PETROLOGIC INVESTIGATION OF GRAPHITIC BRECCIA PIPE ADJACENT TO THE TAMARACK CU-NI-PGE DEPOSIT; CARLTON COUNTY, MINNESOTA

Breccia pipe formation is often associated with ore deposits, and just as commonly the relationship between breccia formation and ore deposit emplacement is unknown. Delineating breccia pipe mineralogy is an important tool in determining its relationship to the ore-hosting rock. This study investigates primary and alteration mineral assemblages of a breccia matrix in order to evaluate the connection between the adjacent intrusion and the breccia, determining which came first and providing data for a time-space model of emplacement.

Predating this investigation, Rio Tinto Corporation intercepted the breccia pipe adjacent to the Tamarack Cu-Ni-PGE hosting intrusion in north central Minnesota. The Keewenawan Tamarack Intrusion is mafic and composed of two discrete rock units: a lower coarse grained feldspathic lherzolite and an upper medium grained lherzolite, both derived from the same source. In map view the intrusion is tadpole shaped, with certain portions containing more assimilated country rock than others. The breccia pipe is adjacent to an assimilant-rich portion of the intrusion. The breccia is characterized by variably sized graphitic clasts set in a highly altered matrix composed mainly of pyrophyllite and chlorite, and makes contact with either the granophyre gabbronorite or assimilated country rocks of the intrusion sidewall or cupola. Petrographic analysis of the breccia matrix yields a relatively high proportion of felsic mineralogies. Compared to the petrology and chemistry of the Tamarack intrusion, the breccia matrix more closely resembles assimilated country rock. Being that country rock assimilation is the direct result of the Tamarack intrusion, the breccia matrix chemistry suggests a syntectonic model for the breccia and intrusion.