How Accurately Can We Date the Duration of Magmatic-Hydrothermal Events in Porphyry Systems?--An Invited Paper

Abstract

Determining the absolute duration of magmatic-hydrothermal events leading to the formation of porphyry systems (i.e., including porphyry copper, skarn, and epithermal deposits) is one of the key questions in ore geology. This is so because the duration of magmatic-hydrothermal events in porphyry systems is instrumental to the development of genetic models necessary to explore a category of mineral deposits that provide most of the copper and significant amounts of base and precious metals to our economy. The problem of determining the absolute duration of magmatic-hydrothermal events in porphyry systems has been addressed through thermal modeling of cooling intrusions and time needed to precipitate specified metal amounts from active hydrothermal systems with known metal concentrations and fluid fluxes. Both these methods have shown that the likely duration of hydrothermal systems is on the order of a few tens of kilo-annum (ka). Isotopic dating in contrast is the only possible way to determine the life span of magmatic-hydrothermal events in fossil porphyry systems. Analytical and methodological developments during the last decade in the fields of the most robust isotopic systems commonly used for absolute dating (U-Pb, 40Ar/39Ar, Re-Os) allow us to date minerals with internal precisions <0.2% (2σ). For a 10-Ma-old mineral this corresponds to a <20-ka uncertainty, which is marginally sufficient to discriminate the duration of hydrothermal systems at the tens of kilo-annum scale. However, many geochronological studies on fossil porphyry systems have shown that these are most often formed through repeated cycles of several intrusion events, which extend the overall life of the porphyry systems to a few 0.X and up to ~2 Ma in some cases. Internal precisions of the above mentioned dating methods allow us, in theory, to comfortably discriminate events at the 0.X scale and the combination of U-Pb, 40Ar/39Ar, and ReOs geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic-hydrothermal events in porphyry systems. In this review we discuss some fundamental problems that are systematically overlooked in most geochronological studies trying to bracket the life span of porphyry systems. We show that if these problems are not adequately taken into account and tackled the result will be that fundamentally wrong life spans of porphyry systems will be estimated. We also provide basic guidelines to follow when trying to resolve the duration of magmatic-hydrothermal events in porphyry systems with the highest accuracy and precision currently achievable.