The amphibolite at Chibuluma Mine Zambia

Abstract

Chibuluma Mine is situated on the southern flank of the Nkana-Chambishi basin in the central section of the Zambian Copperbelt. The amphibolite at Chibuluma Mine occurs exclusively in the Upper Roan Group sediments overlying the economically important Lower Roan Group. The amphibolite occurs in lenticular, sill-like bodies ranging in thickness from less than 10 feet to over 1,000 feet. It is concluded that these sill-like bodies are intrusive sills of basic magma. The largest sill is a multiple intrusion with two phases, both of which exhibit chilled margins. The younger phase was intruded into the older along a plane approximately 50 feet above its basal contact. Grainsize, specific gravity, and micrometric data indicate that each phase of the intrusion has undergone gravitational differentiation during crystallisation. Variations in the relative amounts of amphibole and feldspar distinguish between a lower melanocratic amphibolite, an upper mesotype amphibolite, and a zone of coarse-grained pegmatitic amphibolite schlieren. The amphibolite consists mainly of hornblende, labradorite, albite, and scapolite, with accessory biotite, chlorite, epidote, clinozoisite, sphene, apatite, quartz, micropegmatite, and calcite. Clouding of the basic plagioclase and zoning in amphibole, plagioclase, and scapolite are features of petrological significance. The opaque minerals in the amphibolites and the adjacent sediments are magnetite, ilmenite, pyrite, chalcopyrite, pyrrhotite, cubanite, valleriite, and pentlandite. Intergrowths of these suggest that their temperatures of formation were probably in excess of 450°C. The chemical data show a close resemblance between the amphibolite and a tholeiitic magma-type although minor spilitic characteristics are also evident. The chemical data also confirm the mineralogical evidence of fractionation and gravitational differentiation. The trend in differentiation followed in the development of the pegmatitic and mesotype amphibolite is very similar to the trends followed by other basic intrusions. The main metamorphic affect accompanying the intrusion of the amphibolite is the soda metasomatism evident in both sediments and intrusions. Some evidence exists to suggest that the amphibolites were emplaced before or during the early stages of the Lufilian orogeny when sediments were essentially horizontal