Fluid inclusion noble gas and halogen evidence on the origin of Cu-Porphyry mineralising fluids

The naturally occurring noble gas isotopes (Ar-40, Ar-36, Kr-84 and Xe-129) and halogens (Cl, Br, I) have been determined simultaneously in irradiated quartz vein samples by noble gas mass spectrometry. Quartz vein samples were collected from the potassic and propylitic alteration zones of six porphyry copper deposits (PCD): Bingham Canyon, Utah; and Silverbell, Ray, Mission, Pinto Valley and Globe-Miami in Arizona. In addition, analyses of He-3/He-4 have been obtained from sulphide minerals hosted by the quartz veins at Silverbell, Ray, Pinto Valley and Globe-Miami. The majority of PCD fluids studied have Br/Cl and I/Cl ratios that overlap those of fluids included in mantle diamond, suggesting that the salinity had a juvenile origin. The high I/Cl (121 X 10(-6) mole, M) in samples from the propylitic zone of Silverbell is attributed to the presence of sedimentary formation water. He-3/He-4 ratios have R/Ra values in the range 0.3 to 1.72, and provide evidence for the involvement of a crustal component in addition to mantle volatiles. Ar-40/Ar-36 ratios vary from meteoric values of similar to 317 in the propylitic zone of Bingham Canyon, and 323 in the skarn alteration of Mission up to 3225 in the potassic zone of Pinto Valley. Fluids in both the potassic and propylitic alteration zones of every deposit are a mixture of a low salinity end-member comprising meteoric water and air, and a high salinity end-member consisting of a mixed mantle and crustal fluid. The Ar-40/Cl ratio of fluid inclusions at Pinto Valley (similar to 10(-4) M) is similar to values obtained previously for mantle fluids. The Ar-40/Cl value is two orders of magnitude lower at Bingham Canyon, where a depleted Ar-36 concentration (0.2 X 10(-6) cm(3)/g) below that of air saturated water (ASW), and a range of highly fractionated noble gas compositions ((FKr)-Kr-84 = 13 and (FXe)-Xe-129 = 160) indicate that boiling and pulsed fluid flow have occurred. Copyright (C) 2001 Elsevier Science Ltd.