The nature of early basinal fluids in the Zambian Copperbelt: A case study from the Chambishi deposit

Fluid inclusions from a mineralised lateral secretion quartz vein in the Chambishi open pit in the Zambian Copperbelt represent fluids that immediately postdate stratiform copper mineralisation. Petrographic evidence indicates that this veinset predates deformation in the area. Fluid inclusion microthermometry and Raman spectroscopy of aqueous and aqueo-carbonic inclusions indicate the presence of an early saline H2O-NaCl + MgCl2 ± CaCl2 ± CO2 ± CH4 ± N2 fluid (11.9-23.1 wt.% NaClequiv) at homogenisation temperatures of 86-129 °C. Later fluids occur in fluid inclusion planes, and are linked to early metamorphism of the basin. These fluids are more varied in composition, and are aqueous, aqueo-carbonic, and pure methane inclusions. Aqueous and aqueo-carbonic inclusions are H2O-NaCl ± CaCl2 ± MgCl2 ± KCl ± CO2, N2, CH4, H2S fluids, with two salinity end-members, namely, a high salinity (18-23 wt.% NaClequiv)-lower temperature (∼130-160 °C) fluid, and a lower salinity (∼6 wt.% NaClequiv)-higher temperature (∼140-210 °C) fluid. Later carbonic inclusion planes contain CH4, with N2 and C2H6. Later fluids are complex in composition, salinity, and homogenisation temperatures. These fluids are thought to be late-diagenetic fluids on the one hand, and an early (methane productive) metamorphic fluid on the other. Pressure and temperature conditions in the environment immediately after stratiform copper mineralisation were around 480-800 bar and 130-270 °C.