NEW INSIGHTS INTO PORPHYRY ORE MINERALIZATION FROM HIGH-PRECISION CA-ID-TIMS ZIRCON GEOCHRONOLOGY

Porphyry ore deposit systems are commonly assembled through a series of discrete magma injections over timescales of 10-1,000 ka. Discerning between different intrusive phases in porphyry environments can aid in understanding metallogenesis and mechanisms for mineralization. In order to understand the tempo and duration of porphyry magmatism, it is necessary to precisely determine the timing of emplacement and crystallization of productive and barren intrusions. Zircon U-Pb geochronology is best suited for understanding porphyry development, because it is more robust in the face of alteration than other techniques.

There are a variety of methods to generate zircon geochronologic data, however zircon geochronology using chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) is commonly used for understanding the timing and duration of short-lived geological processes, due to the high precision and accuracy afforded by the technique. However, because porphyry systems are characterized by protracted thermal histories with significant fluid circulation and their intracrustal character, complications such as Pb-loss, presence of antecrystic grains, fluid inclusions and low abundances of radiogenic Pb are common.

We investigated several shallow, Oligocene-Miocene porphyry systems in the Rocky Mountain region, including polymetallic mineralization in the Questa batholith. In these systems, numerous examples of protracted age spectra (> 200 ka) are evident, resulting in overlapping ages between separate samples. The protracted age spectra are the result of unmitigated Pb-loss and/or inclusion of antecrystic zircon. These results alone could be interpreted to indicate that some samples are from the same intrusion; however, when supplemented with cross-cutting relationships and whole rock Nd isotope data we are able to resolve distinct intrusive phases. These pulses correspond to isolated mineralization events of Mo, Au, and Ag in which source magmas reflect individual intrusive units, rather than one upper crustal magma body. Whereas CA-ID-TIMS can generate the most precise, accurate geochronology of porphyry systems, it requires a detailed, multidisciplinary approach in order to reach an informed interpretation.