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

摘要

Volcanogenic massive Cu-Zn-(Pb) sulfide (VMS) deposits occur primarily in subaqueous rift-related environments (e.g., oceanic, fore-arc, arc, back-arc, continental margin, or continental), are hosted primarily by bi-modal, mafic-felsic volcanic successions, and are typically associated with felsic volcanic rocks with specific geo-chemical characteristics. FI alkalic dacites and rhyodacites, despite being abundant in the rock record, are typically barren. Some FII calc-alkalic rhyodacites and rhyolites host VMS deposits, but most are barren. Fill tholeiitic and FIV depleted rhyolites and high silica rhyolites are much less abundant in the rock record but commonly host VMS deposits, regardless of age, and Fill rhyolites appear to host the largest deposits. Most petrogenetic models proposed for the formation of FII and FIII-FIV felsic volcanic rocks link felsic magma genesis to fractionation processes in high-level magma chambers now represented by associated sub-volcanic intrusions, where the magma is also interpreted to have supplied the heat and/or metals required to generate and sustain the VMS-forming convective hydrothermal system. However, the relatively constant compositions of FII and FIII-FIV felsic volcanic rocks within individual areas, the high eruptive temperatures (at or above liquidus) of Fill rhyolites, and the bimodality of VMS-hosting volcanic successions indicate that fractional crystallization within subvolcanic intrusions could not have generated or significantly modified the compositions of FII and FIII-FIV magmas. This, coupled with detailed geological, geochemical, and geochrono-logical studies indicates that many of these subvolcanic intrusions were emplaced in multiple phases and that the later, most voluminous phases often cut ore-associated, hydrothermally altered rocks.