Evaporitic brines and copper-sulphide ore genesis at Jbel Haïmer (Central Jebilet, Morocco)

Abstract

The Jbel Haïmer copper-sulphide mineralisation occurs at 20 km north of Marrakesh in the Variscan Jebilet massif, Morocco. Most of the ores (up to 3.9 wt% Cu, ≤38 ppm Ag, and up to 2.9 ppm Au) occur as impregnations of NE-SW fault/fracture zones and related tectonic breccia. Two independent stages of fluid circulation and mineral deposition are distinguished. First, Late Variscan high temperature-low pressure metamorphism synchronous of granite intrusion induced percolation of C–H-O-N hydrothermal fluids throughout faults and shear zones. The drop in pressure from lithostatic down to hydrostatic values at temperatures around 350 ± 50 °C triggered quartz precipitation associated with minor Sn-As-(Co-Ni) deposits and finally brecciation. The second stage, more recent than Triassic, consists in the deposition of quartz + carbonates, followed by Cu-(Pb-Zn) sulphides (±Ag-Au alloys) in fractures crosscutting 240 Ma microdiorite dikes. Mixing of evaporitic brines likely coming from Triassic formations with low salinity aqueous fluids was responsible for Cu-sulphide deposition at temperatures of around 220–280 °C at a depth of 4–5 km. These base metal-rich brines are similar to those from the nearby Roc Blanc Ag-deposit and several other silver and base metal deposits in Morocco, considered as having circulated during the Central Atlantic Ocean opening.