DIAGENETIC FABRICS IN STROMATOLITES AND COATED GRAINS IN THE PRECAMBRIAN COPPER HARBOR CONGLOMERATE

Non-marine stromatolites in alluvial deposits in the Middle Proterozoic Copper Harbor Conglomerate in the Midcontinent Rift system contain evidence for early diagenesis of carbonate minerals as well as previously described late diagenetic events. The stromatolites have a domal structure over volcanic cobbles and are interbedded with coated grains, such as ooids and grapestones. Fabric analysis based on a petrographic and scanning electron microscopy studies indicate the stromatolites laminae contain Al – Mg silicates and calcite with sweeping extinction. The stromatolites are red due to large amounts of hematite pigment which is interpreted to be early diagenetic in origin. The nuclei of the ooids consist of volcanic rock fragments and quartz grains. The cortices are composed of alternating layers of blocky calcite and a poorly crystalline magnesium silicate (stevensite or sepiolite), and the exterior cortex composed of radial calcite. Based on comparison with Phanerozoic carbonates, the radial calcite suggests a primary fabric whereas the blocky calcite suggests replacement of high-magnesium calcite or aragonite during early diagenesis. The presence of primary radial calcite and blocky calcite suggests rapid changes in water chemistry during formation of the coated grains in a lacustrine setting. Late diagenetic features include, sparry calcite-filled fenestral pores in the stromatolites, authigenic copper minerals, and fluid inclusion homogenization temperatures around 100° C. Also present are calcite filled veins in at least three orientations that crosscut the stromatolites and volcanic clasts, and range in width from 2 mm to 70 mm. Reduction spots are associated with the veins, but also occur as isolated bulbous reduction spots in the stromatolites and surrounding clastics. To better understand the paragenesis, the reddened, reduced, sparry calcite-filled fenestral pores, and the veins are being analyzed for carbon and oxygen isotopes. The petrographic results indicate that in addition to low grade metamorphism and copper mineralization, early diagenetic replacement of metastable carbonate minerals is recorded in the stromatolites and associated deposits.