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Abstract

An initial investigation of the geochemistry of the igneous intrusions which commonly occur spatially associated with lode gold deposits in the southwest part of the Abitibi subprovince shows four main types of intrusion with distinctive geochemical compositions: (1) synvolcanic quartz diorites to tonalites showing characteristics of recent island-arc calc-alkaline suites, (2) feldspar ± quartz porphyries similar in general to tonalite-trondhjemite suites external to the greenstone belts, (3) small plugs of monzodiorite to granodiorite composition which most closely resemble the geochemistry of modern high K calc-alkaline intrusions, and (4) calc-alkaline (shoshonitic) lamprophyre-diorite dikes. Only group 3 shows anomalous primary Au concentrations (G = 7± 24/7 ppb) in contrast to the other groups (G = l±2/1 ppb). Intrusions in groups 1 and 2 played a passive role in the generation of lode gold deposits and host mineralization as a result of the intrusions’ mechanical behavior during subsequent shearing. Zones of dilation within larger deformation zones can be controlled by the positions and sizes of such intrusions. Group 3 intrusions, represented by three plugs in the Lamaque mine, exhibit an extremely close intrusion-gold mineralization association (hosting 90% of the ore) and show an anomalous primary gold enrichment in the least altered samples. This finding suggests that the dikes which coalesced to form these plugs were derived periodically from a parental dioritic magma at depth in which gold (initially ∼6 ppb) was progressively enriched through fractional crystallization processes into the most evolved late melt (∼30 ppb), and therefore, possibly, into a coexisting magmatic fluid.

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