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Geol242 > Ores > Flashcards

Ores Flashcards

1
Q

Ore

A

Any naturally occurring material from which a mineral or aggregate of value can be extracted at a profit.

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2
Q

Types of Ore

A

Oxides
Sulphides
Native elements
Silicates

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3
Q

Ore formation

A

Source - metals are scavenged from somewhere where they can be supplied to an ore forming system
Transport - metals carried by melt or fluid to deposition site
Trap - metals concentrated by physical/chemical processes into an ore deposit

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4
Q

Classification of Ore - Formation

A

Igneous - magmatic process
Hydrothermal - hot, aqueous solutions
Sedimentary - surficial

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5
Q

Classification of Ore - time of deposition

A

Syngenetic - ore deposits that form at the same time as their host rocks
Epigenetic - ore deposits that form after their host rocks

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6
Q

Classification of Ore - hydrothermal ore depth

A

Epithermal - shallow depths and low T
Mesothermal - intermediate depths and temperatures
Hypothermal - substantial depths and elevated T

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7
Q

Hydrothermal ore deposits

A

Magmatic fluid
Seawater
Meteoric fluid - water derived from precipitation, becomes groundwater
Connate fluid - fluid trapped in pores as sediments are deposited

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8
Q

Diamonds

A

Polymorph of C stable under reducing conditions
Subduction of carbonate to transition zone depths and below transport C into the deep mantle. Metasomatism brings mass up to shallower depths.
Form deep in the mantle - deep upper mantle, transition zone and lower mantle (older than kimberlite)
Primary source - kimberlite dikes and pipes (sediments derived from eroded kimberlites or deep in mantle)

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9
Q

Diamond vs Graphite

A

Polymorphs of C
Diamond - very hard, high PT form of C, strong bonds connect closely packed C in tetrahedra
Graphite - very soft, stable at low PT, strong bonds within sheets and weak bonds between laters
Graphite in a high P cell makes industrial diamond

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10
Q

Kimberlites

A

Originate from deep in the mantle, bringing diamonds rapidly up to the surface. Contain mantle xenoliths, a magmatic component and wall rock xenoliths. K-rich, hydrated and carbonated (volatile rich), typically ultramafic.
Diatreme - carrot-shaped body of magma due to expansion of gases in magma as it rises to the surface. High volatile content results in an explosive eruption (assimilation of wall rock).
No kimberlite explosions in last 25Ma, surface expression eroded. Thought to generate a maar - a volcanic crater that forms when basaltic magma contacts groundwater producing a steam explosion.
Powerful eruptions. Magma rises so quickly that diamonds don’t have enough time to convert to graphite (more stable at surface). Once diamonds cool down, they don’t have enough energy to reform their structure into graphite.

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11
Q

Location of diamond-bearing kimberlites

A

Restricted to cratons - cold, thick, stable crust.
Lithostatic keel beneath cratons is the only area cold enough at high enough P to retain diamonds.
Kimberlites younger than diamonds.

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12
Q

Mining diamonds

A

Indicator minerals (high PT)
Most mined diamonds are used for industrial purposes
1-1.4g/t (small amount of diamond per tonne)

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13
Q

Komatiite hosted Ni ores

A

Komatiites - ultramafic lavas and intrusives. Defined by olivines showing spinifex texture.
Commonly in Archean greenstone belts
origin debated, such high MgO (>18wt% MgO) requires high melting T, suggesting higher mantle T in Archean
Characteristic layering of flows produced during crystallisation of ponded lava. Spinifex textured upper part of the flow crystallised through downward growth of crystals from chilled top. Skeletal olivine between spinifex and cumulate layer. Olivines present settle to the base of flow to form the lower cumulate layer.
Syngenetic - high degree melts cause high metal content, assimilates S from country rock, immiscible NiS melt forms in lava, sinks to the bottom and is carried along in channels and deposited where flow slows.

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14
Q

Spinifex texture

A

Believed to develop due to high degree of undercooling, low nucleation and high growth rates. Low viscosity melt

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15
Q

Banded Iron Formations (BIFs)

A

Layered grey ion oxides and red chert. Archean age. Syngenetic (BIFs = sediment).
BIF deposition possibly related to O in atmosphere. Fe is soluble when reduced, but insoluble when oxidised. Not much O in the atmosphere for most of earth’s history. First organisms to produce O (cyanobacteria) evolved ~2.7Ga, initially most O consumed by Fe, resulting in precipitation of Fe minerals. Once all Fe was used up, O could accumulate in the ocean and in the atmosphere.

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16
Q

Cyanobacteria

A

Created O2 as a waste product, but if there was too much O2, they would die. Cycles in BIFs.

17
Q

Layered Mafic Intrusions (LMI)

A

Large sill-like body of igneous rock. Mafic - ultramafic. Characteristic layering of minerals due to segregation by settling and floating from melt. Cumulates on intrusion floor (cumulate texture - phenocrysts with interstitial melt), crystallisation from side and roof (feldspar may float and form cumulate texture), middle last to crystallise. Results in layering - modal (variation in mineral %), phase (change in mineral assemblage) and cryptic (change in mineral composition).
Metals concentrate in discrete layers at mineable grades: Cu, CO, Ni, PGEs. Occur as sulphides (dense), melt must have between S-saturated and sulphides precipitate as an immiscible liquid.
Archean-Proterozoic (there are younger examples).

18
Q

Skaergard intrusion, Greenland

A

Crystallises from the outside surfaces inwards. Layers of crystals. Potentially convection.

19
Q

Chalcophile

A

Sulfur loving (Cu, Zn, Ag, Pb)

20
Q

Siderophile

A

Iron loving (Ni, Co, Pt, Pd, Au), associated with mafic rocks

21
Q

Lithophile

A

Silicate loving (Li, Sn, Zr), associated with felsic or alkaline rocks.

22
Q

Porphyry Cu deposits

A

Mesothermal: intermediate depths and T.
Au-Cu and Cu-Mo deposits.
Low-grade and high tonnage.
Associated with more silicic magmas.
Subduction settings
-island arc setting (oceanic-oceanic) - Cu-Au
-Continental settings (oceanic-continental) - Cu-Mo
Crystallisation of anhydrous high T minerals concentrates H2O, volatiles and incompatible metals in the remaining melt. Hydrothermal fluids and vapour that exsolve from the crystallising melt are enriched in metals. Porphyry deposit form as ore minerals are deposited as a result of fluids separating, cooling, wall-rock reaction and mixing with external fluids.
Epigenetic - ores coming from magma, transported by fluids in magma, deposited in country rock

23
Q

Porphyry Cu ore deposits

A

Typically cylindrical in shape. Outer shell of med-course grained rock and inner zone of rapidly cooled porphyry (igneous rock with phenocrysts and a fine matrix).
Extensive hydrothermal alteration zone (middle (early)=potassic - outside (later) argillic).
Controlled by PT, primary rock composition, fluid composition and fluid:rock ratio.

24
Q

Bingham Cu mine

A

Biggest hole on Earth. Mining into a magma chamber and extracting ores from shells formed around it. Mainly sulphides (some Fe).

25
Q

Epithermal Au deposits

A

Epithermal - shallow depths and low T
More Au and less Cu than porphyry Cu deposits. Also Ag, Hg, Pb and Zn.
Association with active volcanoes and geothermal systems, influenced by shallow meteoric fluids. Ore bodies are varied, can form veins, breccias, disseminations and stockwork. Smaller ore bodies than porphyry Cu, but higher-grade ore.
Most epithermal deposits are young (<50 Ma) due to their shallow depths, older deposits have been eroded away.
High sulphidation (HS) deposits and low sulphidation (HS) deposits.

26
Q

High sulphidation (HS) deposits

A

Occur proximal to volcanic settings, either in or near the vent.
Metals come directly from the magma. Very acidic fluids due to absorption of magmatic vapour by groundwater. Acidic flux that leaches and alters rock.
-eg: Whakaari

27
Q

Low sulphidation (LS) deposits

A

Occur on the fringes of volcanic edifices and in areas without coeval volcanism.
Metals come from meteoric water, with possible mixing with magmatic fluids if volcanism nearby. Fluid pH nearly neutral. Fluid boiling is likely responsible for ore deposition. Geothermal systems.
-eg: Hauraki Gold Field; Martha mine

28
Q

Volcanogenic Massive Sulphides (VMS)

A

Accumulation of metal sulphides (Cu-Zn) that precipitate from hydrothermal fluids below the seafloor. Fluids are corrosive, dissolve metals and reprecipitate them on the surface.
Volcanic associated hydrothermal systems (metasomatism). MOR and subduction zones (back-arc basins and submarine arcs).
3.4 Ga to actively forming
Sulphides - sphalerite, chalcopyrite, galena
VMS lenses can be large, forming sulphide-silicate-sulphide chimneys on the seafloor. Continual growth and collapse forming breccia mound. Metals are deposits within stockwork (network of ore-bearing veins)
Epigenetic (replacement and alteration in subsurface) and syngenetic (ore collecting as sediment on seafloor). Both can occur at the same time.

29
Q

Pegmatites

A

Extremely coarse-grained intrusive igneous rocks, typically granite in composition. Cool very rapidly.
Typically enriched in rare elements Li (batteries), Ta (electronic devices) etc. Associated with lithophile.
Residual melt is concentrated in incompatible non-volatile elements and concentration of incompatible volatiles.
Source (or rare metals) - continental crust that melts forming granite
Transport - granitic magma. Extreme fractionation leads to enrichment in incompatible and volatile elements.
Trap - pegmatite body where crystals with unusual chemistry and size crystallise from low T, volatile rock magma.

30
Q

Crystallisation

A

During crystallization, compatible non-volatile elements enter the silicate minerals.
Incompatible non-volatile elements becomes preferentially concentrated in the residual melt (pegmatite or porphyry Mo).
Incompatible volatile elements partition into a fluid phase (liquid or vapour).

31
Q

Pegmatite rapid crystal grow

A
  • composition - only a few crystals can grow due to melt composition (low nucleation high growth)
  • H2O content - high H2O limits the formation of crystal nuclei
  • fluxing elements - increases diffusivities in melt, allowing cations to quickly move to growing crystals
  • high degree of undercooling - T contrast with surrounding country rock. Crystal is in diseqm and diffusion of elements is unable to keep pace with textural development (skeletal crystals).
32
Q

Aplites

A

Fine-grained equivalent of pegmatites. Form if magma suffers P loss, allowing H2O to escape (transport lost) and lots of crystals nucleate all at once and crystallise into a fine-grained rock.

33
Q

Metamorphic ore deposits - orogenic Au deposits

A

Result of metamorphic process and are hosted in metamorphic rocks.
Metals and hydrothermal fluids must be derived from metamorphic events, NOT igneous intrusions. Generation of fluids to transport and precipitate metals is critical. Metamorphism can destroy, create or concentrate ore bodies.

34
Q

Orogenic Au deposts

A

Large amount of world’s gold. Regionally metamorphosed compression-related deposits, mostly confined to greenschist facies (amphibolite + would be dehydrated to no fluids to transport metals). Wide PT conditions.
Metamorphic fluids are low salinity, near neutral pH and mixed H2O-CO2 compositions. Travel up fracture systems and precipitate out metals at various levels.

35
Q

Macraes Mine

A

Orogenic Au deposits.
Gold in quartz veins within chlorite schist. Formed by hot hydrothermal water in the later stages of metamorphism ~135Ma. Mineralisation began when rocks were being uplifted and were still hot and ductile (brittle-ductile transition in chlorite zone). Once in brittle zone, metals precipitated along faults/fractures. Metals were leached out of surrounding rocks (now depleted). Gold emplaced in mineralised zone along Hyde-Macraes Shear zone (shallow dipping, regionally continuous structure). Shear zone uplifted greenschist to the surface (underlain by normal fault).

36
Q

Reefton

A

Orogenic Au deposit. Lower greenschist facies. Regional metamorphism ~430 Ma. Shear-zone related to quartz veins and breccia-hosted disseminated gold.

37
Q

Tectonic settings of ores

A

Continent - BIFs and Kimberlites
Oceanic arc - epithermal Au, porphyry Cu-Au and LMI
Back arc - VMS Cu-Au
Continental arc - epithermal Au (HS), porphyry Cu-Mo and pegmatites
Back-arc extension - epithermal/hot-spring Au (LS)

Geol242 (2 decks)

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