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 we re collected from the potassic and propylitic alteration zones of six porph yry copper deposits (PCD): Bingham Canyon, Utah; and Silverbell, Ray, Missi on, 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 a t 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 ha d a juvenile origin. The high I/Cl (121 X 10(-6) mole, M) in samples from t he propylitic zone of Silverbell is attributed to the presence of sedimenta ry formation water. He-3/He-4 ratios have R/Ra values in the range 0.3 to 1.72, and provide evi dence for the involvement of a crustal component in addition to mantle vola tiles. Ar-40/Ar-36 ratios vary from meteoric values of similar to 317 in th e propylitic zone of Bingham Canyon, and 323 in the skarn alteration of Mis sion up to 3225 in the potassic zone of Pinto Valley. Fluids in both the po tassic and propylitic alteration zones of every deposit are a mixture of a low salinity end-member comprising meteoric water and air, and a high salin ity 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 wate r (ASW), and a range of highly fractionated noble gas compositions ((FKr)-K r-84 = 13 and (FXe)-Xe-129 = 160) indicate that boiling and pulsed fluid fl ow have occurred. Copyright (C) 2001 Elsevier Science Ltd.