The origin and economic potential of sandstone-hosted disseminated Pb-Zn mineralization in pyritic shale horizons of the Mid-Proterozoic Newland Formation, Montana, USA

Pyritic shale horizons of the Newland Formation (Belt Series, Mid-Proterozoic) show striking similarities to Proterozoic pyritic shales elsewhere that are host to major lead-zinc deposits, such as Mt. Isa and McArthur River. However, in contrast to these deposits the pyritic shales of the Newland Formation contain only minute quantities of lead and zinc. Elevated concentrations of lead and zinc are only found in pore spaces of intercalated sandstone beds. Petrographic and geochemical data indicate that pyritic shale deposition and elevated lead-zinc concentrations in sandstone beds are unrelated, that base metal mineralization is controlled by initial porosity, and that iron and base metals were derived from different sources. Petrographic studies show that base metal sulphides are diagenetic and resemble disseminated mineralization described from sandstone hosted lead-zinc and copper deposits in the Belt basin and elsewhere. The absence of orebodies of disseminated lead-zinc mineralization in sandstones of the Newland Formation may be due to a comparatively thin sedimentary sequence below the sandstone occurrences, as well as to unfavourable geometry and small volume of sandstone bodies. The Mid-Proterozoic Belt basin (Fig. 1) contains several mineral deposits and mining districts of major importance, among them Sullivan (Zn, Pb, Ag, Sri), Coeur d'Alene (Pb, Zn, Cu, Ag), and Troy (Cu, Ag). Numerous occurrences of sediment hosted base metal mineralization were found in a long history of mineral exploration (Reynolds and Harrison 1981; Lange and Eby 1978; Connor et al. 1981; Rye et al. 1981). Huge pyritic shale horizons, recognized in the late 1970's in the Helena embayment (Fig. 1), were intriguing exploration targets because of their great similarity to pyritic shales that host the Pb-Zn-Ag deposits of Mt. Isa and McArthur River in Australia. Yet more than ten years of drilling activity yielded not a single Pb-Zn ore body. A study by Schieber (1985) led to the conclusion that these pyritic shales are probably pyrite mineralized microbial mats, that the iron was carried into the basin by continental runoff (Schieber (Schieber 1987, 1989, 1990), and that pyritic shale hosts of above Australian deposits may have the same origin and are unrelated to Pb-Zn mineralization. In this paper diagenetic mineralization in pyritic shale horizons of the Newland Formation is described. It will be demonstrated that diagenetic Pb-Zn mineralization is porosity controlled and independent of the formation of pyritic shales. Fig. 1. Location of study area. Present day outline of Belt basin indicated by stipple pattern. Enlarged portion of map shows outcrop areas of Belt sediments in the Helena embayment. Star symbol: location of pyritic shale horizons, southern Little Belt