Building geochemical vectors with trace element compositions of sulfides in orogenic gold mineral systems in northern Finland

Abstract

Exploring and discovering new ore deposits that supply the needed metals for our current societal transition to a green-energy and technologically advanced low-carbon future is becoming increasingly difficult. To fully convert the current system however, a significant increase of mining of minerals is required, particularly since current global mineral reserves are insufficient to cover the growing demand for a fossil fuel-free infrastructure. It is therefore even more important to develop and introduce new time-and cost-efficient exploration tools, which can be utilized in the discovery of new mineral deposits and applied in the early, or through advanced stages of mineral exploration. Given that Finland is one of the major EU countries that covers the supply chain for the EU battery industry, its metallogenic provinces are extensively tested and investigated by new exploration technologies. For this purpose, a comprehensive dataset of trace element contents in single grains of pyrite, pyrrhotite and chalcopyrite was compiled on the basis of results of laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analyses. Samples were systematically collected from drill cores from orogenic Au-only and Au-base-metal deposits in the Paleoproterozoic Svecofennian orogenic belts in northern Finland. The objective of this dataset is to serve as a reference tool in heavy indicator minerals-based exploration. By performing correlation analysis with the symmetric pivot coordinates approach for compositional data, as well as principal component analysis, it is demonstrated how all analyzed sulfide species have the capacity to discriminate between Au-only and Au-base-metal deposit types, as well as occurrences of free Au versus refractory Au. Element pairs of symmetric coordinates most suitable for differentiating Au-only systems from Au-base-metal settings are Co/Au and Au/Se in pyrite; Ni/Bi and Se/As in pyrrhotite; and Ni/Ag, Ag/In, Se/Ag, Cr/Ag, Cr/In, Cr/Zn, Zn/Se and Sn/Se in chalcopyrite. Results from principal component analysis reveal that pyrites from Au-base-metal deposits are associated with the positive loadings of Se-As-Co onto PC1 and those from Au-only deposits with negative loadings of Bi-Sb-Te-Au and Ni onto PC1 and PC2, respectively. In the case of pyrrhotite, samples from Au-Co and Au-Cu deposits are clearly distinguished between the positive loadings of Hg-Se-Co onto PC1, and the negative loadings of Sb-Au-Bi-Ni onto PC1. Positive loadings of Ag-Se and Co along PC1 are indicative of chalcopyrite from Au-Co deposits, while negative loadings of Ni and Zn onto PC1 are pointing towards an origin from Au-Cu, as well as Au-only deposits. Additionally, when testing the indicator mineral method with recovered pyrites from till samples from the Peräpohja belt, a potential relationship with bedrock pyrites is successfully demonstrated. This study aims to present the versatility of sulfide trace elements in geochemical exploration, as it could complement the mineral exploration industry with a more efficient and less expensive practice for profitable deposit discovery.