Mineralogy, fluid inclusion, and oxygen isotope constraints on the genesis of the Lalla Zahra W-(Cu) deposit, Alouana district, northeastern Morocco
Résumé
The Lalla Zahra W-(Cu) prospect of northeastern Morocco is hosted in a Devonian volcaniclastic and metasedimentary sequence composed of graywacke, siltstone, pelite, and shale interlayered with minor tuff and mudstone. Intrusion of the 284 ± 7 Ma Alouana concentrically zoned, two micas, calc-alkaline, and post-collisional Alouana granitoid pluton has contact metamorphosed the host rocks, giving rise to a metamorphic assemblage of quartz, plagioclase, biotite, muscovite, chlorite, and alusite, and cordierite. The mineralization occurs in and along subvertical, 20 to 40 cm thick, and structurally controlled tensional veins composed of quartz accompanied by molybdenite, wolframite, scheelite, base metal sulphides, carbonates, barite, and fluorite. Three main stages of mineralization (I, II, and III), each characterized by a specific mineral assemblage and/or texture, are recognized. Quartz dominates in all the veins and commonly displays multiple stages of vein filling and brecciation, and a variety of textures. The early tungsten-bearing stage consists of quartz-1, tourmaline, muscovite, wolframite, scheelite, and molybdenite. With advancing paragenetic sequence, the mineralogy of the veins shifted from stage I tungsten-bearing mineralization through stage II, dominated by base metal sulphides, to stage III with late barren carbonates and barite ± fluorite mineral assemblages. Pervasive hydrothermal alteration affected, to varying degrees, the Alouana intrusion, resulting in microclinization, albitization, episyenitization, and greisenization of all the granitic units. Fluid inclusion data yield homogenization temperatures ranging from 124°C to 447°C for calculated salinity estimates in the range of 0.4 to ~60 wt% NaCl equiv. Similarly, the δ18O values for the three generations of quartz range from 11.7‰ to 13.9‰ V-SMOW. Calculated δ18O values of the parent fluid in the range between −3‰ and +9‰ V-SMOW are consistent either with a mixture of water of different origins, including magmatic water, or an origin from seawater or meteoric water that probably exchanged oxygen with rocks at elevated temperatures. The coexistence of CO2-rich and H2O-rich fluid inclusions reflect the presence of a boiling fluid associated with the deposition of the early tungsten-bearing stage mineralization at relatively high temperature. The general temperature and salinity decrease with advancing paragenetic sequence suggest that the early high temperature, magmatic, highly saline, and boiling fluid mixed with meteoric non-boiling fluid results in the precipitation of base metal sulphide and carbonate-barite stage mineral assemblages, respectively.