The Role of Geophysics in Exploration at Junction Reefs and the Sheahan-Grants Gold Mine, Central N.S.W.

Four economic gold deposits occur in the Junction Reefs area near Orange, N.S.W. Three of these are metasomatic replacement deposits within calcareous sediments. The fourth is a structurally controlled stockwork zone adjacent to a late phase intrusion. The shallow dipping replacement deposits are associated with semi-massive pyrrhotite skarn and are both magnetic and electrically conductive. The stockwork body, which contains disseminated pyrite/arsenopyrite, is recognisable as a coincident magnetic low and radiometric high and is responsive to induced polarisation (IP) techniques.

Early geophysical exploration concentrated on ground magnetic and IP surveys. These surveys were generally unsuccessful in defining either the known mineralisation at the Sheahan-Grants deposit or new deposits, due to the geometry of the mineralisation and strong magnetic and IP responses from surrounding country rock.

An airborne magnetic and radiometric survey was flown over the Junction Reefs area. This survey helped to identify and map a large diorite intrusion which lies approximately three kilometres north of the mine. Within the magnetic response due to this intrusion is the magnetic low and radiometric high associated with the Glendale (stockwork) deposit.

Recent geophysical work in the area has concentrated on detailed ground magnetic and electromagnetic surveys, searching for further gold reserves of the Sheahan-Grants style. The use of electromagnetic techniques has only been possible following the development of software for stripping the responses due to the many fences which cross the prospective area.