Sulfide Piercement Structures in the Selebi-Phikwe Nickel-Copper Deposits, Botswana

Exploration & Mining Geology, Vol. 3, No. 3, 1994

CHRISTOPHER OSTERMAN and RICHARD W. HUTCHINSON, Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado, U.S.A.

The Selebi-Phikwe deposits are hosted by amphibolite to granulite fades gneisses of the Limpopo belt. The sulfide orebodies are generally stratiform and conformable to the foliation of the wallrock gneiss. Sulfide remobilization has occurred during metamorphism and has produced discordant sulfide veins, called piercement structures, which project into the hanging wall and foot-wall gneisses. These piercement structures are compositionally similar to the stratiform sulfides, consisting mainly of pyrrhotite, pentlandite and chalcopyrite. However, texturally the piercement sulfides are coarse grained and inclusion-free, whereas the stratiform sulfides are fine grained and packed with tectonically produced wallrock inclusions. The piercement structure sulfides and adjacent stratabound sulfides were sampled and analyzed for major base metals and PGE's for comparative purposes. Most of the piercement structures were enriched in Cu, Ag and Au, whereas they were generally depleted in Ni, Co, Fe and PGE's. Changing elemental associations were observed from the stratabound sulfides and along the length of the piercement structures. This and the degree of depletion or enrichment of various major and minor elements in the sulfide piercement structures may reflect the relative mobilities of metals under metamorphic conditions. The similarities between sulfide piercement structures and the footwall Cu-PGE veins in some Sudbury deposits are reviewed.