GIS-based identification of areas that have resource potential for lode gold in Alaska

Abstract

Several comprehensive, data-driven geographic information system (GIS) analyses were conducted to assess prospectivity for lode gold in Alaska. These analyses use available geospatial datasets of lithologic, geochemical, mineral occurrence, and geophysical data to build models for recognizing different types of gold deposits within physiographic units defined by stream drainage basins that are approximately 100 square kilometers in area. The analytical methods successfully delineated areas in the State that contain known lode gold deposits and occurrences, providing some measure of confidence in their ability to predict gold prospectivity in areas of unknown lode gold potential. The results of our analyses indicate high prospectivity in a few areas scattered around the State that are not known to contain lode gold deposits. In addition to assessing the potential for lode gold deposits in Alaska, we designed analyses to distinguish different lode gold deposit types, including orogenic, reduced-intrusion-related, epithermal, and gold-bearing porphyry. These can primarily be differentiated using their unique trace element geochemical fingerprints and elemental enrichments, which reflect the characteristics of the geologic environment and chemistry of the ore-forming fluids. We identified multiple parameters that would discriminate the different types of gold deposits, but owing to the limits of available data, the compositional similarity of ore-forming fluids among some types of lode gold deposits, and overlapping geologic environments, distinguishing deposit types at the state scale in Alaska remains problematic. These limitations resulted in overlapping areas of prospectivity for different deposit types, highlighting the challenges for targeted gold exploration in Alaska. Adjustment of some scoring parameters and recharacterization at smaller scales to highlight individual mineral systems for application of prospectivity analyses may be helpful at a district scale. At a regional scale, the aerial overlap of individual deposit type analyses reinforces confidence in prospectivity for a lode gold resource in a drainage basin. Our analysis for undivided lode gold deposits will be the most practical analysis for landuse decisions in which delineation of areas that have confident potential for gold deposits in general is the primary goal. Data-driven GIS analysis for lode gold potential in Alaska, although limited by the size and uneven coverage of available datasets, objectively indicates prospectivity in areas where exposure is good as well as in areas under cover. The results of our analyses show medium to high prospectivity in areas that surround known deposits, indicating an overall expansion of areas that have the potential to contain gold deposits. Exploration in these areas may help improve the balance between the volume of gold produced in placer districts statewide and the relatively low volume of identified lode resources that contribute to these placer deposits. The results of our analyses can help focus future investigations in areas that show prospectivity but are not known to contain gold deposits, as well as in areas where data are lacking and the geology is poorly understood, and acquisition of additional data may help better define and constrain gold prospectivity.