Method
One method for determining the fluid source of gold deposits is by using stable isotopes. Isotopes fractionate during chemical or physical processes that differentiate between isotopes of different masses. This results in a separation between heavy and light isotopes.
For elements with a large atomic number, mass fractionation has a minimal effect, because the addition of one neutron is only a small percentage of the total weight. In elements with a smaller atomic number, however, mass fractionation can be substantial.
The elements commonly used for fluid characterization are: H, C, N, O, and S. These elements have one or more isotopes that can be measured as isotopic ratios using Stable Isotope Mass Spectrometry (SIMS). Samples are dissolved and separated using acids and chemical compounds to isolate the desired elements. These solutions are then heated and degassed to release the component as a gas (e.g., CO2, H2S, N2, etc.). This gas is then ionized and the ions are deflected into detectors by a strong magnet. The trajectory of each isotope is determined by the mass of the ion and is measured as a ratio of light and heavy isotopes.
The ratio of isotopes are compared to a known standard and are reported as δ values. For example, the hydrogen system:
This manner of reporting stable isotope data is commonly used in environmental, climatic, and petrologic studies. By combining the results from different isotopic studies, distinct isotopic “reservoirs” can be determined based on the characteristic isotopic composition of different geologic environments. Some stable isotope systems have been studied for many years in relation to gold and other economic mineral deposits (e.g., Sheppard et al., 1969; Taylor, 1974). Elements such as hydrogen, carbon, oxygen, and sulfur, have been studied widely as an indicator of fluid source. These characteristic signatures can be combined into a sort of isotope “map”. (Sheppard et al., 1969).
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Hydrogen fractionation occurs readily, as protium and deuterium (AMU 1 and 2, respectively) are the natural stable isotopes in the atmosphere and are present in most surface environments. Because it fractionates during precipitation, hydrogen isotopes have distinct latitudinal and geographical controls (Taylor, 1997). Fractionation varies systematically with increasing latitude, but also depends on distance from the coast. As you move higher in latitude, δD values tend to decrease. Measuring δD is a good method for determining surface water influence on a mineralizing system, and is best used to distinguish epithermal ore deposits from deeper seated systems.
The oxygen isotope system is composted of 16O, 17O, and 18O. Because of its abundance in the atmosphere, water, and silicate minerals, oxygen is a valuable indicator for different geologic settings. Instead of measuring and calculating a value for each isotope, general δ18O is reported:
This captures the fractionation record of the heaviest isotope compared only to the light isotope of the system. Accumulation of data from years of study of different geologic environments seems to indicate that the deeper the source of the oxygen, the heavier (more positive) the δ18O ratio. Different geologic environments record different ranges of δ18O values. Oxygen also has a latitudinal dependence, as heavier water (more 18O) rains out closer to the equator. These are commonly plotted against δD to form an isotopic map as seen above (Figure 6).
Nitrogen isotopic ratios (δ15N) are also a valuable tool for distinguishing a geologic source. Ammonium ions, NH4+, can substitute for into the alkali site for K-bearing minerals such as muscovite, K-feldspar, and clay minerals. These minerals are commonly included in the vein alteration zones around mineral deposits, and there is sufficient nitrogen in the system to measure. Because N is so abundant in the atmosphere and surface waters, the total abundance of N in a sample can also be an important indicator of geologic setting. Instead of comparing δ15N to another isotopic system (as δ18O and δD, above), δ15N is generally compared to N in parts per million (ppm) as done by Pitcairn et al., 2005 (Figure 7).
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By combining stable isotope systems, characteristic patterns become more apparent and more conclusive to identify a fluid or rock sample to a given geologic environment. These isotopic characteristics are the primary method for determining fluid source in ore forming environments, because it is possible to distinguish heavy and light isotopic signatures. This page will focus on the δ15N system, but it is also important to consider other systems when identifying a fluid source.