Author: Russ Lord, Geologist/Geochemist

Extract from National Rock Garden Newsletter No. 22, December 2021

Breccias are central to many ore deposits in Australia and around the world. A breccia is a sedimentary rock formed from angular fragments of pre-existing minerals and rock, generally >2 mm in size, cemented together in a finer-grained matrix. The angular nature of the fragments indicates that they have not been transported very far from their source.

The most widely held interpretation is that ore deposits like Mt Isa and Broken Hill were formed by sediments depositing in grabens, a section of crust that lies between two faults that has lowered relative to the blocks on either side. We see this process happening today at Atlantis II in the Red Sea.

An outcropping mineralised breccia is shown below, up dip from the 1100 copper orebody in Mt Isa.

Geologist Chris Gregory examining a breccia that is the outcrop of the 1100 Cu Orebody at Mt Isa, Queensland. Image courtesy J.R. Lord (1989).

Underground these breccias and fracture zones look like the rocks in the next three pictures.

Chalcopyrite with black, fractured siliceous shale or chert clasts in the 1100 Orebody at Mt Isa. A W.D. Smith specimen. Image courtesy J.R. Lord.
Detail of chalcopyrite in silica-dolomite breccia showing clearly discordant mineralisation. A W.D. Smith specimen. Image courtesy J.R. Lord.
Medium grey carbonaceous chert with fracture fill chalcopyrite and silica (white). A WG Perkins specimen. Image courtesy J.R. Lord.

The cross section through the Mt Isa orebodies is very instructive (see next page). It shows the silica- dolomite breccia is widest adjacent to the Paroo-basement or Greenstone Fault, and it progressively thins away from that fault. Likewise the copper orebodies have best grades close to the fault and they decrease away from it. This is a feature not unique to Mt Isa. Other orebodies that demonstrate the same principle are Polaris (Zn) in Canada, Tsumeb (Cu) in Namibia, Carlin (Au) in Nevada, several deposits in east Tennessee (Zn) and several others in Ireland like Lisheen and Tynagh (Zn-Pb).

Silica-dolomite invasive into Urquhart Shale with shale clasts disoriented from their original west dipping orientation; 400 Orebody, 9 Level. The photo is looking south. Image courtesy W.G. Perkins.
Cross section through the Mt Isa Pb-Zn and Cu orebodies (after WG Perkins, 1998).

When galena and sphalerite are precipitated between two competent shale beds, and subsequent tectonics is applied with some heat, they behave like jam between two slices of bread. That is what has happened in the pictures below where one smaller black shale bed became macerated into this galena matrix breccia. Small black shale beds are broken into pieces, but bigger ones are more competent, and are simply folded.

Galena remobilised between black shale beds and brecciating the thinner ones between them; MICAF 6910N & 2830 RL. Image courtesy W.G. Perkins.
Crenulated Pb-Zn ore with black shale interbeds. FP XRF readings indicate the following composition: Fe (orange) 32.8%, Zn 6.22%, Pb 2.9% and Cu 0.13%. HG Schmidt specimen. Image courtesy J.R. Lord.

Oxidised copper minerals form breccias of a different sort, as illustrated in the breccia specimen below.

Cuprite in breccia from the Black Rock Open Cut at Mt Isa. Image courtesy J.R. Lord.

At Broken Hill, the lead-zinc minerals are generally banded in lenses, but they have been severely folded and highly metamorphosed in areas of complex faulting. In the North Mine, those sedimentary characteristics are destroyed, and breccias are the result. These faulted areas are perhaps analogous to the basement fault in the Mt Isa example.

High grade Pb-Zn in brecciated quartz in the Fitzpatrick orebody at the North Mine, Broken Hill. The rectangles are plates for rock bolts. Image courtesy J.R. Lord.
Breccia from the Fitzpatrick orebody, North Broken Hill Mine, NSW. Image courtesy J.R. Lord.

These two famous deposits at Mt Isa and Broken Hill are outstanding examples of major mineralised breccia zones in areas that were once grabens or spreading centres like the one we see today at Atlantis II in the Red Sea.