SCALE AND SIGNIFICANCE OF VARIATION IN SILICA-DOLOMITE ALTERATION IN THE TENKE-FUNGURUME (TFM) CU-CO DISTRICT OF THE CENTRAL AFRICAN COPPERBELT

Ongoing research into the formation and evolution of the Copperbelt’s Cu-Co resource focuses on the type of altering fluids and the scale of flow. To estimate these, paragenesis and composition of alteration minerals were examined at 17 rafted fault blocks of ore-hosting strata over 20 km of TFM. Of these, 13 show the same mineral textures and compositions. Following sedimentary dolomite, the earliest alteration phases were pyrite and chalcedony of uncertain relative timing. The next stage brought dolomite in first a blue-gray, then a white form, accompanied by carrollite, bornite, and chalcopyrite; then quartz replaced much of the dolomite. It was overgrown by a second dolomite with the same composition as the first and like it associated with Cu-Co-Fe sulfides. Over the study area, the dolomites are constant in composition and CL signature, although the second is slightly more Fe-rich than the first and luminesces darker than its medium red. However, both are relatively low in Mn and Fe. A third, crosscutting dolomite, intergrown with malachite, has millimeter-scale banding easily visible in CL, and its composition ranges between Ca_0.964-0.997Mg_0.896-1.031Fe_0.002-0.881Mn_0.001-0.019(CO₃)₂. Its sequence of bands and distribution of compositions are erratic. The other 4 blocks examined lack the ore and the white form of the first dolomite, but their minerals’ parageneses and compositions are otherwise the same as in the other 13.

The low Mn and Fe contents of the early dolomites indicate that they formed from relatively oxidizing fluids, like most sediment-hosted Cu deposits; the consistency of their compositions over space suggests that the formative fluid system extended over at least 20 km with remarkably uniform conditions, like MVT systems. This may help establish a lower limit on the size of individual fluid systems in the ore-forming basin. The absence of ore and one of the early dolomites from 4 blocks suggests that they were occluded during one stage, but the presence of the later alteration minerals indicates that they were afterward accessible to fluids. Local effects dominated later, creating supergene dolomites with strong compositional variations erratically distributed. Thus, basin hydrology may have controlled Copperbelt ore distribution, and local fluid systems its redistribution.