Interpretations of the ore-forming processes at Olympic Dam: previous models

The earliest published article on the Olympic Dam deposit is by Roberts and Hudson (1983). They considered this giant copper-uranium-gold deposit to be a new type of sediment-hosted ore deposit, mainly based on the ‘strata-bound’ chalcocite- bornite-chalcopyrite-pyrite mineralisation that is typical of stratiform copper occurrences. They interpreted the formation of the host breccias (in what is later called the ODBC) to ore in an arid subaerial environment during rifting or strike-slip faulting and suggested that the introduction of ore metals was related to local volcanism, without much strong evidence but spatial distribution of mineralisation relative to volcanic structures and rocks.

With more exposures in underground mining, Reeve et al. (1990) proposed that the primary ore deposition occurred in a shallow (still argued) level environment controlled by coupled redox reactions involving the oxidation of wall rock components and reduction of elements complexed in the hydrothermal fluids, fluid mixing as a consequence of redox reactions, dilution and/or cooling, and boiling. They owed the formation of the ODBC and the ore contained to the interplay of hydrothermal, volcanogenic, sedimentary and tectonic processes.

The breccias, especially the hematite-quartz breccias, are highly enriched in light and heavy rare earth elements (LREE and HREE) (avg. ~5,000 ppm REE in hematite- rich breccias, with La normally reaching 3,000 times chondrite, and maximum 10,000 times chondrite), and this led to special attention on the REE behaviour during the hydrothermal process. Oreskes and Einaudi (1990) suggested that the hydrothermal fluids responsible for hematite alteration and breccia formation, were REE-rich, based on the high abundance of REE in hydrothermal phases, REE enrichment of altered versus unaltered wall rocks, concentration of REE in the centre of the system, variable slopes of chondrite-normalized patterns, and the lack of unusual intrusions. The occurrence of REE phases in the early quartz-sericite veins and barite-bastnaesite fragments, intergrown with hematite, and as inclusions in sulfide grains, led them to conclude that REE minerals were precipitated during much of the hydrothermal history of Olympic Dam. Moreover, evidence for presence of sedimentary rocks in the breccia complex, which might indicate that mineralisation occurred within 1 to 2 km of the Proterozoic paleosurface or possibly at the surface, in contrast with the coarse-grained, plutonic host rock, together with preliminary radiometric dating results including: ca. 1315 Ma of Rb-Sr analysis of sericite from altered granite wall rock (Gustafson and Compston, 1979), a minimum age of 676 ± 200 Ma indicated by Rb-Sr dating of the Woomera Shale, a correlative of the Tregolana Shale which unconformably overlies the deposits (Webb et al., 1983) and a maximum age of ca. 1400 Ma on pitchblende analysed by ion microprobe (Trueman, 1986), inclined them to believe that mineralisation significantly post-dated emplacement of the host granite and, therefore, did not bear a direct link to it. Data from their work also showed an important role for F complexes—in addition to or instead of CO3 complexes—in mobilizing REE in low-

pressure hydrothermal systems.

Oreskes and Einaudi (1992) investigated fluid inclusion, stable isotope and formation temperatures of various ore types at Olympic Dam, concluding that at least two sources of fluids, of contrasting temperature, composition, and oxygen isotope characteristics, were involved in the formation of Olympic Dam. Early magnetite (dominant iron oxide in early hydrothermal assemblages, replaced by hematite during the subsequent hydrothermal process, with only minor localised relicts of magnetite preserved within the hematite-rich breccias) associated with pyrite and siderite was precipitated from fluids of high δ18O (approx. 10 ‰) at temperatures near 400 ̊C. In

contrast, hematite within the ODBC formed at lower temperatures (200-400 ̊C) from fluids of lower δ18_{O (<9 ‰). Waters of surficial origin—seawater, closed basin water,}

or ground water—may have been involved in the later stages of hydrothermal activity that formed the hematite-rich breccias, but this was not a necessity required by their isotopic data. They proposed that the early magnetite was coeval with formation of the Roxby Downs Granite (ca. 1590 Ma), forming at relatively high temperatures in a deep seated hydrothermal environment until the hematite and ore-forming stage began 50 to 150 million years later.

Sm-Nd isotope analyses of pyrite, chalcopyrite, and bornite-chalcocite-hematite- rich ores gave the same initial εNd of -2.5, which suggests that these ore types are cogenetic and contain a contribution from a mantle-derived source rock or magma (Johnson and McCulloch, 1995). In contrast to hematite-rich breccias, volumetrically minor magnetite-rich assemblages have the same initial Nd signatures (εNd of -5) as the host granite, suggesting that they could be cogenetic. The initial εNd isotopic values (up to +4) of the altered mafic/ultramafic dykes within the deposit makes these dykes a potential source for the Nd component of the deposit, and thus a likely source of metals (i.e. Cu) (Johnson and McCulloch, 1995).

Haynes et al. (1995) proposed that fluid mixing was the dominant ore-forming process at Olympic Dam. In their model, the mixing of a hotter magmatic or deeply circulated meteoric water (<300 ̊C) and a cooler meteoric water, which may have originated as saline ground water or playa lake water within the volcanic succession, was thought to be responsible for ore genesis. Moreover, their modelling supported the hypothesis that precipitation of magnetite, hematite, sulfides, and uraninite resulted from coupled sulphate reduction and ferrous iron oxidation and that the cooler meteoric waters were oxidized and were derived from a provenance containing mafic volcanic rocks with or without a felsic component. They also concluded that the ODBC contained a major Cu-U-Au-Ag orebody because it formed within a reservoir of saline ground water in contact with mafic and felsic volcanics and subvolcanic intrusions and the ground water was responsible for transport of Cu, U, Au, Ag and most of the S into the breccia complex, where it interacted with the hotter water which introduced most of the Fe, F, Ba, and CO2 from below. However, they also pointed out that the weak

point of this modelling was that copper and uranium would be supplied to the ore- forming process by hotter waters if their temperature is above 300 ̊C.

The eastern Gawler Craton hosts Australia’s premier uranium-bearing iron oxide copper-gold belt, the >500-km-long Olympic Cu-Au-(U) Province, where there are the Olympic Dam Cu-U-Au and Prominent Hill Cu-Au deposits, together with numerous barren and weakly mineralised IOCG prospects. A general investigation of the whole province was conducive to the exploration in this brownfield terrain (Skirrow et al., 2007). Nd isotope compositions for whole rock samples from barren and weakly mineralised Cu-Au prospects and host rocks in the Olympic Dam and Prominent Hill districts (Skirrow et al., 2007) were compared with previous data from Johnson and McCulloch (1995). Both hematite- and magnetite-rich samples from five weakly mineralised prospects yielded generally similar initial εNd1590Ma values that match

values from fresh and weakly altered Paleoproterozoic metasedimentary and metagranitic rocks (-6.6 to -3.5, including the Donington Suite, the upper and lower Wallaroo Group), as well as from most felsic HS intrusions and GRV in the eastern Gawler Craton (-6 to -4). These data implied that mainly crustal sources (could not be specified, however) contribute REE, and by implication, Cu to the barren and weakly mineralised prospects (Nd isotope signature is roughly correlated to Cu concentration in whole-rock samples of the altered granitoids and mineralised hematite-rich breccias from the Olympic Cu-Au-(U) Province, indicating that Nd and Cu were co-transported in the same hydrothermal fluids, see Skirrow et al., 2007). However, pyrite, chalcopyrite, and bornite-chalcocite-rich hematitic ore samples at Olympic Dam have higher εNd (-2.5, Johnson and McCulloch, 1995), and there is a roughly positive correlation between Nd isotope signature and Cu concentration but no correlation in the plot of SiO2 versus initial εNd, which overall indicates that mantle (primitive) sources

contributed REE and possibly Cu to the Olympic Dam deposit in the way of hydrothermal process rather than magmatic process (Skirrow et al., 2007).

Bastrakov et al. (2007) proposed a two-stage hydrothermal activity model. In this model, early stage pre-existing hydrothermal magnetite with minor associated copper- gold mineralization was flushed by late-stage oxidized brines that had extensively reacted with sedimentary or metamorphic rocks. The reduction of these brines, driven by conversion of magnetite to hematite, resulted in precipitation of copper and gold,

and the oxidized brines may have contributed additional copper and gold to the system in addition to upgrading pre-existing subeconomic Cu-Au mineralization. At last, they assigned the different abundances of ore between the giant Olympic Dam deposit and those weakly mineralised prospects to the diversity of origins of iron oxide-copper-gold systems, even within the same geologic region. Stable isotope analysis was carried out to place further restrictions on the origin of the hydrothermal fluids of different stages. The Br/Cl ratios of the magnetite-forming fluids lie beyond the range of typical magmatic and/or mantle values, allowing for the possibility that the fluids originated as brines from a sedimentary basin or the crystalline basement.

In the most recent synthesis of the Olympic Dam deposit in Ehrig et al. (2012), a detailed analysis of a large dataset of the compositions of the ODBC gathered over years has shown that some elements (Al, Be, Ca, Hf, K, Li, Mg, Mn, Na, Rb, Si, Th, Ti, and Zr) are negatively correlated with Fe, whereas other elements (Ag, As, Au, Ba, Bi, Cd, Co, CO2, Cr, Cu, F, Fe, In, Mo, Nb, Ni, P, Pb, S, Sb, Se, Sn, Sr, Te, U, V, W, Y,

Zn, and REE) are positively correlated with Fe. The former elements are considered as granite-derived elements that are likely to be contributed from the Roxby Downs Granite, whereas the other elements have been named hydrothermal elements, which have a provenance from other lithologies (e.g. mafic dykes, bedded clastic facies).

2.3

Other IOCG prospects and deposits in the Olympic Cu-Au-

(U) Province