcrop and drilling through Kangon Sands cover, white where no bedrock data. Hill contour intervals 25 m, lowest bench level in combined Sabetaung-Sabetaung South pit is 405 m RL. Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 9
1978c) and Kyisintaung (F. B. Lazo, 2006, unpublished data). The andesite porphyries occur as tabular dykes up to 10 m wide, larger bodies are less regular (Fig. 9c) with variable texture. In northwest Sabetaung South fine-grained andesite porphyries with sparse phenocrysts intrude hornfelsed sandstone and are cut by coarsely porphyritic dykes. At Letpadaung R. Marjor- ibanks (2004, unpublished data) has shown that thick sills in pyroclastics can best explain earlier observa- tions (in M. Sheehan, 2007, unpublished data) on the porphyry-pyroclastic relationships. A 20 m thick andesite porphyry sill intrudes pyroclastics northeast of the Sabetaung pit. Andesite porphyries also form Kyadwintaung hill southwest of Kyisintaung (Fig. 8), and the upper part of Taungkamauk (775 m RL) where limonite-veined andesite porphyry is probably a sill or dome more than 200 m thick. Andesite porphyry with only supergene kaolinite alteration intrudes the sedimentary rocks 6 km west of Letpadaung (Fig. 2), but is absent elsewhere in the Wuntho-Popa arc. Flow-banded biotite rhyolites (Fig. 9d) with conspicuous vertical joints and a rhyolite breccia at their margin intrude Mine Pyroclastics in the saddle between Sabetaung and Sabetaung South, locally intrude mineralized breccia, and are cut by rare breccia dykes and sulfide fracture fills. Irregular dacite dykes (Fig. 9e) occur in the same area. Rhyolites form a prob- able sill tens of meters thick around 735 m RL on the southern side of Kyisintaung. A 5.8 Ma K/Ar age was reported by MMAJ-OTCA (1974a, b) on an andesite porphyry dyke from Kyis- intaung. More recently, Dr S. L. Chen (written comm., 2009) obtained a mid Miocene U-Pb zircon age of 13.5 Ϯ 0.2 Ma on a dacite dyke intruding the Mine Pyroclastics (Fig. 9e) in Sabetaung South. Kyaw Win and D. J. Kirwin (1998, unpublished data) reported K/Ar ages on sericite of 13 Ma at Kyisin- taung and 19 Ma at Letpadaung. The 13 Ma alteration age is compatible with the 13.5 Ma determination on the dacite dyke. Fig. 9 Photographs showing ore host rocks and alteration. (a) Mine Pyroclastics (brown) dipping northwest and faulted against megabreccia and andesite porphyry (grey) with Sabetaung South Sandstone in foreground, looking west; (b) chlorite-epidote altered andesite porphyry, 545 m RL, eastern Sabetaung; (c) andesite porphyry dyke (light grey) in sandstone with chalcocite vein (right), all argillised, 535 m RL, Sabetaung South; (d) banded rhyolite dyke in contact with andesite breccia, pit saddle area; (e) chlorite-altered dacite dyke (dark grey) intruding Mine Pyro- clastics, 545 m RL, western Sabe- taung South; (f) ASTER image Monywa copper district, showing alunite (red) and kaolinite (green); K, Kyisintaung; L, Letpadaung; S, Sabetaung and Sabetaung South deposits; Mo, Monywa town (from Earthscan, 2006, unpublished data). A. H. G. Mitchell et al. 10 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
5. Hydrothermal alteration Our understanding of alteration at Monywa is based on recent pit mapping together with, and partly controlled, by results of mid-1990s XRD petrographic and mineralogical work on drill core carried out during feasibility studies, on subsequent petrographic studies, and on XRD and spectral analyses on individual deposits by consultants and research workers including S. Pontual (2001, unpublished data), Maung Maung Naing (2003, unpublished data) and M. Sheehan (2007, unpublished data). In the combined pit (Fig. 10) four main hypogene alteration zones or assemblages are mapped. From oldest to youngest these are a widespread outer chlorite, a localised specular haematite, a quartz-white mica-pyrite or phyllic, and a quartz-pyrite or quartz- pyrite-alunite zone also local in occurrence. Let- padaung and Kyisintaung have more widespread replacement quartz and alunite and less sericite than Sabetaung and Sabetaung South. An Advanced Space- borne Thermal Emission and Reflectance Radiometer (ASTER) satellite image of the Monywa area (Earth- scan, 2006, unpublished data) shows much stronger Fig. 10 Sketch map showing alteration zones, Sabetaung- Sabetaung South, location on Figure 8. Data outside pit are largely from recent drill holes shown by circles; chl, chlorite; hm, haematite; mag, magnetite; py, pyrite; qtz, quartz; ser, sericite. H, high grade zone; WWF, West Wall fault; XF, Cross-fault. Red on blue in Sabetaung South indicates zone of mega-breccia and quartz-replaced sandstone. Red line in southwestern Sabe- taung is quartz-pyrite replacement “vein”. Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 11
alunite alteration at Letpadaung and Kyisintaung than at Sabetaung and Sabetaung South (Fig. 9f). 5.1 Sabetaung and Sabetaung South A major feature of the alteration at Sabetaung and to a lesser extent Sabetaung South is a supergene kaolinite or argillic “blanket” (Fig. 10) which obscures the nature of the hypogene alteration in much of the upper and middle levels of the pit. Clay veins in the northeastern wall at Sabetaung are probably dickite. Chlorite alteration affects andesite porphyry, pyroclastics and basalt breccia in the upper levels of the pit and beyond the pit rim for distances locally exceeding 500 m, and to drilled depths around the rim of at least 200 m. The chlorite mostly replaces groundmass, biotite and hornblende and less commonly cores of plagioclase phenocrysts. It is sometimes accompanied by calcite or galena veinlets and by specular haematite either in veinlets or replacing biotite. Sericite or illite often occurs with chlorite (D. R. Mason, 2004, unpublished data), commonly as rims to chloritised cores of feldspar phenocrysts. Epidote is found only in a small area between the pits, as grains in the andesite porphyry groundmass accompanied by chlorite (Fig. 9b). In most chloritised andesite porphyry, phenocryst biotite is unaltered, brown-pink and lusterless, or replaced by white mica. Chloritised rocks are grey when fresh but rapidly develop a greenish black coating on exposure. Chlorite-altered andesite porphyry with disseminated magnetite forms a competent rock; chloritised porphyry with pyrite is usually argillised. Both host scattered veins of pyrite with or without chalcocite and a silicified and pyritic selvedge best seen where oxidized. Specular haematite is present in two main areas at depth in the Sabetaung pit (Fig. 10), surrounded by phyllic alteration and in the northwest of Sabetaung South. It occurs as grains disseminated in the andesite porphyry groundmass, replacing biotite, and partly replacing plagioclase phenocrysts; mined surfaces have a dark purple colour. The haematite is accompanied by magnetite with or without sericite or chlorite, but with less replacement quartz than in the phyllic alteration; pyrite can occur as sparse grains or veinlets. A zone of specular haematite-chlorite alteration on the 525 m RL in the south of Sabetaung hosts high-grade veins, and in a few drill holes around the combined pit rim specular haematite accompanies chlorite. Quartz-sericite-pyrite or phyllic alteration predominates below about 470 m in Sabetaung and in the deepest part of Sabetaung South. In Sabetaung, where host rocks are andesite porphyry and pyroclastics, feldspar phenocrysts are replaced by sericite and by either pyrophyllite or supergene kaolinite, biotite is altered to brown, pink or yellow mica and the groundmass to quartz and minor clay and sericite. Pyrite occurs as coarse ragged and very fine dissemination in groundmass, as replacement grains in biotite and as stockwork veinlets, and increases with depth where it exceeds 5% of the rock. Destruction of porphyry texture varies from near-total to negligible on a meter scale. In sandstone at Sabetaung South, sericite comprises 20 to 50% of the rock, replacing lithic fragments, some crystals and groundmass (D.R. Mason, 2004, unpublished data). In much of the Sabetaung pit wall, phyllic alteration passes upwards and outwards with disappearance of visible sericite, and decrease in quartz and pyrite, into kaolinised rocks. A quartz-sericite-pyrite-alunite assemblage is widespread within the phyllic alteration at Sabetaung. Petrographic work on a drill core here (M. Simpson et al., 1996, unpublished data) showed replacement quartz and sericite throughout the 200 m core, with sporadic diaspore and rare alunite. In more recent advanced spectral determination (ASD) results on 100 samples from the 455 m RL at Sabetaung, white mica is the most abundant mineral in all samples, with rare alunite, kaolinite and pyrophyllite and minor diaspore. However, among 59 ASD samples on the 435 m RL bench, bladed alunite is the most common mineral together with illite or sericite. Pit mapping shows that while some alunite replaces feldspars adjacent to veins, most occurs within veins and breccia dykes, suggesting that the bladed alunite in bench wall samples occurs in hydrothermal veinlets within sericite alteration. Quartz-pyrite together with quartz-alunite-pyrite replacement of andesite porphyry and pyroclastics form a yellow to black irregular carrot-shaped body up to 40 m wide at the top (Fig. 11), and a 2-m wide replacement vein, both in the Sabetaung pit. A granular mass of replacement quartz and pyrite is cut by breccia dykes with a quartz-pyrite mosaic matrix, by pyritic veins with alunite and barite in vugs, by veins of massive pyrite, and by veins or dykes of fine-grained quartz-pyrite. Alunite also replaces some rock fragments and feldspar phenocrysts. In these bodies copper content is mostly below 0.05%, gold up to 0.3 ppm and pyrite can comprise over 30% of the rock. However, quartz-pyrite-replaced stockworked sandstone hosts some of the mineralization in Sabetaung South (Fig. 12). A. H. G. Mitchell et al. 12 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
5.2 Letpadaung and Kyisintaung Data on alteration in Letpadaung is almost entirely from drill core within the ore body beneath the northeastern half of Letpadaung hill. Here M. Simpson et al. (1996, unpublished data) reported results of XRD, petrographic and mineralogic work on 121 samples from 17 angled drill holes over vertical intervals of up to 400 m. All samples are from quartz andesite porphyry or pyroclastics and have a groundmass replaced by anhedral quartz. Tabular or bladed alunite is present in almost all samples, diaspore is uncommon; pyrophyllite with alunite occurs in a third of the samples. In three of the longer (over 400 m) drill cores, alunite is present in all samples in and below the leached caps, and sericite or sericite-illite accompany alunite in half of the samples. More than 422 short-wavelength infrared spectrom- eter (SWIR) measurements on 5100 m of Letpadaung core (S. Pontual, 2001, unpublished data) demonstrated the predominance of alunite and pyrophyllite with major variations in their relative abundance. M. Sheehan (2007, unpublished data) noted that at Let- padaung alunite is most abundant in hydrothermal breccias, where it replaces phenocrysts, occurs in blades up to 1 cm across, and is surrounded by illite, and that vuggy silica is very scarce. There is no obvious systematic variation in alteration assemblages with depth. From their petrographic work on drill core, largely from Letpadaung, M. Simpson et al. (1996, unpublished Fig. 11 Cross-section, Sabetaung pit, selected drill holes shown, location on Figure 10. Veins and fracture fills are schematic, breccia dykes shown only where recorded in drill core. RL is relative level in meters. Fig. 12 Cross-section, Sabetaung South pit, location on Figure 10. RL is relative level in meters. Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 13
data) and T. M. Leach (1996, unpublished data) inferred initial leaching of most components other than silica, and recrystallization as quartz of magmatic silica in the porphyry groundmass. This was followed by deposition in sequence of quartz, rutile, diaspore, alunite and sulfides; pyrophyllite; and late kaolinite; most sericite was later than alunite and sulfides. M. Simpson et al. (1996, unpublished data) visualized a deposit-scale zoning, with a core of quartz-alunite-diaspore, best developed in breccias, grading out to marginal zones where pyrophyllite fills phenocryst sites and floods the groundmass; beyond this were sericite-illite and rare chlorite. R. Marjoribanks (2004, unpublished data) identified a massive silica-pyrite ore zone from drill core beneath part of Letpadaung (Fig. 13); M. Simpson et al.’s work suggests this zone includes advanced argillic minerals. At Kyisintaung, M. Simpson et al. (1996, unpublished data) described alteration from two drill cores as early replacement quartz associated with tabular alunite, usually with later pyrophyllite, and up to 15% pyrite; sericite is absent. ASD determinations on 121 samples from the same drill holes (Ivanhoe Mines, written comm., 2006) showed alunite as the most abundant mineral in 81 samples and pyrophyllite in 25; in most samples these are the two predominant minerals. A few samples have predominant kaolinite, diaspore or paragonite. A quartz-pyrite replacement body similar to those in Sabetaung is exposed on the western side of Kyisintaung. In general Kyisintaung resembles Letpadaung rather than Sabetaung in the abundance of tabular alunite, very little sericite and absence of reported specular haematite. Fig. 13 Northeastern Letpadaung showing approximate area of grid drilling. (a) geological interpretation map; (b) interpreted northwest-southeast cross- section, RL is relative level in meters, from R. Marjoribanks (2004, unpublished data). A. H. G. Mitchell et al. 14 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
5.3 Alteration zones related to mineralization Alteration associated with mineralized structures is best documented at Letpadaung where S. Pontual (2001, unpublished data) and M. Sheehan (2007, unpublished data) found alunite and pyrophyllite to be most abundant in or close to breccia dykes. In Sabe- taung, alteration zones related to mineralization are seen only at shallow depths, where individual breccia dykes or thick veins in andesite porphyry are surrounded by a bleached supergene argillic zone up to a few meters wide passing outwards into chlorite. On a larger scale, the 130-m wide sheeted vein zone in mostly phyllic alteration at Sabetaung is bordered at upper levels by chlorite alteration. 6. Mineralization styles Copper sulfides are largely confined to planar structures, both breccia dykes and veins (Figs 11, 12). Their geometry is known in most detail from Sabetaung and Sabetaung South, although more petrographic and mineralogical data are available on Letpadaung. The breccia dykes and veins occur in all lithologies, but are rare in the rhyolites, pyroclastics and andesites in the saddle between Sabetaung and Sabetaung South. In Sabetaung South the most productive ore body is a sandstone stockwork and breccia zone. Ore grade gen- erally decreases with depth from a maximum immediately beneath the water table or leached cap, but drill intercepts above 1% Cu occur in the deepest drill intercepts in Sabetaung (170 m RL) and Letpadaung (50 m RL). In overall shape each of the ore bodies resembles an upward-flaring funnel or inverted cone. 6.1 Breccia dykes Breccia dykes are widespread in all four deposits, although forming not more than 2 or 3% of total rock volume. Most are steeply-dipping parallel-sided tabular bodies (Fig. 14a, b) with sharp planar margins but a few bifurcate upwards or are deflected along a low-angle structure for a few meters. Some continue vertically for over 60 m horizontally and vertically in pit walls (Fig. 14c), but others can be discontinuous, and pass up or down into a vertical fracture. In Sabe- taung widths of breccia dykes are up to 2 m in upper and outer parts of the pit but mostly less than 20 cm near the pit bottom (Fig. 14b). No bench wall data are available from upper levels above the pit centres, but we infer an overall vertical rather than lateral variation in dyke width. At Letpadaung estimated true widths of breccia dykes in drill core can exceed 5 m; an early, 1996 map of Letpadaung shows sinuous breccia dykes over 20 m wide but these widths are conceptual. In Sabetaung pit walls non-tabular intrusive breccias are rare. Clasts are up to cobble or rarely block size and many are from the immediate wall rock (Fig. 14a). Most andesite porphyry clasts are angular and retain a porphyry texture (Fig. 14d), but all breccia dykes, even those with jigsaw-fit texture, have few to abundant well-rounded clasts replaced by finely crystalline quartz and pyrite with or without sericite, and with pyritic fractures. Angular to rounded pebbles, cobbles or blocks of fine or coarse-grained massive pyrite also occur (Fig. 14d, e). Breccia dykes have either a mud matrix or variably silicified sulfide matrix. Mud (or argillised rock flour) matrix dykes are most abundant in Sabetaung South Sandstone and Mine Pyroclastics, and are probably phreato-magmatic and hence diatremes. Some sulfide- matrix breccia dykes are probably entirely hydrothermal and were emplaced along fractures, but we believe others, particularly the wider and more continuous dykes, resulted from ascent of hydrothermal fluid within earlier mud-matrix dykes. Most sulfide-matrix breccias appear to be clast-supported with a matrix of pyrite, or pyrite plus digenite-chalcocite and covellite. Where supergene kaolinite alteration is intense, above about 500 m RL at Sabetaung, the sulfide matrix becomes a sulfidic clay. 6.2 Veins and fracture fills In Sabetaung-Sabetaung South sulfide veins, which include fracture fills, are planar structures with widths which on average are greater at shallow depths; where veins and breccia dykes intersect, the breccia dykes are usually, but not invariably, later. Thick shallow veins usually dip steeply, are up to 2 m wide and may decrease dramatically in width within a few tens of meters of the base of oxidation; black sooty chalcocite renders them highly conspicuous. Those with abundant pyrite often include a central sulfide-matrix hydrothermal breccia. Spaced stockwork veins are intersecting mineralized fractures, randomly oriented in three dimensions, and up to tens of centimeters apart. Sheeted veins are close-spaced parallel structures with vertical or steep easterly dips, each up to a few centimeters wide; they are largely confined to and visually dominate a 130-m wide zone in Sabetaung (Figs 11, 15a) where they trend N 40°E parallel to the Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 15
long axis of the pit. The sheeted veins are mostly well- mineralized but in the west wall within the phyllic zone consist almost entirely of pyrite and have distinct silicified selvedges up to several centimeters wide. Through-going low-angle veins dip at less than 40°, can persist for over 50 m and are mostly only a few centimeters wide. A typical bench wall shows a spaced stockwork of planar fracture fills, each at least a few meters in length, and sparse low-angle veins. At Let- padaung, M. Sheehan (2007, unpublished data) reported that vein widths are up to 1.5 m but that most are less than 10 cm; M. Simpson et al. (1996, unpublished data) noted that some breccia dykes at Let- padaung pass into veins. 6.3 Mega-breccia and stockworked sandstone in Sabetaung South The mega-breccia is a NNE-trending zone more than 500 m long and includes part of the main ore body in Sabetaung South (Fig. 8). It has sharp but irregular margins, and is characterized by blocks of wall rock in a weakly lithified to friable clay matrix. In the southwest wall of the pit the mega-breccia is more than 20 m wide with meter-scale blocks of silicified pyritic sandstone and rare slabs of carbonized wood. To the northeast the width decreases to a few m and the breccia lies between stockworked sandstone and Mine Pyroclas- tics or rhyolite. Here cobble clasts are largely of Fig. 14 Photographs showing breccia dykes and veins. (a) 80 cm mud-matrix breccia dyke in argillised sandstone, east side Sabetaung South; (b) Sabetaung 415 m RL, sheeted vein zone with breccia dyke unusually wide for this level—clasts and matrix are pyritic and silicified, below 0.1% Cu; (c) NW-trending 1.2 m breccia dyke (dark grey, right of figure) with chalcocite-pyrite matrix in argillised sandstone and andesite porphyry, above 535 m RL, southern Sabetaung South; (d) close-up of sulfide-matrix breccia in (c) showing clasts of argillised andesite, sandstone and rare rounded phyllic altered porphyry in pyrite-chalcocite-covellite matrix; (e) breccia dyke, Letpadaung core, with angular quartz-replaced andesite porphyry and sericitised fragments in hypogene crystalline covellite matrix. A. H. G. Mitchell et al. 16 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
silicified pyritic sandstone, silicified andesite porphyry and rare mica schist; rhyolite blocks occur adjacent to rhyolite wall rock and there are scattered rafts of black sulfidic ore. 6.4 Disseminated chalcocite and the high-grade zone in Sabetaung Disseminated copper sulfides are almost entirely of chalcocite and economically significant amounts are mostly shallow, extending downwards for a few tens of meters from the base of oxidation. Here chalcocite replaces pyrite or marcasite which itself replaces phenocrysts. Disseminated chalcocite also occurs in or around the currently inaccessible high-grade zone in the east wall of Sabetaung (Mining Geology Section, 2005, unpublished data). The high-grade zone is a vertical pipe-like body about 2000 m2 in area with an average grade of 15% Cu on the 495 m RL (Figs 8, 11) decreasing to around 2% Cu in drill core at 270 m RL. It consists of sulfide-matrix breccia dykes with chalcocite-covellite veins. The andesite porphyry host rock at 455 m RL is black and friable with phenocrysts replaced by chalcocite, probably after pyrite; supergene leaching has transformed part of this to a grey “residual” silica sand. 7. Copper sulfides, associated minerals and metal values 7.1 Minerals in breccias and veins In early mineralogic work on United Nations drill core from Letpadaung, Win Htein (1978, unpublished data) reported that enargite formed coarse aggregates and replaced chalcopyrite and pyrite, and that neodigenite and covellite successively replaced enargite, the Fig. 15 Photographs showing vein mineralization. (a) steep sheeted sulfide veins in quartz-alunite-sericite altered andesite porphyry, stockwork veinlets not visible in photograph, 465 m RL, northeastern Sabetaung; (b) hexagonal blades of pale translucent pink alunite overgrown by enargite, Letpadaung (from M. Simpson et al., 1996, unpublished data); (c) fracture fill showing chalcocite, minor covellite, pyrite and barite, Sabetaung 415 m RL; (d) banded cm wide veins of pyrite with covellite centres (blue) alternating with anhedral quartz, Sabetaung 415 m RL. Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 17
neodigenite retaining the optical features of enargite. R. H. Sillitoe (in United Nations, 1978c) found that most of the copper at Letpadaung occurred as chalcocite, but that the main hypogene copper sulfides were enargite and covellite, in ore with over 10% pyrite. However, among 25 Letpadaung samples with copper sulfides, D. J. Kirwin (1994, unpublished data) reported enargite in only two, suggesting possible over- estimation by earlier workers. Metcon Research (1995, unpublished data) showed that chalcocite replaces pyrite, chalcopyrite and bornite at Letpadaung, and that digenite and covellite can replace all these minerals. From mineralogical studies on drill core from Sabe- taung, Kyisintaung and Letpadaung, E. Iasillo (1995, unpublished data) argued that all chalcocite, whether sooty and coating pyrite, or grey and orthorhombic, and also covellite, are supergene, and that chalcocite replaces enargite and chalcopyrite. K. Wenrich (1996, unpublished data) suggested that most chalcocite at Letpadaung might be djurleite, and also identified spionkopite and yarrowite. E. Iasillo (1995, unpublished data) and Metcon Research (1995, unpublished data) reported traces of molybdenite in Kyisintaung core. The most comprehensive mineralogic study, by M. Simpson et al. (1996, unpublished data), includes description of 34 polished thin sections of drill core in which pyrite, commonly forming 6 to 15% of the section, is accompanied by more than a trace of copper sulfides. Twenty-seven of the samples are from Let- padaung, where chalcocite occurs in fifteen, digenite in seven, and digenite-chalcocite, mostly very minor, in eight; enargite or less common luzonite is present in eight samples, replacing pyrite, covellite and alunite (Fig. 15b), and chalcopyrite overgrowing pyrite occurs in three. In M. Simpson et al.’s (1996, unpublished data) few samples from Kyisintaung with copper sulfides, three have up to 6% chalcocite, accompanied by enargite or covellite. Of their six samples from Sabetaung- Sabetaung South, four contain chalcopyrite and bornite totaling 5 to 10%, partly replaced by a few percent of digenite and chalcocite; three of these samples are from one drill core, suggesting intercepts with a single unusually chalcopyrite-rich structure. Among all 34 of their polished sections, M. Simpson et al. (1996, unpublished data) reported hypogene chalcocite and digenite in only one, where they occur as coarse blades replacing enargite. In the 33 other samples supergene chalcocite or digenite partly replace all sulfides including pyrite and covellite, and mostly very minor chalcopyrite, bornite, and tennantite. In two of these samples supergene chalcocite comprises 35% of the section, replacing enargite, the only primary copper sulfide. Coarse covellite in two samples is regarded as hypogene; this and other covellite over- grows pyrite, a relationship said to be uncommon in deposits elsewhere (Chavez, 2000). Chalcopyrite, bornite, tennantite and pyrrhotite occur as inclusions within pyrite. M. Sheehan (2007, unpublished data) found that in Letpadaung veins, covellite is the main hypogene copper mineral and that chalcocite or covellite is either intergrown with quartz and pyrite or occurs in vein centres bordered by pyrite, and is later than alunite; some pyrite may replace pyrrhotite. He reported little enargite. Chalcocite and covellite in breccia matrix and vugs in Letpadaung are often coarsely crystalline with very coarse pyrite, and the covellite-chalcocite ratio shows some increase with depth. In the combined Sabetaung–Sabetaung South pit walls sulfide matrix breccia dykes and veins have similar gross features. Anhedral to euhedral coarse to fine-grained pyrite usually predominates, forming vein margins and crusts or rims on rock clasts in breccias. At Sabetaung the pyrite is coated or crusted with chalcocite, either steel grey and crystalline or soft, black and “sooty”. Spaces between clasts may be partly filled with alunite, pyrophyllite, and barite. Covellite is very minor relative to chalcocite-digenite. In bench walls preferred breakage along the structurally weak centre line of fracture fills exposes extensive surfaces of pyrite, grey or sooty chalcocite, or rarely covellite. In Sabetaung, barite occurs at all levels as elongate euhedral crystals (Fig. 15c) within fracture fills and as crystals up to 8-cm long in vugs within breccia. Rarely fine-grained marcasite replaces coarse crystalline covellite. Alunite in veins and breccia is mostly tabular but cubic alunite within vugs can be seen in hand specimens from the 415 m RL. The sheeted vein zone near this level includes rare compound veins in which centimeter-wide bands of anhedral grey quartz alternate with thinner pyrite-covellite-alunite veins (Fig. 15d), but in general quartz veins are absent in the Monywa deposits. In the main ore zone in Sabetaung South, chalcocite and pyrite occur as fracture-fills cutting through the breccia and forming stockwork in sandstone and as pods of massive steel grey crystalline chalcocite with enargite and dispersed coarse pyrite. Chalcocite and pyrite also occur in irregular vugs together with A. H. G. Mitchell et al. 18 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
enargite, minor covellite, and barite, alunite and pyrophyllite. Marcasite occurs as later veinlets. 7.2 Mineral paragenesis M. Simpson et al. (1996, unpublished data) defined the mineralization sequence in veins and breccia as pyrite in veins and dissemination, overgrown by chalcopyrite, itself overgrown by bornite, tenanntite and covellite; enargite; coarsely crystalline digenite and chalcocite. They noted that mineralization followed formation of replacement quartz and quartz–alunite, and preceded sequential formation of sericite, pyrophyllite, barite, and kaolinite; they emphasized the overgrowth of pink alunite by enargite (Fig. 15b), a relationship common in high sulfidation deposits (e.g. Muntean & Einaudi, 2001). Maung Maung Naing (2003, unpublished data) suggested that digenite, chalcocite and enargite in veins and vugs were coeval. 7.3 Metal values other than copper Gold determinations are largely from Letpadaung where in 100 drill core samples United Nations (1978c) reported a maximum value of 0.5 ppm with most samples below 0.03 ppm Au; silver averaged below 1 ppm. Ivanhoe reported average values of 0.024 ppm Au and 0.58 ppm Ag in more than 400 core samples, all from Letpadaung. A maximum gold value of 0.51 ppm and average of 0.185 ppm Au were reported from 28 core samples from Kyisintaung; silver at Kyisintaung averaged 27 ppm (Maung Maung Naing, 2003, unpublished data). Assays on nine samples from the quartz- pyrite body at Sabetaung gave a maximum gold value of 0.31 ppm, average 0.08 ppm Au and 3.8 ppm Ag. Among other metals at Letpadaung, a background As value of 185 ppm in soil samples was reported by United Nations (1978c) and D. J. Kirwin (1994, unpublished data) obtained average values of 100 ppm As on 25 core samples. Arsenic in 28 drill core samples from Kyisintaung in 1995 averaged 85 ppm; the same samples assayed 4 ppm Mo. Barium is strongly anoma- lous in these 28 samples, averages 680 ppm Ba, and correlates with copper; three samples from Sabetaung averaged 1800 ppm Ba. In 67 samples from a Let- padaung drill core, zinc ranged from 13 to 470 ppm, increasing with depth; the average Pb value in D. J. Kirwin’s (1994, unpublished data) 25 core samples was 100 ppm. Many early multi-element determination are from around the oxide-sulfide boundary at Sabetaung (Thet Lwin and Hla Win Zaw, 1993, unpublished data) and are omitted here. 8. Structural controls on mineralization 8.1 Dyke trends and mineralization Breccia dykes in Sabetaung mostly trend approxi- mately N 40°E, parallel to the sheeted veins (Fig. 15a), but those in the southwest of the pit and some in the northwest wall trend northwesterly. In Sabetaung South most breccia dykes, and almost all those in the southeast wall, trend northwest (e.g. Fig. 14c). In any one area almost all breccia dykes have roughly the same trend, but high-angle intersections do occur. Most andesite porphyry and rhyolite dykes have the same orientation as breccia dykes in the same area. At Kyisintaung breccia dykes and veins reportedly trend N 35°E and N 15°E (MinCorp and Minproc, 1996, unpublished data); results of later drilling confirmed a broadly NE trend (F.B. Lazo et al., 2006, unpublished data). Surface mapping and drill intercepts in Let- padaung indicated slightly sinuous predominantly northeasterly trends to breccia dykes (Fig. 16) but addi- tional preferred directions were also identified (Minproc Limited, 1998, unpublished data). 8.2 Folding, faulting and mineralization The Sabetaung South Sandstone, exposed in a low- angle NE-trending anticline, is part of the Magyigon Formation which outside the mine area shows upright folds with local steep limbs. Within the deposits, Fig. 16 Conceptual 1996 map of Letpadaung showing mineralized veins and post-mineral porphyry intrusions. Line north of Nache Taung is motor road. Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 19
breccia dykes and magmatic dykes with few excep- tions are within 10° of vertical, even where host rock bedding is inclined at 30° or more. We therefore infer that folding of the Magyigon Formation either accompanied or preceded mineralization. Faults can be defined only from lithological offsets although NE-trending faults have been inferred from drilling along the northwest and southeast margins of the Kyisintaung andesite porphyry (F. B. Lazo, 2006, unpublished data). In Sabetaung South the West Wall Fault or reverse fault (Figs 10, 12) is almost certainly pre-mineral, but the NE-trending cross-fault which drops hanging-wall sandstone 30 m relative to foot- wall basalt breccia, post-dates mineralization. At Let- padaung, faults identified during the 1990s feasibility study include a dominant NW-trending set said to downthrow to the northeast; faults believed to control erosion of the central valley were inferred from topographic features. In northeastern Letpadaung, R. Mar- joribanks (2004, unpublished data) mapped a series of steep easterly-trending faults (Fig. 13a, b) defined by vertical displacements of up to 200 m in the contact between gently-dipping pyroclastics and an underlying andesite porphyry sill. R. H. Sillitoe (in United Nations, 1978c) considered that the mineralization at Monywa took place in a strato-volcano, while Bender (1983) favoured a caldera margin. More recently, R. Marjoribanks (2004, unpublished data) inferred that at Letpadaung hydrothermal fluids ascended along the E-trending faults to the base of the pyroclastics, where they ponded and mineralized the underlying andesite porphyry during inter- mittent pressure release. However, in Sabetaung South numerous veins cut bedding in the host rocks without detectable offset, suggesting that here ascent of miner- alizing fluids may not have been focused in faults. Fault zones identified from breccia and clay intercepts in 2008 drill core (Geology Section, MICCL, 2009, unpublished data) are not mineralized. Steep easterly dips to most of the sheeted veins at Sabetaung (Figs 11, 15a), first reported by MinCorp and Minproc (1996, unpublished data), imply minor rotation of vertical structures and hence post-mineral relative uplift to the southeast. 8.3 Mineralization and the regional stress field Many porphyry and high sulfidation deposits form following a protracted period of arc magmatism, suggesting that the mineralization is dependent on the state of stress or change of stress within the arc. Most arcs are under extensional stress through pull of the subducting oceanic plate, although compression can accompany either oceanward advance of the overrid- ing plate and decrease in slab dip, or collision commonly with a submarine ridge. Takada (1994) and Richards (2005) argued that compression causes magma ponding and development of base of crust sills, and that with stress relaxation, magma ascends to form upper crustal chambers from which volatile and metal- rich magma rises as stocks or dykes. For giant Cu-Mo breccia deposits in Central Chile, Stern and Skewes (2005) infer compressive stress during mineralization; this inhibits dyke formation in the upper brittle crust, allowing brines and vapour exsolved from shallow magma chambers to deposit sulfides as they ascend and cool, rather than being carried to the surface and lost from erupting magma. In the combined Sabetaung–Sabetaung South pit the northwesterly trend of many of the larger breccia dykes and adjacent andesite porphyry dykes implies a least compressional principal stress oriented northeast. However, a northwesterly minimum compressive stress is implied by the N 40°E trending sheeted veins and associated breccia dykes in Sabetaung. The N 40°E structures could be explained if western Myanmar during mineralization were a mega-shear, bounded by the dextral Sagaing Fault in the east and a parallel fault in the Bengal basin, provided that shearing was inter- rupted to allow development of the northwesterly- trending structures. On a regional scale, NNW-trending thrusts in the Minbu basin southwest of Monywa have been related to the collision of the Burma plate with India in the Naga Hills (Pivnik et al., 1998).Alternatively, the thrusts, and folding of the Pegu Group including the Mine Pyroclastics, could speculatively be explained by a decrease in dip of the subduction zone landward of the NNW-trending segment of the trench (Fig. 1). The 22°N uplift between the Chindwin and Salin basins lies directly west of the Monywa deposits (Fig. 4), but whether the uplift and magmatism at Monywa have a common cause is unclear. 9. Post-mineral dykes and diatremes 9.1 Late- and post-mineral andesite porphyry dykes Post-mineral andesite porphyry dykes within mineralized host rocks were first reported from Letpadaung drill core (United Nations, 1978c, R. H. Sillitoe, in United Nations, 1978c), where chloritised or fresh porphyry intrusions with lustrous biotite phenocrysts A. H. G. Mitchell et al. 20 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
were regarded as products of a distinct magmatic event later than mineralization (R. Marjoribanks, 2004, unpublished data) and hence as diluents of ore. Andes- ite porphyry dykes with unaltered biotite, kaolinised margins and chloritised cores intruding limonitic and locally alunite-altered rhyolite on upper benches of the leached cap at Kyisintaung have also been regarded as post-mineral. At Sabetaung and Sabetaung South unaltered or weakly altered andesite porphyry dykes are absent in the deeper parts of the pits which are mostly mineralized and pass upwards and outwards into chloritised andesite porphyry with fresh biotite phenocrysts. Very rarely, mineralized fracture fills terminate against a dyke which like its host rocks shows pervasive phyllic alteration. In the upper and outer argillised levels of northwestern Sabetaung, pyritic andesite porphyry bodies with bleached margins occur within more sulfidic dark grey porphyry, but are cut by mineralized breccia dykes and their boundaries with sulfidic rocks are transitional, not chilled margins (Fig. 17a). Evi- dence for post-mineral porphyry is seen only at the northwest wall of Sabetaung South where an unaltered andesite dyke intrudes mineralized breccia. 9.2 “Diatreme” in Sabetaung South Intrusive breccia dykes or diatremes younger than mineralization have been inferred or at least suspected from surface and pit mapping at Monywa since at least the mid 1990s, although none have been identified in any of the core drilling. R. Marjoribanks (2004, unpublished data) mapped a post-mineral diatreme, a NE-trending body with highly irregular margins, in the then partly covered northwest wall of Sabetaung South. New bench cuts through Marjoribank’s diatreme, southwest of the rhyolites (Fig. 8), show stratified Mine Pyroclastics; the mineralized mega- breccia zone (Section 6.3); and a body of brecciated pyroclastics which could be post-mineral. Fig. 17 Photographs of supergene zone, leached cap and Kangon Sands. (a) northwestern Sabetaung 515 m RL, kaolinised andesite porphyry showing bleached zone with jarositic surface and texture-destroyed margins transitional to sulfide veins and breccia and cut by breccia dyke; (b) breccia dyke in bench wall with marcasite and crystalline and sooty chalcocite within argillised andesite porphyry (light grey), chlorite alteration at right, base of oxide (above dyke) around 553 m RL, southeastern Sabetaung; (c) looking west across Sabetaung, Kyisintaung (K) leached cap in background, base of scree (sc) overlain by western limit of Kangon Sands (ks) around 580 m RL, oxide (ox) base around 570 m RL in pyroclastics and andesite porphyry dykes; alteration: ch, chlorite; ka, argillic (kaolinite); qp, quartz-pyrite; s, sericite-quartz-pyrite; (d) Sabetaung saddle looking east, Kangon Sands cycles 2 and 3 overlying oxidized chlorite-altered andesite porphyry with limonite veins after pyrite: ox, base of oxide; w, waste dump. Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 21
10. The supergene enrichment–argillic zones and leached caps In the Monywa deposits sulfide-bearing rocks are overlain abruptly by an oxidized leached cap, best pre- served at Kyisintaung and Letpadaung. The base of the cap coincides with the present water table indicating that leaching is more or less in equilibrium with uplift. Underlying mineralized structures have undergone supergene digenite-chalcocite enrichment, and their host rocks pervasive argillisation, both of which decrease with depth. 10.1 Supergene enrichment and kaolinisation Supergene enrichment has been recognized at Monywa since the 1970s and up to 75% of total copper has been regarded as supergene (e.g. M. Simpson et al., 2006, unpublished data, M. Sheehan, 2007, unpublished data). Immediately below the base of the oxides at Sabetaung South and Sabetaung, abundant chalcocite or digenite-chalcocite with or without covellite occur in both the wide steep veins (Fig. 17b) and in sulfide-matrix breccia dykes, sometimes with disseminated chalcocite in host rocks. Copper grade and widths of the mineralized structures decrease with depth from the base of oxidation, rapidly at first and then more slowly, implying that much of this copper is supergene. In the central part of the Sabetaung pit the upper levels of the enriched zone provided substantial tonnages of high-grade (2% Cu or above) ore during early mining. In addition, in many drill cores from the pit rim (Geology Section, MICCL, 2009, unpublished data) pyrite or rarely marcasite-rich veins or breccias within chlorite- altered andesite porphyry are mineralized, with grades commonly above 1% Cu, in only the first 5 to 20 m below the base of oxidation. This supergene copper was evidently leached from the same or nearby veins oxidizing at the water table. At Kyisin- taung and Letpadaung, as at Sabetaung, averaged assays on drill cores indicate maximum copper grades immediately beneath the leached caps, and at Let- padaung supergene enrichment to the depths of 530 m or 20 m RL is inferred. Copper enrichment partly through replacement of pyrite can explain the upward decrease in residual pyrite towards the base of the leached cap, at which it falls abruptly to zero. Since marcasite is replaced more readily than pyrite by supergene chalcocite (Chavez, 2000), the marcasite at shallow depths may have facilitated enrichment. The mid 1990s feasibility studies at Monywa con- cluded that black sooty chalcocite is supergene but that crystalline grey chalcocite and crystalline covellite may be hypogene. In some other deposits, for example, the Tyrone and Santa Rita porphyry copper deposits in New Mexico “steel glance” chalcocite, in addition to sooty chalcocite and to covellite, have been described as supergene (Cook & Porter, 2005). Nevertheless, the mineralogic reports on Monywa show that most copper occurs as digenite-chalcocite; that most of this is supergene and replaces enargite, hypogene and early supergene covellite and minor chalcopyrite, bornite and other copper sulfides as well as pyrite; but that significant amounts of hypogene digenite-chalcocite are also present. Copper enrichment and pyrite depletion are accompanied by strong kaolinisation (Figs 10, 17a) superim- posed on earlier hypogene alteration, resulting in structurally incompetent ore. At Sabetaung and Sabe- taung South the kaolinisation is intense and pervasive down to below 500 m RL, and characterized in andesite porphyry and pyroclastics by replacement of phenocrysts and some of the groundmass by kaolinite with or without cubic alunite, and in sandstone by kaolinisation of detrital felspar and matrix. M. Simpson et al. (1996, unpublished data) reported cubic alunite throughout the 70 m of a drill core beneath the base of oxide at Sabetaung, and Metcon Research (1995, unpublished data) noted the common replacement of sericite by kaolinite in Kyisintaung and Sabetaung core. In the upper and outer parts of the combined pit, white argillic zones up to tens of meters wide within chlorite alteration narrow downwards and are clearly supergene. At Letpadaung and Kyisintaung, argillisation is less intense than in Sabetaung and Sabetaung South, and the ratio of incompetent to competent ore is much lower. Nevertheless, at Letpadaung M. Simpson et al. (1996, unpublished data) reported abundant kaolinite in the upper 100 m of drill cores beneath the base of oxide. 10.2 Leached caps The leached cap is more than 200 m thick beneath the hill tops at Kyisintaung (Fig. 17c) and Letpadaung. At Sabetaung the pre-mine leached cap was less than 70 m thick, and in the north and east of Sabetaung it was absent (Fig. 17d). At Kyisintaung and Letpadaung, andesite porphyry or pyroclastics have been leached to a white to grey porous but erosionally resistant rock consisting of hypogene quartz, tabular alunite and A. H. G. Mitchell et al. 22 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
pyrophyllite, and of cubic alunite and kaolinite, with minor limonite on fractures and in breccia dykes, and with jarositic surface stain in benches and drill roads. Much of kaolinite and also cubic alunite replacing plagioclase probably formed in the supergene enrichment zone during uplift. The pre-mining low topographic relief and the high proportion of incompetent ore at Sabetaung–Sabetaung South suggest that these deposits had less hypogene quartz than Kyisintaung, and consequently were easily eroded by the migrating Chindwin River. Supergene chalcedonic silica may also be significant in increasing resistance to erosion. Around the combined Sabetaung pit the base of the leached cap at about 560 m RL, is mostly 20 to 25 m below the level of the plain (Fig. 17c), but around 600 m RL, locally rising to 700 m RL, beneath Kyisin- taung. In detail the base at Sabetaung-Sabetaung South is highly irregular within a 10 m vertical interval, but in most places can be defined within a few tens of centimeters. It is least distinct in drill-holes in chloritised andesite porphyry, where limonitic fractures occur in a transition zone for 10 or 15 m below totally oxidized rock (Fig. 17c). At Letpadaung the oxide base is mostly between 570 and 580 m RL under the central valley but 10 to 20 m lower beneath the hills. The upward change within high-grade chalcocite- rich breccia dykes as they pass through the water table into the overlying oxidized cap can be seen in some southeastern benches at Sabetaung. Here chalcocite, much of it supergene, is oxidized to red haematite, with local haematitic gossan at the base; red haematite continues upwards for 10 or 20 m surrounded by brown or orange jarositic limonite. Oxide copper minerals analogous to those above some porphyry copper deposits (Chavez, 2000) are confined to thin crusts at the base of the water-saturated Kangon Sands cover described below, and to old workings near the base of slope at Letpadaung. 10.3 Leaching and enrichment process The supergene leaching and enrichment process at Monywa was similar in principle to that described from enriched porphyry copper deposits (Blanchard, 1968; Anderson, 1982; Chavez, 2000), whereby residual pyrite is oxidized and supergene and residual hypogene copper sulfides go into solution as they are elevated through the water table. The highly efficient leaching at Monywa, with copper values in the leached caps mostly well below 150 ppm, reflects the high pyrite content, high pyrite : copper ratio, and the non-reactive nature of the hypogene (acid)-altered host rocks. M. Simpson et al. (1996, unpublished data) explained the argillic alteration or kaolinisation at Letpadaung by descent of steam-heated meteoric water from above the water table, during the waning stages of hydrothermal activity. However, kaolinisation must have post-dated all hypogene veining, which required brittle structures. The decrease in intensity of kaolinisation with depth below the present water table suggests that it is related to the base of oxidation and not to a palaeo- surface which was perhaps 1 km above the present surface. We infer that since the late Miocene persistent regional uplift has resulted in elevation of mineralized rocks allowing continuous leaching. Within the leached caps, the bladed alunite and pyrophyllite with goethite and jarositic veins and breccias and local haematite near the base can be explained by uplift and oxidation of mineralized rocks which have passed through the supergene enrichment zone. At the water table residual pyrite was evidently suffi- cient to generate enough acid to remove all the digenite-chalcocite and any residual copper sulfides, producing acid cupric sulfate. This replaced with chalcocite, all or most of the other copper sulphides, and some of the pyrite in the underlying supergene enrichment zone, most intensively within the upper hundred meters. The process is probably self-adjusting: given constant uplift and consumption of acid in argillisation of host rock, more hypogene digenite-chalcocite available for oxidation at the water table consumes more acid, leaving less available to effect supergene enrichment; less hypogene chalcocite at the water table consumes less acid, allowing replacement of more of the residual sulfides by chalcocite. Although residual copper sulfides include more enargite than chalcopyrite, bornite, and tennantite, arsenic values in Let- padaung core average only 100 ppm; therefore either enargite was not a major constituent of the hypogene ore, or arsenic liberated during enargite replacement by chalcocite was highly mobile. Survival of bornite, a sulfide readily replaced by chalcocite, may reflect its occurrence mostly as inclusions in pyrite. Meter-scale intercepts of quartz-replaced sponge- textured sulfide-free andesite porphyry with leached- out felspar phenocrysts occur to depths which at Letpadaung exceed 600 m. This has been explained as residual silica from hypogene acid leaching (e.g. R. Marjoribanks, 2004, unpublished data), and equated with the vuggy silica described from many high sulfidation deposits (e.g. Arribas, 1995). Alternatively, at Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 23
Letpadaung groundmass silicification may have been followed by replacement of phenocrysts by anhydrite, subsequently removed together with sulfides, by ground water. 11. Post-mineral cover and exotic copper potential Around the Sabetaung South and part of the Sabetaung pit the bedrock surface is overlain by the Kangon Sands of MMAJ-OTCA (1974a, b), a more or less horizontal formation which extends eastwards towards the Chindwin River and southwards to Letpadaung (Fig. 2). Prior to mining, the formation covered almost all of Sabetaung South and the flanks of a 50 m high hill at Sabetaung. Along the southern, eastern and northern margin of the combined pit the formation consists of two main fining-upward sedimentary cycles. The southern cycle, with a basal conglomerate around 580 m RL in the southern rim of Sabetaung South, overlies oxidized bedrock. Northwards the conglomerate cuts down to around 565 m RL and is overlain by 10 m of silt-mud alternating beds. This cycle is truncated in the eastern- most rim of Sabetaung by the northern cycle (Fig. 17d) with a quartz-pebble basal conglomerate up to 2 m thick which cuts down to the north to around 548 m RL, below the base of oxidation. The conglomerate passes up into a northward-thickening cross-bedded sand. Overlying black “cotton” soil thickens northwards but continues southwards across the southern cycle. In the northeast of Sabetaung the northern cycle is at least 40 m thick, and forms a major aquifer, in which the elevation of the water table, at around 563 m RL, is similar to the dry season level of the Chindwin River. Towards Kyisintaung the conglomeratic base rises rapidly to 580 m RL, and overlies scree which covers oxidized bedrock (Fig. 17c). The cycles are interpreted as Quaternary fluvial deposits of the migrating Chindwin River channel and flood plain. The sands and basal conglomerate of the northern cycle have a potential for exotic copper deposits, carried laterally in solution from the oxidizing sulfide surface at the topographically higher base of the leached caps. The basal conglomerate in the northeast rim of Sabetaung has an orange limonite cement, locally hosts layered crusts with the upward sequence cuprite-chrysocolla-azurite developed on underlying chalcocite ore and contains up to 1% Cu. However, here and elsewhere around the pit rim, elevated copper values in the conglomerate may be derived from early 1980s waste dumps up-slope. Unequivocal exotic copper mineralization therefore has not yet been found at Monywa. 12. Geological environment during hypogene mineralization The very few research studies carried out since large- scale mining began in 1997 permit only the most super- ficial discussion of the geological environment immediately before and during mineralization. 12.1 Mine Pyroclastics, breccia dykes and the palaeo-surface Within the Mine Pyroclastics the presence of abundant clasts of pyrite and of quartz-pyrite replaced and phyllic-altered rock indicate explosive hydrothermal as well as pyroclastic eruptions, requiring emplacement of andesitic magma below the present exposure level (Fig. 18a). The source of the pyrite and hydrothermally altered clasts lies somewhere beneath the Pyroclastics. Folding of the Sabetaung South Sandstones and Mine Pyroclastics was followed by intrusion of the sub-volcanic andesite or dacite dykes and sills present in all the deposits, and by subsequent emplacement of breccia dykes, mineralization and hydrothermal alteration (Fig. 18b). Some of the breccia dykes are diatremes, perhaps resulting from intrusion of magma into over-pressured sediments; many of these breccias formed preferred structures for ascent of hydrothermal fluids. The association of pyrophyllite with quartz implies a temperature of at least 285°C (Muntean et al., 1990) and is widespread in the ore bodies and overlying leached caps at Monywa. Assuming near- hydrostatic pressure, the boiling-depth curve implies a palaeo-water table a kilometer or more above the pyrophyllite-quartz assemblage in the leached caps. We infer that the eroded rock between the caps and palaeo-water table was similar either to the underlying Mine Pyroclastics or to clastic sedimentary rocks within which tuffs are interbedded outside the mine area. Therefore even if the breccia dykes failed to vent as diatreme and hydrothermal eruptions, it is unlikely that mineralization at Monywa took place within or beneath a strato-volcano. Many epithermal deposits resemble Monywa in the association of mineralization with hydrothermal breccias and reworked products of explosive hydrothermal or diatreme eruptions. For example, at Sari Gunay in Iran (Richards et al., 2006), explosive eruption of A. H. G. Mitchell et al. 24 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
fragmentals, intrusion of “hydrovolcanic” breccias and overprinting by hydrothermal breccias with jigsaw tex- tures, is similar to the sequence inferred at Monywa, and at Kelian in Indonesia (Davies et al., 2008) the widespread “accretionary lapilli” in explosively-erupted rocks are identical to the spherules characteristic of the Mine Pyroclastics at Monywa. However, both Kelian and Sari Gunay are low sulfidation deposits, and there is no reason to suppose that the genesis of the distinctive breccia dykes and pre-mineral Mine Pyroclastics at Monywa was dependent on the sulfidation state of the hydrothermal fluid. 12.2 Hydrothermal fluid and vertical zonation of mineralization The close spatial association of the Monywa mineralization with andesitic to dacitic porphyry intrusions implies a genetic relationship, and we infer that a few kilometers beneath each of the four deposits the dykes are rooted in a cupola or salient in the roof of a large dioritic pluton inferred to underlie the mineral district (Fig. 19). Crystallisation of the dykes and progressive downward growth of the cupola roof inhibited escape of volatiles, resulting in a low-density fluid-saturated cap or column above the underlying magma. Episodic hydraulic fracturing with formation of breccia dykes and veins followed increases in hydrostatic pressure within the low-density column to above lithostatic. Following the proposals of Muntean and Einaudi (2001) and Pudack et al. (2009) for the generation of high sulfidation epithermal gold deposits in Chile, we speculate that at Monywa the magmatic fluid was supercritical, and contracted from vapour to liquid on rising and cooling. Acid sulfate alteration of wall rock resulting from condensation of magmatic SO2 gas in water probably accompanied mineralization in veins and breccias, as argued by Masterman et al. (2005) for zoned alteration around high sulfidation veins in the Collahuasi District in Chile. The Monywa deposits differ from many high sulfidation epithermal deposits in the scarcity of large bodies of replacement quartz or quartz-alunite and in the absence of associated near-economic gold grades. This suggests either a deep erosion level in a formerly vertically zoned epithermal deposit, or more probably exposure of a deep epithermal or sub-epithermal system (Fig. 20) in which hypogene digenite, Fig. 18 Cartoon cross-sections showing two stages in evolution of Sabetaung or Letpadaung deposit. (a) Eruption of phreato- magmatic dykes and hydrothermal breccia and surface reworking to form Mine Pyro- clastics, (b) subsidence with further accumulation of erupted material and sediments, folding, andesitic magmatism, emplacement of breccia dykes, and copper mineralization. BA, Basalt breccia; MB, Mesozoic basement; MP, Mine Pyroclastics; SH, shale; SS, sandstone. Thick wavy line in (b) is present erosion level, grey shading is mineralization. Monywa copper deposits © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology 25
chalcocite, enargite and covellite were not necessarily overlain by gold or enargite-gold mineralization (Kyaw Win & Kirwin, 1998). Mineralization at palaeo- depths below the epithermal range is supported by some similarities to the high sulfidation ore body in the western part of the super-giant Chuquicamata deposit in Chile, where a central enargite zone passing outwards into digenite and then covellite extends vertically for more than a kilometer (e.g. Faunes et al., 2005). However, a close analogy with Chuquicamata would require that much of the digenite-chalcocite at Monywa is hypogene, rather than supergene as inferred from the mineralogical studies by M. Simpson et al. (1996, unpublished data) and others and summa- rized above. Many mineral districts with one or more high sulfidation epithermal copper-gold deposits include slightly older porphyry copper or copper-gold systems at similar or greater palaeo-depths (e.g. Arribas, 1995). The dependence of mineralization style on the depth of cupolas in the roof of an underlying pluton proposed by Proffett (2009) provides a possible mechanism for generating the two deposit types in one district. M. Simpson et al. (1996, unpublished data) inferred porphyry copper mineralization beneath Letpadaung from the presence of saline fluid inclusions in quartz vein fragments within breccias. It could also be argued that within the Monywa district porphyry deposits might exist at shallow depth beneath Kangon Sands or allu- vium as a result of post-mineral tilting and erosion. Alternatively, it may be that only very weak porphyry copper systems developed at Monywa, and were subsequently overprinted by the high sulfidation mineralization. This could explain the chalcopyrite-bornite veins encountered in some drill holes, for example at Sabetaung. Fig. 19 Schematic northwest- southeast cross-section through Monywa copper district showing highly speculative geology at depth and possible intrusion(s) beneath cover. Porphyry copper mineralization could occur above the cupolas in pluton roof. Fig. 20 Zonations in metals and copper sulfides in high sulfidation systems, from D.J. Kirwin (1994, unpublished data) modi- fied from T.M. Leach (1996, unpublished data). A. H. G. Mitchell et al. 26 © 2010 The Authors Resource Geology © 2010 The Society of Resource Geology
Two high sulfidation deposits with some features similar to Monywa but undoubtedly generated at shal- lower epithermal depths are the Quaternary gold- (copper) deposit at Chinkuashih in Taiwan (Tan, 1991) and the Cretaceous copper-gold deposit at Zijinshan in eastern China (So et al., 1998). At Chinkuashih gold and minor enargite mineralization occurs within veins and hydrothermal breccias in silicified intrusive dacites and in sandstones over an 800 m vertical interval, with eruptive dacites at the top. At Zijinshan, sheeted quartz porphyry dykes and their granite host rocks are cut by parallel or sheeted mineralized hydrothermal breccia veins. In upper levels the breccias are within quartz- replaced rock and host ore-grade gold; below, breccias within a larger alunite zone host covellite, digenite and enargite ore. Neither Chinkuashih nor Zijinshan are underlain by a porphyry ore body. 13. Distinctive features of the Monywa copper district We summarise below some characteristic features of the Monywa deposits. Host rocks to the mineralization are distinctive in having as the uppermost stratigraphic unit a sequence of interbedded pyroclastics and eruptive diatreme breccia deposits which was folded prior to intrusion of andesitic porphyries and local rhyolites, subsequent emplacement of breccia dykes, and mineralization. There were therefore two episodes of breccia or diatreme emplacement at Monywa, with copper mineralization confined to the second episode in which breccias may or may not have erupted. The mineralization style differs from that in most high sulfidation deposits in the abundance of hydrothermal breccia dykes of which more than 200 have been mapped on any one of the upper levels in the Sabetaung-Sabetaung South pit walls and almost all of which have one of two orientations, roughly at right angles. The main hypogene ore minerals, with early enargite overgrowing alunite and replacing pyrite, later more abundant digenite-chalcocite and minor covellite, are typical of high sulfidation deposits but distinguished by a scarcity of large massive quartz- pyrite replacement bodies and by the related absence of economic or near-economic gold values. A major feature of the deposits is the very strong supergene leaching, with removal of almost all copper in the leached cap. This highly efficient supergene process at Monywa is best explained by an exception- ally high pyrite content and by uninterrupted uplift, compatible with the coincidence of the present water table with the base of the leached caps. Development of a highly porous but rigid leached cap to form a sea- sonal reservoir for oxygenated meteoric water may have contributed to efficient leaching, and was dependent on hypogene quartz replacement of the porphyry groundmass and perhaps also on deposition of supergene chalcedonic silica. Descent of acid sulfate solutions generated at the water table resulted in the supergene enrichment and argillisation which decrease, probably exponentially, with depth from a maximum immediately beneath the very well-defined base of oxidation. Extensive replacement of enargite, replacement of covellite, minor chalcopyrite and bornite and partial replacement of abundant pyrite and of marcasite by supergene chalcocite and digenite to depths of several hundred meters were partly dependent on the weak neutraliza- tion potential of the hypogene advanced argillic and phyllic alteration below the water table. This replacement, the presence of hypogene digenite-chalcocite, and relative scarcity of enargite explain the amenability of the Monywa ore to heap leaching and hence to production of copper metal through solvent extraction and electro-winning. Acknowledgements We thank Douglas Kirwin for encouragement, discussion, arrangement of ASD determinations, detailed comments on the draft, and provision of Figure 14e, and Win Kyaw for advice on the history of the deposits. References Alam, M., Alam, M. M., Curray, J. R., Chowdhury, M. L. R. and Gani, M. R. (2003) An overview of the sedimentary geology of the Bengal Basin in relation to the regional tectonic framework and basin-fill history. Sediment. Geol., 155, 179–208. Anderson, J. A. (1982) Characteristics of leached capping and techniques of appraisal. In Titley, S. R. (ed.) Advances in Geology of the Porphyry Copper Deposits, Southwestern North America. University of Arizona Press, Tucson, 275–295. 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