DIFFERENTIATING HYDROTHERMAL FLUIDS RESPONSIBLE FOR GOLD MINERALIZATION USING MONAZITE AND XENOTIME GEOCHEMISTRY: IMPLICATIONS FOR THE GENETIC CLASSIFICATION OF THE POGO GOLD DEPOSIT, ALASKA

Orogenic gold deposits form from metamorphic-hydrothermal fluids, whereas reduced intrusion-related gold (and porphyry Cu-Mo-Au deposits) form from magmatic-hydrothermal fluids. Although these deposits involve fundamentally different ore-forming fluids, orogenic gold and reduced intrusion-related systems can have very similar geologic and conventional geochemical signatures. There are currently no geochemical discriminators for these two different deposit types and this can lead to inappropriate application of genetic models to many controversially classified gold deposits, such as the Pogo deposit, Alaska. Potential misclassification can have adverse effects on mineral exploration and mine planning.

In this study, electron microprobe analyses of hydrothermal monazite and xenotime from well-characterized deposits were evaluated to look for geochemical signatures that may distinguish ore-related phosphates precipitated in orogenic gold, reduced intrusion-related gold, and porphyry Cu-Mo-Au deposits. Our test set included samples from the Butte and Pebble porphyry Cu-Mo-Au, the Shotgun and Clear Creek reduced intrusion-related gold, and orogenic gold vein samples from the Sierra Nevada foothills and Klamath Mountains gold provinces in California. Data within deposit types are remarkably consistent, but differences between deposit types are observed. Important differences in monazite geochemistry between deposit types include REE profiles, concentrations of Dy, Er, Pr, Y, and Nd/Sm, and La/Sm. Overall xenotime relative abundances in the deposit and concentrations of HREEs, Ca, and Sc are also distinctive. Phosphate chemistry of the Pogo gold deposit – a deposit classified by some as intrusion-related gold and others as orogenic - is consistent with a metamorphic-hydrothermal fluid (orogenic) origin.

Our initial work indicates that REE-phosphate chemistry can be used to differentiate geochemically similar deposit types formed by different hydrothermal processes – magmatic vs metamorphic fluids. If consistent, such information would be critical for guiding exploration and mine development in the Pogo area and beyond – guiding the exploration focus on either preferential structures for orogenic gold or permissible intrusions for magmatic-hydrothermal deposits.