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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007-5039

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Introduction

Purpose and Scope

As part of its land planning and management responsibilities, the U.S. Bureau of Land Management (BLM) is producing a series of resource management plans (RMPs) for regions of Alaska in which it oversees Federal lands. The U.S. Geological Survey, under the authority of Interagency Agreement # LAI-05-0020, was asked to provide a quantitative assessment of undiscovered locatable mineral resources for inclusion in the Bay RMP area report. The Bay RMP area encompasses parts of eleven 1:250,000 quadrangles in the southwestern part of Alaska (fig. 1). It stretches from the Alaska Range on the East to Goodnews Bay on the west, and includes Lake Iliamna, the Wood River Mountains, and the headwaters of Bristol Bay (fig. 1).

This report and associated digital files are the summary of that quantitative mineral assessment. This information and additional data on known (discovered) deposits will be incorporated by BLM, into the Mineral Occurrence and Development Potential reports and Reasonably Foreseeable Development alternatives in the RMP process.

Terminology

Throughout this report the term “BMPA” refers to the Bay Resource Management Plan area, as defined and used in the Bureau of Land Management’s Resource Management Plan (RMP) process. Other terms used here are modified from USGS assessment language (U.S. Geological Survey, 2000).

A mineral “resource” (or deposit as used in this report) is a concentration of naturally occurring minerals in the Earth’s crust of sufficient size and grade that economic extraction of a commodity from that concentration is currently or potentially feasible (U.S. Bureau of Mines and U.S. Geological Survey, 1980). “Undiscovered” mineral resources are previously unknown deposits postulated to exist 1 km or less below the surface of the ground, and incompletely explored mineral occurrences or prospects that could potentially be of a size and grade to be classified as a deposit or resource. The undiscovered category specifically excludes deposits already known at the time of the assessment. Undiscovered resources include those expected to be similar in type or model to known deposits (hypothetical), and those in favorable geologic settings but of uncertain type or model (speculative).

A “permissive” tract is an area within which, using current information, geologic conditions existed that would permit the formation of deposits of a particular model type. Conversely, areas outside of permissive tracts have a negligible (<1 in 100,000) chance of containing a deposit of a given model type, assuming that the geologic conditions known today are accurate, and the deposit model adequately describes the conditions under which mineralization forms.

A “descriptive model” is a set of information that describes a group of mineral deposits that have similar geologic, mineralogical, and geochemical characteristics. A “grade and tonnage model” is a series of frequency distributions and associations of grades and sizes constructed from data from well explored, often mined, individual mineral deposits of a given type. “EMINERS” is a software program (Duval, 2004) that is used to estimate the metal endowments of specific commodities by combining grades, tonnages, and number of deposit estimates at those same probabilities.

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