Abstract

Volcanogenic massive sulfide deposits in the Flin Flon metavolcanic belt occur in an Early Proterozoic island-arc setting that includes arc tholelite, back arc-ocean floor, and shoshonitic assemblages. Near the town of Flin Flon, Manitoba, these volcanic assemblages are structurally juxtaposed in a tectonic collage. Nearly all of the volcanogenie massive sulfide deposits at Flin Flon are associated with the arc tholeiite assemblage and occur in complex stratigraphic sequences which represent volcanic constructs and associated intravolcanic basins within the former magmatic arc. Back-arc or ocean-floor basalts in the Flin Flon belt host only a few, small, Cu-rich massive sulfide deposits, and consequently, have a lower exploration potential than the arc tholeiite assemblage.Within the arc tholeiite assemblage, volcanogenie massive sulfide deposits occur in association with felsic volcanic units, at major stratigraphic and compositional breaks in the volcanic sequence. The breaks can be recognized by contrasting major element, trace element, and isotopic characteristics of the underlying and overlying mafic rocks. Most volcanogenie massive sulfide deposits are underlain by coarse, mafic, intermediate or felsic volcaniclastic rocks. The best-documented examples (including the Flin Flon, Cuprus, and White Lake deposits discussed in this paper) are clearly localized in basinal structures that vary widely in size.The Flin Flon deposit (62.4 Mt) occurs in a proximal volcanic environment. It lies on the upper flank of a shallow subaqueous basalt volcano, possibly in a fault-bounded depression. The massive sulfides are emplaced within a rhyolite flow complex and are underlain by a disconformable chloritic hydrothermal alteration zone developed in footwall basalt breccia. The massive sulfides were overlain by a thick (>3 km) subaqueous basalt-basaltic andesite succession compositionally unrelated to basalts in the stratigraphic footwall.The Cuprus and White Lake deposits (total 1.3 Mt) occur in a 5.5-km-thick sequence dominated by more than 3 km of basaltic andesitc interpreted as a subaqueous shield volcano. This volcano contains a collapse caldera within which an intracaldera subaqueous rhyolite flow and coeval felsic and intermediate basin-filling volcaniclastic rocks were deposited. Massive sulfides accumulated in subbasins within the caldera, hosted by a thin unit of graphitic mudstone and chert. Overlying formations are laterally extensive (unlike the wedgeshaped caldera-filling deposits) and were deposited in a large, deep-water basin; these rocks are chemically and isotopically unrelated to formations underlying the massive sulfide deposit.

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