week 2 Flashcards
how are the majority of ore deposits around the world formed?
- result from concentration processes arising from the circulation of hot, aqueous solutions through the Earth’s crust
OR - have been significantly modified by such fluids
describe ore minerals from a magmatic-hydrothermal ore deposits that precipitate from high-temperature aqueous solutions
- aqueous implies that the solvent is H2O
- the solutions can have salinities up to several times that of seawater -> brines
- in some brines, salts can form more than 50% solution by mass
- the fluid may be a solution of H2O and dissolved gases (e.g. CO2)
- high-T can range from normal rock temperatures at a few km depth (~100 degrees C) to magmatic temperatures (~800 degrees C): epithermal, mesothermal, hypothermal
- the fluid is a liquid, vapour or gas
how is an ore zone formed
- a hydrothermal fluid serves as an additional enrichment mechanism for a system that is already primed to be an element anomaly and potentially an ore deposit
- the fluid extracts metals from a reservoir, they are dissolved into that fluid and further enriched/concentrated within it. they are than usually sharply precipitated, which again concentrates the metals, creating an ore zone
what is a VMS deposit
- Volcanogenic Massive Sulfide
- the simplest example of a magmatic-hydrothermal ore deposit is a VMS deposit-shallow magma associated with a seafloor spreading centre
- seawater is drawn into the rock, heated by the magma and scavenges metals. it is then pumped out onto the seafloor where the metals rapidly precipitate
describe ligands in seawater
- seawater contains ligands (ions or molecules, e.g. SO42- (sulphate) or Cl- (chloride)) that bind to a metal atom and promote dissolution.
- ligands allow metal cations (e.g. Fe, Mn, Cu, Pb, Zn) to be dissolved. solubility increases with temp: the hotter the fluid, the more metals can be dissolved
how does a VMS form
- finally the fluid is injected into cold seawater which suddenly lowers the temperature
- all the metals dissolved in the fluid are rapidly precipitated and rain down on the seafloor to form a mineral deposit
white smokers vs black smokers
- white smokers are warm and rich in sulphates and carbonates
- black smokers are hot and rich in sulphides. it is modern analogue for VMS deposit formation
VMS deposit zonation (core to margin)
Po = pyrrhotite (FeS), Cp = chalcopyrite (CuFeS2), Py = pyrite (FeS2), Sp = sphalerite (Zn, Fe)S, Gn = galena (PbS), Ba = barite (BaSO4)
hydrothermal vent basics
hot liquids are less dense and therefore more buoyant than cold liquids. so the hot hydrothermal fluids rise up throguh the ocean crust just as a hot-air balloon rises into the air. the fluids carry the dissolved metals and hydrogen sulfide with them
VMS vs SEDEX vs MVT deposits
VMS = Volcanogenic Massive Sulphide deposits (volcanic-hosted, mainly Cu-Zn)
SEDEX = Sedimentary Exhalative deposits (clastic-hosted Pb-Zn)
MVT = Mississippi Valley Type deposits (carbonate-hosted Pb-Zn)
- all form from hydrothermal fluids venting via conduits in extensional settings but SEDEX and MVT deposits lack an obvious or direct link to volcanism
- SEDEX and MVT deposits contain >50% of the world’s known Pb and Zn resources
- SEDEX and MVT are rich in Pb and Ag, with insignificant amounts of Cu and Au. VMS are generally rich in Cu and may have important amounts of Au. Zn occurs in both.
- SEDEX and MVT formed in fault-bounded sedimentary basins on continental crust; VMS mostly are formed on oceanic crust or island-arc crust
- SEDEX host rocks are shales or carbonates; VMS host rocks are volcanic/volcaniclastic
- SEDEC and MVT, 100% of the metals are derived from host sediments/basement; in VMS, metals are mostly derived from intrusive/volcanic rocks
what are porphyry copper deposits
- PCDs derive from metal-carrying fluids associated with subduction-related magmatism and volcanism
they are the world’s major sources of: - copper (~65% of production)
- molybdenum (~95% of production)
- can be a source of gold (Au), silver (Ag) and sometimes Re, Te and PGEs (platinum group elements)
Key characteristics: - intermediate to felsic porphyritic intrusions
- I-type calc-alkaline granitoids
- shallowly emplaced
- hydrothermally altered
what is high tonnage, low grade
metal-bearing sulphides (e.g. chalcopyrite, CuFeS2) are typically confined to veins so the ore:rock ratio is low
stages of porphyry copper mineralisation
can occur in two stages:
1. hypogene mineralisation (driven by magmatic fluids)
2. supergene mineralisation (driven by meteoric fluids)
what is hypogene mineralisation
hypogene minerals (e.g. pyrite, FeS2 and chalcopyrite. CuFeS2) tend to occur in descrete veins filling fractures but can also be disseminated throughout the host and wall rocks of the deposit
key alteration zones used in PCD exploration
potassic - core of PCD. key minerals are biotite and K-feldspar. main source of ore
Propylitic - ubiquitous in the marginal parts of the system. key minerals are chlorite, epidote, albite and carbonate. barren
chlorite-sericite - upper parts of the PCD core zone. key minerals are chlorite, sericite/illite and hematite. significant ore content (pyrite and chalcopyrite)
sericitic/phyllic - upper parts of the PCD above the chlorite-sericite zone. key minerals are quartz and sericite (fine-grained white mica). usually barren
advanced argillic - above PCD in the lithocap (subsurface domain of alteration that it laterally and vertically extensive). key minerals are vuggy quartz, alunite (Al sulphate) and clays (particularly kaolinite). barren
described secondary supergene enrichment
- when exhumed to the surface or near-surface, primary Cu sulphides can be dissolved or “leached” by meteoric water
- Cu is then reprecipitated in a rich blanket of secondary minerals (oxides and sulphides) above and below the water table
- enrichment grades can attain up to 1.5 to >2% Cu (an increase of 2-3x)
what are “leached caps”
a by-product of the supergene enrichment process is Fe-oxide, such as hematite, which gives PCD “leached caps” their distinctive red colour
oxygen + water + chalcopyrite -> Cu + hematite + sulphate + sulphuric acid
describe skarn ore deposits
- used to refer to mineral deposits that result from contact metamorphism and metasomatism associated with intrusion of granite into carbonate rocks
- hot magmatic and metal-bearing fluids dissolve the carbonate rocks, forming pathways
- the addition of carbonate to the fluids causes metal minerals to precipitate out along the pathway
- a wide variety of deposit types and metal associations are grouped into the category of skarn deposits, including W, Sn, Mo, Cu, Fe, Pb-Zn and Au ores
- the metals found in a skarn deposit are a product of the differing composition, oxidation state and metallogenic affinity of the igneous intrusion
- Fe and Au skarn deposits tend to be associated with intrusions of more mafic to intermediate compositions
- Cu, Zn and W are linked to calc-alkaline-type granites
- Mo and Sn are linked to more differentiated S-type granites
types of hydrothermal mineralisation
hypothermal (e.g. porphyry copper deposits) - hydrothermal ore deposits formed at substantial depths (>4.5km) and elevated temperatures (400-600 degrees c)
mesothermal (e.g. gold in metamorphic terranes) - hydrothermal ore deposits formed at intermediate depths (1.5-4.5 km) and temperatures (200-400 degrees c). replaced by “orogenic gold deposits”
epithermal - hydrothermal ore deposits formed at shallow depths (<1.5 km) and fairly low temperatures (50-200 degrees c)
describe epithermal deposits
- low temperature (<300 degrees c), previous - or base-metal deposits temporarily and spatially associated with volcanic centres
- Au and Ag are the major products but they can also contain: Hg (mercury), Sb (antimony), Pb, Cu and Zn
- typically characterised by open spaces (vugs) and the presence of sulfosalts (e.g. tennantite and enargite) - metal + semi-metal + sulphur
describe high and low-sulfidation
2 contrasting styles of mineralisation are recognised in epithermal deposits:
high-sulfidation
low-sulfidation
(also intermediate-sulfidation)
these terms refer specifically to the sulfidation state of sulfur in the ore fluid (a measure of the fugacity or gas pressure, of molecular sulphur in the hydrothermal fluid)
describe high-sulfidation
- high-sulfidation deposits generally occur close to the volcanic vent
- mineralisation fluids are derived directly from the magma
- the fluids are acidic (pH = 1-3) and oxidised, carrying SO2, SO42- or HSO4- in solution
- associated with Au-Cu (lesser Ag, Bi, Te)
describe low-sulfidation
- associated with fluids in areas of high heat flow e.g. hot springs
- the fluids have equilibrated with their host rocks and generally comprise a meteoric component
- but they could have been mixed with an evolved magmatic fluid if active volcanism is located nearby
- the fluids are pH neutral and reduced, carrying HS- and H2S in solution
- associated with Au-Ag (lesser As, Sb, Se, Hg)
describe Carlin-type gold deposits
- large sediment-hosted disseminated gold deposits characterised by invisible gold (typically microscopic and/or dissolved) with pyrite and arsenopyrite
- gold deposition occurs where normal faults intersect a less permeable cap rock, usually ar a shale/limestone contact and in the crests of fault-propagated anticlines. the precipitation mechanism is believed to be related to neutralisation of the acidic ore fluid during carbonate dissolution
describe the mineralisation of carline deposits
- early gas/fluids are hot and highly acid and attack calcite in carbonate rocks making them porous
- later fluids had neutral pH, 120-250 degrees celcius. fluids filled porosity with quartz and pyrite
- gold introduced during one brief pulse towards the end of the event at ~40Ma and deposited as very fine grains in arsenic-rich pyrite
where did the carlin-type gold deposits come from?
perhaps from:
meteoric waters driven by convection to depth as a result of high heat flow during Basin and Range extension
or
fluids released either during contact metamorphism at the time of intrusion of granitic magmas or magmatic fluids derived from granitoid magmas
