handout 7 characteristics of vms deposits

7/31/2019 Handout 7 Characteristics of VMS Deposits

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Handout - 7

VHMS : base metal bearing sulfide accumulationin volcanic se uence of submarine extrusion and

related sedimentary rocks

Very high % sulfides (> 95%), abundant bariteand anhydrite

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Economic for base metal than gold

uro o ype ac cvolcanic host rocks)

(metamorphosed

host rocks)

(ophiolites host

rocks)

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Feature VHMS-Kuroko

Tectonic setting Back arc rift, calc alkaline volcanics

Volcanic structures Calderas and submarine stratovolcanoes locall

synvolcanic faulting can be important control on themineralization

Water depth Deep, > 3500 m

Volcanic rock types Distinctly bimodal : deposits are associated with bothbasaltic-andesitic and dacitic-rhyolitic volcanism

Ore horizon Fine grained rocks: chemical (sil iceous, Fe-richexhalites), volcano-sedimentary (epiclastics, usuallypyritic) and black shales

Ore body shapes Mounds, stratiform sheets, pipes and stacked lenses

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Resembles to high sulfidationepithermal

Meteoric fluids from derived seawater(high chloride & sulfate contents)

Th 175~2350CPb-Zn rich depositform on or near seafloor; Th

270~>3500C Cu-rich, deeper part;

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sa n es : ~ w a eqv.

Sulfur sources : volatiles from magma(yellow ore), and leaches from

surround volcanic rocks (black ore)

Hypogene minerals : pyrite, pyrrhotite, marcasite,arsenopyrite, chalcopyrite, sphalerite and galena. Commonbarite & anhydrite

Alteration : illite-smectite and illite (low T), sericite (higher T),quartz, chlorite and albite. Zeolite present in cooler parts.

Textures : colloform pyrite, growth-zoned sphalerite andclastic bedding and banding of sulfides. Stockwork Fe andFe-Cu in footwall zones

Gold as electrum in baritic deposits, and as inclusion inarsenopyrite associated with sphalerite. In lower part,present as electrum or telluride associated withchalcopyrite and/or pyrite,

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Ore body shapes : stacked, Cu-stocwork/disseminated,Au-Ag-Pb-Zn -stocwork/disseminated, classic mound,sheet, pipe

Typical alteration zonation pattern :

Zone1(outer) : smectite; zeolite, silica (cristobalite), barite, iron oxides

Zone 2 : illite-smectite; barite, anhydrite, quartz, chlorite

one : e, ser c e; ar e, a e, c or e, an y r e

Zone 4 (Inner) : sericite ; chlorite, quartz, pyrite

Example : Lerokis, Kali Kuning Wetar-Maluku

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Named after carbonate-hosted disseminated golddeposit at Carlin, Nevada

Invisible gold in altered carbonaceous silty limestoneor carbonate-rich siltstones

High concentrations of As, Sb, Hg, Tl, Ba, Mo, and W

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Low concentrations Cu, Pb, Zn

Most distinctive feature : disseminated gold hosted insilicified zones

Carlin type different from retrogradezones of carbonate-hosted skarn type

Skarn distinctive feature : calc-silicateminerals

some Carlin type occur close to or

within alteration halos of skarn depositsad acent to intrusives

Carlin type is unrelated to the intrusive-skarn event

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Removed & redistributed

Calcite veining

Enhancement porosity

Removed & redistributed

Calcite veining

Enhancement porosityDissolution of carbonate

Alteration of host rock mineral (detrital K-F & illite)sericite/illite kaolinite/dickite

Alteration of host rock mineral (detrital K-F & illite)sericite/illite kaolinite/dickiteArgillisation-sericitisation

In form of quartz & chalcedony

Early silicification, fossil & original sedimen features andclay are preserved

In form of quartz & chalcedony

Early silicification, fossil & original sedimen features andclay are preserved

Silica introduction

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,silica deposition; quartz vein and silicified brecciascutting early formed silicified zones

,silica deposition; quartz vein and silicified brecciascutting early formed silicified zones

Calcite (contain barite, tetrahedrite, realgar, stibnite,thallium) veining cut silicified zones

Calcite (contain barite, tetrahedrite, realgar, stibnite,thallium) veining cut silicified zonesCalcification

tonnage & grades vary : (Messel 7.8 Mt at 7.3 g/t; Carlin 22 Mt at9.3 g/t; Gold quary 180 Mt at 1.3 g/t)

highest grades associated with intermediate zone of carbonatedissolution and silicification of partially silicified rock (calcite isdisolved but dolomite still present)

mineral noted : pyrite, marcasite, arsenopyrite (rare), native

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,

ore mineral associated with late calcite veins : stibnite,tetrahedrite-tennantite, base metal sulfide, realgar, cinnabar


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rarel native(submicronsize,lowbulk rarel native(submicronsize,lowbulkconc,diss)

Invisiblegold(presencefromassayvalues)

discreteparticles(0.005~0.02mm)inpyrite,cinnabar,quartz

freeparticles(upto0.1mm)withsericite/illite

homogenouslydistributed

within

arsenal

conc,diss) Invisiblegold(presencefromassayvalues)

discreteparticles(0.005~0.02mm)inpyrite,cinnabar,quartz

freeparticles(upto0.1mm)withsericite/illite

homogenouslydistributed

within

arsenal

mostimportantoremineral:gold

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pyritepyrite

most importand controls ofdistribution, alteration patterns and

mineralization : structure & hoststratigraphy (control permability)

mineralizing fluids source : basinalor metamorphic (carbonate diss ~

argilli stage) and meteoricorigin/mixing (ore deposition)

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Ore deposition at : Th 200~3000Cand 1~8 wt% NaCl), rich in CO2 and

H2S, depth formation 2~4 km

mainst lemineralization:

strataboundreplacementwithincarbonateunits

(Carlin)

veinlike structuralstocwork

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in subducted-related extensional basin

mineralization in altered carbonate rocks adjacent to and beneath andesitebody, controlled by steeply deeping fault

highest gold occur at structural intersetions and fault splays

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gold hosted in arsenical pyrite ass with realgar, orpiment, stibnite andcinnabar


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an es ea ere o: esmec e

alterationstages:

Carbonatedissolution

Dolomitization

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Silicification

Argillisation

Decalcification, Dissolution and Dolomitisation

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Geochemicalsignature

Geophysicalsignature om nant ye evate u

butZn,Pb,As,AgandGearekeyindicatorsinrocksamples;

subtleCu

stream

silt

geochemicalanomaliesoccurinproximitytosomede osits.

Resistivity,IPandgravitycouldbeusefulbuttherearenodefinitivetools.

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OtherelementswhichmaybeusefulpathfindersareCo,Ga,Bi,Cd,V,MoandBa

Tectonicallydisturbedzonesandkarsted areaswithincarbonate/oxidizedclastic coupletsofmajorbasinsare

. Dolomitized zonesshouldbecarefullyexamined.Deposits

oftenoccurinclustersand/orinproximitytoassociateddeposittypes.

Depositsoxidize

readily

forming

gossans

with

secondary

CuandFeminerals;manyothersecondaryproductsandmalachitecoatednuggetsofcoppersulphides maybe

resent. Thermalmaturationanomaliesandclaymineralzoning,as

appliedtocarbonatehostedZnPb andunconformityuraniumdeposits,maybeusefultools.

Satelliteimageryisusefultoidentifyregionalstructures.

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