PRESERVATION OF CLAY-MINERALS IN THE PRECAMBRIAN (1.1 GA) NONESUCH FORMATION IN THE VICINITY OF THE WHITE-PINE-COPPER-MINE, MICHIGAN

The Middle Proterozoic (1.1 Ga) Nonesuch Formation, host of the strati form copper deposit at White Pine, Michigan, consists of 200 m of prin cipally dark grey elastic sediments which contain detritus obtained do minantly from underlying mafic to intermediate volcanic rocks. Clay mi nerals from samples collected from the mine area and drill holes up to 100 km away have been studied using SEM, EMPA, TEM and AEM. Two morph ologies of phyllosilicates, both including white mica and chlorite, oc cur in the 'lower' Nonesuch Formation: (1) detrital-shaped and (2) mat rix. Detrital-shaped phyllosilicate grains are up to 450 microns long with long axes parallel to bedding. Matrix phyllosilicates occur as pa ckets typically <200 Angstrom thick and as pore-filling cement. TEM im ages of detrital-shaped chlorite generally display 14-Angstrom periodi city, although 24-Angstrom corrensite-like units occur locally. Most d etrital-shaped chlorite from the mine area samples have a relatively r estricted range of Fe/(Mg+Fe) ratios from 0.52 to 0.58, but the Fe/(Fe +Mg) ratios of detrital-shaped chlorite outside the mine area range fr om 0.27 to 0.64. TiO2 crystals occur within and surrounding the detrit al-shaped chlorite. Matrix chlorite has Fe/(Fe+Mg) ratios of 0.47 to 0 .63, indicating that it is relatively homogeneous and enriched in Fe c ompared to detrital-shaped chlorite. Detrital-shaped white mica occurs as a 2M(l) polytype and generally has a phengitic composition. Matrix illite-rich I/S occurs as a 1M(d) polytype, is K and Al deficient rel ative to end-member muscovite and contains significant Fe and Mg. The data are consistent with homogenization of detrital-shaped chlorite in the White Pine mine area by hydrothermal fluids during copper mineral ization. The TiO2 crystals and corrensite-like units in detrital-shape d chlorite imply that it is at least in part derived from alteration o f biotite. The presence of immature 1M(d) illite-rich I/S and a one la yer chlorite polytype with stacking disorder suggests that the matrix clays are in their original, post-smectite state of formation as consi stent with an authigenic origin during early burial diagenesis; i.e., they have not undergone subsequent transformation even though sediment ation and ore deposition occurred prior to 1 Ga.