The development and application of rock and soil geochemical baselines for environmental studies in mining districts: collahuasi and los bronces mining districts as case studies

Abstract

This thesis presents the first comprehensive, multi-element, geochemical baselines from 1km2 systematic sampling of the Collahuasi (n=237, 500km2) and Los Bronces (n=523, 2300km2) porphyry copper mining districts, defining rock and soil geochemical pre-mining baselines missing in literature.

Element thresholds, separating background and anomalously high concentrations, were determined using robust graphical and statistical analysis. Comparison to international reference concentrations, confirmed that thresholds reflected maximum background concentrations for “normal” rocks and that Co and Ni were regionally depleted. Nonetheless, background (threshold) concentrations of Co (15.5ppm), Ni (14ppm), As (16ppm), Cd (0.3ppm), Cu (80ppm) and Zn (117ppm) exceeded one or more intervention values.

Calculations identified that 80% of analysed rocks were hydrothermally altered. The highest PHEs, As, Ag, Cd, Cu, Hg, Mo, Ni and Pb concentrations occur in the most altered and mineralised rocks. In the PHE anomalism (depletion and enrichment) in the Los Bronces district was constrained within the andesitic basalts, andesites and rhyodacites and in andesitic basalts, andesites, granodiorites, rhyodacites and rhyolites.in the Collahuasi district.

Element soil to rock ratios confirmed enrichment in Collahuasi soils. The ratios of Ni (17.23), Cd (15.60), Co (14.56), As (13.33), Cu (11.21) and Zn (5.98) confirmed that, as a result of weathering and accumulation of elevated chemistry, where rocks exceed intervention values, “paired” soils do also. These findings could inform environmental research in similar, mineralised districts.

Soil to rock ratios (AR: 5.78, WR: 5.36) and relative order of element enrichment (K and Fe least, Ca, Mg, P most) were similar, using concentrations derived from Aqua Regia (AR) and LiBO2 (WR) digestion, confirming AR data successfully measures enrichment and may facilitate repurposing of exploration data for environmental study. Silica concentrations were confirmed as an appropriate proxy of quartz for normalisation of ratios. Normalisation did not alter the element enrichment order. When rock types in Collahuasi were proportionally represented, 1/3 of samples successfully extrapolated the district chemistry, identifying the same relative enrichment, potentially reducing survey cost.

Enrichment factors (EFs), derived from Collahuasi data were used to interpolate mean concentrations of As, Cd, Co, Cu, Ni and Zn in Los Bronces soils. Comparison to measured concentrations, interpolated values were overstated, due to differing rock type abundance between districts. Refinement of this interpolation could provide a desirable method for defining pre-mining soil baselines in metalliferous districts where mining has commenced.

This research provides an evidence-based methodology to evaluate risk, using rock baselines as a benchmark of “natural” chemistry. These findings can identify future, anthropogenic, soil additions, and guide remediation after closure.