Time Scales of Mineral Systems—Advances in Understanding Over the Past Decade

Abstract

The establishment of accurate time scales of mineral systems is essential to construct reliable genetic models about their formation. Time scales of fossil mineral systems are directly determined through radiometric dating of different stages of development of the mineral system. In theory, porphyry systems are, among mineral sys- tems, those whose duration can be bracketed with most accuracy and precision, because of the universal occur- rence of ore and gangue minerals that can be dated with the high precision U-Pb zircon, Re-Os molybdenite, and 40Ar/39Ar dating techniques. Time scales of fossil porphyry systems reported in the literature range between 4 Ma. The long dura- tions (>1 Ma) of magmatic-hydrothermal activity measured in several porphyry systems are likely the result of multiple magmatic pulses in agreement with field observations indicating that porphyry systems are associated with several intrusive events. Nonetheless, estimated long durations could also be affected by methodological problems. One methodological problem is the accuracy of the intercalibration among the three different meth- ods. It has become evident during the last 15 years that 40Ar/39Ar dates are systematically younger compared to U-Pb dates. This has been attributed to incorrect values of the secondary standard (Fish Canyon Tuff sanidine), most commonly used to calculate 40Ar/39Ar ages, and/or of the 40K decay constant. Systematic cross calibrations to check the consistency between Re-Os and U-Pb dates are lacking and should also be carried out. Another possible cause of erroneous long durations of porphyry systems concerns the way to determine the emplacement age of the causative intrusion. The current high precision (≤0.1%) of single zircon U-Pb dating by isotope dilution-thermal ionization mass spectrometry (ID-TIMS) shows that zircon grains extracted from a single sample of intermediate/felsic magmatic rocks do not overlap in age. This is so because zircon grains record a protracted evolution of magmas within the crust lasting several hundreds of thousands of years. Under these conditions, the emplacement age of a magmatic intrusion is best approximated by the youngest ID-TIMS age measured from a population of zircon grains. In contrast, spot ages measured with in situ techniques, due to their lower precisions (1−3%), are not able to discriminate such protracted magmatic evolution recorded by different zircon grains. This allows pooling together spot ages of different zircons, resulting in a statistically significant mean age with a low uncertainty. In reality this is a mixed age that is characteristically older (by up to a few hundreds of thousands of years) than the age of the youngest single zircon grain measured by ID-TIMS. A further problem in estimating the duration of magmatic-hydrothermal activity in porphyry systems derives from the widespread use of 40Ar/39Ar dating. Because this method does not date the crystallization of a mineral but rather its cooling below its closure temperature, 40Ar/39Ar dates may be affected by (hydro-)thermal activity that postdates the mineralization.