PAINTER ET AL. 1000mE. 2000mE. 3000mE. 3500mRL. 3000mRL. M815. Crøss_Laminite. R815 ...... a4S-enriehed parental sulfate relative to the generally ac-.
Economic Geology Vol.94,1999,pp.883-912
Sedimentologic, Petrographic, andSulfurIsotopeConstraints onFine-Grained Pyrite Formationat MountIsa Mine andEnvirons,NorthwestQueensland, Australia MATTHEWG.M. PAINTER,*SUZANNED. GOLDING?
Department of EarthSciences, University of Queensland, Queensland 4072,Australia KEITH W. HANNAN,
MountIsaMinesExploration Pry.Ltd.,GPOBox1042,Brisbane, Queensland 4001,Australia AND MARTIN K. NEUDERT
School ofNaturalResources, Queensland University ofTechnology, Queensland 4001,Australia Abstract
Fine-grained pyriteistheearliest generation of pyriteandthemostabundant sulfide withintheUrquhart Shaleat MountIsa,northwest Queensland. Thepyriteisintimately interbanded withore-grade Pb-Znmineralization attheMountIsaminebutisalsoabundant northandsouthofthemineatseveral stratigraphic horizonswithinthe UrquhartShale.Detailedsedimentologic, petrographic, andsulfurisotope studies of the Urquhart Shale, mostly northofthemine,reveal thatthefine-grained pyrite(834S:-3.3 to +26.3%o) formed bythermochemical sulfate reduction duringdiagenesis. Thesulfate source waslocalsulfate evaporites, pseudomorphs ofwhicharepresent throughout theUrquhart Shale(i.e.,gypsum, anhydrite, andbarite).Deep-burialdiagenetic replacement oftheseevaporites resulted in sulfate-bearing ground waters whichmigrated parallel to bedding. Fine-grained pyriteformedwherethesefluidsinfiltratedandtheninteracted withcarbon-rich laminated siltstones.
Comparison ofthesulfurisotope systematics of fine-grained pyriteandspatially associated basemetalsulfidesfromtheMountIsaPb-ZnandCuorebodies indicates a common sulfursource ofultimately marineoriginforallsulfide types.Differentsulfurisotope ratiodistributions forthevarious sulfides aretheresultofcontrasting formation mechanisms and/ordepositional conditions ratherthandiffering sulfursources. Thesulfur isotope systematics ofthebasemetalandassociated ironsulfidegenerations areconsistent withmineralization byreduced hydrothermal fluids, perhaps generated bybulkreduction ofevaporite-sourced sulfate-bearing watersgenerated deeperin theMountIsaGroup,thesedimentary sequence whichcontains theUrquhart Shale. The available sulfurisotope datafromthe MountIsaorebodies areconsistent witheithera chemically and thermally zoned,evolving Cu-Pb-Znsystem, or discrete Cu andPb-Znmineralizing eventslinkedby a common sulfur source.
Introduction
shallow-water evaporitic conditions (vanden Heuvel,1969; McClayandCarlfie,1978;NeudertandRussell, 1981;Neuderr, 1983) and high-energy current deposition (Neudert, mainsthesubject of debateaftermorethan60 yr of mining 1983). Recently, Neudert (1983) and Painter and Neudert andscientific study. Whereas a syndeformational mechanism (1994) subdivided the Urquhart Shale sequence into different forCumineralization (Perkins, 1984)isnowwidelyaccepted, faciesandnoteda strongfaciescontrolon the opinionon the genesis of the Pb-Znorebodies remainsdi- sedimentary distribution of iron and base metal sulfides. videdamong exhalative-syngenetic (Finlow-Bates, 1979;Finsulfide typeoutside theMountIsamineisan low-Bates andStumpfl,1979),diagenetic (vandenHeuvel, Thedominant extremely fine grained generation of pyritelocallytermed 1969;Neudert,1983;McGoldrick andKeays, 1990),or syn"fine-grained pyrite" (
