Gary asks a big question: "What causes a specific element to concentrate like that? Is it all due to super heated water slowly cooling down?"
That's the topic of several semesters of Economic Geology!
Mineral deposits are the result of many different concentrating processes. Sediments may concentrate heavy minerals into placer deposits. Some deposits are related to their relative solubilities in thermal waters at different temps, pressures, acidity, etc. Another way that minerals are concentrated is during the crystallization of a magma. For example, some metals drop out very early in the cooling process and form nice layered deposits at the base of intrusions. (Chrome and Platinum). Rare-earth elements (and other large lithophile elements), on the other hand, don't crystallize into mineral structures until the very last gasp of a melt, which does contain alot of water. Pegmatites are an example, and are good sources of REE's.
This is a case of the late stages of an unusual magma. The vein I sampled was related to a alkaline intrusive / carbonatite complex in the Wet Mountains of Colorado. These igneous intrusions are chock full of rare-earth elements and strange lithologies. They are believed to originate in the mantle. There is a deposit of pure magnetite associated with this one as well. But mainly it is a small (a couple of square kilometers) intrusion of syenites (quartz poor granite) and gabbro (intrusive basalt). It also shot a lot of mafic veins and Thorium veins into the surrounding host rock. It was probably water rich, but was still mostly molten rock.
More info: Th-Rare Earth Element Vein Depositstext version or pdf:
Ore and gangue mineralogy and zonation
The major ore minerals in thorium-rare earth mineral veins are thorite, thorianite, brockite, monazite, and a group
of rare earth minerals including bastnaesite, parisite, synchysite, and similar rare earth carbonate minerals; sulfide
minerals may be present but are rare. Gangue minerals include quartz, especially smoky quartz, potassium feldspar,
barite, ferric iron oxide minerals, and trace amounts of galena and sphalerite. Many other minerals are found in
specific deposits. Carbonatite dikes contain similar rare earth carbonate minerals and thorium-bearing minerals.
Niobium-bearing minerals, especially pyrochlore and niobium-bearing rutile, may be abundant. Gangue minerals
include calcite, dolomite, ankerite, siderite, sometimes strontianite, and other carbonate minerals. Allanite, monazite,
magnetite, anatase, brookite, rutile, perovskite, barite, and a large number of additional minerals may be present.
Mineral characteristics
Thorium-rare earth mineral veins are chiefly quartz-barite veins with irregularly distributed thorium and rare earth
minerals. In some veins, quartz and barite are coarse grained; euhedral smoky quartz lines vein walls and
paragenetically later barite forms vein interiors. Other thorium-rich veins are fine grained; red, due to ferric iron
oxide content; and have odoriferous fresh surfaces. Ore minerals are irregularly disseminated throughout carbonatite
dikes.
Hope that helps,
jms