Terrestial Hydrothermal and meta

Question 1

Epithermal meaning

Answer 1

This term applies to ores that appear to embedded crystals within vughs or veins where temperature and fluid composition is key. Usually surrounded by low T alterations like clays or sulfosalts (alunite) and are shallow (<2 km). The fluid composition drives the diversity of the types of deposits. These can occur in arc settings or continental settings during rifting and upwelling.

Question 2

Chemical reactions determining HS and LS

Answer 2

(pyrrhotite) Fe₇S₈ + S₂ = 7FeS₂
5CuFeS₂ + S₂ = Cu₅FeS₄ + 4FeS₂
(tennanite) .67Cu₁₂+as₄S₁₃ + S₂ = 2.67Cu₃AsS₄ (enargite)

the right hand side is high sulfidation where S(VI) exists

Question 3

High sulfidation: def, genisis, geometry, tonnage, water composition

Answer 3

These are Au, Ag, Cu within intensely altered rock (sometimes ~volcinclastic) < 1.5 km deep and < 2 km from the magmatic source with grades ~7 ppm Au and ~1 Gt of ore. They oftentimes have a “squished can” geometry based around the lower level of the impermeable lithocap. They form from ~250 C fluids with low salinity (3-10%) but alot of acid (pH ~1-2). They form because H₂S (g) + SO₂ oxidizes to H₂SO₄

Question 4

high sulfidation mineralogy

Answer 4

These usually form in vughy silica where the feldspar has been well leached (think summitville) with alunite clays, calcite (boiling), and some barite

Question 5

Low sulfidation deposits: def, commodities, size, grades, localles, water and genisis

Answer 5

These are deposits where sulfur is reduced S(-2) with Au, Ag +- Pb, Zn, Cu. They are more distal than HS (~2-10 km) from heat source and are structurally controlled with quartz + adularia veins and vertical zoning as agates. Notable localles are Round Mountain, Fresnillo, Creede and Cripple Creek. Fluids are ~175-300 C low salinity (~.5-5%) and deposit ores as a function of boiling point that changes with depth. Water is mainly terrestial with only ~10% from magmas.

Question 6

Key features of terrestial hydrothermal

Answer 6

It is reliant on convection cells of upper crust supercritical fluids. Composition is dependent on the local rock formation and the climate (bc of NaCl). Permeability of rocks, as governed by Darcy’s law. And the solubility of gangue minerals.

Question 7

How does gold complex?

Answer 7

In low pH and oxidized condition gold complexes with Cl and in nuetral waters with mid-O2 levels HS₂

Question 8

How is gold precipitated?

Answer 8

Requires a reducing environment:
AuCl₂- + H₂O = Au + 2H+ +2Cl- +.5 O2
Au(HS)₂- + (9/4)O2 = Au + 2SO₄ + H+ + H₂O

Question 9

Distribution of terrestial hydrothermal systems

Answer 9

These are distributed around magmatic arcs and rifts where there is magma near the surface. In the SW there is a clear distribution of commodities as a function of geograpy. Au in the north (NV), CU/MO in S (AZ) and silver in MX.

Question 10

VMS deposits: locations, fluids, genesis, ore minerals, grades, tonnage

Answer 10

Volcanic massive sulfides are paleo-undersea deposits that form on or around MOR’s or backarc basins (Japan/Cyprus). Fluid temperatures are ~100-250 C at pH ~3-4 (T>BP because of depth of 1-3 km). They create layered deposits above breccias of chalcopyrite, galena, sphalerite, pyrite, and marcasite. Deposits are ~50 Mt at 2-3% Cu and 10% Zn. They are usually metamorphosed (kidd creek ON) when emplaced as ophiolites.

Question 11

Lithophile elements in LS

Answer 11

There are occasionally elements like Be and F in LS deposits. An example being the Mt. Wheeler deposit.

Question 12

What is the difference between black and white smokers?

Answer 12

Black smokers emit sulfide metal grades which are dark and white smokers release sulfate and SiO₂

Question 13

VMS geometry

Answer 13

They are usually seperated 5-20 km apart and have a stockwork with hydrothermal alteration (chloritic in center and sericitic at the edges) then brecciated/disseminated CuFeS₂, pyrhotite, pyrite to sphalertite, then sphalerite, galena, and pyrite. This is overlain by bedded sediments of silica-pyrite-hematite known as “exhalite”.

Question 14

Rxns, extent, and general features of VMS leaching

Answer 14

Generally there is a large area of water filtration and leaching (~2-4 km radius) where metals are mobilized by SO₄ going to S2 and O2 by reducing S and oxidizing iron. Chlorine ligands also have a role.

Question 15

VMS types and effects of temperature and magma types

Answer 15

There are sulfide dominated types. Karako-type which are felsic (backarc within cont. seaway) with more Pb/Cu ratio an example is Bathhurst New Brunswick. Norando type with intermediate crust Cu-Zn with higher Mg/Cu, like Jerome and Quebec. Beshi type (Cu>Zn) which is mafic and has Co like Windy Craggy (the giant BC deposit that is now part of a park). Cyprus type which is a dome (intrusive ores) and encapsulated in ophiolites ~2% Cu. The oxide dominated types are like the Carajas deposits and Algona type of deposits.

Question 16

What is the hydraulic circulation of gluids like for VMS?

Answer 16

In VMS the water is usually fed into the system with faults.

Question 17

Thermochemical sulfate reduction

Answer 17

This refers to the temperature and pressure related reduction of sulfate. This is important for VMS because SO₄^2- in the ocean water goes underground and Fe2+ to Fe0 reduces sulfate to H₂S which then complexes with base metals. This occurs at ~200 C

Question 18

What is the evolution of VMS?

Answer 18

The initial chimneys collapse and then the retarded hot water that filters through them creates characteristic zoning (Cu -> Pb +Zn)

Question 19

What is the role of oxygen within the VMS/O systems and ambient waters?

Answer 19

The area where the VMS deposit forms needs to be relatively anoxic so that the sulfides remain as sulfides. Interesting most VMO systems are paleoproterozoic and thus occur in a time before the oceans are oxygenated. It is thought that the ferruginous waters enabled the BIF-like precipitation.

Question 20

VMS resource size

Answer 20

<100 Mtonne of ore @ 2-3% Cu and <10% Zn

Question 21

Paragenisis and paragenetic sequence

Answer 21

Paragenisis refers to the associations and co-occuraces of minerals in a deposit that grew together. A paragenetic sequence refers to the timing of mineralization within an assemblage.

Question 22

Types of quartz textures

Answer 22

Zoned: 1-5 mm crystals with layers caused by supersaturation with enviromental shifts
Comb: ~1 mm euhedral crytals crusting the surface
Crustiform: ~1 mm crystals alternating with adularia showing changes in fluid composition and multiple growth bands
Colloform: ~2 mm banded aggregates of siliceous gels - agate like - relatively constant fluid composition
Mosaic: <1 mm which is chalcedony or amophorous Si - low T recrystalization

Question 23

Orogenic/lode: def, commodities, geometry, geography, processes, and examples

Answer 23

These are Au, Ag, Cu deposits that are characteristically high grade and have a long strike (multiple km). They form from the de-pressuring of fluids. They can also be formed through favorable hydrofracking of layers in BIFs, slates, quartzites…. They tend to form vertically zoned veins within OLD rocks (Archean +). Examples include mother load in california or Telfer in WA.

Question 24

Orogenic gold mineralogy and genesis

Answer 24

These are dominated with quartz with only a few % of MS, calcite, and albite. There is also some muscovite formed with the calcite from the consumption of albite and clinopyroxene. They are more likely to have native metals. They form from low salinity fluids (2-10% NaCl) with significant CO2 and more reduced sulfur. Ores are deposited at 250-400 C at depths> ~2kBar equivalent (4-16 km) where Au(HS)₂^- reacts with FeO to form FeS + Au. If waters were saline then there would be many more base metals.

Question 25

What is the role of calcite and quartz in orogenic deposits

Answer 25

Quartz increases solutbility in high pressure enviroments and calcite decreases solubility. This means that CO2 behaviour is not as may be expected but is important.

Question 26

Where does the fluid for orogenic gold come from?

Answer 26

~1-10% of the water is dehydration reactions. Dependent upon the presence of organics (like black shales) CO2 <50% and Sulfur species are <5% from decomposition of pyrite and others. Critically there is high degrees of local variance driven by the compositions of distinct layers.

Question 27

Greenstones

Answer 27

These are metamorphosed volcanic rocks and are the primary host for orogenic gold.

Question 28

What is the role of fractures in fluid transport?

Answer 28

The model for water flow at these depths is to have it move upward and at fractures the lower end of the fracture (high P) has locally lower pressure as it sucks in fluid and the higher end releases more fluid creating locally higher pressure. This is important because this means that local shear zones and areas of higher porosity can concentrate fluids over a significant area.

Question 29

Seismeic pumping

Answer 29

This refers to the relatively cyclical overpressuring and depressuring of fluids within impermeable or low-permeability layers. Basically fluids build up at the lower end and then fracture the rock periodically. This is essential for ore formation in veins where the rapid pressure drop precipitates mineral and it also causes fluid mixing that can precipitate mineral.

Question 30

Fluid pressure vs. depth

Answer 30

There are two straight lines, one representing hydrostatic pressure gradient (density of watergdepth) and the lithostatic pressure. When there is pore space fluid pressure is equal to the hydrostatic pressure, when this goes to ~0 fluid pressure=lithostatic.

Question 31

What is the fluid flux like for a seismeic pumping system?

Answer 31

There are rapid spikes of fluid flow. Both the fluid out and fluid in rapidly increase and then decrease.

Question 32

What is the textures that form from seismeic pumping.

Answer 32

These form crack and seal veins and fiber veins. These can be discontinuous as tension cracks (60 degrees from the vein axis) form too. These are seen as “massive” ores in these deposits.

Question 33

Where does ore form in orogenic systems?

Answer 33

Ore forms in higher permeability areas. This can include layers, fold hinges, faults, and localized strain.

Question 34

Orogenic gold: resource estimate and examples

Answer 34

50 MOz of gold at over .5 Oz/ton (like telfer). Mother Lode CA and Juneau district which both formed in the late Cretaceous when plate tectonics shifted from subduction to transform.

Question 35

Slate belt type orogenic gold

Answer 35

These are orogenic gold deposits found within slate belts. Victoria Australia is the famous example. It has evidence for synmetamorphic ore formation where ore is hosted within saddle reefs.

Question 36

Ironstone orogenic gold

Answer 36

These are LARGE deposits because they are great sinks for sulfide (FeO - FeS) but there is some debate about where the gold came from. Some deposits (like Homestake SD) are incredibly uniform across the strata-bound deposit. indicating there may be some type of paleoplacer

Question 37

Carlin Type Gold

Answer 37

Low temperature (150-250 C) epigenetic deposits forming 1-6 km from heat source co-occuring with plutonism and minor volcanism with disseminated Au with As, Hg, Sb, and Tl within replaced carbonate shelfs. There are strata-related deposition in favorable carbonates and fault related fluid flow (Carlin area).

Question 38

Carlin type alteration

Answer 38

There is the decalcification of ores and sometimes the development of jasperoids and argillic alteration aka kaolinitization

Question 39

Genesis of ore in Carlin Deposits

Answer 39

Carlin gold is disseminated within arsenopyrite and was transported by H₂S rich fluids (why there is no base metals)

Question 40

IOCG: shape, examples, commodities, size, texture, and ages

Answer 40

IOCG are large (several Gt of metal) sink-hole like deposits with Fe2O3 at surface and Fe3O4 at depth (with minor pyrite - related to igneous below?) There is also LREE, Ag, As, Co, Mo, Ni, and U at grades of 1-6% copper and .5 ppm Au. Olympic Dam is the mega IOCG in Gawler Craton hosted within a granite and found under cover. The others are less glorious than Olympic Dam. The ore textures are hematite-rich breccias. They are generally Mesozoic.

Question 41

IOCG fluids and genisis

Answer 41

It is believed they are from maar-like igneous features where saline fluids infiltrated and circulated to create the large base metal rich deposits. Fluid temperatures exceed 400 C, low sulfur fugacity, high O2, with widespread alteration. They require that the location is in arid conditions when forming.

Question 42

Under what conditions is gold soluble?

Answer 42

Gold is most soluble with hydrogen sulfide in reduced conditions (low fO2) and nuetral fluids. Gold chloride is solutble in more oxidized solutions and more acidic pH

Question 43

Epithermal evidence for mixed fluids

Answer 43

The ore deposits plot with relatively low dueterium and low oxygen-18 between the VSMOW line and the magmatic deposits region.

Question 44

How do we determine the temperature of formation with inclusions?

Answer 44

The density of an inclusion remains constant and we can heat/cool the inclusion to understand when it is a single phase that indicates T min for the crystal’s formation.

Question 45

Lithophile low sulfidation: general description and examples

Answer 45

These are flourine and beryllium rich deposits like Mt. Wheeler and Spor Mt. where a lithic tufff is altered. These have very little sulfide and K-spar

Question 46

High salinity systems meaning, commodities, and deposit types

Answer 46

These are deposits that require over 10% NaCl in solution to form the deposits and are IOCG and Five-element deposits. They both have characteristic sodic and carbonate alteration without large scale zoning, arsenides, and sulfides.

Question 47

IOA deposits

Answer 47

These are basically IOCG but iron oxide apatite - mined for Fe in Brazil

Question 48

IOCG alteration

Answer 48

Na-Ca, K, and argillic with systematic zoning. The sodium (+- Ca) subs for Ca in plagioclase and alkalai feldspars. They plot on the very sodic side of igneous rocks.

Question 49

Where is the barren part of IOCG?

Answer 49

It is in the center but is in a regular U shape - basically it is upside down compared to porphyries and general thoughts about how these (magmatic) hydrothermal systems do not clearly translate

Question 50

Five-element deposits commodities, mienrals, locations, grade, size, and geometry

Answer 50

These are uncommon with very low sulfur and silver, arsenic, cobalt, uranium, and nickel. They are high salinity and high oxidation - similar to IOCG. The most notable example is Bear Lake in Quebec. They are interesting because they have great dendritic silver, nickel, and cobalt along with interesting arsenides which is mainly their mineralogy.

Question 51

Why is it difficult to determine the origin of ore-forming fluids within Carlin-type deposits?

Answer 51

This is because the fluid paths for meteoric and magmatic fluid intersect at low-T.

Question 52

MB arguements for Carlin not being magmatic

Answer 52

Carlin is too large and long to be magmatic - too regular. Magmatics are based upon geophysics only. He argues the fluids are reduced Eocene lakes like what we see at mono lake which has similar isotope composition. The presence of oils supports that it was reduced and the presence of bio activity.

Question 53

“detachment style” IOCG

Answer 53

This is a Carlin-type-ish deposit but with saline fluids with low sulfur. They are low in value, hematite and magnetite rich, oxidized, with chlorite and epidote alteration along faults. Examples include Copperstone and the yukon. The textures are vein and breccia fills along low-angle detachment faults. Fluids are high salinity, oxidized, and moderate fS2. There is K, sericitic, and propylitic alteration.

Question 54

What are examples of modern epithermal systems?

Answer 54

White Island NZ: it is in a transform from an arc to transform setting in rhyolitic + andesitic rocks. With the current fluxes of gold there would be Moz in long time scales. It is comparable to LS systems (advanced argillic alteration, silicates, carbonates)

Question 55

Western US modern hydrothermal

Answer 55

This ranges from magmatism (Cascades) extensional rifting (Salton Sea). Cascades + Yellowstone have high-T advanced argillic alteration like the LS deposits of W. NV (goldfield, pyramid, paradise). In Great Basin Mono lake has long transport of ambient fluids at the edges of a caldera

Question 56

Salton sea: setting, ore potential, fluid composition, salinity and temperature.

Answer 56

It overlies a trans-tensional gap in the transform fault below W CA. There is ~25% DS in the fluids with IOCG/5-element composition, Li, and Zn.

Question 57

Why do Iceland’s hydrothermal features have low metals?

Answer 57

Iceland’s hotspot is transitory and the fluids are not very saline.

Question 58

Why does IOCG mineralization seem to be independent of magma type?

Answer 58

IOCGs are considered amagmatic or to have their fluids derived from exterior sources meaning that the host rock and water composition is more important than the magma

Question 59

Why does studying Yerrington, the Cordillera, and SW US help us understand IOCG

Answer 59

They help us understand where/when IOCG forms and the continuum between magmatic hydrothermal and terrestial hydrothermal. These examples all show the similar climactic conditions key for IOCG formation and how they can be related to the presence of other systems

Question 60

What is the Chilean iron belt?

Answer 60

It is a belt of IOCG including Candalaria with many smaller IOCG deposits chiefly related to back-arc magmatism and porphyry systems

Question 61

5 element deposit alteration and fluids

Answer 61

Fluids are near nuetral, saline (similar to IOCG) with alot of sodic-carbonate alteration

Question 62

What are the main distinctions and commonalities of magmatic deposits?

Answer 62

The main distinctions come from the texture of ore and the geometry of the deposits. The higher temperature M-H deposits have veins and stockworks. The mid and low T terrestiral and VMS differ in their precipitation of the ores. VMS is the lowest temperature and has fine disseminated ores.

Question 63

What are the major ways that we distinguish VMS?

Answer 63

They are distinguished by changes in the commodity and the ratio of copper to lead.

Question 64

What is the main reason for modern VMS to be between 2 and 3 km below water?

Answer 64

These are the depths where the hydrostatic load is large enough to that the water exiting the vents at 100-300 C are not boiling