CRITICAL ELEMENTS IN ALKALINE IGNEOUS ROCK-RELATED EPITHERMAL GOLD DEPOSITS

Alkaline igneous rock-related gold deposits, primarily of Mesozoic to Neogene age, are among the largest epithermal gold deposits in the world. These deposits are spatially and genetically linked to small intrusions with high alkali-element contents. Important critical-, near-critical, or energy critical elements associated with these deposits include F, REE, Te, and W. Fluorine and tungsten have historically been recovered from some of these deposits. Several hundred thousand metric tons (t) of fluorspar were produced from the Jamestown district in Boulder County, Colorado, where it occurs in breccia bodies and Au-Te veins in and adjacent to the Jamestown monzonite stock. Some alkalic epithermal gold deposits contain scheelite, ferberite, or wolframite, and small tungsten orebodies adjacent to and/or overlapping the belt of deposits in Boulder County were mined historically; however the genetic relations between the tungsten mineralization and Au-Te concentrations are unclear. Deposits in the Ortiz Mountains of New Mexico with micron-sized gold also contain scheelite but recorded tungsten production from these deposits is undocumented. Some alkaline intrusive complexes that contain anomalous gold concentrations also host REE occurrences. These include deposits in the Bear Lodge Mountains in Wyoming and Cu-REE-F (+/- Ag, Au) vein deposits in the Gallinas Mountains in New Mexico, which contain as much as 5.6 % REE. Tellurium is the critical element most commonly associated with alkaline-related gold deposits. Many of these deposits contain >0.5% Te (e.g., Cripple Creek, Colorado) and could be considered a future source as global demand increases and if processing Au-Te ores becomes more efficient. Precious metal tellurides are the most common minerals in these deposits, although native Te and tetradymite (Bi₂Te₂S) have also been reported in some. Assuming that the Dashigou and adjacent Majiagou deposits in Sichuan province, China, are truly alkalic-related epithermal gold deposits, they represent the world’s sole source of primary Te, as tetradymite. Processes that aid transport, concentration, and precipitation of Te in association with epithermal mineral deposit formation remain unclear, although redox and pH likely influence Te solubility.