USGS Info Handout: Stable Isotopes and Mineral Resource Investigations in the United States

Stable Isotopes and Mineral Resource Investigations in the United States

Introduction

The elements oxygen, hydrogen, sulfur, and carbon are important constituents of hydrothermal ore-forming systems and the weathering processes of mineral deposits in the surficial environment. They also play key roles in volcanic activity, ecosystem dynamics, climate change, and hydrologic and atmospheric processes. Therefore, study of the stable isotopes of these elements can provide powerful insights into these processes. This is especially true for ongoing U.S. Geological Survey (USGS) projects in the Eastern United States that are concerned with the origins of base (copper, lead, and zinc) and precious (gold and silver) metal deposits in the Carolina slate belt and northern Maine and with the environmental effects of weathering of mineral deposits (fig. 1).

Isotopes and Mass Spectrometry

Atoms consist of a nucleus of protons and neutrons surrounded by a cloud of electrons. An element is defined by the number of protons in the nucleus of the atom. For example, the element carbon has six protons, whereas the element oxygen has eight. Although the number of protons is fixed for an element, the number of neutrons can vary. Carbon can have six, seven, or eight neutrons. The various combinations of protons and neutrons are called isotopes, which are distinguished on the basis of atomic mass. Atomic mass is the number of protons plus the number of neutrons. Thus, the naturally occurring isotopes of carbon are carbon-12 (6 protons + 6 neutrons), carbon-13 (6 protons + 7 neutrons), and carbon-14 (6 protons + 8 neutrons), which are abbreviated as ¹²C, ¹³C, and ¹⁴C, respectively.

The isotope ¹⁴C undergoes radioactive decay to an isotope of nitrogen (¹⁴N). Because of its decay, ¹⁴C is called a radiogenic or "unstable" isotope. The radioactive decay of ¹⁴C is the basis for radiocarbon (carbon-14) dating. In contrast, ¹²C and 13C do not radioactively decay. Hence, they are called stable isotopes. Stable isotope geochemistry investigates variations in the ratios of stable isotopes such as ²H/¹H, ¹³C/¹²C, ¹⁸O/¹⁶O, and ³⁴S/³²S. Isotopic ratios are measured in the laboratory on an instrument known as a mass spectrometer. Before a sample can be analyzed for its isotopic ratio, the element of interest in the sample must be converted to a gaseous form. For hydrogen, carbon, oxygen, and sulfur isotopes, samples are converted to hydrogen gas, carbon dioxide, and sulfur dioxide, respectively. The extraction of gas from a rock or mineral typically involves a chemical reaction that destroys the rock or mineral.

Causes of Isotopic Variations

In nature, isotopic variations occur because substances (such as minerals, water, and gases) preferentially concentrate one isotope over another, or because organisms can more efficiently metabolize one isotope than another. The preferential concentration of one isotope relative to another between two substances, known as fractionation, also varies strongly with temperature. Thus, natural isotopic variations can arise from numerous common chemical and physical processes, such as cooling of hydrothermal fluids during mineral deposition, the evaporation or condensation of water, mixing of two or more sources of fluid, or the metabolic activity of organisms.

Analytical Capabilities

Mineral resource investigations use Finnigan dual-inlet MAT 251 and continuous-flow Delta Plus mass spectrometers that are configured for automated measurement of hydrogen, carbon, nitrogen, oxygen, and sulfur isotopic ratios. The USGS stable isotope laboratory can analyze most geologic materials, including silicate (¹⁸O/¹⁶O, 2H/1H), carbonate (¹³C/¹²C, ¹⁸O/¹⁶O), sulfide (³⁴S/³²S), and sulfate (³⁴S/³²S, ¹⁸O/¹⁶O) minerals; organic matter (¹³C/¹²C, ¹⁵N/¹⁴N); and waters (²H/¹H, ¹⁸O/¹⁶O, ³⁴S/³²S).

Origins of Gold Deposits in the Carolina Slate Belt

Since the mid-1980's, four new gold mines have gone into operation in the Carolina slate belt of South Carolina. The origins of these four gold deposits -- the Barite Hill, the Brewer, the Haile, and the Ridgeway -- have been the subject of much debate, with proposed models ranging from the ancient equivalents of subaerial geothermal systems, such as those found in Yellowstone National Park, to modern submarine "black smoker" hydrothermal systems found on the ocean floor. The geochemical implications of a subaerial versus a submarine origin are quite different and have a profound impact on exploration strategies and the assessment of mineral resource potential. Geologic features that commonly are used to make these distinctions have been obscured by deformation and metamorphism of the deposits. Therefore, geochemical tools, such as stable isotopes, are needed to see through the deposits' complicated geologic history.

The Barite Hill deposit, at the southwestern edge of South Carolina, is dominated by massive lenses of pyrite (iron sulfide) and massive lenses of barite (barium sulfate) that are hosted by metamorphosed volcanic rocks.

Barite Hill is thought to have formed approximately 550 million years ago in the Cambrian Period. The sulfur isotope composition of the barite is quite distinctive; it is virtually identical to the sulfur isotope composition of Cambrian seawater sulfate. Likewise, the range of sulfur isotope compositions of the pyrite is intermediate between that expected for leaching of sulfide solely from the underlying volcanic rocks and that expected for sulfide produced from the reduction of seawater sulfate. Such sulfur isotope compositions are diagnostic of a family of sea-floor mineral deposits known as volcanogenic massive sulfide deposits.

The Brewer gold deposit, in northeastern South Carolina, is dominated by pyrite and enargite (copper-arsenic sulfide) associated with aluminum-rich alteration and hosted by metamorphosed volcanic and intrusive rocks. The sulfur isotope compositions of the pyrite and enargite are identical to that expected for sulfur in terrestrial geothermal systems, either derived from leaching of sulfide from igneous rocks or derived as subvolcanic emanations. Equally important, the sulfur isotope data do not indicate any sulfur derivation from seawater.

At present, origins of the Haile and Ridgeway gold deposits, located between Barite Hill and Brewer, are enigmatic. Haile and Ridgeway share many common geologic features, but similarities with either Barite Hill or Brewer are lacking. Characteristics of the sulfur isotope data appear to be intermediate between those of Barite Hill and Brewer. More detailed studies of Haile and Ridgeway should resolve these uncertainties. In the meantime, mineral exploration programs must be aware of the genetic implications of a transition from a former sea-floor environment in rocks of the southwestern part of the State to a former subaerial environment in the northeastern part.

Origins of Base and Precious-Metal Deposits in Maine

The Bald Mountain and Mount Chase copper + zinc ± lead ± gold ± silver deposits in northern Maine bear many geologic similarities to world-class volcanogenic massive sulfide deposits in the Bathurst Mining Camp in nearby New Brunswick, Canada. Sulfur isotope data can be used to delineate the conditions under which these deposits formed. All three areas formed approximately 450 million years ago, when oceanic bottom waters were undergoing cyclic variations from oxic (oxygenated) to anoxic (unoxygenated) conditions. The presence or absence of anoxic bottom waters has important implications not only for the formation of sea-floor massive sulfide deposits, but also for their preservation in the geologic record. Under oxic conditions, the hydrothermal fluids must provide both metals and reduced sulfur to form a deposit, and mineralization must soon be followed by a volcanic event or rapid sedimentation to protect the ore minerals from destruction by oxygenated seawater. In contrast, under anoxic conditions, reduced sulfur is abundant in the bottom waters. Thus, the hydrothermal fluids need only supply the metals, and preservation can be accomplished through slower sedimentation because the ore minerals are stable in the surrounding seawater.

The sulfur isotope composition of many of the Bathurst deposits conforms to the known age-dependent sulfur isotope variations of sea-floor sulfide deposits that formed during global oceanic anoxic events in Ordovician time. The sulfur isotope composition of sulfide minerals in the Mount Chase and Bald Mountain deposits does not match that of anoxic Ordovician seawater sulfide. The lack of sulfur-isotope conformity of the Bald Mountain and Mount Chase deposits indicates that they formed under locally unusual conditions, such as continuing volcanism to cover the deposits, that permitted their preservation in an otherwise hostile environment.

Environmental Effects of the Weathering of Mineral Deposits

One of the greatest potential threats to the environment from the mining of mineral deposits comes from the weathering of pyrite and the associated generation of sulfuric acid, otherwise known as acid mine drainage. The oxidation reaction needed to weather pyrite involves water and dissolved oxygen or aqueous oxidized iron to produce dissolved sulfate, aqueous reduced iron, and acid (hydrogen ion). Stable isotopes provide important insights into the formational conditions of acid mine drainage because, in addition to iron, the elements oxygen, hydrogen, and sulfur are the most important ones involved in this process.

Published research indicates that, under normal conditions, stable isotopes can define the conditions of pyrite oxidation such as its location above or below the water table, the role of ironand sulfur-oxidizing bacteria, and the presence or absence of dissolved oxygen. The USGS laboratory is investigating isotopic effects in common geochemical settings that deviate slightly from "normal"; these settings include ponded, evaporating bodies of mine-drainage water and areas where secondary salts, produced from the evaporation of mine-drainage water, are redissolved. Elsewhere, stable isotopes highlight the importance of processes such as evaporation and mixing in deter mining the chemistry of mine drainage. The results of this work will provide important insights to aid reclamation at active and abandoned mines.

Stable isotopes are also proving useful in the investigation of sulfur and water budgets in ground and surface waters in many other areas in the United States. In the vicinity of the Bald Mountain deposit in Maine, sulfur and oxygen isotopes are being used to distinguish between pre-mining contributions of sulfur from the weathering of the deposit and sulfur derived from acid rain and other sources unrelated to the deposit. In West Virginia and Pennsylvania, stable isotopes are being used to fingerprint ground-water contributions to streams affected by acid mine drainage from coal. In the Coeur d'Alene silver mining district in northern Idaho, isotopic data are being used to identify the site of sulfide mineral weathering. In the Mother Lode belt in the foothills of the Sierra Nevada Mountains, California, the weathering of mine waste and its impact on the environment are being studied by using hydrogen, oxygen, and sulfur isotopes. In the Coast Ranges north of San Francisco, stable isotopes are helping to identify the impact of natural geothermal springs on water quality in an gold-mine pit that is no longer in production.

Other Studies

Other recent or current studies include

Analyzing mine drainage in theVermont copper belt, Virginia gold-pyrite belt, North Carolina, Tennessee, and Nevada.
Monitoring modern hydrothermal activity on the ocean floor.
Tracing the sources of dissolved sulfate in the South Florida ecosystem.
Defining the water budget of Lake Baikal, Siberia, the world's oldest, deepest, and largest lake, for global climate change studies in continental settings.
Defining the climate history of the Chesapeake Bay.
Tracing the volcanic evolution of Mount Vesuvius, Italy.
Defining the origin of the world-class Sullivan lead-zinc-silver deposit, British Columbia (Canada), with implications for the occurrence of similar deposits in western Montana and northern Idaho.

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.

For more information, please contact:
Robert R. "Bob" Seal II
U.S. Geological Survey
954 National Center
Reston, VA 20192
Telephone: (703) 648-6290
E-mail: rseal@usgs.gov

U.S. Department of the Interior
U.S. Geological Survey

USGS Information Handout
November 1999

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