Abstract

In the Proterozoic Cosmopolitan Howley gold deposit, northern Australia, quartz vein-hosted gold mineralization occurs predominantly within 50 m of the noncarbonaceous footwall of carbonaceous slates. Although abundant, veins farther away from or within carbonaceous slate are barren. Except for hydrofracture stock-works in the hinge zone, the gold-bearing quartz veins (QV1) are bedding concordant and occur in hornfels and slate in the steeply dipping fold limbs of the Howley anticline. The QV1 veins formed near the peak of contact metamorphism (550 degrees -620 degrees C and 240-200 MPa) following the intrusion of the Cullen granite batholith. The vein-alteration assemblage is K feldspar + biotite + andalusite + or - cordierite, and mineralization was accompanied by large-scale potassium metasomatism whereas sodium and calcium were leached from the country rocks. The QV1 veins were overprinted first by superposed folding and then by reverse faulting. Later, transpressional strike-slip faulting was accompanied by extensive retrograde alteration, the precipitation of sulfide lenses delta 34 S arsenopyrite = -0.3 to +4.8%), quartz veins (QV2) with subeconomic gold enrichments, and barren pegmatitic veins (QV3). The involvement of a magmatic fluid in ore genesis at the Cosmo Howley deposit is indicated by (1) high fluorine contents in biotite from the alteration selvages of the gold quartz veins (QV1: F = 5,573-6,616 ppm; 49,000 ppm at Mt Shoobridge, 7 km west), (2) mutual crosscutting relations of gold quartz veins with aplite dikes, (3) the location of the deposit above I-type granite at a vertical distance of less than 2 km (most likely < or = 1 km), and (4) the high salinity of the mineralizing fluid as estimated from the Cl content of biotite in the alteration halos of the veins. A magmatic derivation of the sulfur in the auriferous sulfides is consistent with isotopic data (-0.3 to +7.2% 5a4S). Calculated oxygen isotope compositions of the fluids which precipitated the successive generations of quartz veins (QVI, delta 18 O (sub (H 2 O)) = 13.6%, QV3 delta 18 O (sub (H 2 O)) = 11.8%) indicate that the isotopically heavy country rock (chert nodules, delta 18 O < or = 18.5%) was infiltrated by an isotopically light...

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