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
\nnorthwest British Columbia. It is hosted by the Red Stock, which has four phases of
\nporphyry intrusions: P1, P2E, P2L, and P3. New U-Pb dating of zircon shows that
\nthese intrusions were emplaced at 211.6 ± 1.3 Ma (MSWD = 0.85), 206.0 ± 1.2 Ma
\n(MSWD = 1.5), 203.6 ± 1.8 Ma (MSWD = 1.5), and 201.7 ± 1.2 Ma (MSWD = 1.05),
\nrespectively. The ore-forming event at Red Chris was a short-lived event at 206.1 ± 0.5
\nMa (MSWD = 0.96; weighted average age of three Re-Os analyses), implying a
\nduration of <1 m.y., as defined by the uncertainty range. This mineralization age
\ncoincides with the emplacement age of the P2E porphyry, and is consistent with crosscutting
\nrelationships that suggest P2E was the main syn-mineralization intrusion.
\nZircons from P1 to P3 porphyry rocks have consistently high EuN/EuN* ratios (mostly >
\n0.4), indicating that their associated magmas were moderately oxidized. The magmatic
\nwater contents estimated from plagioclase and amphibole compositions suggest H2O
\ncontents of ~5 wt. %. Taken together, the P1 to P3 porphyries are interpreted to be
\nmoderately oxidized and hydrous.
\nThe four phases of porphyries are differentiated by sulfur and chlorine contents. The
\nSO3 contents of igneous apatite microphenocrysts from the mineralization-related P2
\nporphyries are higher (P2E: 0.30 ± 0.13 wt. %, n = 34; P2L: 0.29 ± 0.18 wt. %, n = 100)
\nthan those from the pre-mineralization P1 (0.11 ± 0.03 wt. %, n = 34) and postmineralization
\nP3 porphyries (0.03 ± 0.01 wt. %, n = 13). The chlorine contents in
\napatite grains from the P2E and P2L porphyries are 1.18 ± 0.37 (n = 34) and 1.47 ±
\n0.28 wt. % (n = 100), also higher than those from P1 (0.51 ± 0.3 wt. % Cl, n = 34) and
\nP3 (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,
\nsuggesting that elevated magmatic S-Cl contents in the P2 porphyries may have been
\nimportant for ore-formation. Although the process that caused the increase in sulfur
\nand chlorine is not clear, reverse zoning seen in plagioclase phenocrysts from the P2
\nporphyry, and the occurrence of more mafic compositions in P2L suggest that recharge
\nof the deeper magma chamber by a relatively S-Cl-rich mafic magma may have
\ntriggered the ore-forming hydrothermal event.