Elevated magmatic sulfur and chlorine contents in ore-forming magmas at the Red Chris porphyry Cu-Au deposit, Northern British Columbia, Canada

Abstract

The Red Chris porphyry Cu-Au deposit is located in the Stikinia island-arc terrane in northwest British Columbia. It is hosted by the Red Stock, which has four phases of porphyry intrusions: P1, P2E, P2L, and P3. New U-Pb dating of zircon shows that these intrusions were emplaced at 211.6 ± 1.3 Ma (MSWD = 0.85), 206.0 ± 1.2 Ma (MSWD = 1.5), 203.6 ± 1.8 Ma (MSWD = 1.5), and 201.7 ± 1.2 Ma (MSWD = 1.05), respectively. The ore-forming event at Red Chris was a short-lived event at 206.1 ± 0.5 Ma (MSWD = 0.96; weighted average age of three Re-Os analyses), implying a duration of <1 m.y., as defined by the uncertainty range. This mineralization age coincides with the emplacement age of the P2E porphyry, and is consistent with crosscutting relationships that suggest P2E was the main syn-mineralization intrusion. Zircons from P1 to P3 porphyry rocks have consistently high EuN/EuN* ratios (mostly > 0.4), indicating that their associated magmas were moderately oxidized. The magmatic water contents estimated from plagioclase and amphibole compositions suggest H2O contents of ~5 wt. %. Taken together, the P1 to P3 porphyries are interpreted to be moderately oxidized and hydrous. The four phases of porphyries are differentiated by sulfur and chlorine contents. The SO3 contents of igneous apatite microphenocrysts from the mineralization-related P2 porphyries are higher (P2E: 0.30 ± 0.13 wt. %, n = 34; P2L: 0.29 ± 0.18 wt. %, n = 100) than those from the pre-mineralization P1 (0.11 ± 0.03 wt. %, n = 34) and postmineralization P3 porphyries (0.03 ± 0.01 wt. %, n = 13). The chlorine contents in apatite grains from the P2E and P2L porphyries are 1.18 ± 0.37 (n = 34) and 1.47 ± 0.28 wt. % (n = 100), also higher than those from P1 (0.51 ± 0.3 wt. % Cl, n = 34) and P3 (0.02 ± 0.02 wt. % Cl, n = 17). These results imply that the sulfur and chlorine contents of the P2E and P2L magmas were higher than in the P1 and P3 magmas, suggesting that elevated magmatic S-Cl contents in the P2 porphyries may have been important for ore-formation. Although the process that caused the increase in sulfur and chlorine is not clear, reverse zoning seen in plagioclase phenocrysts from the P2 porphyry, and the occurrence of more mafic compositions in P2L suggest that recharge of the deeper magma chamber by a relatively S-Cl-rich mafic magma may have triggered the ore-forming hydrothermal event.