Lecture 5 hydrothermal ore deposits

LECTURE 5:
HYDROTHERMAL MINERAL DEPOSITS
Prof. Dr. Hassan Z. Harraz
Geology Department, Faculty of Science, Tanta University
hharraz2006@yahoo.com
Spring 2020

Hydrothermal
circulation in a
continental setting –
Minerals form in
hydrothermal veins,
disseminated
deposits and grade
into pegmatites.
Pegmatite and Hydrothermal Disseminated Deposits
4

A hot spring on the surface is a sign of deep hydrothermal circulation.
5

Key factors that control the formation of
hydrothermal ore deposits
6

Possible source of hydrothermal ore deposit component
7

3) Hydrothermal processes
❑ Most common way of mineral formation
❑ These processes are the physico-chemicalphenomena and reactions caused by movement
of hydrothermal waters within the crust, often as a consequence of magmaticintrusion or tectonic upheavals.
❑ The foundations of hydrothermal processes are the source-transport-trap mechanism, As following:
i) Sources of hydrothermalsolutions
ii) Metal sources: most metals of economic importance are carried as trace elements within rock-forming
minerals, and so may be liberated by hydrothermalprocesses.
iii) Black smokers:dissolved metals and sulfur precipitation where encounters sea water, changing T, pH, etc.
iv) Transportation:
➢hydrothermalsolution transport often as Chloride (Cl) complex.
➢ Means of transport (ligands)
➢ Transport by hydrothermal solutions usually requires a salt or other soluble species which can form a metal-bearing complex.
These metal-bearing complexes facilitate transport of metals within aqueous solutions, generally as hydroxides, but also by
thiosulfate, chloride and other gold-carrying chemical complexes (notably tellurium-chloride/sulfate or antimony-
chloride/sulfate). The majority of metal deposits formed by hydrothermal processes include sulfide minerals, indicating sulfur is
an important metal-carrying complex.
v) Deposition: precipitation/solution depends on T, pH, Eh, P, Cl, S, Na, K, CO2. Hydrothermalore deposits
precipitation from solution as sulphide (S) complex.
❑ Sulfide deposition within the trap zone occurs when metal-carrying sulfate, sulfide or other complexes become chemically unstable due to one or more of the following
processes:
➢ falling temperature, which renders the complex unstable or metal insoluble
➢ loss of pressure, which has the same effect
➢ reaction with chemically reactive wall rocks, usually of reduced oxidation state, such as iron bearing rocks, mafic or ultramafic rocks or carbonate rocks
➢ degassing of the hydrothermal fluid into a gas and water system, or boiling, which alters the metal carrying capacity of the solution and even destroys metal-carrying
chemical complexes
 Metal can also become precipitated when temperature and pressure or oxidation state favour different ionic complexes in the water, for instance the change from
sulfide to sulfate, oxygen fugacity, exchange of metals between sulfide and chloride complexes, ..etc.
vi) Diagenesis:
➢ low-T modification,recrystallization,cementation. sometimes mineral changes such as dolomite.
➢Cellulose (plant) = loss of S, P, H, concentrationof C, = ranks of coal (lignite, bituminous, anthracite).
➢low cellulose marine plankton (high in lipids, some proteins and carbohydrates)=kerogen = petroleum,
as slowly heated and compressed.
Methane: both Biogenic (CH4 produced as bacteria break down organics, swamp gas, flying saucers), and
Thermogenic(CH4 produced as more complex hydrocarbons break down with heating)
8

Two mechanisms for metals emplacement near granitic intrusions (both occur)
Au, Ag
Metal-rich waters may originate from the magma or groundwater
Heated
groundwater
dissolves metals
Metal ores precipitate
near surface
Metal sources:
9

Magmas can form near
subduction zones when
water causes partial
melting of nearby mantle.
Granitic magmas form by
fractionation of basaltic
magmas and by
assimilation. Once the
granite has frozen, silica-
rich late fractionation
waters with dissolved
metals are left to intrude
nearby rock.
Most searches near continental volcanic arcs
e.g. Andes (Inca Gold) , Sierra Nevada (1849 gold rush)
MOTHER LODE
Au, Ag

Seawater circulates through the ridge basalt. Most minerals form when the hot water
exits into the cold deep ocean water. Sulfide minerals form “black smokers”,
sulfates form “white smokers”. Minerals are typically volcanogenic massive sulfides
(VMS).
Hydrothermal
circulation on the
ocean floor.
Hydrothermal Deposits at Black Smokers
11

❑ Sources of water in Hydrothermal: include seawater and meteoric water circulating
through fractured rock, formational brines (water trapped within sediments at
deposition) and metamorphic fluids created by dehydration of hydrous minerals
during metamorphism.
❑ As Following:
❖ Magmatic water systems (MW): exsolved from a cooling magma;
❖ Crystallization water (MG);
❖ Metamorphic water systems (MM) : by dehydration reactions
❖ Meteoric (rain) water systems (MT).;
❖ Sea water systems (SW);
❖ Sedimentary Rocks: Pore water or Connate water; Formation water; Basinal
water/Hydrocarbon systems (CW); and
❖ Thermal springs
❖ Mixed sources
Origin of hydrothermal fluids
(or Sources of water in Hydrothermal)
12

Magmatic fluids
• Mineralizers:mobile elementsLarge ion lithophile (LIL) such as Li, Be, B, Rb, Cs
• also significant quantities of alkalis, alkali earths and volatiles such as: Na, K, Ca, Cl, and CO2
metals such as Fe, Cu, Zn
• Water is the principle mobile constituentin all magmas, increases in amount with increasing
differentiation and plays an important part in the transportation of many ore components.
• Estimates of water in magmas range form 1 to 15%.
• Micas, clay minerals, zeolites, and amphiboles contain small amounts of chemically bonded water
that has passed through and equilibratedwith the atmosphere is called meteoricwaters.
• Meteoricwaters and Convection cells – circulation
• stable isotope signature - meteoricwater line
Seawater
• Seawater as an ore-forming fluid are best described in the contexts of evaporates, phosphorites,
submarine exhalalites
• Deposits related to seawaters are:
➢ Alpine Peridotitechromite, VMS; BIF; Sedimentaryiron deposits;
➢ Phosphate deposits; Evaporites; and
➢ Manganese modules
Connate waters
• Water trapped in sediments at the time they were deposited is known as Connate Water
• Connate waters are fossil waters
• Observed in oil field exploration
• Rich in sodium and chloride, also considerable amounts of calcium, magnesium, and bi-carbonate,
and many contain strontium, barium and nitrogen compounds
• can also contain light hydrocarbons
• Stable isotope ratios near SMOW
• Mississippi Valley type deposits (MVT)
Metamorphic
fluids
• Recrystallization of hydrous and anhydrous phases
• Volatile and mobile constituents are activated during metamorphism and forced from the rock to
migrate toward cooler

▪ Not all minerals deposits related to igneous activity occur within igneous rock bodies.
▪ Magmas have water and other fluids dissolved in or associated with them.
▪ During the later stages of crystallization, the fluids may escape from the cooling magma chamber, by seeping
through cracks and pores in the surrounding rock (called the country rock).
▪ These fluids carry with them dissolved salts, gases and metals.
▪ These warm fluids can leach additional metals from the rocks through which they pass.
▪ In time, the fluids cool and deposit their dissolved minerals, creating a hydrothermal ore deposit (which can be
obvious veins cutting through the country rock).
HYDROTHERMAL ORE DEPOSITS
❑Epigenetic : Ore bodies not physically associated with the magmatic body that produced the hydrothermal fluids.
❑Hydrothermal activity near volcanically active terrains; hot seawater or groundwater leaches minerals from
permeable rocks.
❑The particular minerals deposited vary with the composition of the hydrothermal fluids and the country rock.
❑Deposits formed from hot aqueous solutions (source not implied).
❑Involve fluids: Released from crystallizing magma (felsic plutons).
❑Minerals precipitate into cracks from metal-rich solutions.
❑Range of temperatures, depths and pressures.
❑Typically forms veins, replacements, and disseminations.
❑Occur as Widely disseminated vein networks.
❑Most diverse group; accounts for a large proportion of metals mined.
❑Majority of ore deposits: Many metals are found in hydrothermal ores, such as copper, lead, zinc, gold, platinum,
silver, mercury, arsenic, bismuth, antimony, and uranium.
Defination

❑ Any concentration of metallic minerals formed by the precipitation of solids from hot mineral-
rich water (hydrothermal solution).
❑ Hot water dissolves and transport minerals, interconnected openings in the rock allows the
solutions to move, and chemical reaction result in deposition. Deposition can be caused by
boiling, by a drop in temperature, by mixing with a cooler solution, or by chemical reactions
between the solution and a reactive rock.
❑ Produce vein deposits:
➢ Typically forms veins, replacements, and disseminations.
➢ Widely disseminated vein networks.
❑ Can occur as disseminated deposits, which are distributed throughout the rock body, rather
than concentrated in veins; called porphyry deposits = (low grade; large volume)
❑ The hydrothermal minerals deposited vary with the variation of composition of the
hydrothermal fluids and the country rock.
❑ Range of temperatures, depths and pressures.
❑Epigenetic deposits: Ore bodies not physically associated with the magmaticbody that produced the
hydrothermalfluids.
❑Majority of ore deposits: Many metals are found in hydrothermal ores, such as copper, lead,
zinc, gold, platinum, silver, mercury, arsenic, bismuth, antimony, and uranium.
HYDROTHERMAL ORE DEPOSITS
15

Hydrothermal veins
@ Hassan Z. Harraz
Metallic Ore Deposits
16

The dark colored layers are chalcopyrite, sphalerite and galena – valuable ore minerals
The white layers are quartz – a useless “gangue” mineral which must be removed
Gangue Mineral Quartz
Popular term “Mother Lode” initial placement Ore Body
@ Hassan Z. Harraz
17

Black Smokers
Circulation of hot water in cracks at mid-ocean ridge dissolves metals in Basalt, (Copper, Iron, Zinc, Lead,
Barium) which are re-precipitated as various ores, often Sulfides. Accumulate in ocean sediments.
http://collections.ic.gc.ca/geoscience
Island of Cyprus made of
Ophiolites with black smokers.
Source of copper that started
bronze age
Cu, Fe
Example:
Sterling Hill

Making Ore Deposits
Black Sulfide Rich Fluid
Artistic Drawing of Smoker Field
A Hot one out of a
Chimney

In order to deposit ores in significant economic concentrations, deposition often has to be preceded by "ground
preparation"; the process of preparing the host rock to "receive" an ore deposit in some concentrated form.
❑Ground preparation:
➢ Ground preparation is either physical or chemical.
➢ Physical ground preparation, which involves changes in the physical properties of the host rock,
applies only to epigenetic ore deposits, whereas chemical ground preparation is often involved
and applied with both epi- and syngenetic deposits.
➢ Physical ground preparation is either primary, developing at the time of formation of the host rock, or
superimposed, which develops after the host rock forms.
1) Physical Ground Preparation:
a) Types of primary ground preparation:
(i) porosity
(ii) crystal lattices: e.g. crystallization of phyllosilicates which allow for exchange of interlayer cations and
enhance diffusion.
(iii) bedding planes
(iv) vesicles
(v) volcanic pipes or tunnels
(vi) cooling cracks
b) Types of Superimposed ground preparation includes:
(i) faults, joints and fissures
(ii) shear zones
(iii) folds: which often result in "saddle reef deposits"
(iv) Brecciation
(v) Solution opening
(vi) development of volcanic pipes.
2) Chemical ground preparation includes:
(i) silicification: which makes the host rocks more brittle and hence more amenable to fracturing.
(ii) dolomitization: which is accompanied by a reduction in the volume of the solid rock, and hence the
development of secondary porosity
(iii)recrystallization accompanying metamorphism is another process that may cause the rocks to become
harder and more brittle.
20

Hydrothermal Vein Types
Shape or form of hydrothermal ore deposits will depend on the type of ground preparation (among other
things). As such, several "types" of hydrothermal veins may be recognized:
1- Fissure veins: (Fig. 1)
a) Simple
b) Complex: multiple layers of the same fracture or fracture system; indicate reopening?
c) Irregular: variable thickness
d) Anastomosing: Are veins that form in a branching fracture system
e) Conjugate: two veins with the same strike but at right angles to each other.
f) Lodes: many small veinlets distributed along a tabular fissure zone.
g) Fiber veins: Which develop in low grade metamorphic rocks by the crack seal mechanism. Fiber
veins are characterized by minerals developing a fibrous aspect and growing perpendicular to the
vein wall. Four types of fibre veins are recognized (Fig. 2):
i) Syntaxial: in which minerals grow from the wall rock to the vein center
ii) Antitaxial: in which the minerals grow from the vein center to the wall rock
iii) Composite: In which some minerals grow from the vein center while others grow towards it
iv) Stretched crystal veins: where the vein has a mineralogy similar to that of the wall rock, and in which
there is optical continuity from one wall to the other.
2. Pinches and swells: (Fig. 3)
3. Stockworks
4. Pipes or chimneys
5. Diatremes and breccia deposits (Fig. 4)
6. Saddle reefs: which result from the folding of the host rock, creating areas of "minimum P" where
hydrothermal fluids deposit ore in the crests and troughs (Fig. 5). Folding may also produce fractures
parallel to the axial planes of the folds that later become filled with ore (Fig. 6).
7. Ladder veins: develop in dykes or other competent strata surrounded by incompetent ones. Accordingly,
the competent strata deform brittley, whereas the surrounding rocks undergo ductile deformation (Fig. 7).
8. Solution cavity fillings.
9. Pitches and flats: form by slumping or gentle synclinal folding of brittle sedimentary beds, giving rise to a
series of connected tension cracks.
10.Shear zone deposits.
21

Nature styles of Hydrothermal deposits
1) Hydrothermal Ore/Vein Deposits
➢ minerals dissolved in hot water that comes from or contacts a
cooling magma body are deposited as they flow into cool
cracks in the surrounding rock
2) Hydrothermal disseminated deposits
➢ mineralized fluid from cooling pluton invades and saturates
permeable rock surrounding pluton
3) Pegmatite/Porphyry deposits
➢ coarse grained intrusive igneous bodies due to preferential
settling of denser ore-rich minerals in a magma chamber -
copper, molybdenum, silver, gold, lead, zinc, tungsten
4) Volcanogenic
➢ mineral-rich fluids vented to surface by volcanic activity black
smoker
@ Hassan Z. Harraz
22

Classification of Orogenic Hydrothermal Ore Deposits
Get gradients and zones due to systematic
variations in T,..etc
1) Hypozonal: Orogenic, hydrothermal ore
deposits formed at depths >10 km and
temperatures between 400 and 6000C.
Au, As, Co, Cu, Mo
2) Mesozonal: Orogenic, hydrothermal ore
deposits formed at depths of 5 to 10 km
and temperatures of 300-4750C. Au, As,
Te.
3) Epizonal: Orogenic, hydrothermal ore
deposits that formed at depths of less
than 5 km and temperatures of 150 to
3000C. Au, Sb, Cu, Zn, Cd, Pb, Ag, Hg, Sb
23

Hydrothermal Ore Bodies
Lindgren's Classification (1920-1930)
Hydrothermal deposits were broadly grouped into five types
whose mineralogy and mode of occurrence indicated
different conditions of origin:
❑ Hypothermal type (600 – 300°C): Sn, W in granites, note commonly
observed associations.
❑ Mesothermal type (300 – 200°C): Cu, Pb, Zn, Au, As, U, Ag, Co, Mo:
➢Gold-Quartz deposits in metavolcanics.
➢Porphyry Copper.
➢Marginal ores around intrusions.
❑ Epithermal type (200 – 100°C ): Au, Ag, Sb, Cu, Zn, Cd, Pb:
➢Outer contact zones
➢Mississippi Valley ore deposits
❑ Telethermal type (<100°C); and
❑ Xenothermal type: Hg
➢ shallow depths, but high T; Hot springs, fault zones.
24

Possible source of hydrothermal ore deposit component
25

i) Hypothermal Type
❑Fairly high temperatures (300-500oC) and
generally at considerable depths (several km)
including porphyry copper type deposits.
26

ii) Mesothermal Type
1) Mesothermal type- moderate temperatures (200-300oC) and pressures,
(approximately 1-5 km depth).
2) sulphides include chalcopyrite, sphalerite, galena, tetrahedrite, bornite
and chalcocite.
3) gangue includes quartz, carbonates (calcite, siderite, rhodochrosite) and
pyrite.
4) most show abundant replacement phenomena.
5) some associated with ultramafic rocks including listwanites (fuchsite
ormariposite (green mica) bearing altered varieties).
6) ribbon structures parallel to vein walls.
7) extensive alteration zones with varying amounts of sericite, quartz, calcite,
doIomite, pyrite, orthoclase, chlorite and clay minerals.
8) closely related to igneous rocks, both spatially and genetically.
❑ Classic 'examples' include: Motherlode District, California; Coeur d'Alene
District, Idaho; Cassiar District, B.C. and Archean lode gold deposits in
Ontario, Quebec and Manitoba.
27

iii) Epithermal Type
1) Epithermal deposits usually form temperature between 100 and 200°C.
2) Epithermal deposits usually form at shallow depths. Deposited normally within 1,000 m
(3,000 ft.) of surface; average 350 m.
3) Most of these deposits form in vein fillings, irregular branching fissures, stockworks or breccia
pipes.
4) Although replacement textures are sometimes recognized, most deposits are characterized by
open space filling textures such as vugs, drusy cavities, crustification, comb structures and
symmetrical banding (generally conspicuous). Colloform textures are sometimes observed.
5) Colloidal textures are characteristic implying free circulation of fluids.
6) Repeated cycles of mineralization are evident, including rebrecciation and multistage banding.
7) The source of fluid is believed to be predominantly meteoric.
8) Majority of deposits are Tertiary in age (esp. SW USA), however, some are much older, e.g.
Toodoggone deposits are early Jurassic (approximately 180 Ma).
9) Wallrock alteration is typically widespread and conspicuous, esp. chlorite, sericite, alunite,
zeolites, adularia, silica, pyrite and calcite (i.e., Wall rock alteration is predominantly argillic
accompanied by silicification.
10) Ore mineralogy includes: sulfantimonides and sulfarsenides (polybasite, stephanite,
pearceite, pyrargyrite, proustite and others), gold and silver tellurides (sylvanite, calaverite
and hessite), stibnite, argentite (acanthite), cinnabar, native mercury, electrum, native gold,
native silver, selenides and minor galena, sphalerite and chalcopyrite.
11) Gangue minerals include: quartz, amethyst, chalcedony, adularia, calcite, rhodochrosite,
barite, fluorite and hematite.
28

iii) Epithermal Type
12) In many places, epithermal deposits appear to be spatially associated with hot springs and
geysers, which may be considered the "surface expression" of epithermal systems.
13) Striking analogies to modern hot springs.
14) Often so diluted with ground water that mineral content is quite low (typical sinters);
however, some do contain sulphides and free gold, (e.g. Steamboat Springs, Nevada).
15) Deposits are formed in extensional tectonic settings with local normal faulting
16) Large scale volcanic collapse structures.
17) Veins are never uniformly mineralized along strike. generally less than 20% of the total vein
is mineralized.
18) Vertical zoning is common. They have a restricted vertical extent controlled by their steep
thermal gradients and/ or boiling of the fluid. Compared to hypothermal deposits, epithermal
deposits show stronger or better developed zoning.
19) Andesites are more common country rocks.
20) Economically, deposits are attractive because they have a high unit value of precious metals
(esp. 'bonanza' types) with generally low or no base metals.
21) Commonly reserves include tonnages less than 1 million tonnes but with good grades (17
g/tonne gold). They have a relatively short but productive mine life, providing a quick payback
and high rates of return on modest amounts of invested capital.
❑ Classic examples include: Creede, Colorado; Toodoggone Camp, B.C.; Blackdome, B.C.;
Premier, B.C.; Comstock Lode, Nevada and Pachuca, Mexico.
29

Magmatic-Hydrothermal Deposits
30

Deposits from Magmatic-Hydrothermal Ore
Forming Processes❑ Magmatic-hydrothermal processes cover transport and precipitation from juvenile water emitted from the melt, including metasomatism at
the contact of intrusive bodies and their country rocks.
❑ Water content of the magma comes from the molten minerals. Not only weathering products or rocks formed on the surface may contain
water. Typical water bearing rock forming minerals also in igneous and metamorphic rocks are chlorite, clay minerals, serpentine and
amphiboles. In arc related volcanism the subducting slab consists of hydrated ocean floor basalts and sediments providing much more water
than mantle derived material of hot spots or spreading zones. Felsic (granitic) magmas are wetter than mafic (basaltic) magmas in general
(Evans 1993).
❑ The exsolution process of an aqueous liquid from magma was studied experimentally. The ongoing crystallization will increase the water
saturation of the remaining liquid phase. On shallow (1-2 km deep) levels this starts under decreasing pressure with relatively low
crystallization grade, so low density gas bubbles are formed in the melt comprising vapor of water and other volatiles, moving upward. This is
called first boiling. On deep levels (2-5 km) the crystallization of water-free minerals must reach a higher grade (80– 90%) for the sufficient
saturation without change of the pressure, which is called second boiling. Of course, temperature, grade of crystallization necessary to boiling
and composition of the phases depend on the initial water content.
❑ Boiling is important because it means a rapid separation of volatile phases and remaining magma mush. It can lead directly to precipitation of
ore minerals along the paths of the upward moving fluids. Most of the halogenide, carbon-dioxide and sulfur content of the magma, light
metals like lithium or beryllium and incompatible elements fractionate into the volatile phases. Elements which are chalcophile also tend to
exsolve forming complexes with halogenides and other anions. Precipitation is controlled by cooling, decreasing of pressure and mixing with
meteoric waters (Robb 2005).
❑ Main magmatic-Hydrothermalore deposits:
I) Pegmatite Ore Deposits
II) Greisens and AssociatedSn – W Deposits:
✓ Tin & tungsten(Sn-W) deposits-associatedwith felsicrocks
✓ Malaysia,Bolivia, Cornwall-England
III) Volcanogenic Massive Sulfide (VMS) deposits (Cu-Zn): Associated with mid-ocean ridge volcanism.
VI) Porphyry Cu, Mo deposits
✓ Alteration of country rock by late, hydrothermal fluids.
✓ Cu, Mo deposited as sulfide minerals in veins.
✓ Associated with plutonic, intermediate igneous rocks (example: Chuquicamata,Chile, largestcopper).
✓ ClimaxMolybdenum (Co), Bingham Copper (Utah), several deposits in Arizona.
VI) Epithermal Au-Ag deposits (meteoric water dominated hydrothermal systems)
VII) Vein & replacement deposits: Butte, Montana (Cu-Zn-Mn)
31

Other Types of -Hydrothermal Deposits
❑ Mississippi Valley Type (MVT) deposits:
➢ Pb-Zn, Missouri, TN.
➢ Pb-Zn and Au-Sb deposits; Upper Mississippi Valley lead zinc district.
❑ Irish Pb-Zn-Ag deposits -Diagenetic
❑ Coeur d’Alene Ag-Pb-Zn deposits (Idaho):
➢ Metamorphic hydrothermal
❑ Seafloor Massive Sulphide (SMS)deposits
❑ Salton Sea geothermal system, CA:
➢ Modern-day hydrothermal activity
33

Porphyry copper replacement deposit – copper minerals are deposited in
fractured, altered igneous rock
34

Volcanogenic Massive Sulfide (VMS) Deposits
❑ VMS deposits are a type of Cu-Zn ore
deposit associated with hydrothermal
events.
❑ They are predominantly stratiform
accumulations of sulfide minerals that
precipitate from hydrothermal fluids in a
wide range of ancient and modern
geological settings (especially the seafloor
but also land-based volcanics).
❑ VMS represent a significant source of the
world's Cu, Zn, Pb, Au, and Ag ores, with
Co, Sn, Ba, S, Se, Mn, Cd, In, Bi, Te, Ga and
Ge as co- or by-products.
❑ VMS deposit consists of over 90% iron
sulfide…!
❑ Pyrite, Chalcopyrite, Galena, Sphalerite,
Barite.
❑ Most VMS deposits show metal zonation,
caused by the changing physical and
chemical environments of the circulating
hydrothermal fluid.
❖ Ideally, this forms a core of massive
pyrite and chalcopyrite around the
throat of the vent system, with a halo
of chalcopyrite-sphalerite-pyrite
grading into a distal sphalerite-galena
and galena-manganese and finally a
chert-managanese-hematite facies.
Or hot springs
Cross section schematically illustrating the characteristic
features of volcanogenic massive sulfide (VMS) deposits.
• Hydrothermal fluids move upwards along fractures in
volcanic rocks towards the sea floor.
• When the hot hydrothermal fluids vent and mix with cold
ocean water, iron, copper, lead, and zinc sulfide minerals
can form and collect as a mound on the sea floor.
• Ore minerals also can form in the fractures underlying the
mound of sulfide materials (After Lydon, 1988).
35

Zonation
in VMS
Anhydrite
+ Sulfides
Cu-Fe Sulfides
Interior zone
of mixing
Mg-silicate + Sulfides

Seafloor Massive Sulfide (SMS) Deposits
VMS Today…!
SMS deposits are forming today on
the seafloor around undersea
volcanoes along many Mid-Ocean
Ridges (MOR), and within Back-arc
basins and Fore-arc rifts (regions of
extension and crustal thinning).
Black Smoker
White Island, New Zealand
37

What are Seafloor Massive Sulphide ("SMS") deposits?
• Seafloor Massive Sulphide ("SMS") deposits form on the ocean floor and contain
appreciable concentrations of copper, zinc, gold, silver and other trace metals. They
are the modern-day equivalents of ancient 'land-based' Volcanogenic Massive
Sulphide ("VMS") deposits such as Kidd Creek in Canada. VMS deposits are a major
source of the world's copper, zinc, lead, gold and silver.
• Formation associated with tectonic plate movement or submarine volcanic activity.
• Mineralization formed from seafloor venting of hot, metal charged hydrothermal
fluids.
• Contact with cold seawater leads to metals precipitating as sulphides.
HOW DO SMS DEPOSITS FORM?
• SMS deposits form on the seafloor in water depths of up to ~4000 m. They form in areas where
new ocean crust is forming, such as seafloor spreading centres. Seawateris drawn down through
fractures in the oceanic crust, towards a hot buried magma chamber at depth (molten rock body).
The heated seawater transforms into a hot acidic hydrothermal fluid and convection causes the
fluid to rise up again towards the seafloor.
• The hot acidic hydrothermal fluid leaches metals from the surrounding rocks during transport, and
may also interact with other hot fluids rising from the magma chamber. When these hot acidic
fluids carryingdissolved metals and sulphur reach the seafloor (up to 400°C), they encounter cold
ambient seawater (typically2°C at 1500 m water depth). The sudden change in conditions causes
metals and sulphur to precipitate out of solution as metal-rich sulphide, forming an accumulation
of sulphide materialon the seafloor. SMS deposits commonly carry high concentrations of copper
(chalcopyrite)and zinc (sphalerite)in addition to gold and silver.
38

Mississippi Valley Type Deposits (MVT)
❑ MVTs are hydrothermal lead-zinc ore deposits that are characterized by:
(1) low-temperature formation (100°-150°C).
(2) epigenetic (forming after) emplacement within restricted dolostone or limestone strata of
sedimentary basins (i.e., stratigraphically controlled).
(3) precipitation from highly saline brines.
(4) the presence of barite and/or fluorite mineralization.
❑ MVT deposits tend to occur in clusters at the margins of sedimentary basins, and they are
intimately associated with the evolution of these basins.
❑ They are also typically
found far from, and lack a
genetic relationship to,
igneous activity or igneous
rocks.
❑The most abundant minerals in
MVTs are sphalerite and galena,
barite, fluorite, calcite, dolomite,
quartz and pyrite.
39

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Lecture 5 hydrothermal ore deposits

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