Use of semi-volatile metals as a new vectoring tool for VMS exploration: Example from the Zn-rich McLeod deposit, Abitibi, Canada

Abstract

Abstract Volcanogenic massive sulfide (VMS) deposits are small targets (100 s of meters) hosted in a hectometric to kilometric-scale zoned alteration halo, comprising narrow proximal chlorite and wider distal sericite zones. For exploration, identification of the distal fine-grained sericite halo related to fertile VMS systems is critical but remains challenging. Sericitization is a low-temperature hydrothermal alteration and its geochemical signature could be very similar to clay assemblages produced as the results of simple low-temperature prolonged seawater interaction with volcanic rocks on the seafloor. Currently, classical lithogeochemical methods (e.g., mass balance, element ratios) are not very efficient to decipher sericite alteration related to distal fertile VMS systems. However, semi-volatile metals have good potential as pathfinders, as they have a wider dispersion than major, or trace elements commonly used due to their enhanced mobility in hydrothermal fluids. New analytical improvements, involving four-acid digestion and advanced ICP-MS technology, now allows the accurate determination of semi-volatile metals at the ppb level. Using the Zn-rich McLeod deposit in the Matagami district (Abitibi, Canada) as a case study, we tested first the representativeness and advantage of the new analytical method, and secondly, the capacity of the semi-volatile metals for vectoring exploration. The new method is both accurate and precise and provided good resolution with low detection limits. The performance of classical lithogeochemical methods was compared with semi-volatile metal distribution (As, Se, Cd, In, Sn, Sb, Te, Hg, Tl, Bi) in the footwall rhyolite. Although the classical tools efficiently identified both the proximal chlorite and distal sericite zones, most of them (except Rb/Sr) cannot efficiently vector, as there is no systematic increase or decrease towards the mineralization. Conversely, overlapping alteration halos were identified using semi-volatile metals. The proximal signature (chlorite zone) is characterized by enrichment of Sb, Li\u202f±\u202fMo, W and U. The distal signature (sericite zone) is defined by successive appearance of higher values of Sn, W, and Tl up to 750\u202fm, 1000\u202fm and 1400\u202fm away from the mineralization, respectively. This zoned spatial distribution of semi-volatile metals clearly allows better localization of a given altered sample within a wide and distal alteration halo, thus providing a solid vectoring tool for VMS exploration. This is especially true since the signal of semi-volatile metals extends far beyond both the visual sericite alteration (up to 200\u202fm from mineralization) and classical lithogeochemical tools (up to 1000\u202fm away). Specifically, the Tl/Co ratio records the widest footprint of the mineralization, up to 1400\u202fm away from the deposit. It is well known that Tl is enriched in Zn-rich VMS. Thallium in the sericite alteration halo is here considered as an indicator for discriminating fertile volcanogenic hydrothermal systems from barren low-temperature convective cells.