Anomalous Iron Distribution in Shales as a Manifestation of "non-clastic Iron" Supply to Sedimentary Basins: Relevance for Pyritic Shales, Base Metal Mineralization, and Oolitic Ironstone Deposits

In previous investigations, nearshore pyritic shale horizons in the Mid-Proterozoic Newland Formation were interpreted to be due to "non-clastic" colloidal iron supply by streams. New data on the chemical composition of shales in the Newland Formation support this interpretation. In these shales, Fe and Al show a positively correlated trend that intercepts the Fe axis above the origin. These relationships suggest control of Fe by clays (via iron oxide coatings on clay minerals), and presence of an additional, "non-clastic iron" component. Shales from stratigraphic intervals during which pyritic shale horizons were deposited plot above the Fe/Al trend typical for the remainder of the Newland Formation. Pyritic shale horizons in sediments are favourable hosts for base metal deposits of the pyrite replacement type. Fe/Al relationships as found in the Newland Formation may help to identify stratigraphic horizons in other sedimentary basins that contain pyritic shale horizons and potentially base metal mineralization. Introduction Shales of the eastern Belt basin (Mid-Proterozoic of Montana) contain sizeable horizons of pyritic shale (up to 60m thick, as much as 20 km2 extent, pyrite Fe on the order of several 108 tons; Schieber, 1987, 1989a, 1990). Most of the pyrite occurs as discrete laminated beds, interpreted as mineralized microbial mats. Study of these pyritic shale horizons (Schieber, 1987), led to the hypothesis that most of the iron was derived from fluvial input in the form of iron colloids (chiefly ferric oxyhydroxides). Iron deposition occurred in partially enclosed coastal embayments (Schieber, 1990), probably due to flocculation caused by mixing with basin waters. An integrated sedimentological and geochemical study (Schieber, 1985) resulted in the hypothesis that pyrite iron was not linked to terrigenous clastic sedimentation (e.g. via iron bearing minerals and oxide coatings on detrital grains), but rather was added as a "non-clastic" sedimentary component ("free" colloidal iron oxyhydroxides). The potential amount of "non-clastic" fluvial iron input was estimated from clues in the sedimentary record (e.g. size of basin marginal drainages, climate, water supply) and the application of knowledge about modern fluvial systems. Even if only very small amounts of "non-clastic iron" (1 ppm or less) are carried in continental waters, the amounts of iron that are supplied to the basin considerably exceed the amounts needed to form the observed pyritic shale horizons (Schieber, 1987). Studies of modern iron transport to the oceans have documented that most if not all of the fluvial "free" colloidal iron is flocculated in estuaries and nearshore environments (e.g. Boyle et al., 1977; Yan et al., 1991). However, some studies also show iron removal to depend on residence time of water in estuaries, with short residence times allowing for offshore transport of a portion of the "free" colloidal iron (Hong and Kester, 1985). These observations suggest a possible way to test the "non-clastic iron" hypothesis proposed for pyritic shale horizons in the Newland Formation. If they indeed originated through nearshore flocculation of iron colloids, a portion of these might have been carried further into the basin to leave a chemical signal in the accumulating sediments. This possibility is investigated in this paper, and new data are presented to demonstrate "non-clastic iron" supply to sediments of the eastern Belt basin. Geologic Setting Pyritic shales occur in two locations along the margins of the Helena embayment (Fig. 1), an eastern extension of the Mid-Proterozoic Belt basin whose sediment fill consists predominantly of sediments of the Lower Belt Supergroup (Harrison, 1972). In the northern Helena embayment pyritic shale horizons occur within the Newland Formation of the southern Little Belt Mountains. They accumulated in coastal embayments that were partially enclosed by offshore sand bars and received terrestrial runoff (details of depositional setting in Schieber, 1990). The Newland Formation is the most widely exposed stratigraphic unit of the Lower Belt Supergroup in the Helena embayment. It consists of a lower member (dolomitic shales) and an upper member (alternating shale and carbonate packages), as well as of a sandstone bearing transition zone between the two members (also referred to as the Newland Transition Zone or NTZ). In this paper, shale and carbonate packages in the Newland Formation are collectively referred to as Newland shales and Newland carbonates. Pyritic shale horizons of the southern Little Belt Mountains occur in the NTZ (Fig. 1). Presence of organic matter and pyrite in these sediments indicates anoxic pore waters. Abundant storm layers and carbonaceous shale beds that are probably fossil microbial mats suggest that the Newland Formation did not accumulate under stagnant bottom water conditions (Schieber, 1986b). From age constraints on sediments of the eastern Belt basin (Obradovich and Peterman, 1968; Harrison, 1972), the Newland Formation probably accumulated at a rate of 0.05-0.2 mm of approxim end of Lowe embayment extensive de graben with (Schieber, 1 enclosed ne accumulatio offshore the study. Methods of Samples of collected fro areas that co Belt Mounta of homogen XRF, follow (1969). Dup and some el against INA methods are sections wer light. A sm by electron backscattere microprobe. Petrograph Figure 1: Location map and stratigraphic overview. Stipple pattern indicates present day outline of Belt basin. In enlarged map dashed lines indicate outcrop areas of Belt rocks within the Helena embayment. Star symbols indicate known occurrences of pyritic shale horizons in the Little Belt and Highland Mountains. The stratigraphic overview is based on data from McMannis (1963), Boyce (1975), and Schieber (1985). It represents a generalized restored cross-section along line AB in the enlarged map portion. /year, and was deposited over a time span ately 10-15 m.y. (Schieber, 1987). At the r Newland deposition the Helena probably changed from a smooth and pression to an east-west trending halfactive faults along the southern margin 985). During NTZ deposition, partially arshore lagoons were sites of pyritic shale n (Schieber, 1990), whereas further shales accumulated that are subject of this Investigation shale (65) and carbonate (23) were m the Newland Formation, outside of ntained pyritic shales (Big Belt and Little ins, Fig. 1). Major element composition ized bulk samples was determined by ing the method of Norrish and Hutton licate analyses were performed by AA, ements (Fe, K, Na) were also checked A data. Data from different analytical in good agreement. Hundreds of thin e examined in reflected and transmitted all number of thin sections was examined microscope in secondary (SEM) and d (BSE) mode, and by electron