System type: Volcanic-hosted, low-sulfidation epithermal gold
Target: High-grade veins and disseminated mineralization
The Seven Devils Project is 55 miles south of Winnemucca, Nevada, and contains extensive volcanic-hosted, low-sulfidation epithermal gold mineralization, with surface values reaching 2.6 ppm (g/t) in preliminary sampling, as well as mineralization in underlying sedimentary rocks.
The gold system lies along a regional, north-trending structural zone which contains several important, low-sulfidation, volcanic-hosted gold deposits north of the project, including the Sleeper, Sandman and Goldbanks deposits. This structural zone, which Redstar terms the Western Nevada Rift, is parallel to and similar to the Northern Nevada Rift, which localizes a series of similar productive volcanic-hosted gold systems about 50 miles to the east, including the Mule Canyon, Fire Creek and Buckhorn gold deposits. All these gold systems represent a class of productive gold deposits in the Great Basin region containing both high-grade veins and low-grade disseminated mineralization of Middle Miocene age hosted in Middle Miocene and locally older volcanic rocks and pre-Cenozoic basement, the best example being the Midas district. Mineralized volcanic rocks at Seven Devils lie beneath basalt flows dated as Middle Miocene, and it is likely that the gold system is a member of this important class.
An area of gently-folded medium-bedded limestone of probable Mesozoic age occurs as a window through volcanic rocks in the central part of the project. The volcanics consists of basal volcanic conglomerate and sandstone, which are apparently laterally discontinuous, and a more extensive section of felsic ash-flow tuff, which also locally lies on the baesment carbonates. Basaltic breccia is common along the unconformity between the basement and the overlying volcanics and is interpreted as intrusive, forming sills and dikes. Mineralization occurs in all these units, which are overlain by an uppermost section of unaltered and umineralized basalt flows that has been dated at about 16 Ma.
Minor areas of silicification (jasperoid) occur in the limestone; the matrix of the volcanic conglomerate as well as clasts are ubiquitously silicified and pyritic; the felsic tuff is locally pervasively silicified and cut by chalcedonic crackle veinlets; and the intrusive basalt is largely argillically altered. Rock-chip sampling has returned gold to 2.65 ppm in the conglomerate, 1.44 ppm in the ash-flow tuff, 435 ppb in silicified sandstone, 636 ppb in argillized basalt and 896 ppb in silicified limestone. The low-sulfidation style gold mineralization is coincident with pervasive chalcedonic silicification, quartz veinlets, hydrothermal brecciation and widespread anomalies in As, Sb and Hg; Ag values are low. Selenium is locally anomalous in altered and mineralized basalt; Se is a trace element characteristic of productive low-sulfidation gold deposits. After Ag, the strongest correlation with gold is Mo, which reaches 411 ppm; this is similar to the high-grade Sleeper deposit. Fluorite is common with gold, both in the basement and volcanics; fluorite occurs in the high-grade veins at Midas. Gold mineralization occurs over a strike length of at least 6,200 feet, with strongly-anomalous trace elements covering 11,000 feet of strike. Structural controls to mineralization are not conspicuous, but evidence points to a north-northwest district-scale control. At the district scale, mineralization is concentrated near the base of the volcanic section.
Previous exploration at Seven Devils from 1988-1990 consisted of 29 shallow drillholes (average depth of 350). The program focused on sediment-hosted mineralization in the basement window. The significance of the volcanic-hosted mineralization wasn’t recognized until the end of the exploration program (final 4 holes), and such mineralization remains poorly explored. Although a number of mineralized intervals were encountered in the drilling, with gold values to 2.52 ppm, most assays were by atomic absorption (AA) without fire assay. The AA analysis may have underestimated grade where gold is associated with disseminated sulfides.
The gold system remains open in all directions, with large areas likely to be underlain by silicified and mineralized volcanics beyond the pre-Tertiary window untested by drilling. Mineralized volcanics are concealed by alluvium and post-mineralization basalt north and east of the window, mineralization is open to the south through a large alluvial basin, and mineralization is open to the west within the lower part of the felsic tuff. In addition to the known gold mineralization and the potential to expand it, the prospectivity of the project is enhanced by the probable Middle Miocene age for mineralization, geologic similarities to the nearby Goldbanks deposit, the location long the western Nevada rift trend and the elevated Se and Mo with gold. By comparison with other Middle Miocene low-sulfidation systems, the project has potential for high-grade veins in addition to the known disseminated mineralization.
It is recommended that the next exploration phase consist of additional detailed mapping to further evaluate the volcanic stratigraphy and structure. Additional rock-chip and possibly soil sampling is warranted to identify and delineate gold and trace-element anomalies. Third, geophysics, such as CSAMT, should be considered across the alluvial basin in an attempt to identify silicified zones and structures.