Cr-spinel tracks genesis of Co-Fe ores by serpentinite replacement at Bou Azzer, Morocco

Abstract

Co-Fe arsenide ores are associated to serpentinite with alteration haloes of carbonation and talcification in the Bou Azzer inlier (Central Anti Atlas, Morocco). These Co-Fe rich ores consist mainly of skutterudite and members of the löllingite-safflorite series, associated to accessory Cr-spinel grains. Cr-spinel grains exhibit modal abundance and texture-typology inversely correlated with the proportion of Co-Fe arsenide in both Co-Fe arsenide ores and their hosting alteration haloes in serpentinite. So, Cr-spinel grains exhibit distinct textures depending on their relation with the Fe-Co ore: (1) Type 1A zoned grains, consist of homogenous cores of Cr- and Fe2+-rich Cr-spinel surrounded by ferrian chromite rims, which are usually found in carbonatized and talc-rich serpentinite, and in disseminated Co-Fe arsenide ores. (2) Type 1B zoned grains, consist of porous cores of Cr-Fe hydroxides rimmed by ferrian chromite with rare irregular bands of Cr- and Fe2+-rich spinel. They are exclusively hosted by disseminated arsenide ores from F55 and F7/5 orebody. (3) Type 2 grains, consist of homogenous ferrian chromite hosted in löllingite and, at a lesser extent in calcite, in serpentinite hosted massive and disseminated arsenide ores. (4) Type 3 grains, correspond to partly dissolved ferrian chromite, and Cr-Fe hydroxide, associated to chlorite from disseminated Co-Fe arsenide ores.Type 1A zoned Cr-spinel was produced by an event of ocean-floor serpentinization that affected these mantle rocks during Neoproterozoic times. In contrast, the others three types of Cr-spinel crystals (Type 1B, Type 2 and Type 3) associated to Co-Fe arsenide ores from F/5 and F55 orebody, were produced during the Co-Fe arsenide ore-forming event(s). So, the infiltration of low temperature, highly oxidizing CaCl2-rich hydrothermal fluids through fractures dissolved partially pre-existing serpentinite, while precipitating Co-Fe arsenides ± calcite assemblage. The first sign of mineralizing fluid infiltration is recorded in the zoned Cr-spinel grains by the replacement of homogenous cores (in Type 1A) by Cr-Fe hydroxide, giving rise to zoned Cr-spinel with porous core (i.e., Type 1B). The increase of Co-Fe arsenides precipitation further promoted dissolution of ferrian chromite and Cr-Fe hydroxide (i.e., Type 3) erasing relicts of former grains, nevertheless some homogenous Cr-spinel grains (i.e., Type 2) persist included in calcite and löllingite in disseminated and massive arsenide ores hosted in serpentinite.