Evidence for a nonmagmatic component in potassic hydrothermal fluids of porphyry Cu-Au-Mo systems, Yukon, Canada

Isotopic (H, Sr, Pb, Ar) and fluid inclusion data for hydrothermal fluids a ssociated with potassic alteration from three Late Cretaceous porphyry Cu o ccurrences, west central Yukon, suggest a nonmagmatic fluid component was p resent in these hydrothermal fluids. Potassic stage quartz veins contain a dominant assemblage of saline and vapor-rich fluid inclusions that have del taD values between - 120 and -180 parts per thousand. Phyllic stage quartz veins are dominated by vapor-rich fluid inclusions and have deltaD values t hat overlap with but are, on average, heavier (-117 to -132 parts per thous and) than those in potassic stage quartz veins. These deltaD values are sig nificantly lower than those from plutonic quartz phenocrysts (-91 to -113 p arts per thousand), and from values typically reported for primary fluids f rom porphyry-style mineralization (-40 to -100 parts per thousand). The ini tial Sr (Sr-87/Sr-86(i),) isotopic values for the plutons are 0.7055 (Casin o), 0.7048 (Mt. Nansen), and 0.7055 (Cash). The Sr-87/Sr-86(i) compositions of hydrothermal K-feldspar ranges from magmatic Sr, values to more radioge nic compositions (Casino: 0.70551-0.70834, n = 8; Mt. Nansen: 0.7063-0.7070 , n = 4; Cash: 0.7058, n = 1). The fluid inclusion waters from potassic qua rtz veins have Sr-87/Sr-86(i) values that are similar to those of co-existi ng hydrothermal K-feldspar. The Pb isotopic compositions of hydrothermal K- feldspar show a weak positive correlation with Sr-i for identical samples. Fluid inclusion waters of phyllic quartz veins also have Sr-i compositions more radiogenic than the plutons. The Pb isotopic composition of pyrite and bornite from phyllic alteration veins are similar to, or more radiogenic t han, hydrothermal K-feldspar Pb isotopic values. Hydrothermal K-feldspar sa mples yield Ar-40/Ar-39 ages (Casino = 71.9 +/- 0.7 to 73.4 +/- 0.8 Ma; Mt. Nansen = 68.2 +/- 0.7 and 69.5 +/- 0.6 Ma; Cash = 68.3 +/- 0.8 Ma) similar to the U-Pb-zircon, K-Ar-biotite and Re-Os-molybdenite ages of the Late Cr etaceous plutons, with the age spectra indicating no excess Ar-40 or distur bance. The Ar-40/Ar-36 values (285-292) of the K-feldspar samples are simil ar to the atmospheric compositions (295 +/- 5) during Late Cretaceous time. The H, Sr, Pb, and Ar isotopic compositions of hydrothermal K-feldspar and quartz vein fluid inclusion waters that characterize the potassic hydrothe rmal fluids show evidence for an exotic component in addition to magmatic w ater (fluid). This component has a low deltaD, radiogenic Sr and Pb, and an atmospheric Ar composition. The inheritance of pre-existing isotope compos itions from the host rocks, postpotassic alteration isotope exchange, or th e replenishment of the magma chamber with magma of different isotopic compo sition cannot explain the isotope data. We suggest that to generate the obs erved H, Sr, Pb, and Ar isotope compositions, crustal fluids must be a comp onent (15-94%) of potassic hydrothermal fluids in porphyry mineralization i n the deposits studied. Copyright (C) 2001 Elsevier Science Ltd.