Water is the dominant constituent of ore-forming fluids, thus obtaining information on its origin is fundamental to understand fluid-rock interaction processes inherent to ore formation. Water shows variations in δ2H and δ18O ratios depending on its isotopic reservoirs or source and processes such as fluid mixing, devolatilization or dehydration during mineral neoformation. Isotopic water information available in ore deposits is limited to the occurrence and access structural water bearing minerals (e.g. OH– inoctahedram layers of phyllosilicates) or water trapped as fluids inclusions in quartz, calcite, or other phases.
Despite the fact that the thermal decrepitation method has been used since 30s years, no formal description of the fluid inclusion water decrepitation method has been detailed until this study. Some studies only mention the use of the technique or it is only described in a broad sense. In this contribution we provide a careful description of a method for fluid inclusion (FI) water extraction, which allows discriminating among different FI families trapped in minerals. Such a technique is lower cost and allows testing reproducibility. The method includes three main steps: FI petrographic and microthermometric analysis, decrepitation analyses and Ultra Vacuum Borosilicate Glass Line FI trapping.
This new multi-disciplinary method allows obtaining sufficient FI water (30 μLt) to measure both δ18O and δ2H isotopes with the Liquid-Water Stable Isotope Analyzer, which might be representative of a particular hydrothermal event providing that the method allows: (1) estimating the amount of sample necessary for a minimum water volume, on the basis of the fluid / crystal ratio; (2) estimating the decrepitation temperature range for a particular FI family; (3) decrepitating and trapping water in a Ultra Vacuum Borosilicate Glass Line Test; (4) analyzing δ18O and δD of trapped water in aLiquid-Water Stable Isotope Analyzer.
We also demonstrate that the obtained δ18O and δD results of FI water of quartz crystals are reproducible and consistent with other geological and geochemical constrains on the different paragenetic events that have occurred in the world class silver-gold multievent of San Dimas District at Sierra Madre Occidental, México, which we used as an example to test our method.
- Montoya-Lopera, Paula12*, Levresse, Gilles1, Fitz-Díaz, Elisa3, Cienfuegos-Alvarado, Edith4, Otero-Trujano, Francisco4, Morales-Puente, Pedro4