A PETROGRAPHIC AND GEOCHRONOLOGIC STUDY OF GOLD MINERALIZATION AT THE LONE TREE DEPOSIT, NEVADA

Lone Tree is a gold mine in the Battle Mountain Mining District, Nevada which was operated by Newmont Mining Corporation from 1997 to 2006. Gold at Lone Tree is structurally controlled in steep faults and disseminated in the sedimentary host rocks. The sedimentary host rock package is crosscut by steep to shallow, east-trending, rhyolitic to felsic porphyritic dikes. Despite years of economic production, the characteristics and genesis of gold mineralization at Lone Tree are poorly constrained. The goal of this study is to determine the nature of the ore and alteration styles at Lone Tree, as well as the ages of the dikes, in an effort to develop a genetic model for the deposit. Several aspects of the deposit have been investigated using petrography, microprobe analysis, and LA-ICPMS. Gold mineralization occurs in microscopic arsenical sulfide rims around pre-ore pyrite cores. These rims show enhanced silver, antimony, copper, thallium, and arsenic relative to the cores, providing a geochemical signature for the gold event. Preliminary U-Pb age estimates have been calculated for zircons from five dikes from LA-ICPMS data. The dikes contain Cretaceous and Eocene zircons. Two age populations of Eocene zircons have been identified, with the older Eocene zircons yielding ages of 42.4 ± 0.8 Ma, 42.3 ± 0.8 Ma, and 45.2 ± 1.0 Ma, and the younger group dated at 39.9 ± 0.5 Ma, 40.0 ± 1.1 Ma, 40.5 ± 0.7 Ma, and 40.6 ± 0.6 Ma. These dates will be followed up using CA-TIMS geochronology. Additionally, geochemical transects across the dikes and into the host rocks will be accomplished to test the relationship between the gold event and the Eocene dikes. Thus far, the results suggest that the deposit has characteristics which could be indicative of either a distal disseminated Au-Ag deposit – such as the spatial association with base metal mineralization and igneous intrusions, or a Carlin-type deposit – such as the geochemical signature and mineralization style. This overlap of characteristics may be due to multiple episodes of mineralization. Alternatively, Lone Tree may represent a middle point along a continuum between Carlin-type and distal-disseminated gold deposits.