An overview of petrochemistry in the regional exploration for volcanogenic massive sulphide (VMS) deposits

Abstract

ABSTRACT Volcanogenic massive sulphide (VMS) deposits are important global sources of base and precious metals. Igneous geochemistry (petrochemistry) of mafic and felsic rocks associated with VMS deposits is extremely useful in delineating potentially fertile ground for VMS mineralization. In mafic-dominated, juvenile environments (e.g. mafic, bimodal mafic and mafic-siliciclastic VMS-types) VMS deposits are associated with boninite and low-Ti island arc tholeiite, mid-ocean ridge basalt, and back-arc basin basalt. These rocks are ultimately sourced from either depleted arc mantle wedge (e.g. boninite, low Ti island arc tholeiite) or upwelling depleted, mid-ocean ridge or back-arc asthenospheric mantle (e.g. MORB and back-arc basin basalt). In evolved environments, those associated with continental crust and typically dominated by felsic magmatism (e.g. bimodal felsic and felsic-siliciclastic VMS-types), VMS-associated mafic rocks have alkalic (ocean island basalt-like) and/or mid-ocean ridge/back-arc basin basalt-like signatures. In these environments alkalic basalt and mid-ocean ridge/back-arc basin basalt-like mafic rocks overlie felsic rocks and mineralization and represent melts derived from lithospheric and asthenospheric mantle sources, respectively. Felsic rocks in Archean sequences are typically tholeiitic, have elevated high field strength elements (HFSE) and rare earth elements (REE), and FIII affinities (low Zr/Y and La/Yb n , flat chondrite-normalized rare earth element profiles). In post-Archean evolved environments, felsic rocks associated with VMS deposits have HFSE- and REE-enrichment and within-plate signatures on discrimination diagrams, like their Archean counterparts, but are more calc–alkalic in composition and commonly have FII affinities. Felsic rocks associated with VMS deposits in post-Archean mafic-dominated, juvenile substrates are associated with trace element depleted rhyolites with tholeiitic to boninite-like signatures and M-type and FIV affinities on discrimination plots. Using mafic or felsic rocks in isolation may lead to erroneous assignments of prospectivity for terrains; however, when mafic and felsic rocks are used in tandem with geological context they are powerful tools in outlining potentially prospective regions. Within VMS-hosting environments there are specific petrochemical assemblages of mafic and felsic rocks. Petrochemical assemblages are specific lithogeochemical associations between mafic and felsic rocks that are common to VMS forming environments and are useful in identifying two key ingredients required to form prospective VMS belts: (1) rifting; and (2) high temperature magmatism.