Les minéralisations aurifères du district polymétallique de Tighza (Maroc central) : un exemple de mise en place périgranitique tardi-hercynienne.

Tighza (or Jebel Aouam) is an important Hercynian Pb-Ag-Zn mining district also rich in gold and tungsten. Major gold-bearing structures are transpressive sinistral quartz veins striking N70°E to N 110°E hosting 1) an early hyaline tungsten-bearing quartz Q1a cataclased into saccharoidal Q1b quartz, and 2) a seldom Q2 quartz with sulphides (lollingite – arsenopyrite) and gold, and 3) a pyrrhotite – sphalerite (chalcopyrite) stage. Perigranitic zoning is obvious, gold grade decreasing quickly when going away from the « granite de la mine”. Skarns associated with « granite du mispickel » display four stages of development: major andradite– F-bearing vesuvianite, hedenbergite, zoned hydrogrossulaire, and retrograde F-bearing actinote. Sulphides indicate mostly an infiltration of gold-bearing hydrothermal fluids rather than a retrograde evolution of skarn. Both hyaline and saccharoidal quartz display calcic aquo-carbonic fluids with minor H2S and Cu, of homogeneous composition (X H2O : 81 mole % ; X CO2 : 18 mole % ; X NaCl : about 1 mole %). Trapping pressure is about 1,5 to 2 kbars with a temperature ranging from 300 to 350°C. These early inclusions have undergone explosions indicating an uplift of Tighza block, and leading to saccharoidal Q2 quartz with multiphased NaCl-saturated fluid inclusions. 40Ar/39Ar dating demonstrate that « granite de la mine », tungsten skarnoid, and scheelite – molybdenite ore vein are coeval, emplaced at 286 +/- 1 Ma. On the opposite, main gold-bearing vein, W1 nord, emplaced before this granite (291,8 +/- 0,3 Ma). Morphology of gold-bearing deposits is moreover dependent of the permeability and reactivity of host-rocks: an open fracture reactivated by a magmatic episode gives birth to a gold-bearing vein, whereas a more reactive rock, such as carbonate, results in disseminations or skarns. Lead isotope and field arguments discard a relationship between Pb-Ag and Au ore deposits at Tighza.