A STRUCTURAL AND GEOCHEMICAL CHARACTERIZATION OF EARLY ARTISANAL GOLD MINES IN THE CENTRAL AND SOUTHWESTERN RAND MOUNTAINS, MOJAVE DESERT, CALIFORNIA

The gold-silver-tungsten ore deposits of the Randsburg Mining District in the Rand Mountains have been heavily exploited and studied. However, mineralization in the poorly explored central and southwestern portions of the mountains has not been studied until now. The purpose of this study is to determine the extent of gold mineralization, its structural control, the geochemical changes from hydrothermal alterations, its relative timing, and its relationship to the ore deposits of the Randsburg District. Field mapping, sampling of ore structures, and generation of assay and XRF data has revealed several clear trends and relationships of the gold mineralization in the central and southwestern Rand Mountains.

Results indicate that gold mineralization is ubiquitous and not confined to the mining district. The highest concentration of gold found was 10 ppm in a prospect 9 km southwest of the Yellow Aster Pit. Concentrations as high as 2.15 ppm Au were found in the Desert Tortoise Wilderness 12km to the southwest. Targeted lithologies were almost exclusively Plate I Rand Schist and Plate III Atolia Granodiorite. The prospects in Plate IV were barren of gold.

Gold is structurally controlled by NE striking faults with moderate to steep dips. This differs from the Randsburg Mining District in which gold is concentrated in NW striking faults (Yellow Aster faulting episode of Morehouse (1986)). The observed northeasterly structural control may be due to clockwise rotation of Yellow Aster faults or entirely be generated by Morehouse’s younger 15 Ma Kelly faulting episode. The age of gold mineralization is narrowed down to two options: 1) 19 Ma during the Yellow Aster Intrusion, 2) 15 Ma during left lateral transtensional faulting accommodating movement along the Garlock Fault to the north.

XRF data indicates that gold correlates positively with altered rocks that record significant LOI, gain in CaO, and leaching of alkalis. It is also evident that within Plate III, gold is hosted in argillic alteration zones and is accompanied by small silver anomalies and moderate arsenic anomalies. Large arsenic anomalies are only observed in mineralized NW trending faults, with the exception of one NE striking fault. Lastly, gold anomalies in schist hosted faults are often associated with high chromium, nickel, and antimony anomalies.