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198 Handbook on the Physics and Chemistry of Rare Earths eg, >11 wt.% REO at Mineville, United States. The Pea Ridge iron deposit in Missouri, United States, contains an unexploited high-grade REE resource in breccia pipes associated with 1.5 Ga granite and syenite. Bulk samples of REE-rich breccia, which contains monazite and xenotime in addition to apa- tite, contain approximately 12% REO which may become a good REE resource (Castor, 2008b). 4.1.4 Hydrothermal Vein Deposits Rare earth bearing hydrothermal phosphate veins have been found in several places in the world. They are Steenkampskraal in South Africa, Hoidas Lake in Canada, Nolans in Australia, and Mushgai Khudag in Mongolia. Among these, the Steenkampskraal deposit once produced REEs as by-product of tho- rium from monazite (total production was 50,000 tons of monazite concen- trate averaging 45 wt.% REO; Andreoli et al., 1994). These deposits are composed of phosphate vein which mainly contain apatite, except for the monazite-dominated veins at Steenkampskraal. The apatite of these deposits generally contain up to 3 wt.% REO. Thus, the REO grades of the ores are generally 1–3 wt.%, and REE are mostly included in apatite and associated monazite. The sources of hydrothermal fluids for these veins have not been well studied yet, except the Steenkampskraal and Hoidas Lake deposits, which are described later. The Steenkampskraal deposit in western South Africa is composed of a monazite–apatite vein hosted by 1.2–1.1 Ga quartz diorite and tonalite of the Roodewal suite in the metamorphic rocks of granulite facies. The vein is about 290 m long and up to 4 m wide. It contains zircon, monazite, apatite, and sulfides (pyrite, chalcopyrite, and galena) in addition to magnetite, ilmen- ite, and quartz. Those minerals are completely recrystallized and/or sheared by the subsequent metamorphic and deformation events (Andreoli et al., 1994). Andreoli et al. (1994) proposed a model that the monazite–apatite vein at Steenkampskraal formed by the fluid exsolved from the host igneous rocks when they crystallized. The Hoidas Lake deposit in northern Saskatchewan, Canada, is a structur- ally controlled vein deposit with allanite-(Ce) and fluorapatite as the main REE carriers. The mineralized veins cut Archean and Paleoproterozoic gneiss and include hyalophane-bearing pegmatitic dikes, REE-mineralized veins with allanite, diopside, hornblende, hyalophane, and titanite, and late breccia veins that contain several stages of apatite. Detailed fluid inclusion study indicates that the Hoidas Lake mineralization involved two fluid types with early car- bonic fluid followed by a mixed Na-Ca-K-rich saline aqueous fluid, the latter of which is attributed to exsolution from an evolving alkaline melt. The temper- ature of the mineralized fluids is estimated to be 310°C. The similarity of the fluids with above characteristics to those of the fluids related to carbonatitic or alkaline magmas suggests that the Hoidas Lake veins are distal pegmatitic part of the carbonatitic or alkaline magmatic system (Pandur et al., 2014, 2015).PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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