Geology and Mineralogical Zonation of the Olympic Dam Iron Oxide Cu-U-Au-Ag Deposit, South Australia -

Rare Earth Deposits & Mining

EHRIG and two co-authors reported in 2012 that the Olympic Dam deposit is a supergiant Fe oxide Cu-U-Au-Ag deposit that is also strongly enriched in a wide range of elements, including F, S, C, As, Ba, Bi, Cd, Co, Cr, Fe, In, Mo, Nb, Ni, P, Pb, Sb, Se, Sn, Sr, Te, V, W, Y, Zn, and rare earth elements (REE). The deposit contains more than 90 minerals. Mineralization was associated with intense, texturally destructive hematite and sericite alteration and brecciation of the primary host rock units, including Roxby Downs Granite, bedded clastic facies rocks, and mafic-ultramafic dikes. Based on comprehensive geological, geochemical, and mineralogical data sets collected during a deposit-scale resource delineation and sterilization drilling program (2003–2008), compiled with historical geological data and interpretations, the authors quantified geochemical and mineralogical associations and distribution patterns.

The granite-derived elements (Al, Be, Ca, Hf, K, Li, Mg, Mn, Na, Rb, Si, Th, Ti, and Zr) are negatively correlated with Fe, whereas the hydrothermal 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; the ore and gangue minerals are also correlated with Fe abundance. There is a strong spatial association of Cu, U₃O₈, Au, and Ag.

From the periphery inward and upward from depth toward the deposit center, the most significant zones are as follows:

1) reduced Fe oxide alteration (magnetite-apatite-siderite-chlorite-quartz) → oxidized Fe oxide alteration (hematite-sericite-fluorite) → hematite-quartz-barite alteration,

2) siderite → fluorite → barite,

3) sphalerite → galena → pyrite → chalcopyrite → bornite → chalcocite → nonsulfide, and

4) distal or paragenetically early (?) base metal-poor (Mo-W-Sn-As-Sb) → base metal-rich (Cu-Pb-Zn) minerals → sulfide-barren hematite-quartz-barite breccia in the deposit center.

Spatially isolated remnants of advanced argillic alteration (sericite + quartz ± Al-OH) have been defined for the first time. Progressive Fe oxide addition to, and sericite replacement of the primary host rocks produced distinctive, albeit complex, hydrothermally altered and mineralized zones in the Olympic Dam deposit.