Abstract

The synvolcanic Mooshla Intrusive Complex intrudes coeval ~2699 to 2696 Ma volcanic rocks of the Blake River Group within the southern margin of the Archean Abitibi greenstone belt. The upper Blake River Group is host to the Doyon-Bousquet-LaRonde mining camp that contains Au-rich volcanogenic massive sulfide (VMS) deposits, possible subsea-floor epithermal-style deposits, and orogenic Au deposits. In total, the camp contains to date in excess of 28 million ounces (Moz) Au, making it a world-class example of Au-rich paleosea-floor environments. The Mooshla Intrusive Complex is spatially, temporally, and most probably genetically associated with all of the above types of mineralization. It is host to parts of the Doyon (5.5 Moz Au), Mouska (0.8 Moz Au), and Mic Mac (0.11 Moz Au) Au deposits and host to the smaller Mooshla A and B Au occurrences. Host volcanic units to the Mooshla Intrusive Complex are intensely deformed, metamorphosed, altered, and mineralized, as is the intrusion itself.

The Mooshla Intrusive Complex was formed by nine distinctive phases of subvolcanic dikes, sills, and stocks. These were emplaced in two stages to form a shallow, multiphase synvolcanic intrusion along the contact between the Hébécourt and Bousquet volcanic formations. The Mouska stage is represented by a preliminary swarm of thin diabase sills, intruded by a well-layered gabbroic sill, a more crudely layered quartz diorite, and tonalite. A period of devolatilization accompanied crystallization of the xenolith-rich top of the tonalite magma chamber, as evidenced by the presence of an aplite dike swarm and associated extensive alteration zones and miarolitic cavities. The younger Doyon stage comprises a series of fine-grained aphyric to porphyritic, tonalite and trondhjemite dikes and sills, which also contain evidence of in situ devolatilization.

The geochemical signatures of the Mooshla Intrusive Complex indicate emplacement during formation of an evolved, extensional oceanic island arc-style succession. Primitive mantle-normalized spider plots suggest a common origin for this island-arc intrusive suite that is similar to that of the volcanic succession of the upper member of the Bousquet Formation. Various element ratio plots used to further define magma origin and emplacement history suggest that whereas the Mouska-stage magmatic phases have a relatively straightforward, coexisting fractionation history, the Doyon-stage tonalite-trondhjemite has a more complex interplay of assimilation-fractionation-contamination, suggesting midcrustal partitioning and interaction with both earlier formed, partially hydrated ~2720 Ma oceanic crust and upper Blake River host strata (~2699-2696 Ma). The protracted and mulitphased magmatic evolution of the Mooshla Intrusive Complex led to the generation of volatile-rich phases that contributed to the development of a submarine magmatic-hydrothermal system that is thought to be responsible for the formation of the Doyon Au-Cu deposit. Geologic and timing relationships suggest that this magmatic-hydrothermal system might also have contributed to the generation of Au-rich VMS deposits higher in the host volcanic succession as part of a large Archean magmatic and hydrothermal center.

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