Interaction between serpentinites and oreforming fluids in the Bou Azzer mining districtclues to understand the genesis of Co-Ni ores

Resum

Discovered for the first time randomly by a French geologist in 1929, The Bou Azzer district (Central Anti-Atlas, Morocco) become the only world producer of Co from primary, hydrothermal Co arsenide ores. Co-Ni -Fe orebodies in the Bou Azzer occurs in strong association with serpentinite (cryogenian time). They occur as (1) contact-type ore mainly distributed along the contact between serpentinite and adjacent cryogenian or ediacaran rocks (quartz diorite, gabbro, volcano-sedimentary rocks); (2) crosscutting ores consisting of N-S to NE-SW veins, cross-cutting the different lithologies (serpentinite, quartz diorite, gabbro and younger volcanic rocks), or (3) serpentinite-hosted ores consisting of flat lenses striking N120E. Ore establishment in contact-type and serpentinite-hosted type was accompanied by the development of an alteration halo. Multiple stages of ore formation were established including a pre-ore stage characterized by the presence of gold mainly associated to chlorite and quartz, a main arsenide stage and a late, post-ore, epithermal stage characterized by the precipitation sulfides associated to quartz and calcite. The main arsenide stage can be subdivided into three substages: (1) Ni-rich, Co ores (i.e., stage IIa) made up of nickeline, rammelsbergite, Ni-rich skutterudite, members of the rammelsbergite-safflorite ¿ löllingite solid solution series and sulfarsenides (gersdorffite and minor cobaltite); (2) Co-Fe ores (i.e., stage IIb) consisting of Co-rich skutterudite, Co-Ni-Fe diarsenides deeply evolving from Co¿ and Ni-rich compositions to the löllingite corner of the rammelsbergite-safflorite-löllingite ternary system, members of the safflorite-löllingite solid solution series and cobaltite; and (3) Fe-Co ores (i.e., stage IIc) composed of a third generation of skutterudite, safflorite, löllingite, cobaltite, alloclasite, arsenopyrite and molybdenite. The ore assemblagre of this late substage further characterize the serpentinite-hosted ores (e.g., F56) as well as the distal, disseminated ores of some contact-type deposits (e.g., F7/5, Aghbar). Nevertheless, the ore assemblage of the studied serpentinite-hosted deposit (F55 orebody) is quite simple since it only consists of of löllingite with minor Ni and Co. Cr-spinel occurs as a minor mineral associated to Fe-Co arsenides. Four textural relationships of Cr-spinel were recorded in the studied serpentinite-hosted ores: (1) zoned Cr-spinel consisting of a homogenous core rich in Cr- and Fe2+ surrounded by ferrian chromite rims (i.e., Type 1A), which are usually found in carbonatized and talc-rich serpeninite and disseminated arsenide ore; (2) zoned Cr-spinel with porous cores of Cr-Fe hydroxides rimmed by ferrian chromite (i.e., Type 1B) identified in disseminated arsenide ores; (3) homogenous Cr-spinel (i.e., Type 2) corresponding to ferrian chromite hosted in löllingite and, at a lesser extent in calcite, from massive and disseminated arsenide ores, and partly dissolved Cr-spinel (i.e., Type 3), mainly consisting of ferrian chromite, commonly with Cr-Fe hydroxide, associated to chlorite, identified in highly carbonated serpentinites hosting disseminated arsenide ores. Co-Ni-Fe arsenide ores post-date the regional Neoproterozoic rocks from the Bou Azzer inlier. This fact was justified by field relations and geochronological dating. Pb, S, Rb/Sr, Sm/Nd isotopes analysis of arsenide and sulfarsenide minerals from five ore deposits (Filon 7/5, Aghbar, Tamdrost, Ightem and Aït- Ahmane) and some whole-rock regional samples show that ores formed during multi-episodic hydrothermal events connected with Hercynian reactivation of Devonian-Carboniferous faults. These isotope data indicate that serpentine-related ophiolite acted as the main source of ore-forming elements, coupled with a number of isotopically different lithologies both from the inferred basement and the volcanic and sedimentary cover. High-temperature ore-forming fluids infiltrated serpentinite from where they leached Co, Ni and Fe. These high temperature (>400 °C) slightly alkaline, oxidized and CaCl2-rich fluids were channelized through faults separating serpentinite and quartz diorite (and locally gabbro and volcano-sedimentary rocks), where they deposited Ni- and Ni-Co-rich ores in these fault-related open spaces (contact-type ores). During the deposition of minerals, fluids became depleted in Ni, and infiltrated serpentinite (far away from the contact serpentinite/adjacent rocks) progressively by intergranular percolation, promoting dissolution of the infiltrated serpentinite and precipitation of Co-Fe ores. Further circulation of Ni-poor forming fluids through intraserpentinite weak fault zones tended to form serpentinite-hosted ores. These fluids evolved at low temperature (¿200ºC), increasing fO2 (CO2 and CO32- increased over HCO3-) and producing dissolution of serpentinite coupled with precipitation of Co-Fe arsenide ores. Homogenous Cr-Fe2+-rich cores from relict Cr-spinels (i.e., Type 1A) associated to disseminated arsenide ores were altered to Cr-Fe hydroxide giving rise to zoned Cr-spinel (Type 1B) under the physico-chemical conditions prevailing during precipitation of these low-temperature, serpentinite-hosted arsenide ores. On the course of Co-Fe arsenide precipitation both ferrian chromite and Cr-Fe hydroxide (i.e., Type 3) tended to be destabilized promoting their dissolution. Only homogenous ferrian chromite grains (i.e., Type 2) included in calcite and löllingite in massive arsenide ores (and at lesser extent in disseminated ores) survived this process.