The Cobar Basin is one of the most important metalliferous regions in Australia and contains some of the largest base metal deposits in NSW. The mining fields have been a source of immense mineral wealth for 140 years since the discovery of the Great Cobar copper deposit in 1870.
Geology and Major Deposits in the Cobar Region
The Cobar deposits are a unique class of large, and commonly high grade, base and precious metal deposits hosted by marine sediments. The typical Cobar-type deposits consist of multiple lenses in steeply plunging, pipe-like clusters. They have great depth extent but only a small surface footprint (typically 250-300 metres across).
Cobar Deposit Model
However, the deposits have a number of unique characteristics which assist geologists in exploring for mineralisation hidden below cover. From an exploration and targeting view, the most important of these is the iron sulphide alteration, which forms extensive disseminated envelopes of iron sulphides, mainly magnetic pyrrhotite, around the deposits.
The successful use of magnetics in exploration is illustrated by the Endeavor (Elura) deposit, which was discovered in 1974 by an airborne magnetic survey. The top of the ore zone barely reaches the surface, but the pyrrhotite content of the ore body was sufficient to generate a classic ‘bulls-eye’ magnetic anomaly. Less than 150 metres below the surface the Main ore zone pipe ballooned to more than 100 metres in diameter and was one of the richest lead-zinc-silver deposits mined in Australia during the century. To date over 50 million tonnes of +12% lead-zinc (+silver) mineralisation has been defined to more than 1.5 kilometres below surface.
Endeavor Magnetics and Long Section of Mineralisation
Thomson Resources has several projects with similar magnetic bulls-eye anomalies.