The Origin of the Kipushi (Cu, Zn, Pb) Deposit in Direct Relation with a Proterozoic Salt Diapir. Copperbelt of Central Africa, Shaba, Republic of Zaire

Abstract

The Kipushi deposit (Zn, Cu, Pb, Cd, Ag, Ge, Ga) is located in the southeastern part of the copper belt of Shaba. It is remarkable for its mineralogical wealth and its economic importance. This is also the case of the uranium deposit of Shinkolobwe (U, Ni, Co, Au, R.E.E., Pd, Pt), which is situated in the central part of the copper belt.

Both are clearly epigenetic and are located in the sedimentary rocks of the Katanga System (Upper Proterozoic). Its lower part, the`Roan Supergroup, is mainly formed by alternating dolostones, siltstones, black shales, and chlorito-dolomitic siltstones. Its upper part, the Lower and the Upper Kundelungu Supergroups, is mainly detritic (tillitic conglomerates, sandstones, shales), but contains also limestones.

The Roan sediments are often of the sabkha or lagoonal type and at several levels of the stratigraphic column show “discontinuities” underlined by interstratified collapse breccias, formed probably by the dissolution of evaporites.

In the copper belt, the rocks of the Katanga System are intensely folded into an arcuate structure (Lufilian Arc), of dysharmonic Jurassic style. The cores of many anticlines are formed by megabreccias containing huge blocks of lower Roan dolomitic rocks of every size up to several kilometers. Many large clasts contain strata-bound Cu-Co sulfides and are actively mined. The matrix is formed by dolomitic siltstones, which may be residual sediments. They contain gypsum and anhydrite pseudomorphs.

The megabreccias often cut abruptly through the flank of an anticline and reach the Upper Kundelungu, with the result that blocks of this formation are mingled with those of the Roan. This is the case of Kipushi.

Anomalous cross-cutting megabreccias also exist along important strike-slip faults which cut obliquely across the folds (extrusion faults). The Shinkolobwe ore bodies are hosted inside such a structure.

Diapiric ascent of evaporites, followed by a long period of selective dissolution and the resulting collapse brecciation seem to be the only possible explanation of such generalized dislocations.

In our opinion, the saturated brines resulting from this dissolution played an essential role in the base metal mineralization, in particular for the formation of post-tectonic epigenetic deposits such as those considered in this paper. They are

able to leach, transport, and deposit heavy chalcophile metals, as is well documented by present-day geothermal fields (Salton Sea, Phlegrean Fields).

To form economic ores, still other conditions are necessary: presence of sulfur and organic matter, high thermal gradient, micro- or macro- permeability, favorable host rock, and structures tending to channel the ascending hydrothermal solutions into a rather narrow space over a long period.

At Kipushi, this condition is created by a huge slab of shaly Kundelungu rocks sunken alongside the wall of the diapir and forming the impermeable roof of the ore bodies. Their footwall is formed by the dolostones of the Lower Kundelungu.

At Shinkolobwe, the rich uranium lodes and stockwerks are hosted by a heavily tectonized block of the Mine Series (lower Roan), which is in abnormal contact with Kundelungu rocks.