Geology and Mineralogy of the El Hammam REE-Rich Fluorite Deposit (Central Morocco): A Product of Transtensional Pangean Rifting and Central Atlantic Opening

Abstract

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.

Mineralogy and Genesis of the Polymetallic and Polyphased Low Grade Fe-Mn-Cu Ore of Jbel Rhals Deposit (Eastern High Atlas, Morocco)

“…Basalts are therefore regarded as one of the metal sources at Jbel Rhals, other sources being for instance the altered/weathered host rocks. to Triassic in age and to have been induced by the thermal heat flow and events triggered by the Permian-Triassic rifting of the Central Atlantic, just as suggested in other mineral deposits of North Africa [29,[74][75][76][77]. Among multiple outcomes, these hydrothermal processes may be responsible for the alteration of basalts, and for the formation of secondary phases such as chlorite and clays, siderite and sulfides.…”

Sulfide and Fluoride Mineralization of the NNE Region of Achemmach (Central Morocco): Paragenetic Sequences and Pyrrhotite-Sphalerite Geothermometry Constraints

“…NaCl) and fairly hot (200 • C to 250 • C), associated with calcite (L Ca ), and found in secondary expression in fluorite and quartz, is responsible for the deposition of late sulfide paragenesis (pyrrhotite II, pyrite II, chalcopyrite II, sphalerite II, hematite). Fluorite brines (S f ) and aqueous fluid (L Ca ) calcite in the sulfide mineralization of this sector have the same thermometric characteristics as those described in the fluorite fluids and paragenesis, with the calcite of late sulfides filling the veins of the El Hammam deposit [36,[65][66][67]. It was deduced that the deposit of sulfide and fluorite ore mineralization in the NNE-A sector is, therefore, the result of the succession of three mineralizing events (Figure 24): (a) an early, high-temperature hydrothermal event related to the placement of basic rocks in the NNE-A sector, with which the primary sulfide paragenesis is associated.…”

“…This fluid would have mixed, during its ascent to the surface, with a fluid of marine origin percolating through the faults and fractures that affected the oceanic crust at depth. This mixture, materialized by the L2 and V fluids of lower temperature and lower salinity, would be at the origin of the precipitation of the metallic charge of these fluids and thus the deposition of the primary sulfide paragenesis around 300 to 450 • C; (ii) a low-temperature hypersaline fluid (brine), specific to fluorite, whose characteristics are identical to those of brines in the fluorite of the El Hammam vein deposit [34,65,66] and (iii) a fluid of medium salinity and moderate temperature L Ca , characterizing calcite and whose traces are found, in secondary inclusions, in fluorite (L f ) and in quartz, thus confirming the posteriority of calcite compared to the other minerals (quartz and fluorite). Fluorite Lf and calcite LCa fluids are associated with the late sulfide paragenesis that also occurs in the fluorite mineralization of the El Hammam district deposits.…”

Plombotectonique des gisements du Maroc

“…La minéralisation en fluorite se compose d'un ensemble de filons puissants de fluorite-calcite exploités par SAMINE depuis les années 1960 et produisant environ 100 kt/an de concentrés à 98 % de fluorite. Le gisement a longtemps été attribué à un événement hydrothermal lié à des granites tardi-hercyniens, mais a récemment été daté par K/Ar à 205 ± 1 Ma, ce qui le relie au développement des bassins contemporains du Trias et du Jurassique lors de l'ouverture de l'Atlantique (Cheilletz et al, 2010 ;Bouabdellah et al, 2016c). Pour d'autres, cette datation apparaît comme une réinitialisation du système lors du soulèvement de la Meseta (Ghorbal et al, 2008 ;Barbero et al, 2011).…”

“…En comparant aux données existantes sur les grands gisements du sud de l'Europe (Arribas et Tosdal, 1994), on remarque que les signatures isotopiques d'Aouli-Zeïda sont semblables à celles d'un groupe de gisements à zinc-plomb-fluorite-(barytine), majoritairement stratiformes, encaissés dans le Trias, tels que Gador (Espagne), Les Malines (France), Bleiberg et Lafatsch (Autriche), Salafossa, Gorno et Raible (Italie). Les minéralisations situées près de la côte méditerranéenne sont les plus jeunes, un fait à relier avec la crise de salinité messinienne et la circulation d'eaux de mer évoluées qui ont favorisé la mobilisation de saumures anciennes stockées dans le socle paléozoïque (Bouabdellah et al, 2016a(Bouabdellah et al, , 2016b(Bouabdellah et al, , 2016c. Les isotopes du plomb reflètent donc la mobilisation progressive des saumures vers le socle crustal marocain.…”