The El Hammam REE-rich fluorite deposit in central Morocco constitutes the largest fluorite deposit in North Africa with an annual production of 110,000 t of fluorite concentrate at 98 % purity. Enclosing host rocks consist of a Silurian to Namurian succession of multiply folded and metamorphosed sedimentary and volcaniclastic rocks locally intruded by late Hercynian granitic stocks. A dike swarm of poorly dated, mostly ENE-striking, steeply south-dipping microgranite and tholeiitic dolerite-diabase intrude all of the supracrustal rocks. Structurally, the mineralized area is dissected by a NE-trending, prominent, crustal-scale strike-slip shear corridor locally known as the El Hammam shear zone along which most of the exploited orebodies occur. Mineralized structures consist predominantly of an array of three major, multi-kilometer-long, transtensional, mostly ENE-trending, subparallel, and steeply dipping veins referred to as the Moufrès-Bergamou, Tlatezma-Achemache, and Mizourza vein systems. The mineralogy of the veins is simple and consists of diversely colored fluorite, accompanied by variable amounts of calcite, quartz, and sulphides ± siderite ± adularia. Texturally, the veins display comb, cockade, laminated, breccia, and crack and seal textures suggesting that episodic, multiple, crack-seal processes were important during vein formation. Irrespective of location, paragenesis, and textural position within the mineralized vein structure, green fluorite is characterized by high total REY contents ranging on average from *249-995 ppm, light REE enrichment, and distinctive positive Y anomalies with or without positive Eu anomalies. Associated pre-ore hydrothermal calcite is strongly REY enriched (average ΣREY = 1164-3192 ppm) and only has a distinctively positive Eu anomaly without a Y anomaly. Preliminary fluid inclusion data indicate that the ore-forming fluids correspond to evolved basin-derived, hot (92-176°C), very saline (up to 33 wt% NaCl equiv), NaCl-CaCl 2 -KCl-MgCl 2 ± BaCl 2 brines. Overall, REY compositions together with fluid inclusion constraints suggest a key role for hot basin-derived brines and fluid-rock interaction in the genesis of El Hammam REE-rich fluorite deposit. Formation of this deposit is probably related to Pangean rifting and subsequent Central Atlantic opening during Permian-Triassic time.
