Ouansimi copper mineralization (Western Anti-Atlas, Morocco): Paragenetic sequence and circulation of gangue hosted paleofluids.

Abstract

The Ouansimi copper deposit, located South of the Kerdous inlier in Morocco, is an example of vein-type mineralization hosted in the Adoudou Formation limestone of the Lower Cambrian Taroudant Group. The copper-bearing veins are mainly controlled by Variscan NNW-trending faults and are subdivided here into Eastern, Western, and Central structures. On the basis of mineralogical, metallographic, and fluid inclusion studies, two mineralizing processes are proposed. A primary process is interpreted for the chalcopyrite, digenite, galena, sphalerite, carrollite, and pyrite with quartz and dolomite as gangue minerals. A secondary process is proposed for chalcocite, bornite, covellite, native copper, copper carbonate and oxide. Microthermometric analysis of fluid inclusions was carried out on quartz and dolomite gangues. Primary fluid inclusions trapped in quartz show the existence of fluids with temperatures of 175°C–275 °C and salinities of 10–15 wt % NaCl eq. However, fluid inclusions in dolomite indicate fluids with temperatures of 200°C–250 °C and salinities of 20–25 wt % NaCl equiv. Combining ore mineralogy and fluid inclusion microthermometry, two main hypogene sulphide mineralization stages can be distinguished in the Ouansimi copper deposit. The first Cu–Fe stage, with quartz as a gangue mineral, was related to hydrothermal fluids with moderate temperature and low salinity. The second Cu–Zn–Pb–Co–Ni–Fe stage was associated with dolomite as a gangue mineral and infilled cavities and fractures cutting quartz. This second phase was related to hydrothermal fluids with moderate temperature and high salinity. The tectonic evolution of the Ouansimi deposit was characterized by thrusting of the south limb over the north limb and by dextral and sinistral offsets of the mineralized structures along late-stage ENE-WSW-trending faults.