Geology and genesis of the George Fisher Zn-Pb-Ag deposit Mount Isa, Australia
Chapman, Lucy H. (1999) Geology and genesis of the George Fisher Zn-Pb-Ag deposit Mount Isa, Australia. PhD thesis, James Cook University.
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Abstract
The George Fisher deposit (107 Mt @ 11.1% Zn, 5.4% Pb, and 93g/t Ag) is the northernmost significant Mount Isa-style deposit hosted by the -1653Ma Urquhart Shale in the Western Mount Isa Inlier. It is distinguished from the Mount Isa and Hilton Mines by a paucity of syn-late tectonic Cu. This has enabled recognition and examination of a previously ·unrecognized, syndiagenetic, hydrothermal Zn-Pb-Ag mineralization-system, despite superposition of intense deformation during the Diamantinan and Isan Orogenies.
George Fisher contains eleven west-dipping, stacked, anastomosing, stratabound ore lenses contained within rhythmically laminated pyritic siltstones intercalated with banded mudstones, and separated by barren stylolitic mudstones. The ore-bearing sequence contains abundant carbonate banding including partly coalesced carbonate nodules in pyritic siltstones and planar white carbonate bands in siltstones and banded mudstones. The deposit is zoned from stratigraphically lower Zn-rich to higher Zn+Pb-rich ore bodies. Subeconomic Cu occurrences are coincident with Zn+Pb zones, but are predominantly restricted to upper portions of the sequence. This zonation was developed within two hydrothermal systems separated by 75-125Ma.
Emplacement of Zn-Pb-Ag at George Fisher was the culmination of a complex, syndiagenetic, hydrothermal alteration system. Calcite is a major constituent of texturally distinct carbonate banding and its formation post-dates earliest dolomite-ferroan dolomiteankerite alteration. At George Fisher, these carbonates are distinguished by unique stable isotope geochemistry within the Urquhart Shale (δ¹⁸O = 17.1-18.5‰, δ¹³C = 4.1 to -1.7‰) and are enriched in (Fe,Mn)CO₃ relative to paragenetically equivalent carbonates located away from economic Zn-Pb-Ag. These carbonates formed during infiltration of warm, basin-derived fluids into cool sedimentary rocks, prior to stylolitization and at depths of a few hundred metres. Chemical zonation of early carbonates over 10-20km provides vectors for Zn-Pb-Ag-bearing fluid influx zones.
Pyritization by thermochemical sulphate reduction of in situ organic material post-dated stylolitization. Infiltrating fluids lacked ore metals at the time. Bedding-parallel carbonate±quartz±celsian-hyalophane-K-feldspar vein development was localized in the immediate vicinity of, preceded and structurally prepared the sequence for, economic Zn-Pb-Ag mineralization. Sphalerite, galena, hydrophlogopite and bitumen were codeposited. Bitumen reflectance data and mesophase textures constrain maximum temperatures of mineralization and peak thermal conditions during burial to 200°C. Metals were transported in slightly oxidized, near-neutral fluids. Sulphur was either sourced from a separate reduced fluid or was transported as sulphate in the same fluid. Metal precipitation occurred via fluid mixing or sulphate reduction. Much sphalerite is preserved in primary depositional sites whilst galena occurs in syn-late tectonic veins and breccias but retains its primary spatial association with sphalerite at deposit scale. Nineteen galena samples, from a range of paragenetic settings, display homogeneous Pb isotope compositions. A Pb-model age of -1653 Ma is interpreted to reflect the timing of Zn-PbAg mineralization.
Key characteristics of the syn-late tectonic Cu system include pyrrhotite, biotite-chloritedominant and ferroan dolomite-ferroan ankerite -dominant alteration. Phyllosilicates occur in stratabound lenses superimposed on Ba-K-feldspar alteration zones, whilst ferroan carbonate alteration is pervasive throughout the deposit hanging wall. These carbonates are texturally, chemically and isotopically (δ¹⁸O = 13.0-17.3‰, δ¹³C = 4.6 to -1.3 ‰) distinct from syndiagenetic carbonates. Copper-bearing fluids had temperatures of 250- 300°C based on phyllosilicate mineral stability relationships.
The temporal-spatial-temperature zonation of the Zn-Pb-Ag and Cu systems at George Fisher is not preserved at Mount Isa and Hilton. Mineralogical and geochemical characteristics of carbonate and phyllosilicate alteration assemblages throughout Zn-Pbbearing strata at the latter deposits are products of syn-late-tectonic Cu mineralization, reflecting greater influx of Cu-bearing fluids at these sites. Poor preservation of syndiagenetic hydrothermal signatures at Mount Isa and Hilton has undoubtedly contributed significantly to the long-standing controversy surrounding the origin of Mount Isa-style Zn-Pb-Ag deposits.
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