A fluid inclusion and stable isotope study of the Moonta copper–gold deposits, South Australia: evidence for fluid immiscibility in a magmatic hydrothermal system

Abstract

The copper–gold ores of the Moonta mineral field, in the eastern part of the Archaean–Mesoproterozoic Gawler Craton, South Australia, are vein deposits occupying fractures and shear zones within the Palaeoproterozoic Moonta Porphyry. The veins show a spatial relationship with Mesoproterozoic granites, equivalents of the Hiltaba Suite granites that are widespread in the Gawler Craton. The ores were deposited in four hydrothermal stages separated by episodes of fracturing; the first two stages were dominated by Fe oxides, the third by Fe sulfides and the final stage by a Cu–Fe–Co–Au–Zn–Pb–S assemblage. Three types of fluid inclusions are present in quartz associated with the oxide and sulfide minerals. Type 1 fluid inclusions are two-phase vapour-rich, with homogenization temperatures up to 473 °C and low to moderate salinity (2 to 22 wt.% NaCl equivalent). Type 2 are two-phase liquid-rich with lower homogenization temperatures (up to 269 °C) and low to moderate salinities (0.7 to 22 wt.% NaCl equivalent). Type 3 are liquid-rich, solid(s)-bearing with homogenization temperatures up to 467 °C and high salinities (28 to 55 wt.% NaCl equivalent). The coexistence of types 1 and 3 suggests that these fluid inclusions resulted either from trapping of boiling fluids or represent two immiscible fluids, most likely derived from a magma. The fluid history, reflected in fluid inclusion characteristics, was complex, involving variable amounts of boiling, cooling and mixing in the vein system. Salinity–homogenization temperature relationships indicate mixing of the hypersaline fluid and the vapour-rich fluid with a surface-derived fluid of low temperature and low to moderate salinity. δD H 2O values (−22‰ to −31‰) and δ 18O H 2O values (2.9‰ to 5.1‰) calculated from analyses of chlorite and δ 18O H 2O values calculated from analyses of quartz (0.6‰ to 8.8‰) on the basis of mean to maximum fluid inclusion homogenization temperatures are intermediate between magmatic water and surface-derived fluids (i.e. seawater or meteoric water). The narrow range of δ 18O values of quartz (10.8‰ to 12.7‰) may be explained by mixing of a magmatic fluid and either seawater or a low latitude, moderately evolved meteoric water, with a coupled decrease in fluid temperature and 18O content. Similar ranges for δ 34S values of ore sulfides (−2.3‰ to 6.4‰) and disseminated sulfides in the Moonta Porphyry and interdigitated Doora Schist (−1.5‰ to 4.6‰) suggest assimilation of crustal sulfur by the granitic magma or ore fluids. The combined fluid inclusion and stable isotope data support previous proposals, based on field and mineralogical evidence, for a genetic relationship between the Moonta ores and the Mesoproterozoic granitoids.