Origin of a fluorite deposit

As a first case study, we borrow from the modeling work of Rowan (1991), who considered the origin of fluorite (CaF2) veins in the Albigeois district of the southwest Massif Central, France. Production and reserves for the district as a whole total about 7 million metric tons, making it comparable to the more famous deposits of southern Illinois and western Kentucky, USA. 

Like other fluorite deposits, the Albigeois ores are notable for their high grade. In veins of the Le Bure deposit, for example, fluorite comprises 90% of the ore volume (Deloule, 1982). Accessory minerals include quartz (SiCh), siderite (FeCCL), chalcopyrite (CuFeSg), and small amounts of arsenopyrite (AsFeS). The deposits occur in a tectonically complex terrain dominated by metamorphic, plutonic, and volcanic rocks and sediments. 

To model the process of ore formation in the district, we first consider the effects of simply cooling the ore fluid. We begin by developing a model of the 

We further specify equilibrium with kaolinite [Al2Si205(0H)4], which occurs in at least some of the veins as well as in the altered wall rock. Since we know the fluid s potassium content (Table 22.1), assuming equilibrium with kaolinite fixes pH according to the reaction, 

By this reaction, we can expect the modeled fluid to be rather acidic, since it is rich in potassium. We could have chosen to fix pH by equilibrium with the siderite, which also occurs in the veins. It is not clear, however, that the siderite was deposited during the same paragenetic stages as the fluorite. It is difficult on chemical grounds, furthermore, to reconcile coexistence of the calcium-rich ore fluid and siderite with the absence of calcite (CaCOs ) in the district. In any event, assuming equilibrium with kaolinite leads to a fluid rich in fluorine and, hence, to an attractive mechanism for forming fluorite ore. 

To model the process of ore formation in the district, we first consider the effects of simply cooling the ore fluid. We begin by developing a model of the fluid at 175°C, the point at which we assume that it begins to precipitate ore. To constrain the model, we use the data in Table 16.1 and assume that the fluid was in equilibrium with minerals in and near the vein. Specifically, we assume that the fluid s silica and aluminum contents were controlled by equilibrium with the quartz (SiO2) and muscovite [KAl3Si3Oi0(OH)2] in the wall rocks, and that equilibrium with fluorite set the fluid s fluorine concentration. 

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Floatability of bastnaesite found in barite-fluorite ores is extremely poor using either fatty acid flotation or sodium oleate. Research work conducted on an ore from Central Asia showed that the floatability of bastnaesite improved significantly after barite preflotation [5]. The flotation of bastnaesite from a carbonatite ore improved with the use of oleic acid modified with phosphate ester. The flotation of bastnaesite from deposits ofpegmatitic origin can be successfully accomplished with several types of collectors, including tall oil modified with secondary amine, and tall oil modified with petroleum sulphonate-encompassing group. 

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