Abstract

Hydrothermal veining at the Musoshi stratiform copper deposit in Zaire has caused extensive alteration in the arkosic footwall sediments and in the mineralized ore shale (argillaceous siltstone) horizon. Pervasive growth of secondary rutile, biotite, and muscovite in the wall rocks accompanied this activity, whereas hematite, biotite, calcite, K-feldspar, barite, and accessory rutile occur with quartz in footwall veins. In contrast, veins cutting the ore shale carry Cu-Fe sulfides, uraninite, and molybdenite, with quartz, calcite, biotite, K-feldspar, and minor rutile. Linear zones of particularly intense alteration and veining probably represent structurally controlled fluid conduits. Fluid inclusions in vein quartz from such zones identify an apparently halite-saturated liquid at 397 degrees + or - 5 degrees C, with approximately 39 wt percent NaCl, 15 wt percent KCl, and minor amounts of CO 2 and other components.Rutile is found pervasively, both as a wall-rock alteration phase and as large euhedra (up to 3 cm) in the quartz veins. The hydrothermal growth of this normally very insoluble mineral is explained by the high temperature and salinity of the aqueous phase.The widespread occurrence of rutile in the deposit makes it suitable for determining the age of hydrothermal alteration. Six samples of rutile picked from footwall and ore-shale veining and alteration give a U-Pb concordia upper intercept of 514 + or - 2 m.y. (2 Sigma ). This value is confirmed on a 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb plot (four of these samples contained common Pb concentrations in excess of blank), which yields a slope age of 513 + or - 5 m.y. (2 Sigma ). A seventh sample from an unusual footwall hematite vein gave a slightly younger 207 Pb/ 206 Pb age (496 + or - 1 m.y.).The 514 + or - 2 m.y. alteration event is considerably younger than the minimum age of the host sediments (602 + or - 20 m.y.), and probably also postdates ore deposition, believed to be older than 600 m.y. No unambiguous evidence was seen at Musoshi to support a relationship between hydrothermal alteration and stratiform Cu mineralization, although sulfide textures were modified at this time (as shown by intergrowths with rutile, dated here at 514 m.y.). The hydrothermal activity is concluded, therefore, to be a late overprint, unrelated to Cu ore deposition, but perhaps related to compressional deformation and metamorphism associated with the Lufilian orogeny.

First Page Preview

First page PDF preview
You do not currently have access to this article.