The Role of Geophysical Data in the Evaluation of the Talapoosa-Appaloosa Epithermal Au-Ag Project, Lyon County, Nevada

摘要

The Talapoosa-Appaloosa Project comprises a 5-mile by 1.5-mile, north-northwest trending, low-sulfidation, epithermal Au-Ag district, located 30 miles east of Reno, Nevada. Talapoosa was discovered in 1863, and historic mining reportedly produced less than 5,000 ounces of gold. Exploration since the 1970s has focused on the Talapoosa mineralization, where over 600 holes have been drilled. Since Gunpoint Exploration Ltd. acquired the property in 2010, additional drilling and re-logging of historic drill core has resulted in reinterpretation of the controls on mineralization and a 40 percent increase in the size of the Talapoosa resource. The Appaloosa Structure, approximately one mile north of Talapoosa, is a well-preserved, high-level, epithermal system that remains underexplored. Acquisition of high-resolution ground magnetic and IP-Resistivity data was undertaken in 2010 and 2011. Evaluation of these data along with historic IP-Resistivity data has helped identify structural controls on mineralization, magnetite destructive alteration, and feeders to epithermal mineralization. Evaluation of magnetic data in a volcanic environment is complicated because the response often includes both remanant and induced components of magnetization. Interpretation can be misleading using conventional imaging, filtering, and modeling techniques. Recently developed Magnetic Vector Inversion (MVI) modeling can effectively identify the location, geometry, and source depths in a complex magnetization environment allowing more accurate interpretation of source position and geometry. This approach has proved effective in defining magnetite destructive alteration over broad areas and associated feeders to epithermal mineralization. Recognition of feeders to epithermal mineralization below post mineral cover or alteration caps, based on interpretation of silicification and sulfide distribution from IP-Resistivity data is a challenge. The feeder is commonly narrow compared to the exploration depth of interest. Large acquisition dipoles required for useful exploration depths cause dilution of any feeder response with interference from the response of adjacent rocks unrelated to mineralization. The data is commonly presented as 2D inversion sections and 3D inversion solids. The inversion models identify coincident IP and resistivity highs defining the footwall and northwest limit to the Talapoosa epithermal system, as well as delineating what are now interpreted as the feeder structures to the mineralization. The geological evidence along the Appaloosa structure suggests that as little as 50 feet of the original system has been eroded from mapped hydrothermal explosion breccias, thus the productive level of epithermal mineralization is interpreted to be at least 300 feet below the current surface. Mineralized clasts of chalcedony, brought up from the underlying feeder structures during the eruptive formation of these vents, assay as high as 0.7 g/t Au. 3D-inversion of IP-Resistivity data shows steeply dipping high chargeability and high resistivity features interpreted to be feeders beneath and continuing along strike of the outcropping hydrothermal explosion breccias (HEB's) and beneath post-mineral cover on the property.