Gold precipitation by fluid mixing in bedding-parallel fractures near carbonaceous slates at the Cosmopolitan Howley gold deposit, northern Australia

Abstract

The Proterozoic Cosmopolitan Howley gold deposit consists of quartz veins on the steeply dipping limbs of the Howley anticline and is hosted by pelitic metasediments within the high-temperature metamorphic aureole of Proterozoic granites in the Pine Creek province of northern Australia. Ore is restricted to a stratigraphic contact, always occurring within noncarbonaceous hornfels within 50 m of carbonaceous slates. Similar veins farther away from, or within carbonaceous slates, are abundant but barren. A companion paper establishes that ore formation occurred in response to hydrothermal activity accompanying granite intrusion and contact metamorphism. The involvement of a magmatic brine in ore genesis is referred on the basis of fluorine concentrations in biotite from the potassic alteration selvages of the gold-bearing veins, from stable isotopic data and from mutual crosscutting relations of veins and felsic dikes. This paper focuses on the fluid flow and chemical processes that formed the gold-bearing quartz veins. It is argued that lateral focusing of fluid flow into the subvertical vein-hosting fractures led to mixing of the ascending magmatic brine (+ or - some metamorphic fluid) with hydrocarbon-rich fluid advected through carbonaceous slate in the steeply dipping fold limbs. Crock-seal textures in some of the gold quartz veins indicate that this fluid flow and mixing were locally episodic. The magmatic brine involved in the mixing was expelled from a crystallizing I-type granite at depth ( or = 0.24. These features indicate a redox state of tile 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 or =100 ppb at > or =500 degrees C) that gold availability in ordinary metasediments may be insufficient to generate an effective ore fluid for high-temperature gold deposition by metamorphic devolatilization. If correct, these predictions further emphasize the inferred role of magmatic fluids in the formation of gold deposits within high-grade contact metamorphic aureoles.