GOLD PRECIPITATION BY FLUID MIXING IN BEDDING-PARALLEL FRACTURES NEARCARBONACEOUS SLATES AT THE COSMOPOLITAN HOWLEY GOLD DEPOSIT, NORTHERNAUSTRALIA

The Proterozoic Cosmopolitan Howley gold deposit consists of quartz ve ins on the steeply dipping limbs of the Howley anticline and is hosted by pelitic metasediments within the high-temperature metamorphic aure ole of Proterozoic granites in the Pine Creek province of northern Aus tralia. Ore is restricted to a stratigraphic contact, always occurring within noncarbonaceous hornfels within 50 m of carbonaceous slates. S imilar veins farther away from, or within carbonaceous slates, are abu ndant but barren. A companion paper establishes that ore formation occ urred in response to hydrothermal activity accompanying granite intrus ion and contact metamorphism The involvement of a magmatic brine in or e genesis is inferred on the basis of fluorine concentrations in bioti te from the potassic alteration selvages of the gold-bearing veins, fr om stable isotopic data and from mutual crosscutting relations of vein s and felsic dikes. This paper focuses on the fluid flow and chemical processes that formed tire gold-bearing quartz veins. It is argued tha t lateral focusing of fluid flow into the subvertical vein-hosting fra ctures led to mixing of the ascending magmatic brine (+/- some metamor phic fluid) with hydrocarbon-rich fluid advected through carbonaceous slate in the steeply dipping fold limbs. Crack-seal textures in some o f the gold quartz veins indicate that this fluid flow and mixing were locally episodic. The magmatic brine involved in the mixing was expell ed from a crystallizing I-type granite at depth (less than or equal to 1.5 km below the deposit). This granite is highly fractionated and ha s I-SC characteristics: hornblende, pink K feldspar, accessory sphene and fluorite and whole-rock Fe3+/Fe2+ greater than or equal to 0.24. T hese features indicate a redox state of the magmatic fluid at or above the NNO buffer. By contrast, gold deposition occurred at 2 to 3 log f (O2) units below NNO, a weakly acid pH of 4.3 to 4.7, 550 degrees less than or equal to T less than or equal to 620 degrees C, and approx 20 0 MPa, from fluids with 1 to 5 in Cl. The vein sulfide paragenesis pyr rhotite + arsenopyrite +/- loellingite indicates redox conditions comp atible with the local carbonaceous units. This implies a redox contras t between the regime of gold deposition and the magmatic fluid source regime. It is inferred that reduction of the magmatic brine by fluid m ixing in the vicinity of carbonaceous metasediments was responsible fo r localized gold deposition. To evaluate this precipitation mechanism, available gold solubility data were fitted to the revised equation of state for aqueous species at high pressures and temperatures. For the high temperatures of gold quartz veining, speciation calculations wit h this dataset indicate that AuCl2- was more important for gold transp ort than Au(HS)(2)(-) and AuHS0. Titration calculations confirm that g old can be precipitated effectively by admixing of carbonic species an d hydrogen derived by pyrolysis of carbonaceous slates. Reaction with carbonaceous matter could thus be a widely applicable gold precipitati on mechanism even at high temperature. However, the predicted solubili ty of gold is so high (typically greater than or equal to 100 ppb at g reater than or equal to 500 degrees C) that gold availability in ordin ary metasediments may be insufficient to generate an effective ore flu id for high-temperature gold deposition by metamorphic devolatilizatio n. If correct, these predictions further emphasize the inferred role o f magmatic fluids in the formation of gold deposits within high-grade contact metamorphic aureoles.