Marymia an Archean amphibolite facieshosted or

Authors: Noreen M Vielreicher John R Ridley David I Groves

Publish Date: 2002/04/24

Volume: 37, Issue: 8, Pages: 737-764

PDF Link

Abstract

The Marymia gold deposit comprising two orebodies Keillor 1 and Keillor 2 is at the northern end of the Plutonic Well greenstone belt in the Marymia Inlier in the southern Capricorn Orogen just north of the Yilgarn craton The Marymia Inlier is a discrete faultbounded Archean gneiss–granitoid–greenstone domain surrounded by sedimentary basins that were formed and variably metamorphosed and deformed during several Palaeoproterozoic orogenic cycles The greenstone sequence at Marymia is stratigraphically and geochemically similar to greenstone sequences in the Yilgarn craton but was subjected to further deformation and metamorphism in the Palaeoproterozoic Late Archean deformation D1–D2 was ductile to brittle–ductile in style whereas Palaeoproterozoic deformation was predominantly brittle Equilibrium mineral assemblages indicate that peak amphibolitefacies metamorphism 540–575 °C 3 kb was overprinted by greenschistfacies metamorphism 300–360 °C Petrographic textures indicate that prograde metamorphism was coeval with D1–D2 with peak metamorphism early to syn D2 Gold mineralisation at Marymia is hosted in metamorphosed tholeiitic basalts and banded iron formation On a gross scale the distribution of gold is controlled by D2 folds and shear zones Lithological contacts with strong rheological or chemical contrasts provide local controls Goldrelated alteration comprises subtle millimetre to centimetrewide zones of silicification with variable amounts of quartz hornblende biotite Kfeldspar plagioclase calcite/siderite scheelite titanite epidote sulfide and telluride minerals Quartz veins are generally narrow and discontinuous with low total volume of quartz Gold is sited in the wall rock at vein salvedges or within stringers of wall rock within veins There are two distinct opaquemineral assemblages pyrite–pyrrhotite–chalcopyrite–galena and hessite–petzite–altaite–Bitelluride–galena Ore samples are variably enriched in Ag Te Pb W Cu S and Fe reflecting heterogeneity of the ore mineralogy Structural timing and temperature of formation of alteration and ore minerals support deposition of gold during late peak amphibolitefacies metamorphism from neutral to alkaline pH=5–6 moderately oxidising log ƒ O2≈–21–22 and CO2bearing X CO2 ≈02 fluids The total sulfur content of the fluid is estimated at 1mΣS Lead isotope compositions support derivation of lead from within the local greenstone sequence Gold lodes were deformed by faults and shear zones in the Palaeoproterozoic with only limited remobilisation Subeconomic carbonate vein and brecciahosted base metal mineralisation is locally hosted within Palaeoproterozoic fault zones which clearly cut gold lodes Basemetalrelated alteration is characterised by intense carbonatisation chloritisation and albitisation of the mafic host rocks Mineral assemblages are consistent with formation at greenschist facies conditions Lead isotope compositions support crystallisation at ca 17 Ga from lead that is similar in composition to earlier goldrelated galena

Keywords: