The composition of magmatic Ni-Cu-(PGE) sulfide deposits in the Tati and Selebi-Phikwe belts of eastern Botswana

摘要

We studied a number of magmatic Ni-Cu-(PGE) sulfide deposits in two distinct belts in eastern Botswana. The Tati belt contains several relatively small deposits (up to 4.5 Mt of ore at 2.05 percent Ni and 0.85 percent Cu) at Phoenix, Selkirk and Tekwane. The deposits are hosted by ca 2.7 Ga, low- to medium-grade metamorphosed gabbroic-troctolitic intrusions situated within or at the periphery of a greenstone belt. The deposits of the Selebi-Phikwe belt are larger in size (up to 31 Mt of ore grade). They are hosted by high-grade metamorphosed gabbronorites, pyroxenites and peri-dotites believed to be older than ca 2.0 Ga that intruded gneisses of the Central Zone of the Limpopo metamorphic belt. The composition of the sulfide mineralisation in the two belts shows systematic variation. Most of the mineralisation in the Tati belt contains 2-9 percent Ni and 0.05-4 percent Cu (Cu/Cu + Ni=0.4-0.7), whereas most of the mineralisation in the Selebi-Phikwe belt contains 1-3 percent Ni and 0.1-4 percent Cu (Cu/Cu + Ni=0.4-0.9). The Cu-Ni tenors of the ores in both belts are consistent with crystallization from a basaltic magma. The Tati ores contain mostly >3 ppm Pt + Pd (Pt/ Pd 0.1-1), with Pd/Ir= 100-1,000, indicative of a differentiated basaltic magma that remained S-undersaturated before emplacement. Most of the Selebi-Phikwe ores have <0.5 ppm Pt + Pd (Pt/Pd<0.1-l), with Pd/Ir=10-500. This suggests a relatively less differentiated magma that reached S saturation before emplacement. The Tati rocks show flat mantle-normalised incompatible trace element patterns (average Th/Yb_N=1.57), except for strong enrichments in large ion lithophile elements (Cs, Rb, Ba, U, K). Such patterns are characteristic of relatively uncontaminated oceanic arc magmas and suggest that the Tati intrusions were emplaced in a destructive plate margin setting. Most of the Selebi-Phikwe rocks (notably Dikoloti) have more fractionated trace element signatures (average Th/Yb_N= 4.22), possibly indicating digestion of upper crustal material during magma emplacement. However, as there are also samples that have oceanic arc-like signatures, an alternative possibility is that the composition of most Selebi-Phikwe rocks reflects tectonic mingling of the intrusive rocks with the country rocks. The implication is that orogenic belts may have a higher prospectivity for magmatic Ni-Cu ores than presently recognised. The trigger mechanism for sulfide saturation and segregation in all intrusions remains unclear. Whereas the host rocks to the intrusions appear to be relatively sulfur poor, addition of crustal S to the magmas is suggested by low Se/S ratios in some of the ores (notably at Selebi-Phikwe). External S sources may thus remain unidentified due to poor exposure and/or S mobility in response to metamorphism.