A FIELD AND LABORATORY STUDY TO EVALUATE THE GENETIC RELATIONSHIPS BETWEEN THE PURVIS PLUTON AND VOLCANIC ROCKS AND VOLCANIC-ASSOCIATED MINERALIZATION IN THE VERMILION DISTRICT OF NE MINNESOTA

The Purvis Pluton is a calc-alkaline, east-west trending, moderate-sized (~3km3), volcanic arc-associated, sill-like multiphase dioritic to tonalitic intrusion with a strike length of 5.7 km and a thickness which ranges from 100-1200 meters. This intrusion occurs near the base of the eastern part of the Neoarchean Ely Greenstone – Lower Member. Cross-cutting relationships, structural fabrics, and an intimate proximal association with regional quartz-epidote alteration suggest that the Purvis Pluton is a synvolcanic intrusion associated with the development of the Ely Greenstone - Lower Member.

Field mapping, supported by subsequent petrographic and lithogeochemical studies, indicates that the Purvis Pluton contains several distinct phases. These include: 1) xenolithic hornblende diorite; 2) xenolithic hornblende tonalite; 3) xenolithic leucotonalite; 4) leucotonalite and trondhjemite; and 5) leucotonalite dikes. Paragenetic relationships between these phases have been determined in the field based and by petrography based on cross-cutting relationships and the xenolith contents of the various phases. Angular, coarse-grained gabbro/diorite lapilli, which have rare earth element characteristics similar to the other phases of the pluton are common in the xenolithic hornblende tonalite and xenolithic leucotonalite phases, and appear to represent an early product of Purvis Pluton crystallization. The xenolithic hornblende diorite, xenolithic hornblende tonalite, and xenolithic leucotonalite commonly contain basalt-andesite lapilli, epidote + quartz-altered basalt-andesite lapilli, and lapilli and blocks of oxide facies iron formation. These three xenolithic phases are intruded by the main phase of the intrusion, leucotonalite/trondhjemite. Leucotonalite dikes are a minor phase of the intrusion, and when present, consistently cut through the other plutonic phases. Phases such as these have been documented in numerous synvolcanic intrusions associated with volcanic-hosted massive sulfide (VMS) mineralization in Canada. The Purvis Pluton appears to represent a felsic, synvolcanic sill-like subvolcanic intrusion that may have been the heat engine which drove subseafloor hydrothermal activity which produced VMS-like Cu-Zn mineralization in the Western Vermilion District.