HYDROTHERMAL & MAGMATIC-HYDROTHERMAL ORESUnderstanding Economic Geology--Eamon McCarthy Earls

FLUIDS IN THE EARTH

HYDROTHERMAL FLUID SOURCESSeawater Na+, K+, Ca2+ Mg2+

Cl-, HCO3-, SO42-

Metal redistribution in crust Expelled through ‘black

smokers’

Meteoric (precipitation) water Main component of

groundwater

HYDROTHERMAL FLUID SOURCESConnate Water Bound in interstitial sed. pores Increases in density & salinity

with depth Bound water in H2O/OH- in clays

Metamorphic Water >200°C At 300°C kaolinite

metamorphoses to pyrophylliteproduces water

At 400°C muscovite & chlorite go to biotitewater, CO2, methane & sulfur

OTHER METAMORPHIC FLUIDS CO2 most common fluid after water High concentrations of CO2 in high-grade metamorphism

Low/medium grade metamorphism—blend of CO2, H2O & CH4

At very high temperatures CO2 & H2O become fully miscible and form a single fluid, dissolved together

VOLUME OF MINERALS PRECIPITATED FROM A FLUID Assumes equilibrium of pore fluid & host rock

Vm=Ftsinβ(dT/dZ)αT/ρVm=volume mineral precipitatedF=fluid fluxt=time (seconds)β=angle of direction of flow & isotherms in rockdT/dZ=geothermal gradientαT=function of solubility & temperatureρ=density of material precipitated

FLUID FLOW IN THE CRUST Pervasive flow in shallow, porous rock Ocean fluid flow driven by heat at mid-ocean ridges—seawater is superheated after migrating into faults

Dilatancy: change in volume of rock due to seismic shifts & shearing

Orogenies can create dilatancy

LITHOSTATIC VS. HYDROSTATIC Lithostatic: pressure gradient of units of rock based on overlying rock densities

Can vary slightly based on the dip of rock

Hydrostatic: pressure on rock at bottom of ocean

MAGMA RELATED HYDROTHERMAL DEPOSITS

QUARTZ & OTHER VEINS Precipitation of silica from hot fluids

Water is a more powerful solvent at higher temp. and pressure

8 wt% silica solubility in water

Need pressures of 7 kbar & 900 C

High temp. water easily dissolves alkali elements

Often similar composition to a silicate melt

Ligands—electron donors are dissolved anions

MAGMATIC SOLUTIONS 10 major oxides make up most rocks Alkali+alkali earth metals dominate in magmatic fluids

Volcanic & geothermal fluids may be best indicators

Generally low in Sulfur—SO2 partitions into vapor at high temperatures and boils away

Oxidized SO4^2- and HS- are mutually exclusive forms

Cu-Mo porphyrys associated with reduced waters, chalcopyrite and pyrite

ANALYSIS Examine fluid inclusions in quartz

High temp-high salinity results common in early stages

Evidence for boiling Microphotographs of daughter crystals—halite, sylvite (KCl)—indicate large amounts of alkali and halogen elements

Laser-ablation inductively coupled mass-spectrometry (LA-ICP-MS)—analyzed tiny quantities of cations

CO2 IN MAGMATIC FLUID Lower solubility in magma than water

Carbonate ions—alkaline magmas

Molecular carbon in felsic/mafic

Early vapors more CO2 rich than later due to volatility

Shallow emplacement—H2O-rich

N2 & CO2 effervesce easily

OTHER MAGMA PROPERTIES Differentiation of magma based on density

Chemically distinct phase in granites due to aqueous saturation

Bubbles of gas form in magma when vapor pressure=load pressure

First boiling—vapor saturation happens because of decreased pressure as magma moves to a shallower point

Second boiling—anhydrous minerals crystallize, leaving H2O saturation

Immiscibility—low salinity/high salinity brine segregation in presence of CO2 or low pressure

PEGMATITES Felsic rocks with large crystals, possibly formed from highly mobilized ions in heated fluid

Very coarse grained—granitic minerals

Quartz, feldspar, muscovite, topaz, tourmaline, beryl

Lithophile alkali and transition metals

Cerny (1991) grouped them by metal concentrations

I-type: Nb-Y-F S-type: peraluminous, Li-Cs-Ta, Boron

One of the least understood processes in igneous petrology

PEGMATITE FORMATION Jahns-Burnham model (1969)—H2O fluid saturation causes the change from granite to pegmatite

Volatile-related solidus depression Mineral zones—density difference between fluid & melt

D. London et. al. (1996)—undercooling granite magmas can also form pegmatites

Pegmatites can be replicated in petrology labs

Uncommon in mafic rocks because mafics nucleate crystals too rapidly

B-F-P+water increase the range of possible crystals that can form at lower temperatures

FLUIDS & TRACE ELEMENT PARTITIONING Holland (1972) showed that Cl- controls most metal solubility in water along with temp. and pH

This ligand must be able to partition into water in the first placeCl-(melt)+OH-(melt)HCL (fluid)+O2-

(melt)

PORPHYRY DEPOSITS Base metal deposits in granite intrusions

Cu-Mo or Sn-W (tin & tungsten)

Calc-alkaline, I-type, arc-related

Near Andean subduction zones S-type granite related to Sn-W—also partial melting of metasediment material

Mo or Cu dominates, in most porphyrys with the other as a secondary metal

Need oxidized I-type conditions near subduction zones

PORPHYRY DEPOSITS Limited fractionation during crystallization due to low water content

Poorly differentiated porphyry rocks like rhyodacite

Early vapor-saturation (first boiling)

Lack of water limits crystallization and keeps Cu in a nearly pure form—ordinarily it’s compatible with granite

By contrast, Mo is incompatible and ends up in the H2O phase—low partition coefficient helps to explain why Mo is usually the secondary metal

TUNGSTEN PORPHYRYS Arc related Reduced ilmenite-rich S-type granites

Deeper below the surface

W is incompatible—it fractionates during crystallization

SKARN DEPOSITS Calc/silicate replaces limestone and other carbonates

Contact metamorphism OR metasomatism

Enriched in Sn, Au, W, Pb, Fe, Cu, Mo

Dolomite vs. limestone origin determines calcium vs. magnesium content

Tungsten mainly from skarns Tasmania MacTung—Yukon Territory Bingham district, Utah—copper skarn related to porphrys

Fe skarns in Sverdlovsk & Sarbai, Russia

EPITHERMAL GOLD Recent volcanics in the Pacific Rim

New Zealand—North & South Island Shallow depths: 50-1000 m Low temp: ~200 C Controlled by high or low-sulfidation

High sulfide: Vapor saturation fluids from magma—highly oxidized and acidic

H+ remains in fluid as CO2 & SO2 partition—leaches most elements

Common in Andes Low sulfide: meteoric water nearly neutral, with low Ph

Common in western US

HYDROTHERMAL ORES

METALS & LIGANDS Ligand: ion or molecule bonded to a metal atom to form a coordination complex

CONTROLS ON PRECIPITATION-TEMPERATURE Temperature: cooling of fluid increases chances of ore-formation

Not true at depth where rocks & fluids have = temp

VMS Deposits: rapid cooling of superheated vent brines in the deep ocean

Enriched in metals—often with chloride or bisulfide complexes

Most efficient ore deposition process for metals

CONTROLS ON PRECIPITATION-PRESSURE Less pressure=more soluble Not generally related to ore-formation Significant in cases of effervescence or boiling

CONTROLS ON PRECIPITATION-PHASE SEPARATION Liquidvapor transformation due to decreased pressures

Depressurization causes effervescence—H2O & CO2 separate

In most metamorphic rocks, H2O & CO2 are mixed, but may separate in cracks leading to ore precipitation

Faults, cracks and fissures create reduced pressure environments needed for deposition

FLUID MIXING Two or more fluids of different temperatures can lead to precipitation when combined

Blended meteoric & magmatic waters produces some of the most economically valuable hydrothermal ores

Connate & meteoric fluids may mix to create sediment hosted ores near continental rifts

Ex) SEDEX deposits in eastern Australia

Fluid Blocks: altered mineral assemblies close to fluid pathways—metals precipitate on walls

Changes in redox & pH

ADSORPTION OH- (alkaline solutions) can promote cation adsorption

Broken or damaged minerals with fracture planes, trace element substitution points or high charge areas invite + acidic solutions tend to adsorb anions

Adsorption usually happens at low temps

Oxides & sulfides play an important role

Ex) goethite+acidic solution= great adsorption

Mercury easily adsorbs to sulfides along with Zn & Cd

BIOMINERALIZATION Many microbes can interact chemically with metals

Possible for microbes to influence local metal precipitation or to take in the metal within their cell membranes

Biologically induced: microbes changes local pH & Eh

Ex) ferric hydroxide (FeOH3) forms due to microbes & can remineralize as goethite/hematite

Several species of bacteria concentrate Fe grains

BIOMINERALIZATION Biological Control: formation within the cell

Ex) magnetite crystals often form in favorable reducing conditions in bacteria

SRB—sulfate reducing bacteria: generate sulfide & accept electrons to organic sulfates—source of pyrite

Birnessite & vernadite, Mn-minerals formed due to bacteria

Phosphorites in Australia Apatite crystals in place of cyanobacteria

Carboniferous sea water reduced around vents to form Irish SEDEX Pb-Ba-Zn deposits

HYDROTHERMAL ALTERATION

PROPYLITIC ALTERATION Most common hydrothermal alteration

Closely resembles greenschist facies metamorphism

Similar suite of minerals

Epidote, chlorite, calcite, clinzoisite, albite

200-350 C H+ metasomatism related

Also Cu-porphyry related

POTASSIC ALTERATION Forms biotite or new K-spars

Sometimes chlorite, sericite or quartz

500-600 C Cu-porphyry related Hydrolysis & K+ metasomatism

Yerington, Nevada—Na/Ca alteration instead of K

SERICITIC/PHYLLIC ALTERATION Wide range of temperatures

Feldspars hydrolyzed Forms sericite (white mica)

Small formations of quartz, pyrite, chlorite

Also related to Cu-porphyry deposits

Potential associations with VMS & mesothermal deposits

ARGILLIC ALTERATION Mostly plagioclase feldspars

Forms clays—smectite, montmorillonite, kaolinite, dickite, alunite

250 C H+ metasomatism Related to several porphyries in deposit margins

Base leaching—removes most alkali elements

Epithermal or near surface Boiling fluids Highly-acidic volatiles

SILICATION (NOT BE CONFUSED WITH SILICIFICATION) Carbonates converted to silicates by replacement

Cation metasomatism Polymetallic skarn deposits

Acidic magma-related fluid invades carbonates

Carbonates easily form deposits due to permeability, susceptibility to acid, and tendency to neutralize solutions

High fluid/rock ratios

SILICIFICATION(NOT BE CONFUSED WITH SILICATION) New quartz or amorphous (glass) silica forms in rock

Alteration haloes around many ore deposits

Isochemical hydrolysis related

Tends to fill in fractures SiO2 leaches out of country rock into hot fluid

Cation metasomatism—high-grade conditions mobilize Si4+

Common in high-grade epithermal zones

GREISENIZATION Related to ‘cupola’ zones in S-type granites

Need highly-differentiated country rock

Sn-W, F, Li, B enrichment Altered sequence of muscovite, topaz, quartz

Less commonly fluorite & maline Form in close proximity to quartz-wolframite-wolfenite zones

HEMATIZATION Related to oxidizing fluids Mineral associations with seracite, hematite, chlorite, epidote & K-spar

Cu-Au-Fe-U deposits in South Australia Cu-Co ores in Central African copperbelt

High salinity, oxidized fluids come in contact with reducing country rock

High Fe2+ & Fe3+

METAL ZONING Zoning: Regular patterns of metals/minerals in ore deposits

Can happen across 100s of kilometers Can be subduction related (Andes) Ex) Cornubian batholith, UK—Sn-W-Pb-Zn-Ag-Sb-U sequence etends laterally

Related to evolving hydrothermal fluid Precipitation follows a defined sequence

PARAGENESIS Guilbert & Park (1968)—paragenetic sequences are minerals/metals related to the same original source

Hypothermal: SN-W-Mo precipitate first Mesothermal: Zn-Pb-Cu-Mn precipitate later

Epithermal: Au-Hg-Sb, near surface fluids

Emmons precipitation sequence developed in 1933

Metal-sulfide presence promotes Emmons precipitation sequence

With metal-chlorides, a smaller sequence of Cu-Ag-Pb-Zn is likely to form

METAL/MINERAL REPLACEMENT Need pore space or microfractures Oxides are less likely to be fully sulfidated—Ex) ilmenite, magnetite

At depth, molar volume is the biggest control on replacement

Small volume minerals must replace large volume minerals

VMS & SEDEX

VMS Volcanogenic/volcanic-hosted massive sulfide deposits

Volcanically related hydrothermal fluids formed during orogenies

Modern analogues or still ongoing “black smokers” discovered in 1977 on ocean floor on East Pacific Rise

400 C—highly reduced with high metal content

Contact 2 C ocean water, oxidized with low metal contents

Tube-like chimneys Cu-Zn deposits like Troodoos Massif, Cyprus related to mid-ocean ridge vents (hosted in ophiolite)

VMS Cyprus-type: common in Norway & Finland, leached from mafic source rock

Kuroko-type: common in Japan, Iberian Pyrite belts, & Newfoundland, arc-related ‘bimodal’—felsic & mafic source rocks

Most fluids from ocean water, small amounts from magma

Clear zonation: Fe-Cu-Pb-Zn Increasingly high temps as ore-body grows

Low temps incapable of dissolving enough metal

Can grow as chimneys collapse, sulfide precipitates out of seawater or dissolved metals arrive from below

VMS Organisms may support sulfate reduction in and around vents

Possibly true in Pb-Zn-Ba deposits in Ireland

Local fluid dynamics control whether the ore-body is dome-shaped (common) or table-shaped (uncommon)

When seawater & hot fluid have similar densities, precipitation hands in nearby low-topography

Rapid quenching before boiling for most metals

In faulted systems, hot fluids brecciate the footwall and deposit metals

VMS—HOW DOES IT GET ON LAND? Ex) Troodos Massif in Cyprus

Obduction—oceanic lithosphere is thrusted onto continental crust

Associated with: Pillowed basalt (common on the ocean floor)

Sheeted dikes Pelagic (mid-ocean) sediments

SEDEX More than half of all Pb & Zn deposits Sedimentary exhalative Paleoproterozoic & Mesoproterozoic British Columbia, South Africa, Australia, India

Modern examples—Salton Sea East Pacific Rise junction with North American plate in Salton Sea area of California—geothermal fields

Man-made lake ‘Salton Sea’ caused after a dam break flooded a massive low-lying salt pan

Brine now mobilizes metals in lacustrine sediments and percolates down to temperatures of 350 C depositing metals

OROGENIC GOLD (ARCHEAN) Largely in greenstone/granite rocks dating to the Archean

Ex) Yilgarn, Zimbabwe, & Superior Provinces/Cratons

Brittle-ductile transformation areas Plate collisions & subduction Magmatic fluids—highly reduced and able to move AuHS2-

Fault ‘valves’ move fluid and are marked by chloritized/carbonated minerals

Large amounts of wall-rock alteration

OROGENIC GOLD (PROTEROZOIC +PHANEROZOIC) Relatively rare in Proterozoic except West Africa, South Dakota, Western Australia, & Sabie-Pilgrim field in South Africa

Housed in thrust faults Clusters in Silurian-Devonian & Cretaceous

Related to converging plates Cut through shales that have metamorphosed to greenschist facies

AKA Slate-belt hosted gold California, Uzbekistan, Ural moutnains, Nova Scota

CARLIN-GOLD Formed due to extension not compression First discovered around Carlin, NV in the 1960s Paleogence 42-30 Ma Also in China Associated with Basin & Range crustal thinning and carbonate aquifer rocks Medium acid, low salinity metamorphic-style fluids 150-250 C Carbonates neutralize fluids along with argillic, silicification, and sulfidation processes

QUARTZ-PEBBLE GOLD Archean & Proterozoic Witwatersrand gold field, South Africa

35% of all gold ever produced Poorly understood Detrital gold? Fluids in sediments causing precipitation?

Large haloes indicate multiple fluid cycles

Slow neutralization and precipitation?

Low salinity, moderate acidity fluids

Closely matched CO2 & H2O

CONNATE & METEORIC DEPOSITS

STRATIFORM SEDIMENT-HOSTED COPPER (SSC) 2nd main source of copper after porphyry deposits Abundant Pb, Zn, Ag, PGE & R Kuperschiefer—Central Europe Central African copperbelt Rift related Oxidized eolian & evaporite sediments buried and modified Neutral, saline, oxidized connate fluid altered sediments Copper leached out of biotite, magnetite, and pyroxene Deposition in shallow marine or red-bed environments

MVT (MISSISSIPPI VALLEY TYPE DEPOSITS) Pb-Zn Happen long after sediments deposited

Mainly sphalerite instead of galena

Exception to the rule—Viburnum Trend is balanced toward galena

Sandstone or carbonate host-rock

Other large deposits in Tennessee, Illinois, Oklahoma and Missouri

Australia, South Africa, Poland, Austria

Devonian-Permian Compression+ orogenies in Pangaea

High rainfall in sabkha zones—high salinity groundwater

METEORIC FLUID Rarely a source of ore transport U6+ uranium ion mobilized best in rainwater

Precipitates as uraninite under low pH conditions

Precipitates due to reduction at low temps

By contrast, U4+ is very incompatible except in monazite and a few other minerals

SANDSTONE-HOSTED URANIUM Colorado Plateau, Southwest, South Dakota & Wyoming

Colorado Plateau: U-V, Jurassic rocks Related to low temp mixing of meteoric water

Uranous silicate & coffinite from plant material

30-40 C related to basinal brines V-rich clay layers welded with dolomite

Coffinite/plant material naturally reducing and helsp to feed sulfate reducing bacteria

REFERENCES

REFERENCES Robb, L. (2005). Introduction to ore-forming processes. Blackwell Publishing.