ZONATION OF LEAD AND SULFUR ISOTOPIC COMPOSITIONS AND TRACE ELEMENTS OF ORE SULFIDES IN THE MOROCCAN HIGH ATLAS MVT ORE PROVINCE: IMPLICATIONS FOR ORE EXPLORATION

The ore sulfides of the Moroccan High Atlas metallogenic province display lead and sulfur isotopic compositions zonation. Sulfide ores show concrete sulfur composition zonation with heavy sulfur isotopic composition ( δ³⁴S ~ +12.2‰) in the center of the High Atlas basin, negative δ³⁴S values (avg. ~ –12.5 ‰) at the northern and western borders of the basin, and intermediate sulfur isotopic signature (avg. ~ +2.3‰) at the southern border. This zonal distribution of sulfur isotopic composition reflects different sulfate reduction mechanisms. Bacterial Sulfate Reduction (BSR) induced the negative δ³⁴S values at lower temperature whereas Thermochemical Sulfate Reduction (TSR) yielded positive δ³⁴S values at higher temperature. The intermediate sulfur isotopic composition reflects the mixing of sulfur-derived TSR and bacteriogenic sulfur.

The west part of the High Atlas near the Precambrian-Paleozoic paleohigh "Le massif ancien" is characteristically more radiogenic (²⁰⁶Pb/²⁰⁴Pb= 18.709 ²⁰⁷Pb/²⁰⁴Pb= 15.658 and ^208Pb/²⁰⁴Pb = 38.808) than the rest parts of the basin (²⁰⁶Pb/²⁰⁴Pb = 18.358 to 18.377, ²⁰⁷Pb/²⁰⁴Pb= 15.636 to 15.668 and ^208Pb/²⁰⁴Pb=38.476 to 38.606). LA Q-ICP-MS analysis of galena samples reveals that galena with radiogenic Pb isotope in the West border are distinguishably rich in silver. The Pb and Zn concentrations respectively in galena and sphalerite are overall similar regardless of the location of the ore deposits.

On the basis of the available data, the grade of the orebodies is independent of the lead and sulfur isotopic compositions. The large size orebodies, however, seem to be generally associated with TSR-derived heavy isotopic sulfur. The center of the High Atlas, characterized by heavy sulfur isotopic signature, is a potential and promising area for ore exploration.