Magmatic hydrothermal

Question 1

Magmatic hydrothermal systems: key feature, main commodities, deposit types

Answer 1

These are systems that have their fluids mainly derived from a magma. They create porphyries, skarns, and replacement deposits which are essential for Cu, Mo, and Sn with Au, Ag, Zn, and Pb being by-products

Question 2

What is a typical P-T range for M-H deposits? What are some of the variables that determine their form?

Answer 2

They are usually at ~900 C and 20 km where post cooling they are at 10 km and 700 C where they hit the wet granite solidus

Question 3

What is the importance of physical characteristics of the exsolved fluids?

Answer 3

Water is usually within a supercritical state (with alot of dissolved solids) and this means that a small change in pressure causes a major change in density. This causes convection.

Question 4

What is the role of water immiscibility in M-H systems in relation to the need for an oxidized magma?

Answer 4

If there is the formation of S2 through having a reduced melt (like LMI’s) then the copper in the melt will be partitioned into the solid. If the melt is oxidized then SO2 or SO4 will dominate and the metals will stay in solution to be precipitated as ore.

Question 5

Components of a mineralizing hydrothermal system

Answer 5

• fluid source: f(magma) or meteoric waters
• source of dissolved ores
• fluid flow pathways
• energy source driving mass transport
• host rock
• chemical driver of ore precipitation
  It necessitates a physical gradient for fluids to congregate and precipatate ore minerals

Question 6

Clarke factor of concentration

Answer 6

This is the V(deposit)/V(leached rock or magma)

Question 7

How does solutbility change as a f(valence, radii)?

Answer 7

high field strength elements and compounds are highly soluble as ions (think alkalai and alkalai E). Transition metals (plotting in the middle) are weakly soluble. The semi-metals are considered solutble. So in comparison to other elements metals are not as soluble. Generally as charge increases and radii decreases solubility increases.

Question 8

Ligands

Answer 8

These are anions and anionic compounds tat increase the solubility of metals by creating a barrier around them in water. They are similar to acid strength and are selective in the metals that they complex. Chlorine (salty brines), H2S, and SO4 are key.

Question 9

What is the composition of magmatic-hydrothermal-solutions?

Answer 9

These are H-O-Si-O…, H2O, CO2, HCl, CH4, H2S, SO2 rich fluids (volatile!)

Question 10

First and second boiling

Answer 10

First boiling refers to the decompression exsolution of fluids when magmas are rising it takes fluids from an entrained state to a vapor or supercritical state. Second boiling is during crystalization where fluids are exsolved chemically. This is when concentration is most significant.

Question 11

Common alteration types and related T and pH

Answer 11

1. Advanced argillic - low pH (1-2) with T 700-150 C - andalusite at high T, pyrophyllite, and kaolinite
2. Sericitic - sericite + chlorite at mid T and mid pH creates characteristic sericite haloes overprinting potassic
3. Intermediate argillic - alumino-silicates - pH ~ 4 T ~ 200-400
4. Propylitic - epidote, sericite ~400 C from the dispersed volatile rich fluids without acids
5. Potassic - high T (700) and K-spar, qtz, biotite - metasomatic

Question 12

Alteration inidicators

Answer 12

Replaced minerals are key so when there is a fine textured mineral within the shape of a common crystal (for example mica within an old hornblende crystal)

Question 13

How to tell if fluids are magmatic vs. hydrothermal for alteration?

Answer 13

Magmatic fluids emplace Qz + Bi + K-spar in potassic alteration, Meteoric fluids replace quartz with albite

Question 14

Skarn: def, commodities, size, geometry, location

Answer 14

A Ca +- Mg silicate-rich metamorphic rock replacing carbonates or, if endoskarns, replacing igneous rocks. They are small (1-10 Mt) but high grade (~2% Cu) with (Cu, Au), (W-Sn), Sn, (Zn-Pb) and form chimneys (cross strata) and mantos (blankets) with increased distance from the heat source they tend to form replacement deposits. These have wollastininite and mass is added to them.

Question 15

Skarn minerals

Answer 15

Ca - wollastonite (CaSiO3), Ca-garnet (Ca3Fe2(SiO4)3), andradite, pyroxene
Mg - pyroxene (XYSi2O6), serpentine (Mg3Si2O5(OH)4), pyrite (FeS2)

Question 16

Primary processes involved with metal precipitation

Answer 16

1. Cooling: As temperature decreases the solubility of metals in solution significantly decreases -> UST
2. Wall rock reactions with acids on carbonates reduce the solution and lower the metal solubility
3. Off-gassing and de-pressurizing

Question 17

Metal solubility trend

Answer 17

Fe > Pb ~Ag~ Zn > Cu > Mo

Question 18

Paragenisis

Answer 18

This is a term used to describe the time-space relationships of events which for porphyries are usually complex with superposition and zoning. Specifically it is for associations and co-occurances of minerals.

Question 19

Equation for approximate cooling time

Answer 19

1 ~ K/(t * a²)
Where: K = thermal diffusivity
t = years
a = 1/2 of the shortest distance from the igneous feature to the surface

Question 20

Types of fluid inclusions

Answer 20

Primary are inclusions that occur during the crystal growth and will not cut accross different growth phases
Seconday are healed cracks and are fragmented lines cross-cutting the crystal growths
“Psuedo-secondary” are healed fractures that only cut through part of the growths

Question 21

Porphyry Copper: general features, magma composition, locations, commodities, and geometry

Answer 21

Porphyry Cu (+- Mo, Au) are very large deposits (Btonnes) that form cupolas above felsic oxidized magmatic intrusions. They are generally hat-shaped with zoning based upon the solubilities of entrained metals. Notable locations include El Salvador (Chile), Bingham, and Yerington

Question 22

What is the importance of oxidation state in porphyry Cu?

Answer 22

If H₂ S is present (S is reduced) then Cu-S compounds will form and become part of the granite but if oxidized so SO₂ is present then the Cu will remain in solution to be deposited.

Question 23

Veins

Answer 23

These are tabular bodies within spaces either via hydrofracturing or pre-existing voids
open veins describe veins that have foreign materials inside and closed veins are shut
It favors the precipitation of minerals that have solubilities dictated by changes in pressure and temperature (like sio2).
Lode deposits are composed of many vein sets.

Question 24

Cupolas

Answer 24

Dome-shaped areas that are clearly influenced by intruding magmatic stocks

Question 25

What is the relationship between magma composition and metal precipitates?

Answer 25

monzo-granitic magmas (LR of ternary) are Mo and Cu is more dioritic/mafic (in the center of the diagram)

Question 26

What are key characteristics of porphyry deposits?

Answer 26

These are defined by a very large volume (1Mt-10Gt) ore within evenly distributed vein systems at <1% Cu, 1 ppm Au, .1% Mo. The intrusions at the center are relatively small

Question 27

Porphyry: distribution and geometry

Answer 27

These are relatively young <75 myr which makes sense because they are on volcanic arcs that are easily eroded and are not incredibly deep. Geometrically they are considered the bases of strato-volcanos and are composed of many heterogeneous intrustions

Question 28

Geometry of ore in porphyry

Answer 28

There are multiple kinds of veins and these tend to form systematic sheets or stocks that indicate relative tension with respect to the intruding magma meaning that they tend to form tangential to the intrusion and normal to the intrusions

Question 29

Unidirectional solidification texture

Answer 29

These are most common within the plutons below the porphyry system and is marked by irregular lobe-like quartz druzy forming layers with quenched magmatic materials. It shows that the growth of the materials was unidirectional.

Question 30

What is the general sequence of events for porphyry formation?

Answer 30

1. magmatic intrusion related to the rise and colling of the hydrated, felsic-int. magmas causing USTs and first boiling
2. Build up + release of fluids related to the convection and collection of fluids in the cupola until reaching critical fraccing pressures
3. Later intrusions and sources of heat continue to repeat the process causing increased ore concentration and the overprinting of prior minerlization events

Question 31

Greisens: def, commodities, size, notable locations

Answer 31

These are hydrothermal alteration of granitic rock specifically related to B and F rich minerals like fluorine, topaz, and/or tourmaline. These are relatively small (10-100Mt) deposits of Sn, W, Cu, Zn, Bi, and Mo like in Cornwall

Question 32

Greisen geometry and geology with respect to porphyry

Answer 32

These are driven by equigranular intrusions that are more fractionated luecogranites that form greisen veins that are sub-tangential to the intrusion - similar to pegamtites but not as complex

Question 33

Fluid composition and function for greisens

Answer 33

Fluids are typically 250-450 C and the ore minerals are complexed with Cl and either replace or alter the minerals to form the typical greisen minerals

Question 34

Carbonate replacement: def, geometry, examples, commodities

Answer 34

These are usually 2-4 km from the heat source and are the complete replacement of CO3 marl-like rocks with epidote, pyroxene, garnet, and sulfide ore minerals. They are in chimneys, karsts/caves, and mantos and form because of the buffering of acidic solutions with the lime at 300-600 C and grades 10-10X that of skarns for Sn, W, Mo Bi, U, Au, Ag, Cu, Zn. Notably the giant silver deposits of N. MX. Replacement deposits have calcite.

Question 35

Common zoning patterns with distance from a magmatic intrusion

Answer 35

Close (600C): W-Bi -> Sn -> Sn-Cu-Pb-Zn-As -> Pb-Ag-Zn-Cu far (250 C)

Question 36

High sulfidation epithermal: def, commodities, major processes, geometry, and examples

Answer 36

This is defined by Au, Ag, Cu within intensely altered rock (where S fugacity is high). It is relatively shallow (<1.5 km) and ~1-2 km from the magma source. They are commonly just below the lithocap of volcanic materials. They are found in fairly heterogenous ore bodies within vuggy silica surrounded by advanced argilic alterations (alunite-kaolinite) that grade into argilic on the edges.

Question 37

Carbonate replacement processes

Answer 37

Basically the fluids dissolve the carbonates and this raises the pH. Upon the dissolution of the carbonates, SiO2 + CaCO3+ H2S + M⁺ = H2CO3 + MS + CaFeSi2O6 and Zn²⁺ + H2_S = ZnS + 2H

Question 38

High sulfidation processes

Answer 38

The fluids are ~250 C with pH ~1-2 with relatively low salinity (2-10% NaCl) and at this temperature the vapors can become liquids which means that H2S and H2SO4 go into solution rapidly lowering the pH, dissolving non-quartz and creating a low-P vugh for ore formation.

Question 39

Paragenetic Sequence

Answer 39

This describes characteristic zones of mineralization related to time synchronous events. For example, sericitiv alteration with type A/B copper veins within a Cu porphyry.

Question 40

What are aqueous complexes and which are most important?

Answer 40

These the the development of metals surrounded by water due to polarity. The most important three are H2S (Cu, Ag, Au, Hg, Ti, As, and Sb), Cl, and OH

Question 41

Hydrtothermal alteration

Answer 41

The process whereby hydrothermal fluids react with rocks to change composition via metasomatism

Question 42

What is vein fill and alteration envelopes?

Answer 42

Vein fill refers to the gangue and small quantities of ore that are within the previously open viens. Alteration envelopes are the areas around the vein that have undergone hydrothermal alteration.

Question 43

skarnoid

Answer 43

This is where similar rocks near one another exchange mass without significant metasomatism. For example, exchanging Si in interbedded clastic and carbonate beds that are exposed to regional meta.

Question 44

What does K/Na for fluids indicate?

Answer 44

This indicates the fluid source as magmatic fluids tend to have more K and terrestial fluids more Na

Question 45

How does iron interact within M-H systems?

Answer 45

Fe is everywhere so it is almost inevitably preceipitated within the deposits. The oxidation/reduction of the fluids dictates the minerals formed.

Question 46

What minerals would form in a limestone skarn vs. dolostone vs. siltstone?

Answer 46

In limestone garnet, wollastonite and calcite would form. In dolostone the 50:50 Mg:Ca ratio forms diopside, tremolite, and talc. Pyroxene within siltstones.

Question 47

How are H and O used to ID the source of fluids?

Answer 47

Rocks have alot more O than H, particularily ¹⁸O, which means that when water percolates through them that they deviate from SMOW and/or when magmatic waters and terrestial waters mix

Question 48

Alunite

Answer 48

This is a low T sheet silicate that forms when the activity of H₂SO₄ is high

Question 49

How do lodes and replacement deposits compare and contrast?

Answer 49

Lodes are different from replacement because they do not necessitate a reaction with the wall rocks like replacement does. They are commonly co-associated (like in MX) and reflective of low to mid T hydrothermal fluids.

Question 50

Mantos

Answer 50

These are CO3 replacement deposits that form flat lying (related to preferrential dissolution) high grade deposits of Cu, Zn, Ag, Au (~2-3%).

Question 51

What are some of the most common skarn minerals and their associated timings?

Answer 51

Garnets: early, high-T, -> andradite is high Ca
Pyroxenoids: early and around the garnet
Amphiboles, sheet silicates, and FeOs: are all later in the process and superimpose the garnets/pyroxene
Sulfides: mid-late in the process and distal
Quartz and CO3 minerals: distal to skarn and late

Question 52

Compare and contrast metasomatism (like skarns) and metamorphism (like hornfels)

Answer 52

Metasomatism has very few minerals involved but high variance across deposits. Metamorphic processes produce a set of very similar diverse minerals regardless of the localle.

Question 53

Chimneys

Answer 53

These are a type of metasomatic deposit which are “dyke like” in geometry, meaning that unlike their oftentimes associated mantos, they are perpindicular to the dominant bedding in a region.

Question 54

Felsic skarns

Answer 54

These are W and lithophile (F, Be, Zn, Mo, Sn, W) skarns that form because of the more fractionated source magma and associated fluids. They are typically reduced magmas that are on the order of 100,000 tonnes (slightly smaller than other skarns)