DATE: Tuesday, 27 July, 1982 @ 4.30 pm.
VENUE: Rugby Club, Crane Place, Sydney
SPEAKER: Dave Roberts, Senior Research Geologist – Olympic Dam Project, Western Mining Corporation, Adelaide.
TOPIC: “The Olympic Dam Copper-Uranium-Gold Deposit, Roxby Downs, South Australia”The Olympic Dam copper-uranium-gold deposit is a new type of strata-bound sediment-hosted ore deposit. It is located 650 kilometres north-northwest of Adelaide in South Australia and was discovered in 1975. It has an areal extent exceeding 20 square kilometres with vertical thicknesses of mineralization up to 350 metres.The deposit occurs in the basement beneath 350m of unmineralized, flat-lying Adelaidean to Cambrian sediments in the Stuart Shelf Region of South Australia. The basement rocks to the deposit are unmetamorphosed and are probably younger than 1580 Ma. The deposit is spatially related to coincident gravity and magnetic anomalies and the intersection of west-northwest and north-northwest trending lineaments.
The Proterozoic sediments comprising the local basement sequence are predominantly sedimentary breccias ranging from matrix-poor granite breccias to matrix-rich polymict breccias containing clasts of a variety of rock types. This sequence is over 1 km thick and has been divided into two main units – the Olympic Dam Formation and the Greenfield Formation. Basement to the Olympic Dam Formation is asssumed to be a granite equivalent to that which occurs outside the known area of mineralisation. Dolerite dykes, plugs and sills intrude the basement rocks but not the flat-lying cover sediments.
The localizing structure for the deposit is a northwest trending trough or graben which is arched about a northeast axis. Arching parallel to the graben long-axis also occurs in some areas. Strike-slip and dip-slip faults occur both parallel to, and at a high angle to, the graben long axis. The Greenfield Formation is preserved in downfaulted blocks in the crest of the structural arch.
Two types of copper sulphide mineralization have been defined – a strata-bound type and a younger transgressive type. Uranium, rare earths, gold and silver occur with both types. Strata-bound bornite (chalcocite)-chalcopyrite-pyrite mineralization occurs in the haematite matrix-rich members of the Olympic Dam Formation. Transgressive chalcocite-bornite mineralization overlies the stratabound type and is confined to a northwest trending linear zone over 6 km long and up to 700m wide in the centre of the graben. In both types of mineralization haematite, quartz, sericite and fluorite are the main gangue constituents with lesser amounts of chlorite, siderite, barite and rutile.
Uraninite, with lesser coffinite and brannerite, is very fine-grained and occurs in a variety of forms. Two rare earths minerals, bastnaesite and florencite, occur in, and adjacent to, sulphide mineralized zones.
It is considered that the graben-fill sediments were deposited in an arid subaerial environment during rifting or strike-slip faulting. The strata-bound sulphide mineralization is syngenetic or syndiagenetic and is probably related to local volcanism. The younger transgressive mineralization is epigenetic and was introduced into favourable structural zones. The uranium and rare earths were deposited during and after the sulphide mineralizing phase.
The deposit is a very unusual example of sediment-hosted mineralization. The most unusual features are the association of copper, uranium, rare earths and gold, the association of reduced sulphur species such as chalcocite and bornite with high haematite concentrations; and the occurrence of strata-bound copper and uranium minerals in sedimentary rocks deposited in a very high energy environment.
Olympic Dam Today – not much geology, well, OK, none…but production stats and a preoccupation with protestors.