AbstractsEarth & Environmental Science

The nature, origin and timing of gold mineralisation in proterozoic rocks of the Peak Hill district, Western Australia

by S. E Thornett

Institution: University of Western Australia
Degree: MS
Year: 1995
Keywords: Gold ores; Geology, Stratigraphic; Mineralogy
Record ID: 1032179
Full text PDF: http://repository.uwa.edu.au:80/R/?func=dbin-jump-full&object_id=34947&local_base=GEN01-INS01


The gold mines at Peak Hill lie within a group of deposits in the western part of the Proterozoic Nabberu Basin, which rests unconformably upon the northern margin of the Yilgarn Craton. The Peak Hill deposits are hosted by mafic and pelitic schist which appears to overlie the southwestern projection of the Marymia Dome. The main objectives of this study were to establish the controls on gold mineralisation, in particular the structural setting, timing, nature and physical conditions of emplacement of the ore-forming fluids. Three deposits, at Peak Hill - Fiveways, Mount Pleasant and Jubilee were studied and compared. All of these deposits lie in different stratigraphic positions in rocks of the Peak Hill Metamorphic Suite. Gold occurs mainly in pelitic schist, in close proximity to bodies of mafic schist or metadolerite, and commonly shows an intimate relationship with graphitic schist. The host rocks are highly deformed, with evidence of early thrusting and layer-pardllel fabric development (Sl). Three later phases of folding are recognised; the earliest is manifested in asymmetric recumbent folds, with the two younger fold generations producing interference folds which define the Peak Hill Dome. D4 folds and associated shear zones are identified as the principal structures controlling gold mineralisation, but earlier (D1) shear zones also appear to be mineralised. In the vicinity of the three deposits studied, regional metamorphism has reached amphibolite facies conditions, with geothermometry on prograde assemblages giving temperatures of 535 to 620°C. Phengite geobarometry, on corresponding assemblages, indicates high minimum pressures of 6.5 to 7 kbar. Intense retrograde alteration accompanies gold mineralisation in D4 shear zones and folds, and comprises hydrous alteration assemblages rich in Fe, K, S, CO2 and possibly Na. Alteration assemblages contain Fe-chlorite, biotite, white-mica, albite, garnet, pyrite, carbonate, tourmaline and fluorite. Thermometric evidence from the Fe-chlorite and low-albite indicates crystallisation temperatures of between 250 to 375°C. The deposition temperature of much of the gold is also inferred to lie within this range. Higher temperatures (530 to 620°C) obtained from apparently retrograde garnet-biotite grain pairs in strongly altered rocks, may signify an extended history of shear zone development and alteration (possibly D1), and perhaps mineralisation. Fluid inclusion petrography on gold mineralised quartz veins indicates that the hydrothermal fluid associated with mineralisation was H2O rich and moderately saline, with some CO2 and possibly minor CH4. Thirty to fifty percent of the fluid inclusions are vapour-rich, suggesting that mineralised fluids were trapped in a relatively lowpressure environment where phase separation was common. Lead-isotope data, from galena and lead tellurides which are intimately associated with gold mineralisation, suggest that the Peak Hill mineralisation is broadly synchronous with other Glengarry and Nabberu deposits at 1.9 - 1.8 Ga. Data…