STRATIGRAPHIC AND LITHOGEOCHEMICAL STUDIES OF THE PRICHARD FM. NEAR PERMA, MT., AND IMPLICATIONS FOR BELT BASIN MINERALIZATION

The Prichard is the basal formation of the Mesoproterozoic Belt Supergroup and is time equivalent to the Aldridge Fm., host of the Sullivan Pb-Zn orebody. The Prichard Fm. has been divided into 8 informal members (A-H) by Cressman (1985,1989). Road-cuts along MT highway 135 near Perma expose portions of each. A detailed stratigraphic log was constructed from the roadcut exposures. Lithogeochemical data were collected through all exposed argillite and siltite units from 20m representative chip samples. Graphic logging confirmed Cressman's interpretation that the Prichard consists primarily of deep water and turbiditic facies; however, in a departure from Cressman, member E also appears deposited in generally deeper water. E exhibits dewatering structures, syn-sedimentary faults and bedding disruption in association with thick turbidite sandstones, as well as evidence of rapid shallowing and deepening. All evidence of deposition in a seismically active sub-basin. The lithogeochemical data show each member can be distinguished, but also detect alteration. Member D is the most altered, and B and D are the most leached. Alkali ratios suggest metamorphism of the Prichard is due to burial. Litho-type comparisons and stratigraphic position relative to the syn-rifting Plains sill (1469±2.5 Ma) suggest that the "Sullivan horizon" could have been within the upper part of member F, but possibly as high as within G. Metal content of F, G and H show they are prospective hosts, and in the Perma region several significant metal occurrences are known in members B, E and F. Dissemminated Pb-dominant mineralization within E quartzites signify fluid-flow pathways. Zn-dominant mineralization in B quartzites may also record fluid passage. Member F hosts the most significant mineralization in the Perma region consistent with the lithogeochemical data. Extensional tectonism with the potential to mobilise metal-fertile basinal brines may have commenced as early as deposition of member E.