Ammonium illite at the Jerritt Canyon district and Goldstrike property, Nevada: Its spatial distribution and significance in the exploration of Carlin-type deposits

Ammoniated hydrothermal fluids interact with potassium-bearing minerals in wall rock material to form ammonium illite alteration patterns along fluid conduits associated with Carlin-type gold deposits. These same fluids transport gold which is disseminated into favorable host rock. Thus, ammonium illite serves as an exploration tool for locating Carlin-type gold deposits.;Ammonium illite alteration patterns at the Waterpipe Canyon area (Jerritt Canyon district) and a similar alteration pattern at the North Screamer deposit (on the Goldstrike property) were mapped and characterized using a combination of remote sensing, GIS, short-wave infrared spectroscopy, and 3-D modeling. Four classes of illite (sericitic, phengitic, potassic, and mixed illitic clays are identified based on the precise wavelength position of the main AlOH spectral absorption feature near 2200 nm. Ammonium occurs primarily in sericitic illite and phengitic illite. Phengitic illite, in particular, is considered a useful indicator of hydrothermal activity and can be a useful guide for gold exploration. On the surface, ammonium illite forms an alteration pattern that mimics hematite alteration in proximity to structure. Both minerals are also associated with anomalous gold thus, providing evidence that ammoniated fluids migrated along the same fluid conduits that deposited gold. The pathways in which fluids migrated are mapped from drill-hole and surface samples. Associated alteration was traced from the deposit to the surface by modeling the distribution of ammonium illite in wall rock material along mapped faults. Three dimensional modeling establishes that ammonium illite forms near gold mineralization and (as alteration halos) around fluid conduits on surface.;Hyperspectral imagery was classified using ENVI hyperspectral image-analysis software to identify individual pixels that most closely match the spectral character of ammonium illite. MNF transformation was applied to the image and a false color composite image was created. The spatial distribution of the classified ammonium illite spectra and the false color composite image correlate well with the ammonium illite alteration map produced from the field spectroscopic data; thus verifying that remote sensing can be used successfully to locate ammonium illite along fluid conduits associated with Carlin-type deposits. When these methods are used in conjunction with field mapping, the ammonium illite maps accentuate mineralized structures exposed at the surface. In these areas, the ammonium illite concentrations are typically in the area of NE- and NW-trending fault intersections.;The combination of hyperspectral analyses and shortwave infrared reflectance spectroscopy provides a cost effective approach for mapping ammonium illite alteration patterns. Fluid conduits that are likely to host mineralization can be identified and evaluated as targets for the exploration of Carlin-type deposits.