PACROFI VI - Electronic Program


A Fluid Inclusion and Stable Isotope Study of the Endako Porphyry Molybdenite Deposit, British Columbia: Implications for P-V-T-X History of Ore and Post Ore Fluids

David Selby and Bruce.E.Nesbitt

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada, T6G 2E3


The Endako quartz monzonite, a Late Jurassic (138 +/- 3 m.a.) pluton of the Francois Lake intrusion suite in the Intermontane Belt, central British Columbia, hosts Canada's largest porphyry molybdenum deposit, the Endako mine. The deposit features pre-mineral dykes of predominantly quartz feldspar porphyry, with lesser amounts of porphyritic granite and aplite dykes, and Eocene post-mineral andesitic dykes. Potassic, phyllic and argillic assemblages characterize the alteration zones. Mineralization is characterized by stockwork quartz+molybdenite veins, bordered by potassic selvages; stockwork quartz+magnetite+pyrite+molybdenite and quartz+molybdenite ribbon veins, bordered by phyllic selvages. Post-ore veins are comprised of calcite and minor chalcedony. Mineralized veins are characterized by liquid-rich and halite-bearing fluid inclusions (Bloom, 1981; Dawson, 1972) . This study has sub-divided these inclusions into 3 types: Type 1, liquid+vapour +/- opaque daughter minerals and liquid+vapour +/- translucent daughter minerals; Type 2, liquid+vapour and Type 3, liquid+vapour+halite. Type 1 and 3 inclusions are further sub-divided into sub-populations by microthermometric behavior and are typically primary, whereas type 2 are generally secondary. Type 1 and 3 inclusions characterize veins bordered by potassic selvages and Type 1 inclusions characterize veins bordered by phyllic selvages. Type 2 inclusions are predominant in ribbon veins, especially in those characterized by brecciation and quartz recovery textures.

Type 1 inclusions have salinities between 5 and 15 wt. % NaCl eq., with modes of 7 to 9 wt. % NaCl eq. The inclusions homogenize by vapour bubble disappearance and by critical behavior at temperatures between 250 and 550oC, with modes at 360 to 370oC, and 390 and 410oC. Salinities of Type 1 inclusions from veins bordered by potassic and phyllic selvages overlap considerably; however, homogenization temperatures of inclusions from veins bordered by phyllic selvages are generally 20 to 50°ree;C lower than those bordered by potassic selvages. Type 2 inclusions have salinities between 0.2 and 5 wt. % NaCl eq. Inclusions in calcite have salinities between 3.1 and 4.2 wt. % NaCl eq. Homogenization by vapour bubble disappearance occurs at temperatures between 130 and 285oC, with modes at 150 and 170oC and 190 to 230oC. Inclusions in calcite homogenize at temperatures between 208 and 210oC. Type 3 inclusions which homogenize by vapour bubble disappearance occurs at temperatures between 375 and 420oC and have salinities of 30 to 45 wt. % NaCl, and those which homogenize by halite dissolution occur at temperatures between 410 and 550oC and have salinities of 45 to 60 wt % NaCl eq (Bloom, 1981).

Fluid inclusion studies of Dawson (1972) and Bloom (1981) and this study suggest that Type 3 inclusions were not generated by aqueous fluid immiscibility. If aqueous fluid immiscibility had occurred to form these inclusions, saline (L+V+H) inclusions should coexist with a low-salinity vapour-rich inclusion, which have similar final homogenization temperatures. However, the latter population was not observed during this study nor by Dawson (1972) or Bloom (1981). Dawson (1972) suggested trapping pressures of 530 to 720 bars for a inclusion of 30 wt % NaCl trapped at temperatures of 500 to 550oC and Bloom (1981) suggested pressure fluctuations between extremes of lithostatic and hydrostatic load.

Stable isotope analyses of potassic, phyllic and argillic alteration phases yield depleted deltaD and enriched delta18O values. Quartz from quartz molybdenite stockwork and ribbon textured veins and hydrothermal K-feldspar and biotite associated with potassic selvages, yield delta18O values, of 8 to 9.3o/oo, 4.2 to 6.6o/oo and 3.1 to 4.5o/oo, respectively. Hydrothermal biotites, which have altered to phlogopite and chlinochore, yield depleted delta18O values of 3.0 to 1.8 and -2.1o/oo. Kaolinite from argillic alteration yields delta18O values of 6.6 to 6.9 o/oo and deltaD values of -156o/oo. Quartz from quartz molybdenite stockwork and ribbon textured veins yield deltaDFInc values of -105 to -157 o/oo. Delta qtz-Ksp and Delta qtz-bio values suggest isotopic equilibrium temperatures between 400 and 490oC. Results from post-mineralization carbonates samples within the Endako mine show three distinct isotopic groups. Samples are divided on the basis of delta18O values: Group 1 has values between 8 and 13 o/oo; Group 2 has values between 2 and 6o/oo and Group 3 has a values of <-3.0o/oo. delta13C values vein carbonate span a range between -2.0 and -6.2o/oo, with a mean of -4.0o/oo.

Results from fluid inclusion (types 1 and 3) studies of veins bordered by potassic selvages suggest the involvement of saline (30-60 wt % NaCl eq, Bloom, 1981) and low salinity (6-15 wt % NaCl eq) magmatic fluids (>480oC, Dawson, 1972) in the origin of the Endako molybdenum deposit. However, quartz deltaDFInc values deviate from generally accepted magmatic isotopic values, suggesting the early involvement of meteoric water in magmatic fluids and ore genesis.