The role of biological and non-biological factors in the formation of gold anomalies in calcrete

Abstract

Calcrete has been shown to contain significant Au, derived from nearby mineralisation, and this has led to its current use as an exploration sampling medium. Calcretes are secondary carbonates, principally consisting of calcite and dolomite that may precipitate in regolith in a semi-arid climate. They overprint existing regolith material and commonly contain this material (e.g. soil, colluvium, laterite, saprolite and rock) on, and within, which they form. Vadose (pedogenic) and phreatic (formed by groundwater) are the two principal types of calcrete. Pedogenic calcretes are those that form in unsaturated soil horizons and are widely distributed in southern Australia and in all continents of the world. They may dissolve and re-precipitate within soil horizons, giving rise to several generations of carbonates, depending on changing climate conditions. This thesis will be restricted to pedogenic calcretes.Despite the common use of pedogenic calcrete as a sampling medium there is a paucity of published research on how Au anomalies actually form in this material. Thus fundamental research is required to be undertaken on the nature of the Au-in-calcrete association, which will promote better understanding of how, when and where calcrete sampling is applicable for the exploration industry. In order to investigate the relationship between Au and calcrete several Au prospects from South Australia were selected for field and laboratory study, including Challenger, Barns, and Edoldeh Tank. Additional samples were investigated from the Bounty Deposit (Western Australia) where Au concentrations in calcrete are an order of magnitude higher than the South Australian counterparts.At Challenger, the origin of the calcrete that is associated with Au was investigated. Pre-Cambrian rocks, such as the Archaean at Challenger, have very high 87Sr/86Sr ratios compared to rocks that have only recently been formed such as those derived from marine sources. Strontium isotopes were determined at Challenger and demonstrated to be derived from a marine source rather than the bedrock, despite the association of Sr (and, by inference, Ca) with Au. It was concluded that pedogenic processes (e.g. capillarity, bioturbation, evapotranspiration) caused exotic alkaline earth metals to become associated with residual Au. At Challenger, more than 95% of the Sr (Ca) was derived from marine sources (dust, rainfall and/or aerosol). The relationship between marine-derived calcrete and Au has never been established for auriferous calcrete before and is significant since it suggests that the origin of the alkaline earth metals (Sr and Ca) is not important for its association with Au; Au is clearly originally derived from the regolith underlying the calcrete. Furthermore, a strong correlation with distance from the coast showed that the nearer to the ocean then the greater the contribution of marine Sr (low 87Sr/86Sr ratio) occurs. In a companion study, stable C isotopes were examined from the same auriferous calcrete samples and it was demonstrated, for the first time, that both C3 and C4 plants (trees and grasses) were equally dominant contributors to the inorganic C in the calcrete. Thus, the calcite in the soil (Ca and carbonate ion) is derived principally from marine and plant sources.At Barns, disseminated Au mineralisation has created a contiguous Au-in-calcrete anomaly developed immediately above and within the saprolite. However, linear (seif) sand dunes have buried the anomaly in places. A biogeochemical survey was undertaken along and across the sand dunes over mineralisation at Barns to examine the role of vegetation in creating the Au-in-calcrete anomaly. A clear biogeochemical anomaly was identified in plant foliage, bark and litter demonstrating that Au was being taken up by roots that were tapping into buried calcrete, or mineralisation beneath it, in some cases, at least 8 m below the surface, and then depositing the Au at the surface on top of the dune. Having demonstrated the uptake of Au by plants, a dune was excavated and powdery calcrete developed in a rhizosphere within the dune was investigated. Significantly, the calcrete was shown to be highly anomalous in Au (five times above background) and thus, for the first time, it was shown that plants have had a key role in the development of a Au-in-calcrete anomaly, and, importantly, in transported regolith. Furthermore, thermoluminescence of quartz grains was commissioned and shown that the emplacement of sand above the calcrete was about 26 000 years old, indicating that the anomaly was younger than this. Mass balance calculations were performed and showed that the Au in the sand dune may have accumulated in less than 10 000 years; calcrete around the root has developed during the life of the tree. Clearly, Au-in-calcrete anomalies can form relatively rapidly compared with the age of sediments themselves.As Au concentrations were generally low (<20 ppb) at Barns, the nature of the Au-in-calcrete was investigated in samples from Bounty where concentrations reach 1 ppm. Having established a causal relationship between Au-in-calcrete and vegetation at Barns, direct evidence of the role of plants at the micron scale was sought. Samples were chosen 6from a soil profile that showed a strong positive correlation between Au and Ca and thus a minimal effect of detrital Au. Using LA-ICP-MS, SXRF (synchrotron X-ray fluorescence) and XANES (micro-X-ray absorption near-edge structure), it was shown for the first time that (i) the distribution of Au-in-calcrete at the micron scale was variable and (ii) Au occurs in both particulate and ionic form. Furthermore, ionic Au associated with Br was found in a root tubule. These observations are evidence of an evapotranspiration model for the formation of the strong, down-profile relationship between Au and calcrete i.e. Au has been mobilised then precipitated with the carbonate as vadose water has been removed from the soil by vegetation. Bromine is a typical element (with chloride and sulphate) found in evaporates. There is no chemical affinity between Au and Ca in the soil profile since further detailed analyses on sub-samples using wet chemical, LA-ICP-MS (laser ablation inductively coupled mass spectrometry) and SXRF techniques show that a Au-Ca relationship is not apparent at the sub-millimetre scale. Gold and Ca are behaving similarly but independently and they do not (at the μm scale) form a chemical bond with carbonate minerals.At Edoldeh Tank (ET) Au prospect the distribution of Au on a tenement-scale was investigated. It was shown that geomorphological factors influence the shape and tenor of Au-in-calcrete anomalies. By their very nature Au-in-calcrete anomalies are dispersed over a wide area (to make them an effective sample medium) and this study served to document the factors involved. The ET prospect (Gawler Craton, South Australia) hosts one of the largest Au-in-calcrete anomalies in Australia and is typical of many such prospects identified in the region. For in situ regolith, Au is concentrated in surficial calcrete and above an upper saprolite zone depleted in Au. The Au anomaly extends downslope from a ridge of calcrete and silcrete covered saprolite into adjacent and transported regolith dominated by thin (~5 m) aeolian sand cover. The anomaly is locally broadened by downslope hydromorphic dispersion and cemented within laminated calcrete in the transported regolith. The laminated calcrete was examined in detail and the nature of the Au was found to be similar to that found at Bounty i.e. the Au in the calcrete occurs in nano-scale particulate and possibly ionic forms. In field experimental studies indicate that Au and also Ag are actively dispersing in the soil. The Ag content may be a means of distinguishing transported anomalies from those developed in situ and the calcrete is much older (120 000 years) than that found at Barns.The combined individual site studies at Challenger, Barns, ET and samples from Bounty advance our understanding of the formation of Au-in-calcrete anomalies. The work has shown that an association between Au and calcrete can form at the sub-micron (ionic) to profile scale from both abiotic (geomorphology, climate, mineralisation style) and biotic (vegetation) factors; abiotic factors can shape the overall form, tenor and evolution of the Au-in-calcrete anomaly. No direct evidence was found of the role that micro-organisms play in the formation of Au-in-calcrete anomalies although experimental results from elsewhere suggest that this may be plausible, at least, in the laboratory. Further work on the role of bacteria on the formation of Au-in-calcrete anomalies in the natural environment should be encouraged. The size and shape of Au-in-calcrete anomalies may be influenced by mineralisation style, hydrology and topography and while these were briefly investigated further work is needed.