Banded ores at the Gamsberg Zn-Pb deposit, South Africa: Insight into ore-forming processes in Broken Hill-type deposits

摘要

mineralogical and chemical characteristics of banded garnet-amphibole mineralization at the Broken Hill-type Gamsberg Zn-Pb deposit are described. Rhythmically alternating garnet- and amphibole-rich mesobands are interpreted as reflecting compositional layering of original metalliferous sediments, similar to those formed at presently active hydrothermal vents (e.g. Red Sea deposits). Garnet-rich mesobands are enriched in Al and Zn relative to the Fe-Mn dominant amphibole layers, suggesting a more pronounce detrital input into the former. If constant sedimentation rates are assumed, the amphibole-rich layers thus suggest rapid precipitation of mostly Fe and Mn. An ore-forming model is postulated, incorporating evidence from the Red Sea deposits. The model envisages precipitation of Zn- and Fe-sulfides from a reducing brine, thereby forming a sulfide mud on the seafloor upon mixing with fine-grained pelagic sediments. Fe and Mn that remained in solution were transported by convection to the upper parts of the proposed brine pool, became oxidized upon mixing with overlying oxic seawater and rapidly precipitated out of solution. It is proposed that these processes continued due to a constant brine supply, thus resulting in alternating sulfide-rich and sulfide-poor sediments on the seafloor. The textural and chemical evidence supports a syngenetic origin of the Gamsberg deposit and allows interpretations of the ore-forming processes in Broken Hill-type deposits.