Many early reports on the occurrence of bastnaesite indicate that the mineral crystallized under a high-temperature environment. The Gallinas Mountain (New Mexico) bastnaesite deposits, however, occur as epithermal veins and breccia fillings in unaltered Permian sandstone in proximity to early Tertiary alkalic hypabyssal intrusives. The low temperature of formation is indicated by the predominance of open-space fillings, little replacement, extensive crustification, and lack of wall rock alteration. In addition, a geothermometric fluid inclusion study indicated a temperature of formation of 175-185 degrees C.This re-examination of a previously-known deposit and a study of other bastnaesite occurrences shows that the mineral is generally indicative of mineralization of alkalic derivation. Bastnaesite, however, is not a critical mineral diagnostic of any particular geological environment or temperature of formation. Bastnaesite occurs in magmatic, hydrothermal, contact metasomatic, and carbonatite deposits. It is both a primary and secondary mineral.