Pyrometasomatic mineralisation of Amensif Cu-Pb-Zn (Ag-Au) distal skarn deposit, Western high Atlas, Morocco: Insights from paragenetic evolution, mineral chemistry, and fluid inclusions

Abstract

The polymetallic Cu–Pb–Zn (Ag-Au) deposit of Amensif is represented by distal skarn and quartz-barite-carbonate vein structures. It is located in the northern part of the Western High Atlas and 5 km southwest of the famous Azegour Mo-Cu-W mine. The Amensif deposit is hosted in Lower Cambrian volcanoclastic-carbonate sequences, intruded by rhyolitic, andesitic and dacitic dykes and sills.

Based on field observations and mineralogical research, a comparative study of the two ore bodies (skarns and veins) was carried out. In the skarn, two main paragenetic stages can be distinguished: (i) prograde skarn and (ii) retrograde stage. The prograde stage was further subdivided into early prograde stage with wollastonite and diopside relics, which are rather discrete, and the late prograde stage, mainly with andradite, grandite garnet and hedenbergite. Based on mineral chemistry from the late prograde stage, garnet compositions evolve from andradite (And_83.97-99.19 Grs_0.00-15.20 Sps_0.41-2.01 Prp_0-0.19) to grandite garnet (And_34.14-65.56 Grs_30.62-64.56 Sps_0.72-6.05 Prp_0-0.16), in association with hedenbergite (Hd_86.83-93.23 Di_1.3-6.54 Jo_0.2-11.9), this evolution indicate a “strongly reduced” or “moderately reduced” condition with low ƒO₂. The retrograde skarn stage is divided into two sub-stages: (1) retrograde stage I, marked by the deposition of the main alteration minerals (e.g., tremolite, actinolite, epidote, quartz I, calcite I and barite) and sulfides I; and retrograde stage II, characterised by quartz and calcite vein-veinlets associated with sulfides II, electrum and Bi-Ag-sulfosalts. The chemistry of garnet and pyroxene supports classification of the Amensif skarn as a Cu-Zn-Fe skarn.

Fluid inclusions from garnets (prograde skarn stage), display high homogenisation temperatures and salinities (468.3–586 °C; 8.27 to 31.6 wt%. NaCl equiv.). By contrast, fluid inclusions associated with quartz I, calcite I and barite (early retrograde stage) record high to medium homogenisation temperatures (300.2–547.2 °C; 8 to 34.29 wt%. NaCl equiv.) for primary fluid I, and low homogenisation temperatures (171.3–288.7 °C) with same salinities for secondary fluid II. The decrease suggest that cooling and mixture are the main mechanisms of ore deposition.

The striking similarity between the silicate and sulfide mineralisation in the retrograde skarn and the vein structures, both in terms of mineralogy and fluid characteristics, suggests a genetic relationship between these two mineralised bodies.