A comparison of selective extraction soil geochemistry and biogeochemistry in the Cobar area, New South Wales

Abstract

In parts of the deeply weathered and semi-arid environments of the Cobar area (NSW, Australia), detection of mineralisation using conventional soil sampling and total metal analysis is impeded. This is due to the intense leaching of trace elements within the weathered profile, discontinuous coverage of transported materials and the existence of diffuse regional geochemical anomalies of ill-defined source. Selective chemical extractions, applied to various regolith components, and biogeochemistry offer a means of isolating localised geochemical patterns related to recent dispersion of trace elements through the overburden. Lag geochemical patterns across the McKinnons deposit (Au) and Mrangelli prospect (Pb–Zn–As) reflect mechanical dispersion processes and minor hydromorphic effects. Concentrations of more mobile elements tend to be higher in the non-magnetic fraction, due to higher proportions of goethite and poorly crystalline hematite than in the magnetic fraction. The subdued soil geochemical responses for metals extractable by cold 40% hydrochloric acid (CHX) and for total element concentration reflect the leached nature of the residual profile, low grade of mineralisation, dilution by aeolian components and disequilibrium of fine fractions with coarser, relict Fe-oxides. The stronger contrast for CHX for most metals, compared with total extraction, indicates surface accumulation of trace elements derived from underlying mineralisation. Enzyme leach element anomalies are intense but generally located directly over bedrock sources or major structural breaks, irrespective of the nature of the overburden. Though mechanisms for the dispersion of trace elements extracted by enzyme leaching are not well established, the lack of lateral transport suggests vertical migration of volatile metal species (atmimorphic dispersion). The strong, multi-element response to mineralisation in cypress pine needles indicates significant metal recycling during the present erosional cycle. However, a comparison of the trace element concentrations in vegetation (cypress pine needles) and selective extractions of soils indicates that recycling by the plants is not the dominant mechanism for transportation of metals through the overburden. The vegetation may be responding to hydromorphic dispersion patterns at depth. The use of selective extractions may be useful in detecting mineralisation through deeply leached profiles, but offers even greater potential when integrated with biogeochemistry to detect targets buried by significant thickness of transported cover.

Introduction

Single step and sequential, selective chemical extractions are used in geochemical exploration to characterise element dispersion mechanisms, to improve geochemical contrast by screening out trace element components not derived from local mineralised sources, and to detect subtle leakage haloes emanating from deeply buried mineralisation. In many geochemical terrains, weak geochemical dispersion haloes are actively forming in the top few metres of overburden due to groundwater movement, capillary action, diffusion of volatile compounds (atmimorphic¹ dispersion), natural electrochemical gradients, vegetation recycling and the action of organisms. Metals transported by these mechanisms would typically be associated with recently developed, transient and metastable secondary minerals, such as amorphous Fe and Mn oxyhydroxides, especially metals that are not readily incorporated into the crystalline mineral phases. Such element accumulations may represent only a small fraction of the total trace element content of the overburden; hence, geochemical contrast would be suppressed if total metal extractions were used.

In the absence of detailed mineralogical studies, most extractions are, in effect, only “operationally defined” (Hall et al., 1997), the amount of trace elements released being dependent on the degree of crystallinity and purity of the mineral phases present, dissolution conditions and digestion time. The newly developed enzyme leach appears to selectively dissolve amorphous manganese oxides and other soluble phases, and yet is slow to attack crystalline manganese oxides, amorphous or crystalline iron oxides or carbonates (Clark, 1993; Cohen et al., in prep). In the early stages of reaction, strong solutions of cold hydrochloric acid (CHX) primarily attack manganese oxides and poorly crystalline Fe oxides, rather than more crystalline Fe oxides. The latter, including hematite, are more gradually attacked by CHX (Xie and Dunlop, 1998).

Geochemical anomaly detection in the Cobar area is impeded by the presence of transported overburden, truncation of weathered profiles, removal of zones of trace element palaeo-accumulations and development of diffuse regional geochemical haloes. Although stream sediments and lag have been used in regional reconnaissance surveys (Dunlop et al., 1983), conventional sampling procedures have had to be adapted to accommodate the problem of Quaternary and recent siltation of drainage channels (including augering to zones containing buried lag; Schmidt, 1990). At the prospect scale, various soil fractions and regolith components have proven effective sampling media in erosional landforms dominated by residual overburden (Scott et al., 1991; Chaffee and Scott, 1995; Cairns et al., 1995; Cohen et al., 1996). In many cases, sampling has focussed on an ill-defined `B-horizon', developed in the massive red soils of the region, with the objective of detecting metals suspected to be accumulating under the present cycle of weathering. Selective extraction on soils by weak hydroxylamine hydrochloride (Chao and Zhou, 1983) has provided interpretable patterns, particularly for Au, near mineralisation at McKinnons (Rugless and Elliot, 1995). Although such approaches have proven effective over some truncated profiles and in areas with thin transported cover, it is less certain that such an approach will delineate metal sources under thick transported cover.

An alternative to regolith sampling is the analysis of groundwater (Giblin, 1995). This, however, is dependent on the availability of water bores, which are sparse in the Cobar area. Another alternative to regolith sampling is biogeochemistry. Phreatophytes assimilate trace metals from large volumes of soil and weathered bedrock, via interaction between their extensive root systems and groundwater or contact with soil particles. Biogeochemistry has been able to delineate mineralisation in various Australian terrains (Baker, 1986; Cohen et al., 1995; Lintern et al., 1995; Marshall and Lintern, 1995), in some cases in the absence of strong anomalies in adjacent soils (Smith and Keele, 1984).

This study compares the response of a relict medium (lags) with that of vegetation and the total, CHX and enzyme leach soil geochemistry over the McKinnons deposit (Au) and the Mrangelli prospect (As–Pb–Zn). This provides a basis for discussion of the nature and extent of dispersion processes, and future planning and interpreting geochemical surveys in areas of transported cover in regions with characteristics similar to those of the Cobar area.