Exploration rock geochemistry in the Rosebery Mine area, western Tasmania

This study reports on the role of rock geochemistry in massive sulphide exploration in the Rosebery mine area. The study involved four aspects: (1) distinction between the host rock and the footwall pyroclastics; (2) geochemical zonality associated with the Rosebery deposit; (3) geochemical haloes around the Rosebery deposit; (4) geochemical evaluation of the host rock.

The difficulty in distinguishing between the host rock and footwall, which have similar lithological characteristics because of the alteration effect within the Rosebery mine area, may be removed by examining their geochemical signatures. The host rock is characterised by the enhancement of TiO₂, Al₂O₃, Fe₂O₃, MgO, K₂O, CaO, MnO, P₂O₅, Pb, Zn, Rb, Ba and Tl, and depletion of SiO₂, Na₂O, Cu and Sr. The footwall pyroclastic rocks have enrichment in SiO₂, K₂O, Cu, Rb, and possibly MgO and MnO, and depletion in TiO₂, Al₂O₃, CaO, Na₂O, P₂O_S, Sr, Ba and Tl. Distinction between the host rock and footwall pyroclastics can be made by using the relations between TiO₂, Al₂O₃ and SiO₂. Higher concentrations of MgO, CaO, MnO, P₂O₅, Ba and TI in the host rock compared to the footwall also play a major role to distinguish it from the footwall

The Rosebery deposit is associated with significant lateral and vertical (stratigraphic) geochemical zonalities. The lateral zonality sequence (from upper level to lower level along the dip direction) is defined as As-Ba-Sb-Pb-Zn-Cu. The stratigraphic zonality sequence may also be arranged (from top to bottom) as (As, Ba, Sb)-(Pb, Zn)-Cu.

The Rosebery deposit is associated with well-defined surface geochemical haloes (at least 3000 x 1200 m) of Ba, Mn, Fe, and F which are much larger than the mineralogical alteration zone and which also extend into hangingwall.

The dispersion haloes of K, Rb, Na, Ca, Sr, K/Na and Rb/Sr outline tire footwall alteration zone and tire host rock of the Rosebery deposit, and also extend into the hangingwall pyroclastics.

Ba, Tl, Ga and V together with Pb, Zn, As, Sb and Ag may be the element association to determine the mineralisation potential of the host rock.