Application of lithogeochemistry to gold exploration in the St Ives goldfield, Western Australia

Abstract

Lithogeochemistry is being routinely applied to gold exploration programmes at St Ives, Kambalda. Sampling of fresh rock for multi-element analysis is undertaken in all ongoing exploration drilling, and supplementary sampling of archived drill hole pulp samples has also been implemented. Samples are digested by a four-acid digest (hydrofluoric, nitric, perchloric and hydrochloric acids) in Teflon tubes using a specially designed microprocessor controlled digester and analysed by ICP-MS/OES for 36 elements (Ag, As, Ba, Bi, Ce, Cs, Hf, La, Li, Mo, Nb, Pb, Rb, Sb, Ta, Te, Th, W, Zr, Al, Ca, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, S, Sc, Sr, Ti, V, Zn). Multi-element results undergo two stages of assessment: (a) utilization of trace elements to characterize lithological and stratigraphic domains, and (b) utilization of major and pathfinder elements to characterize samples that are altered or enriched above background levels. Bi-plots of trace elements are used to constrain rock type and stratigraphy. Diagnostic inter-element ratios are apparent, which enable clear differentiation of Kambalda Komatiite, Devon Consols Basalt and Paringa Basalt. Condenser Dolerite can be differentiated from other mafic units except Lunnon Basalt. Fractionated dolerite sills are highly prospective host rocks for gold mineralization in the St Ives field. In all cases the fractionated portion of dolerite sills is readily identified by a Ti concentration greater than 6000 ppm. Unaltered and altered ultramafic, mafic, felsic–intermediate rocks have been assessed using molar Na/Al and K/Al alteration indices to define areas of sodic, potassic and strong potassic alteration. The distribution of potassic and sodic altered samples favourably correlates with the distribution of observed biotite and albite alteration and known gold mineralization. Pathfinder elements (As, Bi, Sb, W, Mo) have been assessed at the scale of the whole field. Antimony is decoupled from Mo and Bi. Bismuth displays good association with existing gold deposits and areas of known intrusions. Areas of As enrichment correlate spatially with Sb although they are not as widely distributed. Tungsten is the most constrained of the pathfinders and displays excellent correlation with major structures and existing gold deposits.