TOURMALINE AS AN INDICATOR OF DYNAMIC FLUIDS ASSOCIATED WITH ORE DEPOSITS: EXAMPLES FROM WESTERN NEVADA

Tourmaline (tur) preserves evidence of the dynamic and evolving fluid systems associated with three types of ore deposits in the Humboldt Range of Nevada, based on new mineral chemical data. Lincoln Hill, a gold/silver deposit, is developed in a metasomatically altered rhyolite that locally contains abundant quartz, dumortierite and tur. Here, tur exhibits oscillatory growth zoning, but is generally foititic in composition. Foititic tur implies formation in a low Na, Ca aqueous fluid. Fibrous dumortierite growth post-dates tur and is associated with cross-cutting fractures suggesting a later B-rich fluid influx. Buena Vista Hills, an iron deposit, is associated with albitized and scapolitized gabbroic and volcanic rocks. A tur-rich sample with late quartz veins exhibits two-stage development of tur: larger brown-green compositionally heterogeneous prismatic grains (low-Al dravite/oxy-dravite) with late blue caps on earlier-formed tur or blue fibers (foitite). The contrasting compositions of these tur imply early tur growth in an oxidizing, saline fluid followed by later fracturing with quartz veining and influx of low Na, Ca aqueous fluids. Majuba Hill, a copper/tin deposit, is associated with a tourmalinized Tertiary rhyolitic porphyry intrusion. The tourmalinized sample preserves igneous quartz phenocrysts with some feldspars that are extensively replaced by tur. Tur commonly nucleates in fan-like radial clusters and individual crystals display both oscillatory and sector zoning. The interior of the zoned tur crystals is foitite and the external zones are schorls of variable composition. These compositions suggest that invasive B-bearing aqueous fluids evolved from a low-Na fluid to one that was progressively Na-enriched. A common feature in all of these locations is the tourmalinization of the deposits, and this requires an early metasomatizing fluid that is B-rich. Because tur is typically stabilized under acidic conditions, it is likely that tur was precipitated from B-rich fluids upon acidification in this environment. Chemical compositions of tourmalines associated with ore deposits provide an additional constraint for understanding a sample’s formation history, evolution and fluid interactions that may be unique to ore formation.