Halogen and Ar-Ar age determinations of inclusions within quartz veins from porphyry copper deposits using complementary noble gas extraction techniques

Extension of Ar-Ar methodology has been used to determine mineralisation ag es from mica inclusions and to simultaneously evaluate the noble gas and ha logen composition of inclusion fluids within irradiated quartz vein samples from five porphyry copper deposits. Samples have been collected from the p otassic and propylitic zones of Bingham Canyon, Utah, and four Arizonan dep osits; Silverbell, Globe-Miami, Pinto Valley and Ray. Data obtained using t hree noble gas extraction techniques (laser ablation, in vacuo crushing and stepped heating) are compared with each other. Laser ablation provides a m eans for the analysis of individual fluid inclusions but is limited by blan k levels and detection limits. Stepped heating and in vacuo crushing are bu lk extraction techniques that preferentially release gases from solid and f luid inclusion phases, respectively, and can be used in combination to obta in accurate and meaningful ages of mineralisation. Ages obtained for porphy ry copper deposits are as follows: Bingham Canyon 37.1 +/- 0.5. Ray 65.3 +/ - 1.5, Globe-Miami 61.7 +/- 3.4, Pinto Valley 63.2 +/- 8.0 and Silverbell 5 5.8 +/- 1.8. The age of mineralisation at Ray is of particular interest as it enables a reported discrepancy in K-Ar ages to be understood. Halogen da ta for the Bingham Canyon samples gives insight into the partitioning of th e heavy halogens (Br and I) between solid and liquid phases. (C) 2001 Elsev ier Science B.V. All rights reserved.