Identifying Mineralogical and Geochemical Vectors towards the Epithermal Au-Ag Correnso Mine, Waihi

Abstract

The Au-Ag rich Correnso vein is a low sulphidation epithermal deposit located east of Waihi and is named after the clay mineral corrensite. Hydrothermally altered andesitic and dacitic host rocks of the Waipupu Formation, informally subdivided into Upper Andesite and Lower Andesite units, contain Au-Ag rich quartz veins. The interaction of hot, dominantly meteoric water with the host rocks causes mineralogical and geochemical changes. Hydrothermal alteration manifested as mineralogical alteration and geochemical signatures can help in identifying vectors to constrain potential areas of enhanced mineralization for exploration purposes. Hydrothermal alteration has been suggested to have potential as exploration vector for epithermal deposits in particular as alteration halos can extend up to considerable distances (Christie et al., 2001; Simpson and Mauk, 2004). Visual core logging, petrography, pXRF, and Aqua-Regia/2-Acid Digest (ICP-MS) analysis were used in this study to analyse and quantify alteration zonation patterns and trace element metasomatism in order to identify vectors towards the mineralisation. Results suggest a clear mineralogical and geochemical alteration zonation pattern around the vein system. The unaltered, moderately altered and highly altered rocks are often adjacent to one another, implying that hydrothermal fluids have migrated through the host rocks in a highly heterogeneous fashion. The migration of fluids is likely controlled by both primary and secondary (e.g. fault/fracture) permeability. The overall mineralogical pattern can be described as quartz ± adularia ± sericite assemblage which is similar to the adjacent deposits of the Waihi area. Alteration zonation in the host rocks consists of potassic alteration proximal to the vein system surrounded by sericitic alteration, with illite-smectite dominated argillic assemblage overprinting both alteration types. Propylitic alteration is more prominent to the outward zones and distal to the mineralisation. Geochemical analysis show enrichment of pathfinder elements such as As, As, Sb, Hg, Zn, Pb, Se, and K and depletion of Cu, Te and Se.