Selective and partial extraction analyses of transported overburden for gold exploration in the Yilgam Craton, Western Australia

Seven different sites on the Yilgarn Craton (Harmony, Fender, Bronzewing, C urara, Safari, Steinway and Apollo Au deposits) have been used to test the usefulness of various selective and partial extractions for Au exploration, particularly in areas of extensive transported overburden. Soils were samp led along traverses, air dried, sieved to <2 mm, and extracted with various reagents, these being: (I) Sequential Selective extractions [(1) pH 5 acet ate, (2) 0.1 M hydroxylamine, (3) 0.25 M hydroxylamine]; (II) 4 M HCl (15 d egrees C/4 h) and 10 M HCl (25 degrees C/4 h) with Au extracted using an ox idizing agent); (III) Iodide (0.1 M KI, pH 7.4); (IV) Mobile Metal ion (MMI ); (V) Enzyme Leach. The Sequential Selective and HCl extractions showed co mparable results for a wide range of elements. MMI analysis includes: (1) a n acid extraction for Cd, Cu, ph, Zn, with similar comparative results to s elective and HCl extractions, except for carbonate-rich soils which give mu ch lower MMI responses; (2) an alkaline extraction for Ag, Au, Co, Ni and P d, with virtually identical Au results to the Iodide and HCl extractions. I n particular, the HCl, Iodide and MMI extractants dissolve 70-80% of the to tal Au from carbonate-rich soils. MMI also dissolves most of the Ag and the refore does not give any additional exploration information for Au or Ag th an total, aqua regia or BLEG analyses. Enzyme Leach results were lower and showed different distribution patterns to the other methods. For those site s with thin cover (e.g., Harmony, Fender and Safari) there has been suffici ent mechanical or biological mixing to bring some Au to the surface, and MM I, HCl and Iodide extractions give useful results, though total Au is equal ly effective. In contrast, Steinway has >25 m transported cover, but also h as a strong soil Au anomaly (155 ppb) approximately overlying mineralizatio n, and much lower Au solubility than all other carbonate-rich soils. Silver and W also show a well-defined peak, though 80 m to the west of the main A u peak. These anomalies may be due to physical transport of Au-bearing detr itus. Drainage channels accumulate Mn oxides and amorphous Fe oxides, which are commonly enriched in many trace elements. This leads to major soil ano malies, which may be enhanced by the use of selective or partial extraction s, but appear unrelated to buried Au mineralization. Aside from Au, no othe r elements appear to give useful exploration data, with the exception of En zyme Leach I and Cu and MMI Ni for one of two traverses at Harmony and Enzy me Leach Cu at Bronzewing. The poor repeatability, absence of any clear rea son for I, Cu and Ni correlating with mineralization, and lack of success a t other sites suggest that these minor correlations are not significant. Th e extraction methods are highly site-specific, possibly related to soil min eralogy, landscape position and drainage channels. The depth of transported cover appears particularly significant; sites with more than 10 m of trans ported material do not show any Au anomaly at surface associated with the m ineralization. Generally, partial extractions showed false positive anomali es, with little ability to show buried mineralization except where totals w ould work anyway. They do not appear to offer any major advantages for Au e xploration in areas of transported overburden in the Yilgarn Craton. (C) 19 99 Elsevier Science B.V. All rights reserved.