Non-Traditional Stable Isotopes

Science Center Objects

Understanding the genesis of ore deposits and their behavior in the environment is a subject of great importance to the Nation. A relatively new tool to aid in these efforts to investigate the origin and environmental effects of ore deposits is the use of "heavy" metal stable isotopes. Our research objectives are to utilize various isotopic systems to advance our understanding of ore genesis and the weathering of mineral deposits. Our studies focus on two themes: 1) use of stable isotopes as source indicators, and 2) use of isotopes as process indicators.

Science Issue and Relevance

periodic table with isotopes

Periodic table shows which elements have multiple isotopes, which elements are currently being run routinely in Denver, and which elements we are currently evaluating for potential future use. (Public domain.)

Understanding the genesis of ore deposits and their behavior in the environment is a subject of great importance to the Nation. Precious and base metals and critical elements may become enriched in the Earth’s crust by a wide range of processes. However, the formation of an ore deposit requires a sequence of extraordinary conditions and/or events, which remain poorly understood in many cases. A relatively new tool to aid in these efforts to investigate the origin and environmental effects of ore deposits is the use of "heavy" metal stable isotopes. These non-traditional stable isotopes are tracers of specific geologic and biologic processes and can be used to further advance our understanding of metal cycling within magmatic, hydrothermal, and low-temperature systems.

Methodology to Address Issue

Our research objectives are to utilize various isotopic systems to advance our understanding of ore genesis and the weathering of mineral deposits. Our studies focus on two themes: 1) use of stable isotopes as source indicators, and 2) use of isotopes as process indicators.

SEM image of carbonatite

SEM backscattered electron image of REE fluorocarbonates in barite beforsite unit of Elk Creek carbonatite. From Verplanck and others, 2015, BCGS Paper 2015-3. (Public domain.)

Isotopic and Geochemical Systematics of Rare Earth Element Ore Deposits: With the increased demand for rare earth deposits outside of China, understanding the processes that control ore-grade enrichment of rare earth elements is critical for not only identifying exploration targets, but also determining which zone of a particular target may be enriched in specific elements of interest. The processes responsible for 1) formation of rare earth element-enriched magmas and 2) ore-grade concentration of rare earth elements are complex and poorly understood. Our objectives are to utilize stable and radiogenic isotopes coupled with trace element geochemistry to constrain the processes responsible for ore-grade enrichment of rare earth elements. We are focusing on carbonatite-related deposits because these deposits:

  1. are the world's primary source of light rare earth elements (La, Ce, Pr, Nd, Sm, and Eu) and Nb,
  2. have been mined for PO4, F, Cu, V, Ti, and Ta,
  3. are potential sources of other critical elements (Th, Y, and other rare earth elements).

Radiogenic, stable, and non-traditional stable isotopic systems can help constrain important questions including:

  1. what is the source of rare earth elements for these deposits;
  2. what processes control ore-grade enrichment; and
  3. why are there over 500 known carbonates but only 9 currently producing ore.
Flowchart of organism reaction to metal exposure

Flowchart depicting mechanisms by which an organism may react to excess metal exposure. Green lines represent possible pathways taken by N. triangulifer; dashed red lines are pathways taken by bacteria in hydrozincite biomineralization. (Public domain.)

Stable Metal Isotopic and Geochemical Signatures of Element Dispersion from Ore Deposits: Non-traditional isotope ratios may be used to identify sources of metals as they migrate from a mineral deposit or altered area in the weathering environment. These isotopic ratios also may be affected by processes that occur during weathering and transport, including biological uptake. By examining the ratios of selected metal isotopes we hope to resolve the effects of source and process to increase our understanding of the weathering of mineral deposits and surrounding altered areas. Our objectives are to conduct isotopic analyses of several metals, semi-metals, and non-metals, including copper, iron, zinc, and molybdenum to determine the range of values observed in natural systems, and to evaluate the conditions and processes that lead to isotopic fractionation.

We have completed research on the following:

  • Method development for copper, iron, and zinc isotopes in complex aqueous samples,
  • Fate of metals in mining-affected areas,
  • Adsorption of metals onto Fe-oxyhydroxides,
  • Using metal isotopes to examine background/baseline conditions and to estimate pre-mining baseline in mined areas.
  • Fate of metals in the biosphere - uptake of Zn by aquatic insects.

We are currently working the following:

  • Multi-isotope characterization of mineralized areas - what do Fe, Cu, and Zn isotopes tell us?
  • Refining methods - lowering blanks for Zn and Cu
Zn isotope chart and hydrozincite photomicrograph

A) Concentration and Zn isotope signature of water in the Rio Naracauli as a function of distance downstream. As hydrozincite precipitates, favoring the heavier isotope of Zn, both the Zn concentration (open symbols) and the value of δ66Zn (solid symbols) decrease. B) Photomicrograph of hydrozincite forming on the surface of bacterial exopolysaccharide secretions. (Public domain.)

sphalerite sample

Banded sphalerite sample from Pomorzany mine, Poland. The sample (right) is 10 cm by 6 cm. From Ridley and others, 2015, doi:10.1016/j.proeps.2015.07.028.
​​​​​​​(Public domain.)

Simultaneous Measurement of Trace Elements and Isotopes as Constraints on Ore Forming Processes: In many non-magmatic ore systems the sources of metals and ligands (ion or molecule attached to a metal atom) remains incompletely understood yet is at the heart of determining transport pathways, processes of deposition, and genetic relations between deposits in a mining District. In many deposits the ore minerals are frequently chemically zoned, suggesting complex precipitation mechanisms. Hence, bulk rock compositions, or even bulk chemical properties of individual minerals, cannot reveal the details of the ore forming process or the sources of metals and ligands.

The goal of this study is to evaluate the genetic relations (sources and transport) of metals and ligands that ultimately precipate to form an ore deposits. This task will be looking at the application of the split-flow method to economic geology for simultaneous isotope and trace elements analysis. 

Insights into the Genesis of Magmatic-Hydrothermal Ore Deposits and Related Active Mineralizing Systems From "Heavy Metal" Stable Isotopes: The formation of an economically important metal-rich ore deposit requires a sequence of extraordinary conditions and/or events, the details of which remain poorly understood in most cases. The discovery of new ore deposits, and the full production of existing deposits, depends heavily upon clear knowledge of the mineralizing processes and their geologic context. Heavy metal stable isotopes are a relatively new tool to aid in efforts to investigate the origin of ore deposits.

The advent of multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has made it possible to measure variations in the isotopic composition of a wide array of metals from lithium to uranium. High-precision isotopic measurements of these elements in geological materials have revealed mass-dependent variations due to a range of natural processes, a development that has revolutionized the field of stable isotope geochemistry. These heavy metal stable isotopic systems are particularly well suited to studies of the genesis of ore deposits and related mineralizing systems due partly to the relatively high concentrations of many transition metals and metalloids in these systems. In addition, many of the elements that are amenable to stable isotopic investigations are economically or strategically important (directly, or indirectly because they are key elements that are used to understand the origin of mineral and energy resources). Our primary research objective is to apply “heavy metal" stable isotopic systems to case studies of magmatic-hydrothermal ore deposits, and spatially and genetically related modern (active) natural analogs of ore-forming processes.

Sulfur isotope scan chart

Mass scan for the three sulfur isotopes (32S, 33S, 34S) resolving the isobaric interference of 16O + 16O on 32S and 32S + 1H on 33S as well as peak coincidence. (Public domain.)

Multicollector-Inductively Coupled Plasma-Mass Spectrometry Methods Development: The high resolution multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) laboratory supports the research of this project and other priority research endeavors in the Geology, Geophysics, and Geochemistry Science Center. The laboratory currently houses two MC-ICP-MS units, a quadruple ICP-MS, and a laser ablation system. Isotopic data can assist geologists and geochemists to gain a better understanding in ore genesis processes, metal sources, and metal migration. We focus on continually improving isotope capabilities by improving or developing new methods in support of novel applications for isotope systematics, with current focus on ore genesis and metal mobility and migration.

Application of Lithium Isotopic Studies to Ore Genesis: Lithium is a rare metal with importance in the nuclear, electronic optical, medical, ceramic, and glass industries. Lithium is identified as an energy-critical element due to the growing demand for lightweight rechargeable lithium-ion batteries to power small electronics, power tools, and vehicles. The current lithium supply is projected to fall short of meeting the near future demands, such that mineral exploration for new lithium resources is quite active. Substantial lithium resources occur in pegmatites, continental brines, and hydrothermally altered clays. Lithium-rich brines currently represent the most economically recoverable lithium source and contribute three-fourths of the global lithium production.

This project is exploring the application of lithium isotopes to constrain geologic processes responsible for the mobility and concentration of lithium, particularly in ore deposit formation. The initial focus is on technique development to measure lithium isotopes in solution and in-situ by (multi-collector) inductively coupled plasma mass spectrometry (ICP-MS) and establishing a procedure for quantitative separation of lithium from rocks, minerals, and waters. Once technique development is complete, efforts will focus on utilizing lithium isotope systematics to understand ore genesis of lithium and other deposits.

cinnabar with mercury beads

Cinnabar ore from Almadén mining district, Spain. Cinnabar (HgS, red mineral) is the dominant ore mineral at the Almadén mine, whereas elemental Hg (Hg0, gray beads) is abundant locally. (Public domain.)

Completed Activities

Isotopic Compositions of Mercury in Ore Deposits and Evaluation of Mercury Isotope Fractionation During Mining Processes: Mercury is a common contaminant in the environment due to both natural and anthropogenic sources. It is a heavy metal of environmental concern because elevated concentrations can be toxic to living organisms. Tracing natural and anthropogenic sources of mercury is critical to the understanding of mercury contamination in various ecosystems. Measurement of mercury isotope fractionation during various geochemical processes is also critical in order to facilitate tracing of various sources of mercury. Our objectives are to evaluate mercury isotopic variability during mine processes and historical deposition from anthropogenic sources through the measurement of mercury isotopes in 1) ore samples from several mercury mines, 2) ore processed mine wastes, and 3) lake sediment cores.

 

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