Pyritic shales and microbial mats: Significant factors in the genesis of stratiform Pb-Zn deposits of the Proterozoic?

Extensive horizons of pyritic shale occur in Mid-Proterozoic sediments of the eastern Belt basin, Montana, U.S.A. These pyritic shales are of striped appearance. Laminated pyrite beds alternate with non-pyritic shale beds. Laminated pyrite beds have wavy-crinkly internal laminae and are interpreted as mineralized microbial mats. Pyrite is essentially the only sulfide mineral in these shales. Pyritic shale horizons occur along the basin margins, and it is feasible that colloidal iron was introduced by rivers into basin marginal lagoons and then incorporated into microbial mats and reduced to pyrite. The pyritic shales in the Newland Formation show great similarity to those that host the Pb-Zn deposits of Mt. Isa and McArthur River. It is suggested that pyritic shales of this kind are relatively common in Mid-Proterozoic shales, and that the processes that led to the occasional formation of Pb-Zn ore bodies in these shales are not related to those that formed the pyritic shales themselves. Pyritic shale horizons described in this study occur along the margins of the Helena embayment, an eastern extension of the Mid-Proterozoic Belt basin. The sediment fill of the Helena embayment consists predominantly of rocks of the Lower Belt Supergroup (Harrison 1972), and pyritic shale horizons are known from two localities: 1) The Highland Mountains and 2) The southern Little Belt Mountains (Fig. 1). Gossans of these sulfide horizons contain anomalous amounts of Pb and Zn. Drilling by Anaconda Minerals Co. showed that these pyritic shales consist of thin beds of pyrite-rich shale that alternate with beds of normal shale. Thus, these pyritic shales are of striped appearance and bear great similarity to those that host the large Mid-Proterozoic Pb-Zn deposits of Mt. Isa (Queensland, Australia) and McArthur River (Northern Territory, Australia). However, the pyritic striped shales of the Belt basin were disappointingly unmineralized with regard to Pb and Zn. Individual pyritic shale horizons represent a rock volume on the order of 0.5-1 kin3, and to explain the paucity of Pb and Zn in in such vast volumes of pyritic shale that elsewhere host major Pb-Zn deposits was the starting point for the author's Ph.D. research (Schieber 1985). A brief description of stratigraphy, sedimentary setting and evolution of the Helena embayment is given in Schieber (1986 a). Belt sedimentation commenced with deposition of the Neihart Quartzite (Weed 1899), which is in turn overlain by the Chamberlain Shale (Walcott 1899; Schieber 1989) and the Newland Formation (Fig. 1). The Newland Formation of the southern Little Belt Mountains can be subdivided (Nelson 1963) into a lower (dolomitic shales) and an upper member (alternating shale and carbonate packages). In the transition between lower and upper member a sandstone bearing unit occurs, informally named the "Newland Transition Zone" (or NTZ; Schieber 1985). Deposition of the NTZ marks a major regression, and the Helena embayment changed from a smooth depression to an east-west trending half-graben with active faults along the southern margin (Fig. 1). The pyritic shale horizons of the Southern Little Belt Mountains occur in the upper portion of the NTZ (Fig. 1). The purpose of this paper is to discuss the possible origin of pyritic shale horizons in the Newland Formation of the Little Belt Mountains and to evaluate their significance for Proterozoic stratiform base mental deposits that are associated with texturally very similar pyritic shales. Pyrite mineralized microbial mats in the Newland Formation Pyrite in the Newland Formation has been described in detail by Schieber (1985). It occurs as tiny spherical grains or euhedral crystals (0.001-0.01 mm in size) that are scattered irregularly, are clustered in framboids (0.02-0.25 mm), or form fine wavy-crinkly laminae (0.01-0.2 mm thick). Bundles of such laminae form laminated pyrite beds (Fig. 2). Scattered and framboidal pyrite occurs throughout the Newland Formation in low abundances (1-4%) and is identical in appearance to early diagenetic sedimentary pyrite observed elsewhere (Berner 1970; Love 1964; Sweeney and Kaplan 1973). In contrast, laminated pyrite beds are only found in distinct horizons of pyritic shale (Schieber 1985). Pyritic beds are some mm to several cm thick, contain up to 50% pyrite (Fig. 3), and are separated by be .. of dolomitic clayey shale (Fig. 2). The latter contain only small quantities of pyrite (1-4%, of the scattered and framboidal variety), have in many places silt at the bottom, and form graded silt/mud couplets (Fig. 4). The lower silty portion of these couplets may contain clasts, and may show cross-lamination, parallel lamination, and graded rhythmites (Fig. 2). Intercalation of laminated pyrite beds with silt/mud couplets gives these shales a distinct striped appearance (Fig. 2). Striped shales of identical texture, but of much smaller pyrite content, have been described from the Newland Formation in an earlier publication (Schieber 1986b). Thin intervals (0.1-1 m thick) of these "normal" striped shales are found interbedded with pyritic striped shales in the pyritic shale horizons of the Newland Formation, and also constitute the lateral equivalents of pyritic striped shales. "Normal" striped shales consist of beds of carbonaceous silty shale that alternate with silt/ mud couplets (Schieber 1986 b; and Fig. 3). The latter are identical to silt/mud couplets in pyritic striped shales (Fig. 4). Upon close inspection (see Figs. 4 and 5) it also turns out that in both shale types the beds that are intercalated with silt/mud couplets (carbonaceous silty shale and laminated pyrite beds respectively), show identical textural features. Both show wavy-crinkly internal laminae (compare Figs. 4 and 5) and alternating dark carbonaceous silty laminae and light coloured sediment drapes consisting of clay, dolomite and silt. However, the Fig. 1. Location map and stratigraphic overview. Present day outline of Belt basin indicated by stipple pattern. The Helena embayment portion of the Belt basin is essentially contained in the enlarged portion of the map. Star symbols point out the occurrences of pyritic shale horizons. The stratigraphic overview for the Helena embayment 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 carbonaceous laminae are strongly enriched with pyrite in the case of the pyritic striped shales (Figs. 3 and 4). The match of sedimentary structures and textures between the two shale types, as well as their interbedding and lateral equivalency, suggests that the conditions of sedimentation were the same in both cases. Carbonaceous silty shale beds in "normal" striped shales were examined in considerable detail by Schieber (1986b) and interpreted as fossil benthic microbial mats. The main reasons for that interpretation are listed below: a) irregular wavy-crinkly laminae in carbonaceous silty shale beds (Fig. 5) resemble stromatolitic laminae. One would not expect to find such laminae in a compacted shale that was deposited from suspension in a stagnant environment. Shales deposited under the latter conditions typically are characterized by parallel laminae; b) during soft sediment deformation and erosion the carbonaceous silty shale beds behaved like a tough leathery membrane, rather than like a soupy organic muck (see Schieber 1986b; his Fig. 11); c) abundant filamentous microorganisms, probably remnants of filamentous bacteria and cyanobacteria, were observed by Horodyski (1980) in ripped up fragments of carbonaceous laminae. The cumulative indirect evidence of microbial mat activity (Schieber 1986 b) justifies the interpretation of carbonaceous silty shale beds as microbial mat deposits. Because carbonaceous silty shale beds and laminated pyrite beds show the same textural features (compare Figs. 4 and 5), and because conditions of sedimentation were probably the same for "normal" and pyritic striped shales (see above), it is a reasonable extension of previous conclusions (Schieber 1986b) that laminated pyrite beds represent mineralized microbial mat deposits. Fig. 2. Drillcore specimen of pyritic striped shale. Laminated pyrite beds indicated by arrows. Note wavy-crinkly internal laminations of laminated pyrite beds, and sharp boundaries to non-pyritic interbeds. Thick storm layer with pebbles at the base and parallel laminated silt and dolomitic shale towards the top is visible in the lower half of the specimen. Scale has mm subdivisions Fig. 3. Photomicrograph (reflected light) of laminated pyrite bed. Note wavy-crinkly internal texture and wavy-lenticular sediment intercalations between pyrite laminae. Scale bar is 0.5 mm long Fig. 4. Photomicrograph of pyritic striped shale (combined transmitted and reflected light). The figure shows a laminated pyrite bed (lower two thirds) with approximately 25% pyrite that is overlain by a graded silt/mud couplet. Pyrite grains appear as tiny white dots. Several pyritic laminae are pointed out with arrows. Note wavy-crinkly nature of laminae and compare to carbonaceous silty laminae in Fig. 5. Scale bar is 0.5 mm long Fig. 5. Photomicrograph of "normal" striped shale. Carbonaceous silty shale beds are dark and show wavy-crinkly internal laminae. These beds were interpreted as microbial mat deposits by Schieber (1986b). A silt/mud couplet is pointed out by an arrow. Scale bar is 0.5 mm long Erosional channels in striped shales can be as deep as 50 cm and are filled with sandstone and conglomerate. In the pyritic striped shale facies these channel fills contain ripped up fragments of laminated pyrite beds. Laminated pyrite beds also show load casts, caused by overlying silt, sand, and conglomerate beds. These observations indicate that the pyrite formed very early in diagenesis (Schieber 1985).