Résumé
The Achmmach tin deposit, located in the Moroccan Central Massif, is supposed to be related to late Hercynian granite because of systematic relationships with metasomatic tourmaline alteration halos. The main structures are (1) tectonic breccia within tourmaline-rich rocks, and (2) hydrothermal breccia containing the cassiterite-rich mineralization. Both are overprinted within the main structures during the late orogenic extensional setting. Relationships analysis between orebody geometry, internal filling and texture have been carried out with the aim of understanding ore formation process. Three steps have been defined: The Premineralized step is dominated by tourmalines. The firsts (metasomatic) exhibit replacement textures. The seconds constitute with rare quartz, the matrix of the primary breccia. Variable-sized clasts, angular shape and rotated fragments argue in favor of tectonic breccia. The presence of intermediary structures between dextral shear bands and breccia, are indicators of a transpressional context developed at the limit of ductile to brittle domains. The Main Mineralized step comprises two tin stages. The first is defined by quartz, arsenopyrite and cassiterite assemblage forming the matrix of the secondary breccia; the abundance of matrix and the puzzle-like geometry attest for an hydrothermal origin. The second one shows the main mineralogy but essentially expressed as filling of channeling veins. In both cases, quartz exhibits many habits as epitaxial, geodic, euhedral and comb texture with overgrowth, outlined by fluid/solid inclusions. All suggest that crystallization occurs in open-space in an extensional context. The occurrence of such filling within extensional pull-apart structures controlled by normal faulting confirms such interpretation. The Postmineralized step consists of fluorine and carbonates that lately fill veins. Banded, geodic and collomorph textures are the main expressed occurrences, consistent with open-space filling processes. The internal texture of mineralized structures herein analyzed reflects that orebody formation process integrates intimately linked tectonic and hydrothermal activity. Open-space textures demonstrate that fluids circulation and ore deposition at Achmmach tin deposit were realized in an extensional context.