Hematite Breccia-Hosted Iron Oxide Copper-Gold Deposits Require Magmatic Fluid Components Exposed to Atmospheric Oxidation: Evidence from Prominent Hill, Gawler Craton, South Australia

Schlegel, T.U.; Wagner, T.; Wälle, M.; Heinrich, C.A.

Economic Geology 113(3): 597-644


ISSN/ISBN: 0361-0128
DOI: 10.5382/econgeo.2018.4564
Accession: 068506172

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The Prominent Hill deposit is a large iron oxide copper-gold (IOCG) resource located in the Olympic Iocg province of South Australia. The deposit is hosted by brecciated sedimentary rocks and structurally underlying lavas of the ca. 1.6 Ga Gawler Range Volcanics. Both rock units are altered and mineralized, forming characteristic hematite breccias. They are located in the footwall of the Southern overthrust separating the host-rock package in the footwall from Paleoproterozoic metasedimentary rocks in the hanging wall. The metasedimentary rocks were intruded by the Hiltaba Suite granites, which are comagmatic with the Gawler Range Volcanics and show widespread magnetite-rich alteration. Economic mineralization was formed through a two-stage process. Early pyrite and minor chalcopyrite were deposited from moderately reduced fluids during sulfide stage I and are hosted in subeconomic magnetite skarns and in the brecciated sedimentary host rocks. This preore stage was overprinted by the economically important stage Ii sulfides, deposited from hypogene, oxidized fluids ultimately sourced from the paleosurface. The high-grade Cu ores contain dominantly chalcocite, bornite, chalcopyrite, and gangue minerals including fluorite, barite, and minor quartz hosting mineralization-related fluid inclusion assemblages. Petrography, microthermometry, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) microanalysis were used to characterize pre-, syn- and postmineralization fluid inclusion assemblages. The results permit discrimination of four fluid end members (A, B, C, and D). Fluid A is the main ore fluid and hosted in fluorite and barite intergrown with Cu sulfides in the breccia matrix. It is weakly saline (-10 wt % Na Cl equiv) and contains low concentrations of K, Pb, Cs, and Fe (600 ppm) but is rich in Cu (1,000 ppm) and U (0.5-40 ppm). A magmatic origin of the salinity is supported by the low molar Br/Cl ratio of 0.003. We suggest that the solute inventory was derived from shallow fluid exsolution and degassing of late Gawler Range Volcanics and subsequent complete oxidation of the fluid via contact with atmospheric oxygen. Fluid A migrated through oxidized aquifers to the site of the Prominent Hill deposit, where it became the main driver of stage Ii copper mineralization. Fluid B occurs in fluid inclusions in siderite + quartz-bearing veins crosscutting the hematite breccia. It is the most saline fluid, with a total Na Cl + Ca Cl2 concentration of 36 to 45 wt % and a low Ca/Na mass ratio of 0.3. Fluid B is rich in K, Fe, Pb, and Cs and contains modest Cu (~70 ppm). Its composition is typical of a moderately reduced magmatic-hydrothermal brine modified by fluid-rock interaction. Fluid C is hosted by fluid inclusions in fluorite and barite within bornite + chalcocite-bearing ores. It is a calcic-sodic brine with 16 to 28 wt % Na Cl + Ca Cl2 and has an elevated Ca/Na (0.6) and high Br/Cl ratios characteristic of basin brines of residual bittern origin. It is quite rich in Cu (~200 ppm) and likely contributed metals to economic mineralization. Fluid D is hosted by inclusions in fluorite in late veins crosscutting the hematite breccia. The total Na Cl + Ca Cl2 salinity ranges between 19 and 30 wt %, and the concentrations of K, Mn, Cs, and Pb are similar to those of fluid C. Fluid D is a basement brine with characteristically high Ca/Na ratios of about 2 and contains modest Cu (~100 ppm). The high-grade copper mineralization in the Prominent Hill deposit originated dominantly from magmatic components, including both the metal inventory (Cu, minor Au, and U) and sulfur. The essential factor controlling the high-grade Iocg mineralization was wholesale oxidation of magmatic Cu and S from Gawler Range volcanic eruptions; volcanic gases reacted with atmospheric oxygen at or near the Earth's surface- for example, in an acidic volcanic lake environment. Oxidized surface fluids charged with Cu, U, and sulfate and high acidity then infiltrated the host rocks at Prominent Hill and became reduced by ferrous iron in residual magmatic brine, in nonmagmatic basin brine, and by ferrous iron-bearing minerals. Copper sulfides and hematite precipitated in a zone of fluid mixing and by interaction of the fluids with acid-neutralizing host-rock strata. Limited sulfide for the low-sulfidation ore mineral assemblage was supplied, to presently unknown proportions, from magmatic-hydrothermal pyrite derived in a preore stage of mineralization and from magmatic sulfate reduced by aqueous Fe2+ to precipitate hematite. Exploration-relevant factors for Prominent Hill-style high-grade copper ores are the combination of (1) active volcanism during the late stages of the Gawler Range Volcanic Province, (2) a fluid-mixing interface between deep brines and an acidic, surface-oxidized but magmatically charged water as the main ore fluid, and (3) a mappable gradient of acid neutralization in brecciated and chemically re active host rocks, ideally containing carbonates and some earlier pyrite.