Pyrite mineralization in microbial mats from the mid-Proterozoic Newland Formation, Belt Supergroup, Montana, U.S.A.

Abstract

The Mid-Proterozoic Newland Formation, a shale-dominated unit of the Belt Supergroup, was deposited in an eastern extension of the Belt basin, the Helena embayment. A variety of different shale facies types can be distinguished. Of particular interest for this study is a shale facies that has been interpreted to be the result of microbial mat growth (resulting in carbonaceous shale beds) interrupted by storm deposition (causing deposition of graded silt/mud couplets). Alternation of carbonaceous beds with silt/mud couplets gives these shales a characteristic striped appearance. Along the basin margins a pyrite-rich sub-facies of these striped shales is found locally, consisting of laminated pyrite beds that alternate with non-pyritic silt/mud couplets. Laminated pyrite beds in pyritic striped shales are interpreted as mineralized microbial mats because of wavy-crinkly internal laminae and because of the direct association with unmineralized striped shales that contain microbial mat deposits. Excess iron in pyritic shale horizons was probably supplied by terrestrial runoff in colloidal form. Iron hydroxides, introduced by rivers into basin marginal lagoons, flocculated, and were then incorporated into microbial mats and reduced to pyrite upon burial.