Weathering of the zinc-lead lode, Dugald River, northwest Queensland: I. the gossan profile

Abstract

Oxidation of the relatively iron sulfide-poor Dugald River zinc-lead lode in northwest Queensland and reaction of the acid solutions with carbonate has resulted in an undifferentiated gossan profile. The gossan is composed predominantly of quartz, goethite, hematite, barite, adularia, plumbian jarosite, plumbogummite and minor mica, chlorite, kaolinite and montmorillonite. Barite and adularia are formed by the breakdown of the barium feldspar hyalophane (K, Na, Ba)[(Al, Si) 4 O 8 ] which occurs in the lode. Lead in the gossan is contained within the secondary minerals plumbogummite and plumbian jarosite, and in traces of anglesite and cerussite, whereas Zn occurs in the barite, secondary lead minerals and coronadite structures, and is adsorbed by iron oxides, phyllosilicates and carbonaceous matter. Only traces of zinc minerals smithsonite, goslarite and hemimorphite were detected. Use of Gresens' general metasomatic equation has enabled quantitative determination of compositional changes resulting from the oxidation of the ore. Silicon, Al, Ti and Ba are essentially immobile under the mildly acidic oxidizing conditions. In decreasing order of mobility Cd, Ca, S, Na, K, Mn, Mg, Zn, Ni and Cu, together with CO 2 and Tl, have been leached from the gossan profile, while Ag, Sb, Se, As, Fe and Pb appear to have been added to the gossan, notably in a zone of solution-deposited secondary minerals where they have been concentrated, possibly as a result of leaching from the surface gossan.