Deciphering the hydrothermal evolution of a VMS system using trace elements in pyrite by laser ablation ICP-MS: an example from the Bracemac-McLeod deposits, Abitibi, Canada and implications for exploration.

English Abstract:

Textural and chemical characterization of pyrite was used to reconstruct the hydrothermal evolution of the Bracemac-McLeod archean VMS deposits in the Matagami district, Abitibi, Canada. The mineralization is divided into a 1) zinc-rich zone, concordant to the Key Tuffite - a marker horizon at the district-scale, 2) a copper-rich zone and 3) some localized magnetite-rich zones replacing sphalerite and pyrite. Petrographic and LA-ICP-MS studies of whole pyrite grains from the Key Tuffite and from the different ore zones of the deposits provided the opportunity to investigate the distribution of trace elements in pyrite over a wide range of sulfide assemblages related to temperature. The study has revealed five pyrite types. Nodular pyrites (I) in the Key Tuffite and pyrites from the low temperature (250°C) zinc-rich zone (II) have the same chemical signature (significantly enriched in Sb and Tl) suggesting that both precipitated from similar physicochemical conditions. Pyrites from the copper-rich zone (III) have a different signature, enriched only in Se ± Co. These pyrites are related to later higher-temperature fluids (300°C). Pyrite preserved during a later magnetite-replacement (>350°C) stage (IV) are also characterized by enrichment in Se, but are depleted in all other elements. Similar signature is found in idiomorphic metamorphic pyrite (V), which occurs at the district-scale. During recrystallization of pyrite IV and V, only Ni, Co, As and Se are preserved whereas other base metals are expelled from the pyrite structure. Consequently, only Ni, Co, As, Se, Sb and Tl are 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), from pyrites associated with Cu-rich zone (10<Se/Tl<10000) and pyrites from non-economic parts of the deposit (replaced by magnetite or metamorphogenic; Se/Tl>10000).