Origin of rare earth element-enriched hematite breccias at the Olympic Dam Cu-U-Au-Ag deposit, Roxby Downs, South Australia

Abstract

Hematite breccias, the host rocks to Cu-U-Au-Ag ore at the Olympic Dam deposit, occur as steeply dipping, northwest-striking, dikelike bodies within fractured granite. The breccia complex has a strike length of over 5 km and extends to depths greater than 1 km. Both the deposit as a whole and the individual breccia bodies are zoned from weakly brecciated, sericitized and hematized granite on the margins, through heterolithic breccias, to hematite-quartz microbreccia at the center. Hematite occurs as euhedral laths and fine-grained aggregates in the matrices of all breccia types, in veins and fragments, as pseudomorphs after feldspar, and rarely as pseudomorphs after siderite and magnetite. Relict magnetite is rare, and most iron oxide was deposited as hematite. Heterolithic breccias include fragments of earlier breccia and veins. Minor sedimentary rocks, including bedded hematite and laminated barite, also are present as clast components. In the upper portion of the deposit, volcaniclastic conglomerate and siltstone occur in a fault-bounded block approximately 500 X 500 m in plan view.Copper-iron sulfides occur as interstitial grains (most common), microveinlets, and rare clasts. They are most abundant in heterolithic breccias and are zoned laterally from chalcocitebornitc to chalcopyrite-pyrite. Zonal boundaries are subparallel to steep breccia contacts. Copper-iron sulfides commonly are intergrown with fluorite and locally rim corroded quartz grains. The breccias are highly enriched in light (LREE) and heavy rare earth elements (HREE) (avg = approximately 5,000 ppm REE in hematite-rich rocks). Total LREE content and La/Lu ratios are correlated with hematite abundance; maximum La values (10,000 X chondrite) occur in hematite-quartz microbreccias from the geographic center of the deposit. Five hydrothermal REE phases have been identified: bastnaesite, florencite, monazite, xenotime, and britholite(?). They occur in all breccia types and in a variety of habits: disseminated in quartz-sericite matrices of granite breccia, in quartz-sericite veins, interstitial to hematite laths, intergrown with barite in laminated sedimentary rocks, and as inclusions in hematite and in sulfide grains.The hematite breccias formed by progressive hydrothermal brecciation and iron metasomatism of the granite host. Minor sedimentary components preserved as blocks and fragments within the upper portion of the deposit point to a near-surface environment for the breccia complex. Textural relations indicate that most of the copper was introduced late relative to hematization and brecciation. Based on spatial relations between barren zones, oxidized sulfide grains, and high-grade chalcocite ore, some supergene redistribution of copper probably occurred. The abundance of REE in hydrothermal phases, REE enrichment of altered relative to unaltered wall rock, concentration of REE in the center of the system, and variable slopes of chondrite-normalized patterns, combined with the lack of evidence of intrusion of unusual magmas at the present level of exposure, attest to extensive transport and deposition of REE by hydrothermal fluids responsible for hematization and breccia formation. This study demonstrates hydrothermal REE mobility on a scale previously undocumented and suggests that REE may be particularly mobile in F-rich hydrothermal systems.