New near-mine prospecting approach using multivariate analysis and reflectance spectroscopy to define surface footprint: A case study of the Pequizão Gold Deposit, Crixás Greenstone Belt, Central Brazil

Abstract

The Pequizão gold deposit in the Crixás greenstone belt, central Brazil, is structurally-controlled mineralization hosted mainly in carbonaceous phyllite with pervasive hydrothermalized zones and large amounts of disseminated sulfides and gold. Usually, regoliths of Au deposits are either unstudied or understudied in exploration surveys because of lower Au content and the difficulty and complexity of soil profiles. However, such investigation can be beneficial in brownfield exploration to assess larger areas than drilling. In this work, results of clustering and machine learning of soil geochemistry and reflectance spectroscopy integrated with a multivariate approach to determine soil footprints and to target new deposits in the surface of the Crixás greenstone belt. Reflectance spectroscopy was applied in 939 soil samples and is a valuable tool in mineral exploration for an immediate investigation of mineral assemblage from the target. It was calibrated with X-ray diffraction, geochemical, and multivariate approaches providing consistent vectors toward mineralization, with Pequizão soil samples developing gibbsite and phengitic-white mica as minerals related to mineralized samples. Multivariate analysis reveals that the deposit has a typical orogenic gold deposit chemical signature. Principal components and factor analysis first defined samples derived from carbonaceous phyllite and dolomite as the main ore hosts. The chemical aspects of hydrothermal alteration are As, Ag, Te, Ca, and Mg, with enrichment of Sb, V, Na, Ba, and W and depletion of Zn, Ga, and Pb that, according to ensemble learning, they have significant importance in the detection of gold. Understanding surface footprints of known deposits can be an exploratory guide for finding new soil geochemical halos related to mineralization zones. The research revealed new associations of minerals and chemical elements that can be determined as exploration vectors. It was possible to clarify further the knowledge about the footprint of mineralization in soil. Combining the soil-related methods applied in this study and a broad-coverage spectral approach in soil and drill cores can enhance success in prospecting in brownfield areas and expand to greenfields.