GIS Prospectivity Analysis for Critical Minerals in Ore-Forming Systems in Alaska

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Alaska is dominated by a history of tectonic events that foster mobilization and concentration of a wide variety of mineral commodities that are critical to the US economy and are vital to national defense, renewable-energy, and emerging electronics technologies.

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Map of Alaska showing estimated mineral-resource potential

Estimated mineral-resource potential and levels of certainty, indicated by shading of colors, for rare earth elements, thorium, yttrium, niobium, uranium, and zirconium in alkaline igneous rocks.
(Credit: Keith Labay, U.S. Geological Survey. Public domain.)

The project’s current objective is to quantify and understand the distribution of critical elements in ore-forming systems in Alaska.  It uses a recently developed, data-driven geographic information system (GIS)-based method to evaluate the potential of various mineral deposit types across the state of Alaska. The method systematically analyzes pre-existing, geospatially referenced datasets to generate a map, which provides a visual indication of the estimated potential (high, medium, low) and the certainty of that estimate (high, medium, low) for each watershed in Alaska for a mineral commodity, mineral deposit type, or group of mineral deposit types. Probabilities are calculated with set criteria based on lithology, geochemistry, and geophysical data. The datasets used include the USGS Alaska Geochemical Database, the Alaska Division of Geological and Geophysical Surveys web-based geochemical database, data from the USGS geologic map of Alaska, the USGS Alaska Resource Data File, USGS airborne magnetic surveys, and radiometric surveys from the National Uranium Resource Evaluation.

GIS-based method

The GIS-based method applies a scoring system to geospatially referenced geologic data in subwatersheds, which are hydrologic units defined by the National Hydrography Dataset and Watershed Boundary Dataset. The scoring system uses multiple parameters based on key characteristics specific to each deposit type. The key characteristics are chosen based on known occurrences of the mineral in deposits around the world, known occurrences in Alaska, and previous research relating to the geologic processes responsible for the formation of these deposit types. Each deposit type has uniquely defined parameters and different point values assigned to the parameters. The parameters are then scored and weighted for each of the different mineral deposit types to generate a score for potential for a concentration of the mineral or deposit type in each watershed. These scores are classified using natural statistical breaks and assigned colors for each watershed for portrayal on maps as shown in the figure: high (red), medium (yellow), low (green), or unknown (gray, owing to lack of data) potential.  The confidence or certainty of the score is then calculated taking into consideration factors such as quantity of data and completeness of the datasets. Levels of confidence are shown as dark (high), medium (medium), or light (low) shades of red, yellow, or green, respectively, for each watershed on the maps.

As an example, the scoring system for placer gold deposits uses five parameters: pan concentrate mineralogy, sediment geochemistry, rock type, geographical location, and whether the Alaska Resource Data File has any reports for the specific area mentioning placer gold as a keyword. Each parameter has a set number of possible points which allows for the parameters to weigh in differently to the total score. For example, pan concentrate data were assigned point values of up to 10, while the presence of plutonic rocks in the watershed could be assigned a maximum of 3 points. In other words, the presence of gold in pan concentrate data indicates a high probability of finding gold in the area; however the presence of plutonic rocks in an area may often, but not always, correlate with placer gold deposits, and thus carries less weight in the overall score. After each parameter is assigned a score and a total score is calculated for each watershed, the certainty of the overall score is determined based on the number of datasets that contributed to the score for each of the parameters in that watershed.

The method is highly adaptable for the needs of diverse users, including scientific researchers, industry, land mangers such as the Bureau of Land Management and the State of Alaska, and Alaska Native corporations. Examples of how this GIS analysis can be applied include:

  • Identification of areas with high mineral potential supported by abundant data
  • Identification of understudied and undersampled prospective areas
  • Topical investigations to improve current deposit models
  • Development and modification of resource evaluation techniques
  • Identification and recognition of new pathfinders and different types of data combinations linked to processes of ore genesis
  • Constraint or expansion of the footprint of known mineral belts
  • Discovery and definition of new mineral trends or belts

 

This project is a continuation of the Alaska Critical Minerals Cooperative Project.