PETROGENETIC EVOLUTION OF FELSIC VOLCANIC SEQUENCES ASSOCIATED WITH PHANEROZOIC VOLCANIC-HOSTED MASSIVE SULPHIDE SYSTEMS: THE ROLE OF EXTENSIONAL GEODYNAMICS

Abstract

Felsic-volcanic hosted massive sulphide deposits form in many different geotectonic environments usually associated with episodes of extension. However, empirical geochemical links between felsic volcanism, geotectonic environment, and extensional geodynamics have not been adequately documented so as to distill the fundamental relationships between these features and formation of volcanic-hosted massive sulphide (VHMS) mineralization, which is important considering most VHMS deposits are hosted in felsic volcanic-bearing sequences. Using felsic geochemical data from six different VHMS mineralized Phanerozoic geotectonic settings, it seems that the felsic geochemical data are consistent with the overall geotectonic interpretations for these areas, which in turn helps in stratigraphic and geotectonic interpretations for detailed VHMS exploration. In extensional geodynamic settings, bimodal volcanism is common compared to the continuum of compositions observed under compressional to neutral (high-stress) regimes in calc–alkaline, arc-building environments. The empirical geochemical–metallogenic (VHMS) relationships evident in the Archean felsic volcanic rocks, in particular, the low Zr/Y (<7) and low LaN/YbN (<6), are also apparent in the Proterozoic and Phanerozoic felsic volcanic rocks. These low ratios are consistent with minimal fractional crystallization of hornblende (±clinopyroxene) through much of the lower and middle crust, (i.e., except the subvolcanic environment) and are related to low crustal residence times of magmas in extensional environments (i.e., rapid adiabatic ascent of magmas), low oxygen fugacities and low water fugacities of the magmas. These elemental ratios should closely reflect partial melting reactions that are a function of P, T, p(H2O) conditions in different source areas and geotectonic environments. They seem to be the predominant controls on the compositional evolution of felsic magmas in extensional settings, because of the limited degree of assimilation and fractional crystallization (AFC) affecting their compositions with rapid intrusion into the uppermost crust to positions were they achieve neutral buoyancy in the volcanic to subvolcanic environment. Within-plate felsic (or A-type/`Anorogenic' type) and anomalous ocean ridge (OR-type) felsic magmas that have tholeiitic affinities (relatively low alkali elements) are common within parts of the volcanic sequences hosting massive sulphide deposits reflecting, in part, a relationship to extension. The high contents of high-field-strength elements (Zr, Ga, LREE, HREE, Y, Nb, etc.) and other lines of evidence indicate they are relatively high-temperature, relatively reduced magmas probably derived from previously dehydrated granulitic or melted oceanic (simatic) or continental (sialic) crust (low P granulites or amphibolites), but also in some cases possibly mantle, rocks such that they are significantly water undersaturated during melting (i.e., a(H2O)≪1). Reduced viscosities typical of high temperature melts enable rapid intrusion to higher levels (diking vs. diapirism). Higher rates of heat advection from mantle mafic magmas into the thickened oceanic and/or continental crust during lithospheric extension are needed to facilitate melting of these rock to form A-type and/or OR-type partial melts (tholeiitic felsic). This usually results in bimodal magmatism as magma mixing is not favored in extensional settings, especially at high magma production, intrusion, and extrusion rates, i.e., high heat flow. This would inhibit ferromagnesian phase, in particular hornblende, and magnetite fractionation (especially at low fO2) in the felsic magmas that would deplete the melt in Y, HREE, as well as ore-related elements, particularly Zn, which is 2 to 3 times higher in A-type and OR-type rocks compared to other magma types. Therefore, the empirical felsic igneous geochemical–metallogenic relationships observed by others [low Zr/Y (<7) and LaN/YbN (<5) with VHMS mineralization] may simply represent the degree of fractional crystallization of ferromagnesian phases including magnetite and residence time in the deep to middle crust (i.e., rapid ascent rates) in extensional environments. The relationship between extensional geodynamic settings in intra-continental and intra-oceanic intra-arc, inter-arc, and back-arc rift environments and VHMS formation is simply related to extensional faulting, subsidence and submergence, high rates of heat advection to the near-surface environment and resultant seawater convection/reaction, and, in some cases, ocean basin redox conditions and possible contributions of magmatic–hydrothermal solutions.