Staged formation of the supergiant Olympic Dam uranium deposit, Australia

The origins of many supergiant ore deposits remain unresolved because the factors responsible for such extreme metal enrichments are not understood. One factor of critical importance is the timing of mineralization. However, timing information is commonly confounded by the difficulty of dating ore minerals. The world’s largest uranium resource at Olympic Dam, South Australia, is exceptional because the high abundance of U allows U-Pb dating of ore minerals. The Olympic Dam U(-Cu-Au-Ag) ore deposit is hosted in ca. 1.59 Ga rocks, and the consensus has been that the supergiant deposit formed at the same time. We argue that, in fact, two stages of mineralization were involved. Paired in situ U-Pb and trace element analyses of texturally distinct uraninite populations show that the supergiant size and highest-U-grade zones are the result of U addition at 0.7–0.5 Ga, at least one billion years after initial formation. This conclusion is supported by a remarkable clustering of thousands of radiogenic 207Pb/206Pb model ages of Cu sulfide grains at this time. Upgrading of the original ca. 1.59 Ga U deposit to its present size at 0.7–0.5 Ga may have resulted from perturbation of regional fluid flow triggered by global climatic (deglaciation) and tectonic (breakup of Rodinia) events.

Supplemental Material: Staged formation of the supergiant Olympic Dam uranium deposit, Australia

Items S1 (mine-scale maps of Cu, U, Pb grades), S2 (methods), and S3 (interpretation of Pb isotopes in Cu sulfides); and Tables S1 and S2 (U-Pb dating and REE compositions of uraninite, respectively).<br>

Supplemental Material: Staged formation of the supergiant Olympic Dam uranium deposit, Australia

Items S1 (mine-scale maps of Cu, U, Pb grades), S2 (methods), and S3 (interpretation of Pb isotopes in Cu sulfides); and Tables S1 and S2 (U-Pb dating and REE compositions of uraninite, respectively).<br>

Olympic Dam: BHP thinking big about the future

Olympic Dam is one of the world’s most significant polymetallic orebodies producing copper, uranium, gold, and silver in remote South Australia. The polymetallic deposit is located 520 km north-northwest of Adelaide, South Australia and has an inferred resource of 2660 Mt at 1.2% Cu, 1.4 kg t−1 U3Os, and 0.5 g t−1 Au. Ore is mined from the underground operation at a rate of approximately 10 mt year−1, and is processed on site through a concentrator and hydrometallurgical facility, smelter, and electrolytic refinery. Olympic Dam is one of the only sites in the world to claim the ‘mine to market’ title. Protection of the workforce and the environment has been a primary focus for the operations through its 30+ year life and will continue to be into the future. Broken Hill Propriety Company (BHP) believes that its most important asset is its people. With such a large orebody and a very long potential mine life, it is important to think strategically about the future to ensure the viability of the operation. This requires development of mine and surface processing facilities in a staged manner. Importantly, it also involves the development of people. This presentation provides an overview of BHP’s work at Olympic Dam and outlines development plans for Olympic Dam into the future. © 2020 ICRP. Published by SAGE.

OPENING THE MAGMATIC-HYDROTHERMAL WINDOW: HIGH-PRECISION U-Pb GEOCHRONOLOGY OF THE MESOPROTEROZOIC OLYMPIC DAM Cu-U-Au-Ag DEPOSIT, SOUTH AUSTRALIA

Establishing timescales for iron oxide copper-gold (IOCG) deposit formation and the temporal relationships between ores and the magmatic rocks from which hydrothermal, metal-rich fluids are sourced is often dependent on low-precision data, particularly for deposits that formed during the Proterozoic. Unlike accessory minerals routinely used to track hydrothermal mineralization, iron oxides are dominant components of IOCG systems and are therefore pivotal to understanding deposit evolution. The presence of ubiquitous, magmatic-hydrothermal U-(Pb)-W-Sn-Mo–bearing zoned hematite resolves a range of geochronological issues concerning formation of the ~1.6 Ga Olympic Dam IOCG deposit, South Australia, at up to ~0.05% precision (207Pb/206Pb weighted mean; 2σ) using isotope dilution-thermal ionization mass spectrometry (ID-TIMS). Coupled with chemical abrasion-ID-TIMS zircon dates from host granite and volcanic rocks within and enclosing the ore-body, a confident magmatic-hydrothermal chronology is defined. The youngest zircon date from the granite intrusion hosting Olympic Dam indicates magmatism was occurring up until 1593.28 ± 0.26 Ma. The orebody was principally formed during a major mineralizing event following granite uplift and during cupola collapse, whereby the hematite with the oldest age is recorded in the outer shell of the deposit at 1591.27 ± 0.89 Ma, ~2 m.y. later than the youngest documented magmatic zircon. Hematite dates captured throughout major lithologies, different ore zones, and the ~2-km vertical extent of the deposit support ~2 m.y. of hydrothermal activity. New age constraints on the spatial-temporal evolution of the formation of Olympic Dam are considered with respect to a mantle to crustal continuum model. Cyclical tapping of magma reservoirs to maintain crystal mushes for extended time periods and incremental building of batholiths on the million-year scale prior to main mineralization pulses can explain the ~2-m.y. temporal window temporal window inferred from the data. Despite the challenge of reconciling such an extended window with contemporary models for porphyry deposits (≤1 m.y.), formation of Proterozoic ore deposits has been addressed at high-precision and supports the case that giant IOCG deposits may form over millions of years.