Application of Remote Sensing in Mineral Assessments of Arid Regions: Desert National Wildlife Range, Nevada

摘要

In 2014, the Nevada Bureau of Mines and Geology (NBMG) updated a mineral assessment of the Desert National Wildlife Range (DNWR). The DNWR is a vast, remote area, encompassing nearly 3,000 km~2 in Clark and Lincoln Counties. Given time and budget constraints, NBMG utilized Landsat ETM+ and ASTER satellite imagery to assist in identifying areas of hydrothermal alteration and guiding field work. Detailed analysis of the Landsat ETM+ and ASTER satellite imagery generated a number of band ratios, band indexes, and mineral likelihood maps highlighting five key mineral classes (silica, clay, limonite, dolomite and calcite) that are commonly spatially associated with hydrothermal ore deposits elsewhere in Nevada. When compiled with available geologic and geochemical data sets, these intermediate products provided guidance for targeted field evaluation. By combining field observations with spectral data collected using an ASD Terraspec and ASTER imagery, parameters for the Spectral Angle Mapper algorithm were defined, and distribution maps of the five mineral classes were generated. Results indicated no evidence for large hydrothermal alteration zones indicative of significant mineral potential that may have been missed in past assessments of the DNWR. More detailed analysis of Zn-Pb-Cu-Ag mines in the historic Gass Peak district, which had limited production, showed only spotty dolomitization rather than widespread, more-pervasive dolomitization characteristic of large MVT deposits. Further analysis, focused on detecting Carlin-style gold mineralization in carbonates along the Ordovician-Cambrian boundary, did not detect any significant limonite, silica, or clay, except for the Lake mine area. ASTER imagery allowed precise mapping of the limits of hydrothermal alteration in the Lake mine area and focused sampling of decalcified and silicified rocks from which analyses showed anomalous As, Sb, Ag, Cu, T1, Pb, Zn, and Mo, but yielded no Au assays higher than 44 ppb. The satellite imagery of the DNWR mainly mapped lithologies. For example, silica mineral maps showed nicely the distribution of chert and quartzite. The remote sensing study of the DNWR can be viewed as a pilot study for time- and cost-effective regional mapping of lithologies and hydrothermal alteration in large remote arid regions.