Mapping Where Alaska's Lode Gold May Lie: A USGS Geospatial Analysis

U.S. Geological Survey (USGS) scientists used data-driven geospatial analysis of geological data to map where Alaska has potential for lode gold. The analyses point to possible lode sources for known gold placer districts and to new areas prospective for lode gold. The same adaptable techniques can focus future investigations, data collection and exploration for gold and for critical minerals that come with it as byproducts, such as arsenic, tungsten, antimony and tellurium.

How the analysis works

The team identified measurable characteristics of gold deposit types in seven geologic, geochemical and geophysical datasets and scored each for how strongly it indicates gold. The scoring was tailored to three conventional lode gold deposit types, two types of gold ore-forming system, and one undivided group covering all lode gold, and applied statewide to drainage basins of about 100 square kilometres each. The criteria and weights can be changed to answer other questions.

A Python script adds each basin's parameter scores into a cumulative score, which is sorted by natural statistical breaks into High, Medium or Low potential; basins without data are marked Unknown. Because some datasets are unevenly spread, each result also carries a certainty score based on how many datasets contributed. Each analysis produces a map on which red, yellow and green mean high, medium and low potential, dark to light shades mean high to low certainty, and gray means unknown. Users can query the scores for any basin in the digital maps and compare them with known gold occurrences.

Estimated potential and certainty for undivided lode gold deposits: red high, yellow medium, green low; darker shades mean greater certainty. Credit: U.S. Geological Survey.

Deposit types

The analyses were designed to tell apart the main conventional lode gold types expected in Alaska — orogenic, reduced-intrusion-related gold (RIRG) and epithermal — which have different trace-element signatures reflecting the settings where they formed and the chemistry of their ore fluids. But limited data, similar fluid compositions, complex geologic history and environments that overlap in space and time make separating them at the scale of the whole state difficult. Where several types score highly in the same basin, that overlap strengthens confidence that gold is present.

The main findings:

• the undivided analysis, compared with known mineralisation, shows large areas of Alaska with medium to high potential that may be underexplored for lode gold;
• the analyses for orogenic, RIRG and epithermal deposits overlap considerably, because of the limits of the geospatial data currently available.

Ore-forming systems

To sharpen the results, the next analyses scored the environments in which ore forms, above all the oxidation state of the mineralising system. Only basins rated Medium or High in the undivided analysis were scored; the rest are shown in gray.

Reducing systems. Orogenic and RIRG deposits, though not genetically related, often form in reducing environments in thickened crust during and after collisional events, and may overlap in time and place. They share reducing conditions, carbon dioxide–rich fluids and cryptic alteration assemblages. The combined model scores characteristic element suites such as tungsten–bismuth–tellurium, all soluble in carbon dioxide–rich fluids, typical of structurally controlled quartz veins in collisional settings — for example, statewide tungsten concentrations at several levels above background.

Oxidizing systems. Gold-bearing porphyry and epithermal deposits form in oxidising conditions, typically with calc-alkaline magmatism in subduction-related arcs, and grade into one another depending on temperature, pressure and depth. They share abundant copper and sulfur with the gold and characteristic alteration minerals. The model scores (1) hypabyssal and plutonic rock types; (2) epithermal pathfinder elements — arsenic, antimony, mercury, selenium, tellurium and copper; (3) alteration mineral assemblages; (4) mineral suites including molybdenite, high-sulfidation copper minerals and silver-bearing sulfosalts; and (5) aeromagnetic data, in basins containing hypabyssal and granitic intrusive rocks.

Estimated potential and certainty for orogenic and reduced-intrusion-related gold ore systems. Credit: U.S. Geological Survey.

Estimated potential and certainty for gold-bearing porphyry and epithermal ore systems. Credit: U.S. Geological Survey.

Comparing the two system models showed far less overlap than comparing the individual deposit types — evidence that the system approach defines prospective areas more precisely.

Gold-bearing quartz vein in granite of the Fort Knox pluton. Photograph by G. Case, U.S. Geological Survey.

Conclusions

• The prospectivity models pick out known deposits and show areas with strong potential for new discoveries.
• With the data available, telling deposit types apart for regional exploration is difficult; distinguishing ore-forming systems works better.
• The undivided lode gold analysis is the most practical for land-use decisions, where the goal is to mark areas with reasonable potential for gold.
• The models can be tailored — for example, to emphasise pathfinder minerals and critical byproduct commodities such as silver, arsenic, bismuth, antimony, tellurium and tungsten — and applied at smaller scales to target districts and focus future data collection.

Visible gold, telluride and sulfide minerals in quartz from Kensington Mine. Photograph by S. Karl, U.S. Geological Survey.

Sources

Based on "Geospatial Analyses Delineate Lode Gold Prospectivity in Alaska," by Susan Karl, Doug Kreiner, George Case and Keith Labay, U.S. Geological Survey Fact Sheet 2022–3008, prepared with the Alaska Division of Geological & Geophysical Surveys and the Bureau of Land Management, published by the U.S. Geological Survey; rewritten in hubnx's own words.

• The introduction, findings, conclusions, maps and photographs come from the fact sheet's PDF: https://pubs.usgs.gov/fs/2022/3008/fs20223008.pdf
• Full details are in Karl and others (2021), USGS Open-File Report 2021–1041, and its data release; the geologic map is USGS Scientific Investigations Map 3340.
• The fact sheet's photographs of Pogo Mine ore, used with permission, are not reproduced.