Geology, mineralogy and stable isotope geochemistry of the Kabwe carbonate-hosted Pb-Zn deposit, Central Zambia

摘要

The carbonate-hosted Kabwe Pb-Zn deposit, Central Zambia, has produced at least 2.6 Mt of Zn and Pb metal as well as minor amounts of V, Cd, Ag and Cu. The deposit consists of four main epigenetic, pipe-like orebodies, structurally controlled along NE-SW faults. Sphalerite, galena, pyrite, minor chalcopyrite, and accessory Ge-sulphides of briartite and renierite constitute the primary ore mineral assemblage. Cores of massive sulphide orebodies are surrounded by oxide zones of silicate ore (willemite) and mineralized jasperoid that consists largely of quartz, willemite, cerussite, smithsonite, goethite and hematite, as well as numerous other secondary minerals, including vanadates, phosphates and carbonates of Zn, Pb, V and Cu. Galena, sphalerite and pyrite from the Pb-Zn rich massive orebodies have homogeneous, negative sulphur isotope ratios with mean delta~(34)S_(CDT) permil (per thousand) values of -17.75 +- 0.28 (1sigma), -16.54 +- 0.0.27 and -15.82 +- 0.25, respectively. The Zn-rich and Pb-poor No. 2 orebody shows slightly heavier ratios of -11.70 +- 0.5 per thousand delta~(34)S for sphalerite and of-11.91 +- 0.71 per thousand delta~(34)S for pyrite. The negative sulphur isotope ratios are considered to be typical of sedimentary sulphides produced through bacterial reduction of seawater sulphate and suggest a sedimentary source for the sulphur. Carbon and oxygen isotope ratios of the host dolomite have mean delta~(13)C_(PDB) and delta~(18)O_(SMOW) values of 2.89 per thousand and 27.68 per thousand, respectively, which are typical of marine carbonates. The oxygen isotope ratios of dolomite correlate negatively to the SiO_2 content introduced during silicification of the host dolomite. The depletion in ~(18)O in dolomite indicates high temperature fluid/rock interaction, involving a silica- and ~(18)O-rich hydrothermal solution. Two types of secondary fluid inclusions in dolomite, both of which are thought to be related to ore deposition, indicate temperatures of ore deposition in the range of 257 to 385 and 98 to 178 deg C, respectively. The high temperature fluid inclusions contain liquid + vapour + solid phases and have salinities of 15 to 31 eq. wt. percent NaCl, whereas the low temperature inclusions consist of liquid + vapour with a salinity of 11.5 eq. wt. percent NaCl. Fluid transport may have been caused by tectonic movements associated with the early stages of the Pan-African Lufilian orogeny, whereas ore deposition within favourable structures occurred due to changes in pressure, temperature and pH in the ore solution during metasomatic replacement of the host dolomite. The termination of the Kabwe orebodies at the Mine Club fault zone and observed deformation textures of the ore sulphides as well as analysis of joint structures in the host dolomite, indicate that ore emplacement occurred prior to the latest deformation phase of the Neoproterozoic Lufilian orogeny.