Chemistry of low-temperature hydrothermal gold, platinum, and palladium (+ or - uranium) mineralization at Coronation Hill, Northern Territory, Australia

Abstract

Coronation Hill is a low-temperature, hydrothermal Au-Pt-Pd(-Se-Sb + or - U) deposit in the Paleoproterozoic Pine Creek inlier of the Northern Territory, Australia. Similar to unconformity-related uranium deposits elsewhere in this province, precious metal (+ or -U) mineralization at Coronation Hill occurs below a major unconformity that separates a deformed and metamorphosed basement (composed of igneous intrusions, volcaniclastic rocks, and calcareous and carbonaceous sedimentary units) from a cover sequence of little-deformed hematitic quartz sandstone and sedimentary breccias. Geologic, fluid inclusion, stable isotope, and thermodynamic data indicate that mineralization at Coronation Hill occurred by reaction of highly oxidized and acid basin brines originating from the cover sequence and hydrosphere, with feldspathic and reducing rocks and fluids of the crystalline basement. The two styles of metal enrichment occurring in separate parts of the Coronation Hill deposit--dominant gold and platinum-group elements (PGE) ores, and subordinate mixed U-Au-PGE ores --can be explained as a result of varying degrees of acid neutralization by feldspars and reduction by carbonaceous matter and ferrous iron in various host rocks.Transport of Au, Pt, Pd, and U in hydrothermal fluids at Coronation Hill was investigated by studying fluid inclusions in both mineralized and unmineralized quartz and carbonate veins. Fluid inclusions in mineralized veins generally have irregular shapes and show evidence of necking. Four types have been recognized; type I are vapor-rich inclusions, type II are two-phase aqueous inclusions containing approximately 5 vol precent vapor, type III are single-phase, liquid inclusions, and rare type IV inclusions contain up to four solid phases and 10 vol percent vapor. Type II inclusions are common but many microfractures contain only type I vapor-rich inclusions. Type III inclusions always coexist with type I inclusions and are probably related by necking down. Type IV inclusions represent early, premineralization, regional fluids whereas all other inclusion types are relics of the ore-forming fluid.Microthermometry and low-temperature laser Raman spectroscopy indicate that the ore fluids were saline and calcium dominated (ca. 26 wt % CaCl 2 equiv) and that mineralization occurred at around 140 degrees C. No gases could be detected by Raman spectroscopy in the vapor phase of most inclusions. However, O 2 was detected in inclusions close to pitchblende mineralization indicating that radiolysis may have occurred. Raman spectroscopy was also used to identify some of the solid phases and calcite, hematite, and white mica were identified in the multiphase inclusions. Fluid and mineralogic data indicate that the ore-bearing fluid was initially a highly oxidized, acidic, and calcium-rich brine. At these conditions, transport of Au and PGE would involve mainly chloride complexes whereas U may be transported as the oxychloride species, and thus, all species could be transported in the same fluid. Thermodynamic calculations indicate that mineralization may occur by reduction of the oxidized fluid and/or by an increase in pH. Oxygen isotope and mineralogical evidence suggests that the ore fluid originated as saline ground water which traveled down subvertical structural faults before reacting with the feldsparhie and reduced lithologies below the little-deformed, hematitie quartz sandstone.The chemistry of ore deposition has been modeled by the numerical equilibration of the fluid with potash feldspar plus a very small amount of graphite. This results in a moderate decrease in fO 2 , an increase in pH from below 4 to 5.1, and precipitation of Au,Pt, and Pd with little or no U (< or = 1% of total UO 2 ). A greater decrease in fO 2 and/or an increased pH is needed to precipitate the U-Au-PGE ore and this may result from either the direct interaction with more highly carbonaeeous or ferruginous units or from mixing with reduced fluids derived from these rock units. This model explains the observed distribution of the Au-PGE-only ore across a number of feldspathic lithologies and the restriction of the U-Au-PGE ore to carbonaceous-chloritic host rocks.