A regolith geochemical anomaly formed by accumulation and corrosion of resistant minerals from Scuddles VMS primary ore, Western Australia

Abstract

Cassiterite, rutile and gahnite from Scuddles VMS primary ore have accumulated residually in silcrete and in ferruginous lateritic zones (duricrust and gravel) that cap the weathering profile at Scuddles. Cassiterite and rutile contain inclusions of base metal sulfides and sulfosalts which are stable under alkaline and reducing conditions in the supergene sulfide enrichment zone, and alkaline to neutral conditions in the supergene oxide enrichment zone. However, cassiterite and rutile have been intensively corroded in the silcrete and lateritic zones under oxidizing conditions by acidic fluids derived by infiltration of acidic meteoric water and the oxidation of sulfide inclusions. This has resulted in releasing a suite of pathfinder elements, including Sn, Cu, Zn, Cd, Hg, Pb, Sb, Bi, W, Mo, Se and Te. The released Cu, Zn and Cd tend to be mobile under these conditions and are leached from the silcrete and lateritic zones. However, these elements are elevated at certain stratigraphic zones in silcrete and form surface anomalies in exposed saprolite. The other, less mobile elements, particularly Pb, Sb, Bi, W and Mo are concentrated in silcrete and ferruginous duricrust to form a variety of secondary microcrystalline phases of mixed or of unestablished mineralogy, including QRAZ (quartz, rutile, anatase, and zircon) and more complex Pb-, Ag-, Bi- and Sb-bearing phases. These pathfinder elements form a narrow, near-surface anomaly that delineates the Scuddles VMS deposit. This paper also reinforces findings that silcrete, ferruginous duricrust and ferruginous gravel are anomalous in pathfinder elements over the Scuddles deposit and, furthermore, are important sampling media in mineral exploration in general, particularly where gossans may not have formed or have been eroded.