METHODS, MODELS AND MICROANALYSIS: NEW DEVELOPMENTS IN THE APPLICATION OF GEOCHRONOLOGY TO UNDERSTANDING ORE DEPOSITS

Geology is fundamentally a 4-dimensional science and is highly dependent on geochronology to provide information on the timing and rates of processes, including those involved in the generation of economic deposits. To this end, proliferation of reliable radiogenic isotope-based methods began 3 decades ago, ostensibly with the near-simultaneous development of U-Pb micro-analysis and the ⁴⁰Ar-³⁹Ar variant of K-Ar. However, the initial focus on reducing analytical error, driven by now-overcome shortcomings of instrumentation, has recently been replaced by a renewed focus on the accuracy of the age interpretation. In part, this has been a result of access to new instrumentation that provides high-spatial resolution analyses (HR-SIMS, laser ablation on MC-ICP-MS and noble gas instruments) for a range of isotopic systems. Integrating complementary datasets such as U-Pb, ⁴⁰Ar-³⁹Ar and more recently, Re-Os, requires detailed knowledge of their relative accuracy. Dating of multiple phases and materials, especially apparent with studies of ore deposits and the sensitivity of genetic models to improper use of intercalibrated standards and constants, has led to worldwide interlaboratory efforts to overcome these deficiencies and give more accurate delineation of closely-timed events. Additionally, these new instruments, with improved methodologies have resulted in a substantially increased understanding of isotopic systematics at the sub-grain scale. Although results have shown a need to re-evaluate the older simplistic models that represent dates as specific events, they have also allowed development of more robust ways of deriving thermochronological information. Specifically, thermochronology is evolving from the concept of dating discrete events to dating continuous histories, a paradigm shift that is most apparent in low to medium thermal regimes. Ore deposits, with their complex interplay and wide range of thermal and hydrothermal regimes are natural environments where the multiple analytical approaches coalesce. Current approaches recognize inherent strengths and limitations of all aspects of analysis, from suitability of analyzed material to appropriateness of instrumentation to external factors that limit interpretation of results.