TRACE ELEMENT GEOCHEMISTRY AND PETROGENESIS OF FELSIC METAVOLCANIC ROCKS ASSOCIATED WITH VOLCANOGENIC CU-ZN-PB MASSIVE SULPHIDE DEPOSITS

Volcanogenic Cu-Zn-Pb massive sulphide (VMS) deposits are preferentially associated with certain types of subaqueous felsic volcanic rocks (FVR: FII “calc-alkaline” and FIII “tholeiitic”), regardless of age. It was previously assumed that FII and FIII FVR were formed and/or extensively modified in high-level magma chambers / subvolcanic intrusions associated with many of the deposits. However, recent studies of these intrusions indicate that many, at least in part, post-date mineralization. Two factors fundamental in VMS formation are: 1) there must be a heat source, and 2) it must be located at a high enough level in the crust to generate and sustain a high-temperature convective hydrothermal system.

Our model relates the continuum of FVR compositions (FI alkalic thorough FII calc-alkaline to FIII tholeiitic) to the depth of melting and resulting systematic changes in the mineralogy and composition of the crust, pressure, temperature and, to a lesser degree, to subsequent fractionation processes. Thus, rather than being directly related, it appears that high heat flow results in partial melting of the crust (with consequent generation, eruption, and intrusion of FII and/or FIII felsic magmas), generation and emplacement of high level subvolcanic intrusions, and convection of fluids in the crust (with consequent formation of a VMS system). Rifting is believed to be fundamental in generating high heat flow and in allowing melting to occur at high crustal levels within the zone of convective fluid flow, explaining why VMS deposits occur in a wide variety of subaqueous rift environments (e.g., oceanic, back-arc, fore-arc, arc, continental margin, or continental).

Many large tonnage VMS deposits from a variety of tectonic environments are hosted by FIII FVR (e.g., Kidd Creek: primitive arc; Neves Corvo: continental arc/fore arc/strike slip collision; Flin Flon: primitive arc) regardless of geologic age. The greater abundance of ore-associated FIII rhyolites in the Archean may reflect a thinner, more mafic crust, whereas the greater abundance of ore-associated FII FVR in the Proterozoic and Phanerozoic may reflect a thicker, more felsic crust.