Rare earth element lithogeochemistry of granitoid mineral deposits

CIM Bulletin, Vol. 76, No. 860, 1983

P.P. TAYLOR and B.J. FRYER, department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland

As a monitor of the processes involved in the formation of granitoid mineral deposits the coherent group behaviour of the rare earth elements (REE) actively reflects changing fluid characteristics. For example, in the porphyry environment, magmatic-hydrothermal fluids produce potassic alteration with strong enrichment in the light rare earth elements (LREE), reflecting their high pH, low fluid/rock ratios, and the dominant role of Cl- complexing in metal (i.e. Cu, Au) transport. With increasing fluid/rock ratios and decreasing pH accompanying the progressive involvement of meteoric fluids (and the production of propylitic, argillic, and phyllic alteration) anionic species such as F- and CO32- become important in metal (e.g. Mo, W,) transport through complexing, and their activity in the hydrothermal fluids is illustrated by mobilization of the heavy rare earth elements (HREE). The relative involvement of Cl- versus F- versus CO32- in metal transport in other granite-related systems can also be monitored through REE behaviour. Hence granitoid tin-tungsten mineralization and associated greisenization typically exhibit heavy rare earth enrichment and evidence the importance of F- and CO32- in metal transport. Similarly, heavy rare earth element enrichment in hydrothermal uranium deposits can be related to the transport of uranium as carbonate complexes. REE are widely accepted as powerful tools in the study of rock petrogenesis, but their use has been neglected in the investigation of mineral deposits. The recognition of the systematic variation of REE distributions in granitoid mineral deposits suggests that the application of REE geochemistry, particularly when integrated with fluid inclusion and isotope studies, can provide (1) an effective method for identifying the physicochemical controls of metal transport and (2) a useful criterion for elucidating metal distribution.

Keywords: Rare earth elements, Geochemistry, Mineral deposits, Porphyry deposits, Granophile deposits, Peralkaline granites, Fluid inclusions, Isotope studies, Ore genesis, Mineral exploration