Assessing microbiological surface expression over an overburden-covered VMS deposit

Abstract

An integrated biogeochemical sampling approach, consisting of soil geochemistry, plant biogeochemical analyses and investigation of microbial community structure and function, was used to characterize a surficial geochemical signature of the buried Jaguar volcanogenic massive sulfide (VMS) Ag-Cu-Zn deposit in Western Australia's NE goldfields region. The study is a proof of concept in using multiple science disciplines (microbiology, geochemistry, regolith) to evaluate surface anomalies related to exploration. In a factorial design, soil from the surface (1–5 cm) and sub-surface (10–20 cm) horizons, was sampled under trees in locations directly adjacent to the stem base and also further under the canopy. Samples of mulga foliage and litter resting on the soil were sampled. Soil and plant material were collected from trees above the projected zone of mineralization and from background areas. Significant differences ( P < 0.05) in elemental composition of soil, foliage and litter collected from between mineralized and background samples exist. The soil anomaly exhibited elevated concentrations of Cu and Zn, which were concentrated in the top soil horizon. Concentrations of Ag, Sr, Hg, Na and Ca were also significantly higher in soil sampled over the mineralization. Mulga foliage (phyllodes), and particularly fallen mulga litter, had elevated concentrations of Cu and Zn ( P < 0.05) over the mineralization. Microbial processes involved in organic matter cycling (litter decomposition) under the mulga were investigated using BioLog EcoPlates. Compared to background samples, the patterns of substrate utilization were significantly different in soil ( P < 0.05) collected from under plants growing above the mineralization, however the rates of microbial processes were higher in soil adjacent to the base of the plant stems. Utilization of the carbohydrates glucose-1-phosphate, β-methyl- d-glucose, and α- d-lactose were significantly increased in the mineralized soils, whereas the utilization of various carboxylic acids, polymers, phenolic and amine compounds were suppressed. The genetic structure of the soil bacterial, archaeal and fungal communities, determined using PCR-DGGE fingerprinting of rRNA genes, were also significantly different above the buried mineralization. Changes in the bacterial community were most pronounced, and were associated with Gemmatimonadaceae, Acidobacteriaceae, Oxalobacteraceae, Bradyrhizobiaceae, Caulobacteraceae, TM7 genera incertae sedis, and unclassified bacteria. Thus, the biogeochemical anomaly associated with the buried mineralization is also expressed in soil microbial communities. We propose that the formation of the surficial soil anomaly is a product of plant concentration of elements via scavenging from the surrounding regolith profile followed by the microbially-mediated release of metals to the soil surface following plant litter fall. The methods employed and the discovery of anomalous microbial processes and communities has implications for mineral exploration. Future investigations using these techniques will provide mechanistic support to models describing the formation of biogeochemical anomalies in general. Results have also shown that biogeochemical approaches to mineral exploration are viable in arid ecosystems with low net primary productivity.