Basinal Brines at the Origin of the Imiter Ag-Hg Deposit (Anti-Atlas, Morocco): Evidence from LA-ICP-MS Data on Fluid Inclusions, Halogen Signatures, and Stable Isotopes (H, C, O)

Abstract

The Imiter Ag-Hg deposit is located in the Precambrian volcano-sedimentary formations of the Saghro inlier (eastern part of the Anti-Atlas Mountains, Morocco). The orebodies consist of northeast-southwest to east-west veins and lenses hosted by Cryogenian black shales and graywackes and Neoproterozoic conglomerates, and are controlled by an east-west fault network, the so-called Imiter fault. Mineralogical and paleo-fluid geochemistry investigations (microthermometry, Raman spectroscopy, LA-ICP-MS on individual inclusions, bulk crush-leach analyses, and stable isotope data (O, H)) show that the main Ag ore stage is related to circulation of deep-basinal sedimentary brines (Na-K-(Mg) (salinity = 16.7 to >= 26 wt % NaCl equiv, molar Cl/Br = 330, delta O-18 = 2.15-2.35%(SMOW), and delta D = -53.8 to -65.5 parts per thousand (SMOW)), at temperatures of about 180 degrees to 220 degrees C and hydrostatic pressures. The main driving mechanism for silver ore deposition is the dilution of ore-bearing brines by a low-salinity meteoric fluid containing a low-density volatile component (N-2 > CH4 > CO2), T-h = 180 degrees to 220 degrees C, delta O-18 = -1.4 parts per thousand(SMOW), and delta D of about -28.2 parts per thousand(SMOW). Silver content of the brines ranges from 2 to 30 mmol/kg solution (up to 3,200 ppm Ag, avg Ag concentration about 900 ppm), whereas the maximum Ag content found in dilute waters is about 0.4 mmol/kg solution (40 ppm). The ore-forming model proposed for the Imiter deposit is (1) Ag extraction from the basement by the penetration of deep-basinal brines, and (2) deposition in a structural trap through fluid mixing with recharge fluids. This model is comparable to that described worldwide for the origin of Pb-Zn, F, Ba, and U deposits near unconformities between basement and sedimentary basins. Similarities among the major Ag deposits from the Anti-Atlas (Imiter, Zgounder, Bou Azzer) strongly suggest that they resulted from a unique event, likely related to the opening of the Atlantic Ocean. The silver ores are superimposed on the same lineament as a preexisting uneconomic Pan-African Co-Ni-As system linked to magmatic intrusions, but Ag ores have no genetic relationship with them.