Two lithogeochemical approaches to the identification of alteration patterns at the Elura Zn–Pb–Ag deposit, Cobar, New South Wales, Australia: use of Pearce Element Ratio analysis and Isocon analysis

Abstract

Lithogeochemical approaches to the identification of ore-related alteration can be obscured by closure effects and/or pre-existing lithological variations. Two mass balance approaches that can address these problems are Pearce Element Ratio analysis (PER) and Isocon analysis (IA). Both approaches have been applied to sedimentary rocks hosting the Elura ore-body, north of Cobar, in central-west New South Wales, Australia. Elura is an epigenetic, zinc–lead–silver deposit, (45 Mt with 8.5% Zn, 5.3% Pb, and 69 ppm Ag), hosted within Devonian siltstone–sandstone turbidites of the Cobar Basin. The ability of PER and IA to identify alteration at Elura and to navigate towards ore within altered zones is evaluated. PER identified mineralogical controls on calcium, carbonate, potassium and aluminium. Molar Ca/Ti v. C/Ti plots segregate calcite-bearing background samples from iron–magnesium–carbonate altered rocks closer to ore. Molar K/Ti v. Al/Ti and K/Ti v. (Al–Na)/Ti diagrams show altered rock bulk compositions to be consistent with the development of muscovite, while background samples have an illite ± albite precursor. The precursor lithotype has a major influence on the intensity of alteration and resultant mineralogy of altered samples. IA shows that carbonate carbon, Ag, As, K, Pb, Rb, Sb, Tl and Zn are strongly added to host rocks, while Na is strongly leached. Other elements show variable trends towards ore, with lithotype strongly affecting the patterns. Samples have greater mass gains closer to ore. Selection of an appropriate parent rock is critical in gaining meaningful results in IA.