Pedogenic carbonate sampling for Cu exploration on the Yorke Peninsula, South Australia

Abstract

We report on the geochemistry of 215 regolith carbonate rocks collected across the Yorke Peninsula, South Australia in order to evaluate the potential of regolith carbonates as a geochemical sampling media for buried Cu mineralization. The Yorke Peninsula forms the southern part of the Olympic Domain iron-oxide‑copper‑gold (IOCG) mineral province and is host to a number of Cu-bearing mineral deposits and prospects including Hillside and the historic Moonta-Wallaroo mining district. The majority of the Yorke Peninsula is covered by a veneer of Cambrian to Quaternary sedimentary rocks and regolith, including marine limestones and regolith carbonates. We present a new regolith map of the Yorke Peninsula derived from geophysical data and field observations and use it to provide landscape context and regional characterization of the sampled regolith materials. Whole rock geochemistry of the sampled carbonates is used to demonstrate that major element chemistry is dominated by CaO (33%–53%), reflecting the calcite component. Other major components include SiO2 and Al2O3, reflecting quartz (typically quartz sand in carbonate indurated quaternary sands) and clay minerals respectively. Ca/Sr ratios vary between 179 and 1324 with 86% of the samples <650. The Ca/Sr < 650 group of samples forms a continuous array on a plot of Ca vs Sr which overlaps with the range of values expected from sea water derived Ca and Sr subjected to variable degrees of meteoric fractionation. These samples are interpreted to be regolith carbonates with little input of marine carbonate. A single sample has Ca/Sr 1324, consistent with a significant component of marine limestone. Twenty nine samples have Ca/Sr >650 and <940 which may indicate mixing of dominantly meteoric sources with a lesser component of marine carbonate. We recommend that geochemical results from samples with Ca/Sr >650 be treated with caution as the regolith carbonate trace element geochemical signal may be diluted by the marine carbonate component. Copper concentrations found in the carbonate rocks with Ca/Sr <650 range from 1.4 ppm to 36 ppm and form a slightly right-skewed, near log normal population. Samples with elevated log10Cu concentrations have a broad spatial coincidence with areas of known Cu enrichment in the basement and not related to dust or windblown contamination. Copper concentrations show no systematic relationship with common dust-born contaminants (Al2O3, Fe2O3, Zr) which would be expected if the Cu was derived from windblown spoil from the historic mining operations. We conclude that regolith carbonates are a potentially useful sampling media for Cu exploration on the Yorke Peninsula and that they can be easily differentiated from less useful marine carbonates on the basis of Ca/Sr ratios.