Handout 7 Characteristics of VMS Deposits
-
Upload
prahastomi-muttahari-s -
Category
Documents
-
view
219 -
download
0
Transcript of Handout 7 Characteristics of VMS Deposits
-
7/31/2019 Handout 7 Characteristics of VMS Deposits
1/7
9/29/2007
MFR@2007 1
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
2MFR@2007
Economic for base metal than gold
uro o ype ac cvolcanic host rocks)
(metamorphosed
host rocks)
(ophiolites host
rocks)
3MFR@2007
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
MFR@2007 4
-
7/31/2019 Handout 7 Characteristics of VMS Deposits
2/7
9/29/2007
MFR@2007 2
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;
MFR@20075
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,
6MFR@2007
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
7MFR@2007
MFR@20078
-
7/31/2019 Handout 7 Characteristics of VMS Deposits
3/7
9/29/2007
MFR@2007 3
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
MFR@2007
9
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
10MFR@2007
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
MFR@2007
11
,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
MFR@200712
,
ore mineral associated with late calcite veins : stibnite,tetrahedrite-tennantite, base metal sulfide, realgar, cinnabar
-
7/31/2019 Handout 7 Characteristics of VMS Deposits
4/7
9/29/2007
MFR@2007 4
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
MFR@200713
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)
MFR@200714
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
MFR@200715
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
MFR@200716
gold hosted in arsenical pyrite ass with realgar, orpiment, stibnite andcinnabar
-
7/31/2019 Handout 7 Characteristics of VMS Deposits
5/7
9/29/2007
MFR@2007 5
an es ea ere o: esmec e
alterationstages:
Carbonatedissolution
Dolomitization
MFR@200717
Silicification
Argillisation
Decalcification, Dissolution and Dolomitisation
MFR@200718
MFR@2007 19 MFR@2007 20
-
7/31/2019 Handout 7 Characteristics of VMS Deposits
6/7
9/29/2007
MFR@2007 6
MFR@2007 21 MFR@2007 22
Geochemicalsignature
Geophysicalsignature om nant ye evate u
butZn,Pb,As,AgandGearekeyindicatorsinrocksamples;
subtleCu
stream
silt
geochemicalanomaliesoccurinproximitytosomede osits.
Resistivity,IPandgravitycouldbeusefulbuttherearenodefinitivetools.
23MFR@2007
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.
24MFR@2007
-
7/31/2019 Handout 7 Characteristics of VMS Deposits
7/7
9/29/2007
MFR@2007 7
MFR@2007 25
TobeContinue