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The tools used by economic geologists to study the formationand genesis of ore systems.

Petrography - determine the mineralogy and textures of both sulfideand non-sulfide minerals

Geothermometry - the temperature of ore depositionFluid inclusionsisotopic studiesmineral assemblage stabilities

hydrocarbon maturation

Geobarometry - the pressure of ore deposition, few reliable techniques

Isotope studies (H, O, C, Sr, S, Pb) to determine the sources ofmineralizing fluids and metals.

Geochronology - determining the age of mineralization

GEGN 401 - Mineral Deposits

Lecture 8 - Research Tools for Ore Deposits

M. W. Hitzman

Petrography Petrography (and micro-analytical techniques such as

microprobe, SEM, QEMScan) are used to determinemineralogy.

Petrography also allows observations of texturalrelationships between minerals.

Petrography is conducted in transmitted light (for non-opaque minerals) and in reflected light (for reflective opaqueminerals such as sulfides and oxides).

Mineralogy and textures are utilized to derive the parageneticsequence of a sample (in conjunction with megascopicobservations).

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Ore Petrology

Ore petrology is a cheap and relatively fastmethod of determining a variety of importantinformation:

mineralogy of a deposit mineralogy as regards metallurgy texture of sulfide minerals for metallurgical/

environmental parameters

paragenetic sequence of ore deposition to allow us toconstrain genetic models for ore formation

M. W. Hitzman

Ore Petrology - Mineralogy Determine major (and minor) mineral species Why is this important?

What metals of interest occur in the deposit - good and bad (Cu, Au good; As, Te possibly bad) Why not just assay the rock?

companies are often cheap - dont do a multi-element analysis - only find what you are looking for. Even if the company does a multi-element analysis, why is this not enough?

A sample may have high As - where is it? It could be in arsenopyrite or it could be in the main oremineral (enargite - Cu3AsS4). Likewise, if the rock has a high iron content - where is the iron? Itcould be in iron sulfide or maybe the iron is a substitution in sphalerite. If it is in the sphalerite thenyour zinc concentrate may have a lower zinc content than you expect. Both these possibilities (Asand Fe) can be easily determined with polished section work. Thus, ore microscopy can provideimportant (and cheap) clues as to the metallurgical processing that will be necessary.

Assay will just tell you elements - ore may contain abundant iron sulfide. Is it pyrite, pyrrhotite, ormarcasite. This makes a big difference. Marcasite is highly reactive and will oxidize easily(environmental problems). Pyrrhotite oxidizes much more slowly.

Is the metal of interest metallurgically available? In copper (zinc, lead, etc) deposits the Cu can occur as sulfides (cp, bn, cc, en, etc) or as oxides,

carbonates, chlorides, etc.

Flotation will work only on the sulfides (and different minerals have different flotation rates withdifferent chemicals).

Solvent extraction technology will only work on oxides, carbonates, etc.(and on chalcocite!). Oxidesleach with less loss of acid than carbonates.

But you can look at the rock and see if it is dominantly sulfide or oxide. Sometimes. In many cases, oxides coat sulfide particles. Thus the material will not leach well, but

because of the oxide coating it will not float. Need to understand the proportion (and texture) of themineralogy.

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Ore Petrology - Textures

Why are textures important? Grain boundaries - Do the sulfides show straight, regular boundaries? If so they may fall apart easily

when they are ground up.

What if the sulfides have complicated, embayed or irregular boundaries? These sulfides may be more difficult to separate with grinding - will probably need a much finer grind

(more expensive). In the worst cases the mineral of interest may form small particles completelyenclosed (encapsulated) in another mineral. If this mineral is a sulfide and may float duringprocessing (example gold in pyrite) then you will need extremely fine grind or another way to liberatemineral of interest (i.e. roasting).

Textures are especially important in gold deposits. A gold assay just tells you what the total amount of gold is in the rock - not how the gold is situated. It

can be associated with sulfides (need to roast the ores) or i t could be with oxides (heap leach).Alternatively it could be included as free gold in silica - may need a very fine gr ind to extract it -expensive. A relatively few polished sections may tell you much (early on) about the potentialeconomics of the property.

Why is grain size important? How large are the sulfide grains, particularly the sulfide of interest? This will tell you how fine you will

have to grind the ore to liberate the minerals. Fine-grained iron sulfides have a tendency to oxidize quickly - they can oxidize so fast that they catch

on fire and the mine has a real problem. Textural analysis with polished section work can alert thecompany to the potential for this problem.

Environmental - The grain size of the ore will also effect the size of the waste (waste dumps andtailings). The grain size will effect how the material will weather and how quickly it might generateacid mine drainage.

M. W. Hitzman


Texture indicative of simultaneous growth -

If minerals display relatively flat contacts and angles between contactsapproach 120, then the assemblage is moving toward equilibrium. Suchtextures can occur in hydrothermal ore deposits where several mineralsare precipitating together. However, this texture is more common in hightemperature magmatic deposits or in metamorphosed deposits.

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Exsolution -The migration of one or more components in an existing mineral bydiffusion to form a new mineral. The new mineral generallynucleates on cleavages and crystallographic planes. Exsolution iscommon upon cooling.

A) Seriate exsolution grains of hematite in ilmenite; B) exsolution lamellae of ilmenite in magnetite.(from Evans, 1993, p. 85)

M. W. Hitzman

Ore Petrology - TexturesOpen Space Filling - Colloform banding

Banded layers of one mineral or several minerals filling an openspace. It commonly displays a botryoidal texture (numerousinterfering circles or ovals). Texture may have formed as stalactitesor stalagmites.

Colloform sphalerite with concentricbanding, interpreted to be colloidal

in origin; Katowice, Poland. (fromGuilbert and Park, 1986, p. 154)

Colloform banding of different minerals fromLisheen, Ireland. Early pyrite is intergrown

with marcasite. There are late, thin dolomitebands and then sphalerite. Reflected light

image.

marc

dol

sph

0.5 mm

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Open Space Filling - Vein/vug Filling

Open space in rock filled by mineral growth. Often minerals displayunidirectional crystal faces. This type of banding is commonly, butnot always, symmetrical across matching vein walls.

Vein with inward-growing quartzcrystals and a centerline of calcite.

Note, vein offsets earlier vein (fromGuilbert and Park, 1986, p. 153)

Progressive infill of vug by pyrite (with minordolomite), sphalerite, and finally dolomite.

The sphalerite displays color banding.Note how crystals grow into vug. Lisheen,

Ireland. Transmitted light image.

py sph

dol

0.5 mm

M. W. Hitzman

Ore Petrology - TexturesReplacement textures

Textures indicative of replacement are important for determining theparagenetic sequence of samples. Replacement is indisputable if theform of a pre-existing mineral or rock component, such as a fossil, ispreserved in the later mineral.

Replacement of ooliths in limestone by pyrite which is itself being replaced by sphalerite.Lisheen, Ireland.

pysph

0.5 mm

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Replacement textures

Textures indicative of replacement are important for determining theparagenetic sequence of samples.

Oriented islands of pyrite (which define anoriginal large cube) replaced by

chalcopyrite. The texture suggests an earlypyrite-bornite assemblage which is then

replaced by chalcopyrite (from Guilbert andPark, 1986, p. 140)

Originally colloform marcasite with pyrite beingReplaced by sphalerite. Note replacement cuts

Largely across the colloform banding. Lisheen,Ireland. Reflected light image.

marc

py

sph

0.2 mm

M. W. Hitzman

Ore Petrology - TexturesReplacement textures


Assemblage of intergrown tetrahedrite andsiderite cut by chalcopyrite veins. The cpy

vein shows enlargement at a vein intersectionsuggesting replacement of adjacent minerals.

(from Guilbert and Park, 1986, p. 147)

Fragments of tetrahedrite within a chalcopyritevein. If the tetrahedrite appears to be optically

continuous this texture may indicate replacementof tetrahedrite by chalcopyrite. (from Guilbert and

Park, 1986, p. 147)

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Replacement textures


A precipitation sequence in which the minerals become progressively richer in an ore constituent mayindicate replacement. Here the sequence is from early chalcopyrite (cp) to bornite (bn) to chalcocite

(cc). The assemblage cp-cc is not stable at most hydrothermal conditions. Thus, the bornite is areaction rim separating the two minerals. (from Guilbert and Park, 1986, p. 145)

M. W. Hitzman

Ore Petrology - TexturesReplacement textures - Chalcopyrite Disease

In many volcanogenic massive sulfide (VMS) deposits, especially those which have not beenseverely metamorphosed, we observe sphalerite with numerous inclusions of chalcopyrite.Originally these were thought to be exsolution grains from a Cu-rich sphalerite. However,the amount of chalcopyrite is too high for there to have been this much Cu in the sphalerite.We find this texture near the base of the Zn-rich portions of VMS deposits. It is nowinterpreted to result from progressive heating of the sulfide body by hydrothermal fluidswhich progressively replace the lower temperature sphalerite assemblage with chalcopyrite.

Sphalerite-rich ore from the Furutobe mine, Japan which displays a myrmekitic intergrowth ofsphalerite in some crystals and very weak, diffuse chalcopyrite on sphalerite crystal edges

(difficult to see). Left - transmitted light; right - reflected light. (from Eldridge, Barton, and Ohmoto, 1983,in Ohmoto and Skinner, eds., The Kuroko and related volcanogenic massive sulfide deposits, Econ. Geol. Mon. 5)

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QEMScan

SEM technology that allows quantitative mineralogy and textural analysis and isexcellent for determining mineralogy for metallurgical studies. Available in CSMGeometallurgy Lab.

M. W. Hitzman

QEMScan - Mineralogy and Mineral Volume Data

Allows determination of volumes to be shown visually and then elementalabundances calculated numerically.

Na

Mg

Cu

K

Ksp

Alb

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QEMScan - Mineralogy and Mineral Volume Data

Allows determination of volumesof total sulfides (and visualpresentation - above) andmineralogy (visual and numerical -left).

M. W. Hitzman

Fluid Inclusions

Fluid inclusion Liquid and/or gas trapped within imperfections ordislocations in crystals while they grow. Fluid inclusions range insize from infinitesimal to several cm in diameter.

(from Goldsteinand Reynolds, 1994)

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Fluid Inclusions

Types of fluid inclusions: Primary: Fluid inclusions dispersed

through a mineral with no clearrelationship to any structure (such as afracture) that would permit the escape orentry of either gas or liquid. They may beconcentrated in primary crystallizationfeatures such as growth zoning. Theyoften have negative crystal shapes.

Secondary: Fluid inclusions that formedby any process after the primarycrystallization of the host crystal. Theclassic example is inclusions formedalong a fracture in a mineral byprogressive healing.

Psuedosecondary: Have the linearappearance of secondary inclusions(along a fracture) but formed in a crackwhich was produced while the crystalwas growing.

Primary fluid inclusions in quartz

(from Goldstein and Reynolds, 1994)

Secondary and psuedosecondary trappedAlong healed fractures. To right, secondary

Fluid inclusions in calcite. (from Goldstein

and Reynolds, 1994)

M. W. Hitzman

Fluid Inclusion Contents Fluid inclusions commonly contain liquids

an aqueous (H2O) fluid with dissolved salts (Na, Cl, K, Mg, etc.) liquid CO2 liquid hydrocarbons

Fluid inclusions commonly contain gases: H2O vapor CO2 methane

Fluid inclusions may contain daughter minerals: halite (NaCl) sylvite (KCl) anhydrite (CaSO4) hematite (Fe2O3), magnetite (Fe3O4) chalcopyrite (CuFeS2) solid hydrocarbons

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Fluid Inclusion Types

Five types of inclusionshave been described*:

I. liquid with small vaporbubble, no daughterminerals

II. liquid with large vaporphase and no inclusions

III. Polyphase inclusionswith small vapor bubble

IV. two liquids with vaporand daughter minerals

V. liquid CO2 with vapor

* Nash, J.T. and Theodore, T., 1971, Ore fluids inthe porphyry copper deposit at CopperCanyon, Nevada: Econ. Geol., v. 28, p.744-750.

M. W. Hitzman

Working with Fluid Inclusions (I)

Most work with fluid inclusions is done optically - hence needinclusions in transparent or translucent minerals:

most common: quartz, sphalerite, fluorite commonly used but often problems because of weakness of minerals (leads to

leaking or breakage over geologic time): barite, calcite, dolomite, halite

good minerals if available: apatite, topaz, cassiterite Standard thin sections are utilized for the initial search for fluid

inclusions. If this is successful a quick plate - a thicker section ismade to refine the petrography of the fluid inclusions.

Once, a sample with good inclusions has been identified, thinwafers (up to 1 mm thick) are produced and polished on both sides(to reduce light scatter and improve visibility).

These doubly polished sections are placed on a microscopefitted with a heating and freezing stage.

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Working with Fluid Inclusions (II)

Heating experiment Thetemperature is raised until theinclusion (if liquid + vapor) isrestored to a single homogenousfluid. This is the homogenization orfilling temperature. Themeasurement assumes that theinclusion trapped only a liquid andthat the liquid has separated intoliquid + vapor (+daughter minerals)during cooling.

The filling temperature is theminimum temperature of formation.The actual trapping temperature mayhave been higher depending on the

pressure on the system at the timeof trapping and the salinity anddensity of the fluid.

Frequency histogram of homogenization temperatures(Th) for fluid inclusions in different minerals. Temperature

Reached at least 150C. (from Goldstein and Reynolds, 1994)

M. W. Hitzman

Working with FluidInclusions

The salinity of the solution in vapor + liquidtwo phase inclusions may be determined byfreezing the liquid in the inclusions and thenobserving the temperature at which the icemelts upon warming. Since the freezing pointdepends on the amount of salts in thesolution, freezing-point depression tables areused to calculate salinity. Note that it isgenerally assumed that all the salinity iscaused by NaCl.

Freezing-point depression measurement isnot necessary if the inclusion contains NaCldaughter minerals. In this case the volume ofthe mineral relative to the inclusion volume ismeasured and the salinity is calculated on thebasis of the amount of halite and salt-

saturated liquid present. Note that with other phases (CO2,

hydrocarbons, etc.) other phase relationshipscan be determined.

Current technology, using a laser (laserablation) allows us to open fluid inclusionsand measure the composition of thecontained fluids.

Frequency histogram of first melting temperatures (Tm)for fluid inclusions in different minerals. Note that primaryall-liquid fluid inclusions in dolomite span a narrow rangethat does not include the range of the later fluid inclusions(in calcite and fluorite). This suggests that some of the 2phase inclusions in dolomite that appear primary haveleaked and refilled during thermal reequilibration. (fromGoldstein and Reynolds, 1994)

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Evidence of Boiling from Fluid Inclusions

Boiling of hydrothermalfluids can be established ifprimary inclusionpopulations include twotypes of inclusions onethat is vapor-rich andhomogenizes to a vaporupon heating, and anotherthat has a very small vaporbubble which dissolves byhomogenizing to a liquid.

This indicates that two fluidswere trapped nearlysimultaneously: a vapor anda liquid. This condition canonly occur if thehydrothermal fluid itself wastwo phase (i.e. boiling).

M. W. Hitzman

Fluid Inclusions Provide Evidence of theComposition of Hydrothermal Fluids

Fluid inclusions provide us with our best source ofinformation concerning the chemical composition of orefluids since they are trapped representative samples of suchfluids.

We sample these fluids by decrepitation: breaking open theinclusions either en-mass (crushing of rock or mineral) andmeasuring the composition of the fluids or by using a laserto burn a hole into a single inclusion and vaporize the liquidand analyze it.

Other less invasive techniques are also being developed toanalyze the contents of single inclusions.

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Examples of Fluid Inclusion Data

When combined with textural paragenetic sequence information, fluidinclusion data can provide constraints on the changes in fluidtemperature and/or composition through time.

Tin - Tungsten Ores, Bolivia

Paragenesis of different minerals plotted against homogenization temperatures of fluid inclusionsIn those minerals. (from Kelly and Turneaure, 1970 in Evans, 1993, p. 43).

M. W. Hitzman

Geobarometry

Determining the pressure at which different minerals formed isdifficult. While some minerals or mineral assemblages arespecific to pressure environments (e.g. glaucophane - highpressure amphibole), most minerals associated withhydrothermal ore deposits form at a wide range of pressures.

Work has been done looking at the amount of trace elementssome minerals, such as sphalerite, can contain at differentpressures - however, there has been controversy about theapplicability of this work.

We generally assume either lithostatic or hydrostaticconditions.

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Pressure

Lithostatic (geostatic) pressure Thevertical pressure at a point in the Earthscrust, equal to the pressure that would beexerted by a column of the overlying rock orsoil.

Hydrostatic pressure Stress that isuniform in all directions beneath ahomogeneous fluid (causes dilation ratherthan distortion). Implies opening of fracturesto Earths surface.

M. W. Hitzman

Isotopic Studies - Stable Isotopes

Stable isotopes of an element are atoms which have the same number ofprotons, but differ in the number of neutrons in the nucleus. Stable isotopesdo not undergo spontaneous decay.

The most useful pairs of stable isotopes are: hydrogen - deuterium (Z the number of isotopes of an element = 1; 1H = 99.985%,

and 2H = 0.015%)

carbon (Z=6; 12C = 98.89%, and 13C = 1.11%) oxygen (Z = 8; 16O = 99.756%, and 18O = 0.205%) sulfur (Z = 16; the most useful isotopes being 32S = 95.05%; and 34S = 4.21%)

Stable isotope analyses are conducted on: present day fluids fluids extracted from fluid inclusions minerals precipitated from a hydrothermal solution minerals altered by a hydrothermal solution

Isotopic values of minerals are generally reported in permil. Permil meansper thousand (10 permil = 1%).

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Variations in the geologic distribution of the stable isotopes depend onslight differences in the physical and chemical behavior in naturalenvironments. Fractionation, or stable isotope separation, occurs becausethe thermodynamic properties of ions, radicals, and molecules depend onthe mass of the atoms of which they are composed. The different isotopesrepresent slightly different masses.

In general, bonds formed by lighter ions (lower numbered isotopes) areweaker, so the lighter isotopes ions are more mobile and reactive thanequivalent heavy-isotope ions.

Processes that cause stable isotope fractionations include: evaporation and condensation oxidation-reduction dissolution - precipitation from aqueous solutions absorption - desorption reactions microbially catalyzed reactions such as in sedimentary - diagenetic environments diffusion

M. W. Hitzman


The utility of stable isotope studies is in howthey help us understand natural processes.

For example, vapor evaporated from seawaterhas lower18O/16O and deuterium (2H)/hydrogen(1H) ratios than seawater itself because watermolecules containing light isotopes can escapefrom liquid to vapor (evaporate) preferentially.

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Stable Isotope Studies

Minerals precipitating from a solution will retain a memory of theoxygen, hydrogen, carbon, and sulfur characteristics of the fluid fromwhich they formed by precipitating ions of those isotopes. The stableisotopic ratio of a mineral is a function of the isotopic value of the fluidand the temperature of mineral formation.

Note that an individual mineral mayhave a number of isotopic values indifferent growth zones or thatalteration of that mineral to anothermineral may introduce otherisotopic values.

Map of an individual galena crystal from the Viburnum Trend, MO showing 134 individual ionmicroprobe 208Pb/206Pb ratios. These have then been contoured (note 913 means 1.913). This

shows the zoning of lead isotopes within the individual crystal.(from Hart et al., 1981, Economic Geology)

M. W. Hitzman

Stable Isotope Studies

Minerals which have been altered by a hydrothermal solution can have complexisotopic signatures. The isotopic value of such minerals is the result of:

the stable isotopic composition of the original mineral prior to alteration the extent of isotopic interaction between the fluids and the rock.

The extent of the interaction between hydrothermal fluid and minerals in thehost rock is typically expressed as a water/rock (w/r) value which is the totalmass of water interacting with a total mass of rock over time.

If w/r values are substantially greater than 1, then extensive isotopic exchange occursbetween the fluids and the host rocks and the stable isotopic composition of the hostrocks will approach equilibrium with the original isotopic values of the fluid.

If w/r values are significantly below 1, then the stable isotope composition of the rockswill remain essentially unaltered and the stable isotope composition of the fluids wi llshift toward equilibrium with the host rocks.

Minerals may also have a memory of post-depositional exposure to fluidsother than the original hydrothermal fluids that are isotopically different.

It is critical that extreme care is taken to ensure that the samples usedfor isotopic analysis are well known in terms of their context andrelationship to paragenesis.

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Stable Isotopic Studies:Oxygen and Hydrogen

An isotopic standard is need against which to measure isotopic shifts. The isotopic near-uniformity of theoceans makes it a logical choice. Therefore standard mean ocean water (SMOW) is the value against whichother values are plotted.

With SMOW as a standard, other fluids can then be analyzed. For meteoric water (rainwater), the firstrainwater falling will be isotopically heavy relative (enriched in 18O and D relative to vapor) to later rainwater(the reverse of evaporation).

As an air mass progresses inland across a continent, or to higher latitudes and elevations, it becomessteadily depleted in the heavier18O and D ions. Thus we can contour changes in isotopic variation ofrainwater (D or18O). How could you use O , H isotopes for forensic science? [Note how low the isotopicvalues for meteoric water are for Colorado]

Also, meteoric water becomes lighter towards the poles and heavier near the equator.Average D values Average 18O values

(after Taylor, 1974, in Guilbert and Park, 1986, p. 274-275)

M. W. Hitzman

Since the fractionation ofmeteoric water is proportionalit can be described by astraight line (meteoricwaters on the figure).

Through isotopic analysis ofvarious types of fluids indifferent environments (aswell as analysis of mineralsformed in thoseenvironments) we candetermine the isotopic

composition, in terms ofDand 18O, of different types of

fluids - magmatic,metamorphic, sedimentary.

Stable Isotopic Studies:

Oxygen and Hydrogen


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If we know thetemperature of mineralformation and theminerals contain both Oand H, or if we have co-existing minerals one ofwhich contains O and theother H, we can calculatethe isotopic compositionof the fluids responsiblefor the mineralprecipitation.

These values for the fluidcan then be compared to

known fluid compositionsto see if the deposit wasformed in a magmatic,metamorphic, orsedimentary environment.

Stable Isotopic Studies:Oxygen and Hydrogen

M. W. Hitzman

Oxygen (and hydrogen)isotopic analysis can alsobe utilized in exploration.For example, in theNoranda volcanogenicmassive sulfide district ofCanada, Cathles (1993)identified a 2 to 4 km widering of low whole rock 18Ovalues (

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Stable Isotopic Studies:

Oxygen and Carbon An isotopic standard for carbon is need against which to measure isotopic

shifts. The standard that is used is that of carbon in CO2 evolved byphosphoric acid digestion of a particular fossil, the Pee Dee Belemnite andthe standard is termed PDB. This standard is used for carbon and has beenused in place of SMOW for oxygen - particularly among carbonateresearchers.

The ultimate isotope composition of a carbon compound precipitating from afluid depends on:

the carbon isotope ratio of the fluid pH which affects speciation between CO, CO2, H2CO3, HCO3-, CO3-2, and CH4 (each

of which affects 12/13C distribution

oxygen fugacity temperature

The principal sources of carbon include: marine carbonates (around 0 permil) atmospheric CO2 (13C = -8 permil) magmatic (13C = -2 to -10 permil) biogenic carbon (13C = -25 permil)

M. W. Hitzman

Measurement of thestable isotopes ofcarbon and oxygenare used todifferentiatedifferentgenerations ofcarbonate mineralsin carbonate-hosteddeposits or thosewith carbonate

mineral gangue.

Stable Isotopic Studies:Oxygen and Carbon

...

5"

In the Irish ore system analysis of hydrothermal dolomite gives insightsinto the degree of water/rock interaction and hence the volumes ofhydrothermal fluids that passed through an individual prospect or deposit this can be used as a proxy for the size of the associated ore deposit.

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Sulfur isotopes are particularly useful in economic geologybecause so many ore minerals are sulfides and because sulfateminerals are common gangue.

The sulfur isotope standard is the sulfur in troilite (FeS) fromfragments of Arizonas Canyon Diablo meteorite.

The measured isotope ratio of32/34S in the troilite of themeteorite is 22.22.

The 34S of another mineral is defined as:34S = 34S/32S sample -

34S/32S standard /34S32S standard.

Therefore, Canyon Diablo troilite has 34S of zero.

Stable Isotopic Studies: Sulfur

M. W. Hitzman

Sulfides from different geologicenvironments have different sulfurisotopic compositions.

Sulfide from meteorites clustersaround 0 permil.

Sulfide from magmatic rocks alsoclusters around 0 permil but shows awide dispersion around this averagevalue.

Sulfide in sedimentary environmentsgenerally has negative sulfur isotopic

values. The most common values arebetween 0 and -20 permil.

Marine sulfate has varied throughgeologic time which provides a meansof dating evaporites and providesclues as to the source of sulfur in somedeposits which may have involvedseawater.

Stable Isotopic Studies:Sulfur Isotope Variations in Nature and through Time


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Fractionation among sulfur isotopes in nature occurs either organically or inorganically. Organic fractionation:

Several anaerobic bacteria consume SO42- (break S - O bonds to generate energy) and excrete gaseous H2S. Bacteria can break 32S - O bonds more easily than 34S - O bonds so the SO42- consumed and the H2S

generated are preferentially isotopically lighter.

The isotopic values may lightened to 34S values of -20 to -30 permil. The H2S produced through these biogenic reactions is free to react with Fe (or other metals Zn, Pb, Cu) to

form sulfides which would be isotopically light (called bacteriogenic sulfides).

Stable Isotopic Studies:Sulfur Fractionation

Inorganic fractionation: The ion species of sulfur important

in hydrothermal fluids attemperatures between 25 and 500C include H2S, HS

-, S2-, SO42-, HSO4

-,NaSO4

-, KSO4-.

The activities of these ions are pHand oxygen fugacity dependent H2S, HS

-, S2- are largely pHdependent, while oxygen fugacity

controls SO42-

relative to reducedspecies.

Inorganic fractionation betweenindividual sulfide and sulfateminerals is also temperaturedependent. If these mineralscrystallized in equilibrium then it ispossible to utilize sulfur isotopicresults for geothermometry.

(from Evans, 1993, p. 88)

M. W. Hitzman

Understanding of thedistribution of sulfurisotopic values in adeposit (or district) canrefine the model of howthe deposit formed andlead to betterexploration models.For instance in Ireland,sulfur isotopic studiesindicate that majordeposits formed frommixing of a metal-rich,sulfur-poor ore solutionwith a sulfur-rich fluidderived from overlyingseawater. Without thesecond, sulfur-richsolution, sufficientsulfur will not exist toprecipitate largeamounts of sulfide.Hence prospects withonly isotopically heavysulfides are likely not tohave potential for largedeposits.

Stable Isotopic Studies:Sulfur Isotopes in Exploration

KilloranFaultZoneLK3N S

+18

+15

+5

-35

Deepsulfur

Carboniferousseawater sulfate

-20-40

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Radioactivity is a function of time and the decay constant only.Temperature, pressure, chemical mass, or element activity donot affect it.

Five radioactive decay series are widely used in either sourceor geochronologic studies:

rubidium-strontium (87Rb - 87Sr) uranium - thorium - lead (235U - 207Pb, 238U - 206Pb, and 232Th - 208Pb) potassium - argon (40K - 40Ar) samarium - neodymium (147Sm - 143Nd) carbon - nitrogen (14C - 14N)

Isotopic Studies - Radiogenic Isotopes

M. W. Hitzman

The isotopic value of Rb-Sr occurs because of the difference in ionic size ofthe two atoms:

Rubidium has so large an ionic radius that it cannot be accommodated in mostminerals, thus when the earth was formed it was rejected from the mantle andconcentrated in the crust.

Strontium has a much smaller ionic radius and is similar in size and geochemistry tocalcium. Strontium is preferentially taken into calcium sites in plagioclase,pyroxene, garnet, and carbonate minerals.

There is a negligibly low value of Rb/Sr in the mantle,thus the strontiumisotopes of mantle materials have undergone almost no change by decay ofrubidium.

Because of this mantle-derived intrusions have low 87/86Sr values whileintrusions that have assimilated crustal rocks have higher values (more 87Srderived from Rb decay) and intrusions derived exclusively from crustalmelting have still higher values. Thus we can finger-print the source ofintrusions associated with mineralization.

Isotopic studies of alteration feldspars, carbonates, and sulfates (all with traceSr substituting for Ca) can also provide evidence of the pathway taken byhydrothermal fluids responsible for precipitating the Sr-bearing phases(through primitive mantle materials or through crustal materials).

Radiogenic Isotopic Studies:Rubidium - Strontium for Sourcing

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Lead consists of four stable isotopes: 204Pb, 206Pb, 207Pb, and 208Pb. 204Pb has no known radioactive parent and is a stable isotope. Thus its

abundance does not change.

206Pb, 207Pb, and 208Pb increase as a result of decay of uranium and thorium. Lead isotope measurements are generally reported as ratios relative to 204Pb:

206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb.

The percentage difference between isotope masses is so small that naturalphysiochemical fractionation can be disregarded.

Lead isotope ratios change systematically as a function of time and 204Pbinheritance from different crustal regimes.

In the crust and mantle, the three lead isotope ratios increase with passingtime as more uranium and thorium decay into 206Pb, 207Pb, and 208Pb (and204Pb remains constant).

A mineralizing event leaches lead from silicate minerals (and separates itfrom associated U and Th) and fixes it in sulfide (galena), thus freezing in alead isotope ratio. Note that younger formed galena should be moreradiogenic (have larger ratios).

Radiogenic Isotopic Studies:

Uranium - Thorium - Lead

M. W. Hitzman

Isotopic analysis can determinewhether the leads are derived from asingle-stage or two-, three-, ormultiple-stage lead isotopeevolution.

Model-lead ages can be measuredwhen lead is removed from a single-stage evolving source (the mantle orthe crust) and is deposited asgalena which contains no U andthus isotope ratios are locked in atthe time of isolation. These isotoperatios provide an exact age ofgalena formation.

However, if the lead has beenleached from one source, depositedin another, and then leached againand deposited (ex. erosion of aPrecambrian mountain belt to formsediments which cycle prior tobeing leached by basinal brines)then the original U-Th-Pb ratios willalmost certainly have been modifiedand calculated ages will reflect morethan one 238U/204Pb source.

Radiogenic Isotopic Studies:

Uranium - Thorium - Lead for Geochronologyand Sourcing


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A number of dating techniques are used in economic geology: U-Pb (238U to 206Pb; half-life of 4.5 x 109 years) used for dating uraninite,

pitchblende, zircon, sphene, titanite, apatite, epidote, and whole rock.

Rb-Sr (87Rb to 87Rb; half-life of 5.0 x 1010 years) used for dating mica, feldspar,amphibole, sphalerite, whole rock.

K-Ar (40K to 40Ar; half-life of 1.31 x 109 years) used for dating mica, feldspar,amphibole, pyroxene, whole rock.

Ar-Ar step-heating. Used primarily for dating mica, K-feldspar, hornblende. Re-Os used to date sulfides (187Re to 187Os half life of 1.3 Ga) Molybdenite yields

best results, but sometimes possible to date other sulfides (pyrite, Cu-Fe sulfides)and recently utilized on organic matter.

14C (14C to 14N; time scale to short - only back to 60,000 years - for most economicgeology applications) used for dating charcoal and organic remains.

Paleomagnetics. Utilization of magnetic minerals to determine paleo magnetic poleand through comparison of results with polar wandering curves for the plate inwhich the deposit occurs determine time of magnetization. Minerals generallyused are magnetite and hematite.

Geochronology

M. W. Hitzman

Assignment #5

CSM Career Day and Mining/Exploration Companies

Attend Career Day (Sept. 13) at CSM. Prepare a list of the number of mining/mineral exploration companies, consulting companies, and other groups(U.S. government agencies for instance) hiring mineral deposit geologists.Talk to representatives of at least three of these groups. You should get thename and title of the individuals you speak with as well as their contactdetails (address, phone, email). What types of people are they looking for(undergrads, grads, only GE majors, etc.). What background (classes,internships, other experience) are the companies looking for in new hires?What are the job opportunities with this company or agency? Is is US-basedwork, overseas work, field work, office based work, etc.?

Write a two to three page paper based on your findings this should includea list of the companies seeding mineral deposit geologists, the names and

addresses of the three (or more) you spoke with and a description of whatthey are looking for and what opportunities there are with this company orgroup.

Please email me ([email protected]) your final work by Monday, Sept. 19.

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What are Companies looking for now?

The ore deposit types which are currently (2011) hotinclude:

Porphyry copper deposits Epithermal gold (silver) deposits Carlin-type Au Mesothermal Au Iron oxide-Cu-Au deposits Ni (Cu-PGE) deposits Iron deposits REE deposits

M. W. Hitzman

Hot Deposits (2011) -Associated with Intermediate to Felsic

Magmatic Rocks

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Hot Deposits -

Associated with Intermediate to FelsicMagmatic Rocks

These deposit types are associated with intrusions related to subduction. Theywill be the second group of deposits we will consider in the class.

Porphyry copper deposits - relatively common deposit type, oxidized deposits arerelatively cheap to produce and can be mined at a large scale. New exploration(Mongolia, China) and older areas previously closed to western companies (SouthAmerica) are attractive. Many major producers.

Epithermal gold deposits - gold exploration boom in 1980s, earliest 1990s.Price declined into the new century but are currently high (now ~$990/oz).Companies have consolidated in the past 5 years to relatively few major producers(Anglo Gold Ashanti, Newmont, Barrick).

Carlin gold deposits. Still very active in Nevada.

M. W. Hitzman

Hot Deposits (2011) -Ultramafic / Mafic Magmatic

Deposit type Companies Exploring Main Exploration Areas Researchers

PGE AngloAmerican, Vale,Norilsk, Lonmin

South Africa, Finland,Canada, USA, Brazil

Laurentian Univ.,Univ. of TorontoUniv.Witwatersrand

Ni-(Cu-PGE) Vale, BHP-Billiton, RioTinto

Australia, Canada, Brazil Laurentian Univ.

Diamonds DeBeers, Rio TInto, BHP-Billiton, juniors

Canada, southern Africa,Brazil, west Africa,

(Australia)

UBC, Univ.Witwatersrand

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Hot Deposits (2011) -Other Types

These deposit types will be discussed later in the course. Some are well-knowndeposit types (VMS) and mesothermal (orogenic) Au, while others IOGC are newdeposit types with much left to learn about their genesis.

Iron oxide-Cu-Au deposits (IOGC) - Relatively new deposit type (deposit typerecognized by Hitzman et al., 1992). Aggressive exploration by a number ofcompanies - discoveries have increased in past 5 years.

Mesothermal (orogenic) gold - Resurgence of interest by major gold companies. Iron deposits - Incredible price increases for iron (prices rose 71.5%, 19%, and

9.5% over the past 3 years!) have spurred new looks at mothballed deposits andnew exploration.

Focus on renewable energy has lead to new exploration for trace metals used inconstruction of these technologies Rare earth elements (REEs) Molycorpand other elements such as tellurium (Te) First Solar

M. W. Hitzman

Hot Deposits (2011) -Other

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Bowen (1928) publishes The Evolution of the Igneous Rocks and lays thefoundations for modern igneous petrology. Speculates on the origin of oredeposits associated with magmas.

Graton (1940) expands the concept of a hydrothermal fluid produced as aresidual (immiscible) fluid during crystallization of a felsic magma.

Sales and Meyer, 1948, Wall rock alteration at Butte, Montana: AIME Tr., v.178, p. 9-35. Conclude that the expansion of the total alteration envelope ofthe Butte veins took place by continuous reaction between wall rock andore fluid and that each alteration zone maintained its identity and positionrelative to the others by growing at its outer edge as it was encroached, andbest, at integrating careful detailed geological mapping and observationwith chemistry.

Lovering et al., 1949, Rock alteration as a guide to ore East Tintic district:Econ. Geol. Mon. 1, 64p. Concluded that the various zones of hydrothermalalteration at East Tintic were produced by distinct and different solutions atdistinct times. Integrated careful field observations with good laboratorywork to come to conclusions different from those of Sales and Meyer.

History of Economic Geology 1920s - 1940s

M. W. Hitzman

History of Economic Geology,

Classification of Ore Deposits

Agricola, De ReMetallica (1556)

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History of Economic Geology

16th through 17th Century

16th Century: Economic Geology as a science begins with the publication

ofDe Re Metallica (1556) by Georgius Agricola in Germany.Agricola classifies ores as either alluvial or in-situ. Hefurther subdivided in-situ deposits into: veins, bedded orhorizontal deposits, impregnations, stringers, andstockworks. Made two major observations:

that ore channels are secondary features, youngerthan the country rocks

ores have been deposited from solutions circulating inthese channels.

17th Century: Based on observations in Italy, Nicolas Steno in 1669 writes

that ores are a product of condensation from vaporsascending through fissures.

M. W. Hitzman

History of Economic Geology 18th Century

18th Century: Both Henkel (1725, 1727) and Zimmermann (1746) based on observations in central European mines stressthe importance of hydrothermal solutions, or vapors of deep-seated origin, (i.e. hot fluids) in carryingdissolved metals. They recognize metasomatism of wall rocks (hydrothermal alteration) and replacement ofwall rocks by ore minerals.

Delius (1770, 1773) recognized the near-surface alteration of sulfides and the development of enrichedcopper and silver beneath a weathered zone (i.e. supergene enrichment). Unfortunately he believed thatsuch enrichment was do to solar heating rather than meteoric water.

Charpentier (1778, 1779) published results of his observations in the Freiberg Ag-Pb mines which includedthe suggestions, based on the gradational contacts between some veins and wall rock, that the metals in theveins were the result of alteration reactions between the country rocks and the water that had passedthrough them. He likened this process to that of silicification of wood. Gerhard (1781) considered theseveins to be open fissures which were filled with material leached from adjacent country rock. These writingsintroduced the concept oflateral secretion (that the metals in ore deposits are leached from adjacent wallrocks by circulating hydrothermal fluids).

James Hutton (1788, 1795; popularized by John Playfair, 1802) observed the textures of cooled metals andslags from smelters and those of igneous rocks and ore deposits. Based on these observations heproposed that both igneous rocks and ore deposits were derived from molten magmas and were transportedin the liquid state to their present positions. He did not believe that aqueous hydrothermal solutions wereinvolved in ore deposition. This school of thought was termed Plutonist.

A. G. Werner (1791) argued that basalts, limestones, and ore deposits were formed as sediments in aprimeval ocean. Envisioned veins as forming as cracks on the ocean floor during earthquakes or slumpingand filled with chemical precipitates derived from aqueous fluids. This school of thought was termedNeptunist.

Invention of the Cornish water pump (1798) which permitting mining (and observation of rocks) below thewater table.

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Elie de Beaumont (mid- 1800s) developed the idea of hydrothermal ore-depositformation. Recognized that hydrothermal solutions could be related to igneouscenters and that some deposits formed by replacement of carbonate rocksadjacent to intrusions. Also described magmatic segregation of sulfides.

Though it is difficult to pinpoint an individual, during the mid-19th centurychemistry began to play an important part in economic geology with chemicalprinciples becoming critical parameters for ore genesis.

Most important during the 19th century was the mapping of ore deposits - muchof it carried out by government geologists in the U.S..

Early reports were on the copper and iron deposits of the upper Midwest (Michigan and Minnesota -1850s).

In 1867 the U.S. Geological and Geographical Surveys of the Territories were established. In 1879 the various government surveys were consolidated as the U.S. Geological Survey. Geologists

S.F. Emmons and G. F. Becker, both trained at Freiberg Univ., Germany, initiated a policy of

comprehensively studying mining districts (mainly in the western U.S.) in order to understand theorigin of the ore deposits.

The first major publication resulting from this work was by G. F. Becker, 1882, Geology of theComstock Lode and the Washoe District, U.S. Geol. Survey Mon., v. 3.

It was soon followed by S. F. Emmons, 1886, Geology and mining industry of Leadville, U.S. Geol.Survey Mon., v. 12 and W. Lindgren (also educated at Freiberg), 1896, The gold-quartz veins of NevadaCity and Grass Valley districts, California, U.S. Geol. Survey 17th Ann. Rept., pt. ii, p. 1-262.

History of Economic Geology 19th Century

M. W. Hitzman

From 1901 to 1906, Reno Sales and H.V. Winchell, together with D.M. Brunton,worked out the essential methods of procedure for detailed surface andunderground geological mapping, including mapping of structure, alteration, andsulfide assemblages at Butte, MT for the Anaconda Company.

In 1905 the Society of Economic Geologists was founded to facilitate the studyand discussion of ore deposit genesis.

In 1907 Lindgren published the first edition ofMineral Deposits which include hisclassification of ore deposits and which represents the first modern oreclassification.

In 1910 the Kennecott Company began large-scale, bulk mining at BinghamCanyon initiating a revolution in mining.

1912 eruption of Katmai volcano, Alaska and subsequent study of the ash flowsand fumaroles by Allen (1923) provides evidence of transport of metals byhydrothermal fluids and gases.

In the 1920s J.H.L. Vogt initiated the rigorous application of physical chemistryto petrology and economic geology. This work lead to the modern field ofgeochemistry.

History of Economic Geology early 20th Century

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1950s - development of fluid inclusion technology by E.Roedder and others. Resulted in recognition that many orefluids are highly saline fluids which abundant chlorine.

1950s - Application of aqueous geochemistry to metal transportand ore genesis by Barton, Holland, Krauskopf and others.

mid- to late-1950s development of exploration geophysicaltechniques.

late 1950s Development of sulfur isotopic techniques (Jensen,Thode and Munster) and application of this technology to thestudy of ore deposits.

White (1957) produces description of the geology andgeochemistry of western U.S. hot spring systems.

History of Economic Geology 1950s

M. W. Hitzman

Baass Becking (1961) produced simple sulfides through biological methods, thusdemonstrating that biological processes may be important in some ore-forming situations.

late 1950s to mid-1970s Development of the syngenetic theory regarding volcanogenicmassive sulfide deposits by Japanese geologists (first papers on the subject in 1920),Canadian geologists (notably R.W. Hutchinson), Australian geologists (notably R. Stanton),and U.S. geologists (notably Kinkel).

early to mid-1960s Experimental work on sulfide stabilities notably by Craig and Kullerud. Discovery (1964) of hot, metal-rich brines in the Red Sea. Opening of the Carlin Mine, Nevada by Newmont and development of heap leach gold

technology.

Early to late 1960s - the Plate Tectonics Revolution. Helgeson (1964) publishes results of computer calculations of solution equilibria in the NaCl

- HCl - H2O system and demonstrates the value of computer-aided geochemical analysis toore systems.

early to late-1960s Development of leached cap models (geology, geochemistry) by theKennecott Company.

History of Economic Geology 1960s

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Lowell, J.D. and Guilbert, J.M., 1970, Lateral and vertical alteration-mineralization zoning inporphyry copper deposits: Econ. Geol., v. 65, p. 373-408. The paper provides an integratedgeological model for the genesis of a major ore deposit type.

Development by Taylor (1970s) of stable isotopic techniques for studying fluids and mineralsand their application to ore deposits.

Discovery in 1970s of mid-ocean ridge hot springs culminating in 1979 discovery of activeblack smokers.

Development (1970s and into early 1980s) of the sedimentary exhalative (SEDEX) conceptfor many sediment-hosted mineral deposits.

Application of solvent extraction / electrowinning (SXEW) technology to low grade copperoxide resources (1980s).

Development of pressure leach technology for nickel oxides (1990s). Development of GIS technology; development of software and hardware to construct large 3-

D models of ore deposits (1990s).

Development of solvent extraction / electrowinning technology for zinc oxide resources.

History of Economic Geology 1970s - 2000

M. W. Hitzman

Two end member classification schemes: Occurrence Models (descriptive based)

Petrotectonic setting and age Regional geological setting Ore body characteristics

Genetic Models (theory based) source of components mechanisms of transport mechanisms of concentration / depositional processes

The occurrence model classifies things while the geneticmodel classifies processes. In reality we most commonlyutilize a hybrid of both.

Classification of Ore Deposits

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Historical Classification Schemes

Agricola (1556) - subdivided into alluvial or in-situ Beck (1904) - genetic classification Lindgren (1911) - genetic classification emphasizing depth of

formation

Niggli (1929) - Minero-chemical classification Schneiderholn (1941) - classification by metal Bilibin (1955) - classification by orogenic stage Plate tectonics classification - many authors, utilize Guilbert

and Park (1986)

Modern genetic + occurrence classification (Einaudimodified by Hitzman)

M. W. Hitzman

Modern Genetic + Occurrence Classification (I) Deposits formed at the earths surface by processes involving

oceanic or meteoric water Involving chemical or clastic sedimentation

placers and beach sands: Au, Sn, Pt, Ti, diamonds, Cr Witwatersrand-type Archean quartz pebble conglomerate: Au, U Evaporites and brines: K2O, NaCl, gypsum, Na2CO3, B2O3,, nitrates, Sr, Br, I, Li,

CaCl2, MgO

Precambrian banded iron formation: Fe Phanerozoic ironstone: Fe Marine-transgressive Mn Submarine Mn-(Ni-Co-Cu) nodules; Fe-Mn oozes.

Weathering Lateritic Ni Lateritic bauxite: Al, Ga Oxidized, enriched Cu (supergene enrichment blankets) Saline, alkaline lakes: zeolites

Deposited from groundwaters in sandstone aquifers roll-front sandstone U (V) tabular sandstone U

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Modern Genetic + Occurrence Classification (II)

Deposits formed at or near the earths surface by low to moderate T (100-300C) hydrothermal fluidsdominantly of meteoric, ocean, or connate origin.

Contemporaneous with submarine volcanism Ophiolite (Cyprus) - type massive sulfides: Cu (Zn, Au) Besshi-type (basalt+greywacke) massive sulfides: Cu, Zn Archean (Superior)-type massive sulfides: Zn, Cu, Au Kuroko-type massive sulfides: Zn, Pb, Cu, Ag (Au) Eskay Creek-type (submarine epithermal system) massive sulfides: Ag, Au, Zn, Cu, Pb, As

Penecontemporaneous with cratonic sedimentary basin (rift) or rifted continental margin, prelithification Shale hosted deposits (SEDEX): Zn, Pb, Ag, Ba

Replacement of semi-lithified or lithified sediments in cratonic sedimentary basin (rift) or rifted continental margin Mt. Isa type carbonate shale hosted: Zn, Pb, Ag Irish-type carbonate hosted: Zn, Pb, Ag, Ba, (Cu) Red bed copper deposits: Cu, Ag, Co, (Zn, Pb) Mississippi Valley-type: Zn, Pb Proterozoic unconformity (Athabaska-type): U

Continental hot spring environment (meteoric water dominant) hot spring type Hg hot spring type disseminated Au

jasperoid-carbonate Au volcanic sulfur

Replacement, open-space filling in continental volcanic-sedimentary environments volcanic hosted disseminated Ag (Cu, Pb, Zn, Mn, Ba) volcanic alunite (non-bauxite Al source) epithermal base and precious metal Ag, Au, Pb, Zn, (Cu, U) lithophile elements in high-silica rhyolite: Li, Cs, U, Th, Be, F, Sn

M. W. Hitzman

Modern Genetic + Occurrence Classification (III)

Deposits formed at shallow levels in the earths crust (1-5 km) by moderate to high T (250-550C) hydrothermal processes in ornear intermediate to felsic igneous intrusions. Waters of mixed meteoric - magmatic - metamorphic origin

In or near cupolas of small porphyry stocks Cordilleran base-metal lodes, veins: Cu, Pb, Zn, Ag (Au, Mn) Enargite-gold veins Tourmaline-quartz breccia pipes: Cu, Mo, (W, Au) Cordilleran-type porphyry: Cu, Mo, Au Island arc-type porphyry: Cu, Au Quartz monzonite-type porphyry Mo: Mo, Cu Climax-type porphyry Mo: Mo (Sn, W, F, Zn, Pb) Cornwall-type greisen: Sn, W, Mo, F, Zn Base metal sulfide skarns: Cu, Zn, Pb Iron skarns: Fe, (Cu, Au) Tin skarns: Sn, Be, F

In batholithic environment W skarns

Above zones of basaltic underplatting forming felsic melts with hydrothermal brines that become highly saline due tointeraction with evaporites or highly evolved sedimentary brines

Olympic Dam type iron oxide deposits: Fe, (Cu, U, Au, REE) Deposits formed in metamorphic environments from 1 to 10 km in the earths crust by moderate to high T (200 - 500 C)

hydrothermal processes at variable pressures. In shallow metamorphic environments

Greenstone copper deposits: Cu, Ag Saddle reef Au: Au

In deep (amphibolite) metamorphic environments Shear zone gold quartz veins Homestake-type iron formation hosted Au

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Modern Genetic + Occurrence Classification (IV)

Deposits formed by magmatic processes Involving magmas of high-volatile content

pegmatites - Li, Sn, Ta, Nb, Bi, Be carbonatites - Nb, P, REE, U, Cu

Involving fractional crystallization stratiform (Bushveld-type) chromite podiform (Alpine-type) chromite stratiform magnetite: V

Involving immiscible oxide liquid anorthositic Ti

Involving immiscible sulfide liquid Merensky Reef-type: PGE, Cu, Au

Zoned Alaskan-type ultramafic: PGE, Fe Komatiitic-type: Ni, Cu, (PGE) Sudbury-type impact: Ni, Cu Mafic type: Ni, Cu, PGE

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