Abstract

Polymetallic (Fe-Zn-Cu-Pb) ore veins at Aïn Barbar are the result of the activity of a high energy liquid-dominated terrestrial palaeogeothermal field between 16 and 15 Ma ago (Langhian). Fluid inclusion studies yield data on both the conditions of ore deposition and the characteristics of fluid circulation within the field. Pressure was essentially hydrostatic in nature, corresponding to a depth of about one kilometre at the beginning and reduced to no more than 0.5 km towards the end due to erosional unroofing. Chalcopyrite deposition resulted from a break in fluid circulation, allowing the mixing of two kinds of fluid inside the veins ; according to a preferred interpretation, these are : — hot (330°-340 °C), "saline" (≥6 wt. % eq. NaCl) aqueous fluids (A) equilibrated with pyrrhotite, coming from the deeper parts of the aquifer, and drained along the ore veins ; — cooler (300° -320 °C) less saline (~ 1.5 wt. % eq. NaCl) fluids (B') equilibrated with pyrite, coming from the upper and external parts of the aquifer. These diluted B' fluids became progressively more abundant, and cooled down to about 250 °C ; they were responsible for the intense pyritization of earlier pyrrhotite in the veins. Further cooling, associated with a renewed increase of salinity (3-4 wt. % eq. NaCl) is recorded in late-stage quartz. Then, a new cycle of fluid circulation is characterized by : — a transient reheating of the system (in response to a recurrence of magmatic activity) up to 300°-310 °C, the salinity being unchanged ; — a new inflow of diluted fluids (~ 1 wt. % eq. NaCl), associated with a renewal of sulphide deposition. Finally, the system cooled down to 150°-100 °C, and eventually died away.

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