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

Abstract

The Prominent Hill deposit is a large iron oxide Cu-Au (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 old 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 co-magmatic 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 pre-ore stage was overprinted by the economically important stage II sulfides, deposited from hypogene, oxidized fluids ultimately sourced from the paleo-surface. 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 LA-ICP-MS microanalysis were used to characterize pre-, syn-and post-mineralization 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.% eqv. NaCl) and contains low concentrations of K, Pb, Cs, and Fe (600 ppm), but is rich in Cu (1000 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 NaCl + CaCl2 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 16-28 wt.% NaCl + CaCl2 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 i...

Trace element catalyses mineral replacement reactions and facilitates ore formation

“…Early high temperature (>400 °C) hydrothermal magnetite mineralisation was flushed by cooler, later-stage brines that had reacted with sedimentary or metamorphic rocks, and the reduction of these brines, driven by conversion of magnetite to hematite, was suggested to have resulted in Cu–Au precipitation. A modified two-stage model with early high temperature magnetite overprinted by retrograde hematite has also been recently suggested for Prominent Hill 9 .…”

“…The replacement of magnetite by hematite is commonly considered to reflect the influx of oxidizing fluids 9 , 26 . The involvement of oxidised, sulfate-bearing fluids in the ore stage at Olympic Dam is indicated by the abundance of barite in the ores (~1.2 wt%) 11 and is consistent with the location of the orebody at or near the paleosurface 9 , 32 . The proposed Ce-catalysed hematitisation reaction is nominally redox-independent (reaction ( 2 )).…”

“…(ii) The observed magnetite replacement textures and Ce enrichment strongly suggest that Ce-catalysed redox-independent replacement was widespread in the province. (iii) Sulfate is the most likely oxidant invoked for replacement via the oxidative pathway 9 . However, a previous study also observed that even in the presence of a much stronger oxidant (O 2 ), the replacement of magnetite by hematite proceeded via a combination of redox-independent and oxidative pathways, as a result of the faster kinetics of the redox-independent pathway 4 .…”

“…Economic iron deposits mostly consist of large accumulations of these minerals. In some important deposit classes, including iron oxide copper gold (IOCG) and rare earth element (REE) deposits, it is commonly observed that early magnetite is replaced by hematite in the main ore-forming stage [6][7][8][9] , and voluminous iron oxide-rich ores are associated with enrichments in light REE (LREE), in particular, Ce (refs. 7,10,11 ).…”

Iron oxide copper-gold (IOCG) deposits – A review (part 1): Settings, mineralogy, ore geochemistry and classification

“…Such zones also may represent the most acidic conditions (e.g. Prominent Hill, Schlegel et al, 2015Schlegel et al, , 2018. Moderately oxidised hydrothermal assemblages generally contain hypogene bornite ± chalcocite with hematite ± pyrite and white mica (at low pH) or potentially K-feldspar (at high pH), as shown in Figure 9.…”