ORE DEPOSITS RELATED TO MAFIC, ULTRAMAFIC, INTERMEDIATE TO FELSIC IGNEOUS ROCKS

1. ORE DEPOSITS
ORE DEPOSITS RELATED TO MAFIC, ULTRAMAFIC, INTERMEDIATE TO
FELSIC IGNEOUS ROCKS:
Kunwar Shruten Chauhan
Department of Geology
University of Rajasthan
Jaipur

2. PAGE 1
ORE DEPOSITS RELATED TO MAFIC AND ULTRAMAFIC ROCKS
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Ore deposits formed during fractional crystallization of magmas are designated as magmatic
segregation deposits (also known as orthomagmatic deposits). These deposits are the direct
crystallization products of magma, usually form in the magma chamber and thus constitute deep-
seated intrusive bodies. However differentiated or immiscible melts and crystal mushes can be
driven into the walls or roofs of the magma chamber to form ore bodies in the form of dikes, sills,
and even extrusive flows. A magmatic segregation deposit may constitute an entire intrusive
body, form a single compositional layer within the igneous rock body, or occur as disseminated
minerals.
Mode of occurrence:A magmatic segregation deposit may occur in any of the following forms:
(i) constitute an entire intrusive body
(ii) form a single compositional layer within the igneous rock body
(iii) may be defined by the presence of disseminated minerals.
Orthomagmatic deposits of Cr-Pt-Ti-Fe and Cu-Ni-Fe (-Pt) associated with basic and ultrabasic
rocks emplaced in cratonic and active orogenic settings. The worlds major Orthomagmatic
deposits are associated with
(1) Layered igneous intrusive (Bushveld, Great Dyke and Sudbury)
a) Bushveld layered complex contains about 75% of the world’s chromium reserves.
The Bushveld complex is large reserves of platinoid ores (composed of
Pt+Pd+Rh+Ru+Ir+Os) and the average ore grade is 8.27ppm. Another significant
resource of the Bushveld complex is the layered magnetite – rich units associated
with titanium and vanadium.
b) The Great Dyke of Zimbabwe is a layered igneous intrusion, 532 km long, 5 to 9.5
km wide and consists of chromite layers that occur along the entire length and the
individual layers extend across the entire width.
c) The Sudbury basin is Shock metamorphic features favours a meteorite impact
hypothesis for the eruption of magma. Sudbury Igneous complex (1849 Ma) consists
of a lower zone of augite - norite, a thin middle zone of quartz–gabbro and an upper
zone of granophyres and these three rock units are reported to be comagamatic. In
steep to vertical, radial and concentric dykes that appear to penetrate downward into
the foot wall from the base of the complex and referred to as offsets,the inclusion–
rich sulphide bearing rock is quartz diorite. Most of the ore bodies occur in the
sublayer (inclusion- and sulphide- rich norite and gabbro) whose magma was rich in
sulfides with inclusions and peridotite, pyroxenite and gabbro. The sublayer and
offsets are at present the world’s richest source of nickel as well as an important
source of copper, cobalt, iron, platinum and 11 other elements.
(2) Intrusions related to flood basalts of cratonic areas (Norilsk–Talnakh)

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Noril’sk deposit , Russia, occurs in the differentiated layered, dominantly gabbroic
intrusion. The copper-nickel sulfides form breccia and disseminated and massive ores at
the base of the intrusion and vein ore bodies developed in the footwall rocks and the basal
portion of the intrusion.
(3) Bodies emplaced in active orogenic areas (Eastern Gold Field Province)
Eastern Goldfields province, Yilgran craton, Western Australia, is bestowed with several
occurrences of orthomagmatic copper-nickel sulfide deposits associated with komatiitic
suite of lava flows and related shallow dyke-like or sill-like dunitic intrusions.
Assosiated rocks;
1) KIMBERLITES AND LAMPROITES
About 20% of the world production of diamonds is derived from kimberlites and
lamproites and the rest is extracted from beach and alluvial placers deposits
containing diamonds.
 Kimberlites and lamproites are carrot shaped and are generally regarded as having
been intruded upwards into cratonic areas through a series of deep–seated tension
fractures,often in terrains of regional doming and rifting.
2) CARBONATITES
 Carbonatites are generally found in:
(1) stable cratonic regions along major faults and fault intersections and
(2) rift valleys.
 Carbonatites are exploited for phosphorus (from apatite), magnetite, niobium (from
pyrochlore), zirconia, Rare earth elements (from monazite and bastanasite), barite,
strontianite and vermiculite. Only one carbonatite body (Palabora complex, South Africa)
is known for large reserves of copper ore.
3) ANORTHOSITES
 Anorthosite massifs are essentially Proterozoic in age and confined to terrains of
anorogenic intercontinental rifting or incipient rifting tectonic setting in a broad belt
across North America, Britain and Scandinavia.
 Anorthosite massifs are divisible into two groups (Herz 1976) based on their
plagioclase and oxide compositions; they are:
(1) Labradorite anorthosite massifs (e.g., Michigamau anorthosite, Labrador; Duluth
Gabbro complex, Minnesota), and
(2) Andesine anorthosite massifs (e.g., anorthosite massifs of Adirondack Mountains in
New York; Allard Lake region in Quebec,Canada and Rosland in Virginia, USA).
 Anorthosite massifs are the major source of titanium minerals (ilmenite and rutile).
ORE DEPOSITS ASSOCIATED WITH INTERMEDIATE TO FELSIC IGNEOUS
ROCKS:

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Orthomagmatic deposits associated with felsic volcanism
Felsic magmatic melts are rarely in a position to generate significant Orthomagmatic deposits.
But iron ore deposits, consisting magnetite ± apatite encountered at severalplaces are considered
as of magmatic segregation origin. These deposits occur both as lava flows and dyke-like bodies
Pegmatites and their mineral deposits
Pegmatites are coarse-grained igneous or metamorphic rocks, generally of granitic composition.
Pegmatite developed at higher structural levels and spatially related to intrusive late tectonic
granitic plutons, are marked by minerals with volatile components (OH,F, B) and a whole range
of accessory minerals containing rare lithophile elements, including Be, Li, Sn, W, Rb, Cs, Nb,
Ta, REE and U.
Pegmatites are mined for:
(a) Sn, Nb-Ta,REE, Y, Zr, Be and U ores,
(b) Gem quality beryl, Chrysoberyl, topaz and tourmaline; optical grade fluorite crystals;
piezoelectric quartz crystals,
(c) feldspar required for ceramic industry
(d) Li-minerals (spodumene, petalite, lepidolite) for the glass industry and manufacture of
Licompounds, (e) book-mica (muscovite) for the electrical industry and electronic tubes.
Greisen deposits
Greisens are granoblastic aggregates of quartz and muscovite (or lepidolite) with accessory
amounts of topaz, tourmaline and fluorite formed by post-magmatic metasomatic alteration of
granite.
Greisens are usually developed at the upper contacts of granitic intrusions and mineralization
occurs as large irregular or sheet-like bodies immediately beneath the upper contact of late stage
Greisen deposits are mined for tin, tungsten and beryllium. In future, greisens may constitute
important sources of Be, Nb-Ta, REE, Y and other elements.
Skarn deposits
Skarn deposits have been referred to variously in literature as (a) hydrothermal metamorphic, (b)
igneous metamorphic, (c) contact metamorphic and (d) pyrometasomatic deposits. They are
developed at the contact of intrusive plutons and carbonate country rocks. The latter are
converted to marble, calc-silicate hornfelses and/or skarns by contact metamorphic effects. The
majority of the world’s economic skarn deposits occur in calcic exoskarns.
Skarn deposits are mined for Cu, Fe, W, Pb-Zn, Mo, tin etc; these deposits are generally smaller.
Most copper skarn deposits are associated with calc-alkaline granodiorite to monzogranite stocks
emplaced in continental margin arcs. A small number of copper skarns occur in oceanic island
arcs associated with quartz diorite to monzonite plutons.
Giant skarn type iron deposits at Sarbai in Turgai iron ore province, Kazakhstan are associated
with diorite and contain 715 Mt of iron ore with 46% Fe. Iron skarn deposit at Iron Springs, Utah
(USA) is an ideal example of Skarn development at the boundary between intrusive and
limestone.

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Bulk of the skarn tungsten (scheelite) comes form a few relatively large deposits,
Dolomite hosted skarn deposits of talc are encountered in France (Trimouns Mine) and
Austria. Minor amount of graphite production comes from skarns from Skaland Mine,
Norway. ‡ Sn- and Mo- skarns are typically associated with high-silica (~75% SiO2) well
differentiated plutons, while Au- and Fe- skarns relate with relatively less-evolved, low
silica (62% SiO2), high – iron plutons. Cu-Zn-and W- skarns are associated with plutons
having silica content ranging from 63-69%.
Skarn deposits are associated with magmatic rocks emplaced under diversified tectonic settings;
Oceanic back arc basin, subduction zone beneath continents and post-subduction or continental
rifting environment.
Porphyry deposit
The term “Porphyry system” relates to a composite (occasionally single), mediumto silicic
igneous intrusive complex wherein at least one member displays porphyritic texture, with
~25vol% of phenocrysts of alkali feldspar and/or quartz.
Three main types of “Porphyry” ores are recognised:
1) Porphyry copper with two sub-type Cu-Mo and Cu-Au (these two types of deposits
represent the Lowell-Guilbert model and Diorite model respectively)
2) Porphyry molybdenum which is subdivided into the “Climax-type” with Sn and W
and “Quartz-Monzonite type” (Endoco-type) which grades into Cu(Mo) porphyry
with decreasing Mo/Cu ratio.
Porphyry-tin, which contains minor/trace amounts of W, Mo, and Bi. Major
concentrations of porphyry Cu and Mo deposits are confined to Phanerozoic island arc
settings and Andean type continental margins,Porphyry Cu and Moores are also known
from Palaeozoic fold belts. Porphyry Mo deposits are mostly of Mesozoic and Tertiary
ages. Porphyry Sn deposits are mostly of Tertiary age ; Sn – granites of Asian countries are
of Mesozoic age.