Abstract:

The Flin Flon camp, in the Flin Flon Belt of the Paleoproterozoic Trans-Hudson Orogen, hosts three world-class volcanogenic massive sulphide (VMS) ore bodies: the Flin Flon, Callinan and Triple 7 deposits. The deposits are hosted in heterolithologic<br/>breccias and rhyolitic flows, domes and volcaniclastic rocks of the Millrock member of the Flin Flon formation, on the western limb of the Hidden Lake syncline. The VMS deposits and rhyolites have been interpreted as emplaced into a depositional basin created during collapse of a caldera during intra-arc rifting. The Myo member, of the FJin Flon formation, is on the west limb of the Beaver Road anticline and consists of rhyolite and rhyodacite sills and flows (“Myo rhyolites”) with aphyric, amygdaloidal, massive to pillowed basaltic flows, and thinly bedded to laminated mafic tuffs. Geological, petrographic and geochemical evidence indicate that the Myo rhyolites are the timestratigraphic equivalent o f the Millrock member and thus are of particular interest for their VMS potential.<br/>Three Myo rhyolite lithofacies are identified based on phenocryst type and abundance as well as rare earth element (REE) and high field strength element (HFSE) abundances: sparsely quartz porphyritic (SQP) rhyolites, quartz porphyritic (QP) rhyolites and feldspar porphyritic (FP) rhyodacites. The Myo rhyolites display primitive sNd values (+3.2 to +4.2) and have volcanic arc signatures (relatively flat REE patterns and depleted HFSE abundances, negative Nb and Ti anomalies) but are interpreted, based on geology, to have been emplaced into a non-arc geological setting. This, along with their FIII-FIV classification, suggests that the Myo rhyolites were derived from partial melting of hydrated, REE- and HFSE-depleted basaltic arc rocks in response to the ponding of basaltic magma at the base of the crust during intra-arc rifting.<br/>Two extrusive phases of Myo rhyolite are identified and, based on the presence of transported and resedimented autoclastic breccias, are interpreted as vent-proximal suggesting nearby vents or fissures. The majority of the Myo rhyolites, however, are<br/>interpreted as intrusive: contacts with bounding massive to pillowed mafic volcanic flows are sharp and chilled and lack breccias or hyaloclastite. In contrast with the volcanic environment of the Millrock member, the Myo rhyolites are interpreted to have been emplaced into a basaltic, flow-dominated lava shield. Observations supporting this include the continuity of the Myo rhyolites for 6 km along strike, lack of both synvolcanic faulting and dike swarms, and absence o f volcaniclastic rocks in strata surrounding the Myo rhyolites. A feature that is essential to VMS-forming environments that is found associated with the Millrock member, but is missing from the Myo member, is the fault/fracture system associated with rifting +/- caldera development. The absence of abundant faults and fractures results in fewer and less focused pathways for metal-rich fluids to ascend and reach the seafloor, thus leaving the Myo rhyolites with a lower potential for hosting economically significant mineralization.