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 = '--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, fiorencite, 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 exposu...

Rare Earth Element Behaviour in Apatite from the Olympic Dam Cu–U–Au–Ag Deposit, South Australia

“…Total acquisition time was 60 s for each analysis comprising a 30 s background measurement, 30 s of ablation and a 40 s delay between analysis allowing for adequate wash-out, gas stabilization and processing time. The following isotopes were analysed 23 Na, 24 Mg, 27 Al, 28 Si, 31 P, 34 S, 39 K, 43 Ca, 45 Sc, 47 Ti, 51 V, 55 Mn, 56 [19]). The insert map shows the location of Olympic Dam with respect to the Gawler Craton and South Australia.…”

“…The recent work of Migdisov et al (2016) [6] has emphasized the variability of REY behaviour brought about by the dominance of certain REY complexes such as REY-Cl and -SO 4 over others, notably REY-F. Such variability can be tested against changes in fluid characteristics, as suggested from fluid inclusion studies-i.e., from early, high-T and salinity fluids to late, lower-T and salinity fluids, corresponding to magnetite and hematite stages, respectively, at OD [43]-and in other IOCG prospects from the Cu-Au Olympic Province [28]. Of particular relevance to apatite is how, under different conditions, LREE may be partitioned relative to HREE due to the increased stability and solubility as LREE-Cl complexes, thus explaining the LREE-enriched nature of many hydrothermal deposits, including IOCG systems [6].…”

The Midcontinent of the United States: Permissive terrane for an Olympic dam-type deposit?

“…Here, we have shown (Oreskes and Einaudi, 1990) that the heterolithic hematite-granite breccia bodies, many of which are elongate and dikelike, strike north-northwest, parallel with regional photolinears. These breccias commonly display steeply dipping, wavy foliation broadly parallel with the orientation of the breccias.…”

“…A thick sequence of volcanic rocks is preserved at Acropolis; extensive hematization and chloritization mask the original rock types, but they are porphyritic and probably are felsic to intermediate flow rocks and ash-flow tuffs (Paterson, 1986b ). The granite of Olympic Dam is too coarse grained to represent a subvolcanic intrusion, and, inasmuch as all the evidence points to a near-surface origin for the ore deposit (Oreskes and Einaudi, 1990;Reeve and others, 1990), we do not believe that there is a close temporal and genetic relation between ore-forming hydrothermal activity and the granite of Olympic Dam. Local felsic porphyry dikes, altered and dismembered by faults and breccia columns, may be an expression of the volcanic link, but nothing is known about the original composition of the volcanic rock.…”

“…Trueman, oral commun., 1985), relative lead and strontium isotope closure dates for apatite and biotite (Mortimer and others, 1988), and geologic relations (see later) suggest that the hydrothermal activity that produced Olympic Dam could be 100--200 m.y. younger than the granite host (Oreskes and Einaudi, 1990). Such an age would overlap with 1,525-1,450-million-year silicic volcanic and plutonic rocks exposed in the Gawler Ranges to the southwest (Webb and others, 1986) and thought to extend under the Stuart Shelf (see later).…”