INTRUSION-RELATED, HIGH-TEMPERATURE GOLD QUARTZ VEINING IN THE COSMOPOLITAN HOWLEY METASEDIMENTARY ROCK-HOSTED GOLD DEPOSIT, NORTHERN-TERRITORY, AUSTRALIA

In the Proterozoic Cosmopolitan Howley gold deposit, northern Australi a, quartz vein-hosted gold mineralization occurs predominantly within 30 m of the noncarbonaceous footwall of carbonaceous slates. Although abundant, veins farther away from or within carbonaceous slate are bar ren. Except for hydrofracture stock-works in the hinge zone, the gold- bearing quartz veins (QVI) are bedding concordant and occur in hornfel s and slate in the steeply dipping fold limbs of the Howley anticline. The QV1 veins formed neat; the peak of contact metamorphism (550 degr ees-620 degrees C and 240-200 MPa) following the intrusion of the Cull en granite batholith. The vein-alteration assemblage is K feldspar + b iotite + andalusite +/- 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, transpressi onal strike-slip faulting was accompanied by extensive retrograde alte ration, the precipitation Of sulfide lenses (delta(34)S(arsenopyrite) = -0.3 to +4.84 parts per thousand), quartz veins (QV2) subeconomic go ld enrichments, and barren pegmatitic veins (QV3). The involvement of a magmatic fluid in ore genesis at the Cosmo Howley deposit is indicat ed by (1) high fluorine contents in biotite from the alteration selvag es of the gold quartz veins (QV1: F = 5,573-6,616 ppm; 49,000 ppm at M t Shoobridge, 7 km west), (2) mutual crosscutting relations of gold qu artz veins with aplite dikes, (3) the location of the deposit above I- type granite at a vertical distance of less than 2 lan (most likely cl 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 cons istent with isotopic data (-0.3 to +7.2 parts per thousand delta(34)S) . Calculated oxygen isotope compositions of the fluids which precipita ted the succussive generations of quartz veins (QVI, delta(18)O((H20)) = 13.6 parts per thousand, QV3 delta(18)O((H20)) = 11.8 parts per tho usand) indicate that the isotopically heavy country rock (chert nodule s, delta(18)O less than or equal to 18.5 parts per thousand) was infil trated by an isotopically light component such as magnetic fluid (appr oximate to 9 parts per thousand). We interprete our observations to in dicate that the QV1 veins are tile product of focused flow of magmatic and metamorphic fluids rising through the metasediments of the Howley anticline. The dependence of the vein gold content on proximity to ca rbonaceous slate permits the hypothesis that mixing within tile veins of the rising magmatic fluid with methane- and CO2-rich fluid from the carbonaceous slate was instrumental in gold deposition. The broader o re-forming scenario involving interaction of magmatic fluid and carbon acesus lithologies in inner-contact aureole settings is widely observe d and may therefore allow relationships to be drawn with other synplut onic gold deposits.