In situ pyrite thermoelectricity and geochemistry: Implications for exploration in the Longhua Ni-Co vein deposit, Dayaoshan area, South China

Abstract

Pyrite thermoelectricity has been widely utilized in predicting deep/concealed quartz-vein-related gold orebodies. However, whether this methodology can be utilized as vector for Ni-Co mineral exploration remains to be explored. This study investigates the major element composition, trace element composition, and thermoelectric characteristics of pyrite across a vertical profile (531 m, 492 m, and 446 m above sea level, a.s.l.) along the No. 3 orebody of the Longhua Ni-Co deposit to evaluate their potential for Ni-Co exploration. In this deposit, Ni-Co mineralization is dominated by arsenides (e.g., niccolite), sulfarsenides (e.g., gersdorffite and cobaltite), and sulfides (e.g., pentlandite and bravoite), which are primarily hosted in quartz veins. Field observations indicate that pyrite from different elevations is closely associated with Ni-Co ore minerals. Time-resolved LA-ICP-MS depth profiles of Co, Ni, and As in pyrite from different elevations exhibit gentle curves, suggesting that these elements primarily incorporate into the pyrite lattice in an isomorphic form. Despite a relatively low Co/Ni ratio (mostly <1), the quartz-vein-controlled occurrence of pyrite points to a hydrothermal origin. The trace elements exhibit a cyclic variation along the vertical profile: from 531 m to 492 m and then to 446 m a.s.l., the (Co + Ni) content initially decreases then increases, while As content shows an inverse trend. Correspondingly, pyrite thermoelectricity changes from N-type conductivity at 531 m a.s.l. to P-type at 492 m a.s.l., and reverting to N-type at 446 m a.s.l. This variation is attributed to a direct link between (Co + Ni) content and N-type conductivity, and between As content and P-type conductivity. The observed association of arsenide ores with P-type pyrite and sulfide ores with N-type pyrite establishes these conductivity types as indicators for distinct mineralization characteristics. Furthermore, the increase in sulfur fugacity below 446 m a.s.l., combined with the presence of N-type pyrite at 430 m a.s.l. (from drill-hole along the inferred extension of the No. 3 orebody), indicates the potential for deeper sulfide-rich mineralization below currently investigated 446 m a.s.l. drift. This study highlights the feasibility of using the pyrite thermoelectricity and geochemistry to guide the Ni-Co mineral exploration.