Mineralogical and Isotopic Characteristics of Sodic-Calcic Alteration in the Highland Valley Copper District, British Columbia, Canada: Implications for Fluid Sources in Porphyry Cu Systems

The Highland Valley Copper porphyry Cu (+/- Mo) district is hosted in the Late Triassic Guichon Creek batholith in the Canadian Cordillera. Fracture-controlled sodic-calcic alteration is important because it forms a large footprint (34 km(2)) outside of the porphyry Cu centers. This alteration consists of epidote +/- actinolite +/- tourmaline veins with halos of K-feldspar-destructive albite (1-20 X-An) +/- fine-grained white mica +/- epidote. The distribution of sodic-calcic alteration is strongly influenced by near-orthogonal NE- and SE-trending fracture sets and by proximity to granodiorite stocks and porphyry dikes. Multiple stages of sodic-calcic alteration occurred in the district, which both pre- and postdate Cu mineralization at the porphyry centers. The mineral assemblages and chemical composition of alteration minerals suggest that the fluid that caused sodic-calcic alteration in the Guichon Creek batholith was Cl bearing, at near-neutral pH, and oxidized, and had high activities of Na, Ca, and Mg relative to propylitic and fresh-rock assemblages. The metasomatic exchange of K for Na, localized removal of Fe and Cu, and a paucity of secondary quartz suggest that the fluid was thermally prograding in response to magmatic heating. Calculated delta(18)O(fluid )and delta D-fluid values of mineral pairs in isotopic equilibrium from the sodic-calcic veins and alteration range from 4 to 8 parts per thousand and -20 to -9 parts per thousand, respectively, which contrasts with the whole-rock values for least altered magmatic host rocks (delta O-18 = 6.4-9.4 parts per thousand and delta D = -99 to -75 parts per thousand). The whole-rock values are suggested to reflect residual magma values after D loss by magma degassing, while the range of hydrothermal minerals requires a mixed-fluid origin with a contribution of magmatic water and an external water source. The O-H isotope results favor seawater as the source but could also reflect the ingress of Late Triassic meteoric water. The Sr-87/Sr-86(inital) values of strongly Na-Ca-altered rocks range from 0.703416 to 0.703508, which is only slightly higher than the values of fresh and potassic-altered rocks. Modeling of those data suggests the Sr is derived predominantly from a magmatic source, but the system may contain up to 3% seawater Sr. Supporting evidence for a seawater-derived fluid entrained in the porphyry Cu systems comes from boron isotope data. The calculated tourmaline delta B-11(fluid) values from the sodic-calcic domains reach 18.3 parts per thousand, which is consistent with a seawater-derived fluid source. Lower tourmaline delta B-11(fluid) values from the other alteration facies (4-10 parts per thousand) suggest mixing between magmatic and seawater-derived fluids in and around the porphyry centers. These results imply that seawater-derived fluids can infiltrate batholiths and porphyry systems at deep levels (4-5 km) in the crust. Sodic +/- calcic alteration may be more common in rocks peripheral to porphyry Cu systems hosted in island-arc terranes and submarine rocks than currently recognized.