Isotope and Chemical Methods for Mineral and Geoenvironmental Assessments and Support of USGS Science Strategy

Science Center Objects

This Project integrates several geochemical tools—stable isotope geochemistry, noble gas geochemistry, active gas geochemistry, single fluid inclusion chemistry, and fluid inclusion solute chemistry—in studies of the processes that form mineral deposits and the processes that destroy them during mining or natural weathering. Research is directed toward fundamental scientific questions or, in collaboration with other Mineral Resources Projects, toward case studies of individual deposits, deposit types, or districts. The ultimate objective is to improve the scientific basis for mineral deposit models, and thereby improve the accuracy of assessments of the Nation’s mineral wealth.

scientists standing on outcrop

Field work at the Pilot Knob iron mine, Missouri.
(Credit: Craig Johnson, USGS. Public domain.)

Science Issue and Relevance

The core mandate of the Mineral Resources Program is to inform decision-makers on matters related to mineral resources on the Nation’s lands, including the consequences of mining and consequences natural weathering. To fulfill this mandate, genetic and geoenvironmental models must be developed for the various types of mineral deposits based on the best-available scientific understanding. This Project integrates several geochemical tools—stable isotope geochemistry, noble gas geochemistry, active gas geochemistry, single fluid inclusion chemistry, and fluid inclusion solute chemistry—in studies of the processes that form mineral deposits and the processes that destroy them during mining or natural weathering. Research is directed toward fundamental scientific questions or, in collaboration with other Mineral Resources Projects, toward case studies of individual deposits, deposit types, or districts. The ultimate objective is to improve the scientific basis for mineral deposit models, and thereby improve the accuracy of assessments of the Nation’s mineral wealth. The tools supported by this Project are applicable over a broad spectrum of Earth science research, so the Project also performs reimbursable work for other Programs consistent with the mandate of the USGS Science Strategy to leverage USGS skills in integrated studies that examine the Earth as a whole. This Project succeeds a similar Project that was summarized in U.S. Geological Survey Circular 1343.

Methodology to Address Issue

Several labortories support the project objectives:

Stable Isotope Laboratory: Stable isotope geochemistry involves isotopic analysis of carbon, hydrogen, nitrogen, oxygen, and sulfur. These elements are abundant in common minerals and rocks, and they are the building blocks of most geologic fluids (surface waters, magmatic waters, hydrocarbon fluids, and others) and most biological compounds. Geologic metal deposits are in most cases precipitates from hot fluids. Stable isotope measurements can help to determine the source of the fluids, the sources of dissolved constituents, physicochemical parameters of ore formation such as temperature, and the trigger for metal precipitation. Stable isotope analysis can also reveal the broader geologic environment of ore formation, an essential part of any mineral deposit model.

zinc ore

Zinc ore from the Sterling Hill mine, New Jersey.(Credit: Craig Johnson, USGS. Public domain.)

zinc ore under uv light

Ore from the Sterling Hill mine luminescing under ultraviolet light.
(Credit: Craig Johnson, USGS. Public domain.)

Noble Gas Laboratory: Helium, neon, argon, krypton, xenon, and radon are inert "gases" that have multiple isotopes. The relative abundances of the isotopes can reveal whether rock or water constituents came from the mantle, the deep crust, the shallow crust, or the atmosphere. In studies of mineral deposits, noble gas analyses are complementary to other types of chemical or isotopic analyses because they reveal how ore-forming systems fit into larger frameworks of crustal evolution and magma generation.

Active gases contained in hydrothermal minerals also give insights on ore formation. Active gases that are routinely measured include N2, CO2, CH2, H2, H2S, SO2, HCl, HF, H2O, and the light hydrocarbons. The data can reveal volatile evolution in hydrothermal systems, magma degassing histories, and fluid-rock chemical buffering.

fluids within crystal

Preserved samples of ore-forming fluids within a quartz crystal; note vapor bubbles and daughter crystals.
(Credit: Gary Landis, USGS. Public domain.)

Single Fluid Inclusion & Melt Inclusion Laboratory: Inclusions trapped in hydrothermal minerals can contain remnants of the waters from which the minerals precipitated. Chemical and isotopic analysis of these miniscule inclusions provides a wealth of information on ancient hydrothermal systems and their role in the formation of mineral deposits. A variety of important parameters can be determined, including the mass of fluid required to produce the deposit, the chemical species that carried the metals, and the trigger that led to metal precipitation. 

Fluid Inclusion Solute Laboratory: Certain cations and anions in fluid inclusions within hydrothermal minerals can be diagnostic of the source and history of the mineral-forming fluid. Particularly insightful are the abundances of the alkali metals lithium, sodium, and potassium, and the halides fluoride, chloride, bromide, and iodide. Analyses of these ions can reveal periods of evaporation, water-rock reactions within aquifers, and mixing of multiple fluids, all important inputs for mineral deposit models.

V-rich rocks with pen

Vanadium-rich rocks of the Gibellini prospect, Nevada.
(Credit: Craig Johnson, USGS. Public domain.)

heap leach operation

Heap leach operation at the Standard Hill gold mine near Mojave, California.
(Credit: David Grimes, USGS. Public domain.)

 

mine near lake

The Red Dog open pit mine in northwestern Alaska, a major zinc producer.
(Credit: Karen Duttweiler Kelley, USGS. Public domain.)

microscopic view of skarn minerals

Microscopic view of skarn minerals associated with the Sterling Hill zinc deposit, New Jersey.
(Credit: Craig Johnson, USGS. Public domain.)