ABSTRACT Soils from the vicinity of the Endeavor Mine, in the Cobar region of western NSW, have been sieved into the (a) 2–4 mm, (b) 1–2 mm, (c) 0.5–1 mm, (d) 250–500 μm, (e) 125–250 μm, (f) 63–125 μm and (g) <63 μm fractions, with a magnetic fraction (m) also prepared from the coarser soil fractions. The chemical compositions of these soil fractions show that the coarse (2–4 mm) and magnetic fractions contain the highest As, Fe, and Pb contents whereas the fine (<63 μm) fraction is enriched in Al, Ca, Cu and Zn. Phosphorus and S have bimodal distributions.
Elevated Pb and As contents in the coarse or magnetic fraction of soils in the Endeavor Mine area reflect the underlying weathered Pb–Zn mineralization and may give rise to a larger anomaly than that defined by conventional magnetic lag sampling. Within this area of Pb and As anomalism, the fine fraction of the soils directly above the Northern Pods mineralization (c. 400 m below) exhibit enriched Zn, S and soluble salt contents (the latter measured as electrical conductivity). The association of elevated Zn, S and EC (electrical conductivity) suggests sulphide-derivation and such anomalism appears more prospective than Zn-only anomalism in fine soil fractions from transported material to the SW of the mine.
Unfortunately the preparation of the fine soil fraction is time consuming but, because the fine fraction of the Cobar region soils consistently represents 40–50% of the <2 mm soil fraction, the latter can be used as a surrogate for the fine soil fraction. Thus, cost effective exploration for base metal mineralization in the Cobar region could utilize the >2 mm soil fraction for determining Pb and As and the <2 mm soil fraction for S, Zn and EC. Use of the <2 mm soil fraction discriminants should be considered during exploration for Zn-only mineralization in semi-arid regions like that at Cobar.
- Received September 18, 2009.
- Accepted February 3, 2010.
- © 2011 AAG/Geological Society of London