Deciphering the Hydrothermal Evolution of a VMS System by LA-ICP-MS Using Trace Elements in Pyrite: An Example from the Bracemac-McLeod Deposits, Abitibi, Canada, and Implications for Exploration

Textural and chemical characterization of pyrite was used to reconstruct the hydrothermal evolution of the Bracemac-McLeod Archean volcanogenic massive sulfide (VMS) deposits (~6 Mt) in the Matagami district, Abitibi, Canada. The mineralization, hosted in a bimodal volcanic sequence, is divided into a (1) Zn-rich zone, concordant to the Key Tuffite— a marker horizon at the district-scale, 2) a Cu-rich zone which locally crosscuts the stratigraphic pile, and 3) some localized magnetite-rich zones replacing sphalerite and pyrite. Detailed petrographic and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) studies of whole pyrite grains (including inclusions and zonation) from the Key Tuffite and from the different ore zones of the deposits, has revealed five pyrite types. Nodular pyrites (I) in the Key Tuffite and subidiomorphic pyrites from the low temperature (250°C) Zn-rich zone (II) have the same chemical signature, suggesting that both precipitated under similar physicochemical conditions. A wide range of trace elements (Cu, Zn, Ag, Sn, Sb, Te, Au, Pb, Bi, Tl) are present in both, but compared to the other types of pyrite, they are significantly enriched in Sb and Tl. Subidiomorphic pyrites from the copper-rich zone (III) have a different signature, enriched only in Se ± Co, In, Pb, and Bi. These pyrites commonly overgrow the nodular pyrites and are related to later higher-temperature fluids (300°C). Pyrite preserved during a later magnetite-replacement (>350°C) stage (IV) also are enriched in Se, but are depleted in all other elements. A similar signature is found in idiomorphic metamorphic pyrite (V), which occurs at the district scale. During high temperature recrystallization of pyrite IV and V, only the contents of Ni, Co, As, and Se are mostly preserved, whereas other base metals are expelled from the pyrite structure. Consequently, only Ni, Co, As, Se, Sb, and Tl were useful to reconstruct the hydrothermal evolution of the VMS system and used to build a suite of discrimination diagrams. Specifically, the ratio Se/Tl can discriminate pyrites from mineralized Zn-rich zone (Se/Tl 10,000). The innovative approach of this study, based on LA-ICP-MS acquisition of the chemistry of entire pyrite grains, makes our results directly applicable for exploration. Consequently, pyrite concentrate samples, analyzed for As, Tl, and Se by more classical mass-spectrometric techniques, have the potential to be used for vectoring to ore by using the proposed discrimination diagrams.