Abstract

U-Pb geochronology and detailed field mapping has resulted in a new subdivision of the Kamiskotia Volcanic Complex into Lower and Upper Kamiskotia strata with a 3.5-m.y. volcanic hiatus between the two. The hiatus is interpreted as an unconformity, in common with newly recognized unconformities throughout the greenstone belt. The Genex, and other volcanogenic massive sulfide (VMS) deposits in the area, occur within the Upper Kamiskotia strata. Volcanic facies mapping and reconstruction indicate that the Genex VMS deposit formed within a volcanic graben where synvolcanic mafic and intermediate sills and dikes, with peperitic and locally pillowed contacts, define a proximal volcanic vent area. Sulfide mineralization at Genex occurs in three zones, the first hosted in pillow breccia and hyaloclastite, the second occurring at the contact between felsic tuff and an intermediate intrusion, and the third hosted within the intermediate intrusion. The mineralization represents subsea-floor replacement sulfides localized within zones of higher primary permeability.

Although the synvolcanic mafic intrusions are not directly related to the Genex VMS mineralization they are indicative of a high heat-flow thermal regime. The synvolcanic faults that controlled the emplacement of synvolcanic dikes also provided conduits for the hydrothermal fluids responsible for the mineralization and alteration at Genex. This high-level mafic dike and sill complex was previously correlated with the upper zone of the Kamiskotia Gabbroic Complex but is now interpreted to be younger and correlative with the Upper Kamiskotia Volcanic Complex.

New geochronological data also indicate that the Kamiskotia Gabbroic Complex is older than the VMS-hosting Upper Kamiskotia strata and was emplaced into the Lower Kamiskotia Volcanic Complex. This negates the possibility of a genetic relationship between the Genex and other Kamiskotia VMS deposits within the Kamiskotia Gabbroic Complex. The small size of the synvolcanic intrusions indicates that they were also not the heat source for the Genex hydrothermal alteration system. However, their spatial coincidence suggests that this part of the Kamiskotia Volcanic Complex was the focus of long-lived intrusive activity and high heat flow that defined a thermal corridor encompassing the VMS deposits in the Kamiskotia Volcanic Complex.

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