Preservation of Clay Minerals in the Precambrian (1.1 Ga) Nonesuch Formation in the Vicinity of the White Pine Copper Mine, Michigan1

Gejing Li, Jeffrey L. Mauk2 and Donald R. Peacor
Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063
1 Contribution No. 500 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann Arbor, MI 48109-1063.
2 Present address: Geology Department, University of Auckland, Private Bag 92019, Auckland, New Zealand.

Abstract: The Middle Proterozoic (1.1 Ga) Nonesuch Formation, host of the stratiform copper deposit at White Pine, Michigan, consists of 200 m of principally dark grey clastic sediments which contain detritus obtained dominantly from underlying mafic to intermediate volcanic rocks. Clay minerals 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 morphologies of phyllosilicates, both including white mica and chlorite, occur in the ‘lower’ Nonesuch Formation: (1) detrital-shaped and (2) matrix. Detrital-shaped phyllosilicate grains are up to 450 microns long with long axes parallel to bedding. Matrix phyllosilicates occur as packets typically <200 Å thick and as pore-filling cement.

TEM images of detrital-shaped chlorite generally display 14-Å periodicity, although 24-Å corrensite-like units occur locally. Most detrital-shaped chlorite from the mine area samples have a relatively restricted 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 from 0.27 to 0.64. TiO2 crystals occur within and surrounding the detrital-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 compared to detrital-shaped chlorite.

Detrital-shaped white mica occurs as a 2M1 polytype and generally has a phengitic composition. Matrix illite-rich I/S occurs as a 1Md polytype, is K and Al deficient relative 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 mineralization. The TiO2 crystals and corrensite-like units in detrital-shaped chlorite imply that it is at least in part derived from alteration of biotite. The presence of immature 1Md illite-rich I/S and a one layer chlorite polytype with stacking disorder suggests that the matrix clays are in their original, post-smectite state of formation as consistent with an authigenic origin during early burial diagenesis; i.e., they have not undergone subsequent transformation even though sedimentation and ore deposition occurred prior to 1 Ga.

Key Words: Chlorite • Diagenesis • Illite • Mixed-layer illite/smectite • Nonesuch Formation • White mica

Clays and Clay Minerals; June 1995 v. 43; no. 3; p. 361-376; DOI: 10.1346/CCMN.1995.0430311
© 1995, The Clay Minerals Society
Clay Minerals Society (www.clays.org)