Hydrothermal alteration and fluid chemistry of the Endako porphyry molybdenum deposit, British Columbia

Hydrothermal alteration and fluid chemistry data of the early Cretaceous En dako porphyry molybdenum deposit, British Columbia, provide new information on the hydrothermal fluids associated with low-fluorine molybdenite minera lization. Molybdenite mineralization and hydrothermal alteration occur as e arly quartz +/- molydenite stockwork veins with K feldspar-bearing salvages and paragenetically later quartz-molybdenite ribbon veins with sericite-be aring selvages. Late hydrothermal alteration is associated with the develop ment of kaolinite and postore (Tertiary age) calcite veins. Fluid inclusions in early-formed quartz +/- molybdenite stockwork veins wit h K feldspar-bearing alteration assemblages are dominated by moderate salin ity (5 to 15 wt % NaCl equiv), liquid-rich (type 1) and rare high-salinity (30 to 45 wt % NaCl equiv), halite-bearing (type 3) fluid inclusions. Type 1 and type 3 fluid inclusions in early veins homogenize between 390 degrees and 430 degrees C and 375 degrees and 420 degrees C, respectively. Seconda ry fluid inclusions (type 2) of low salinity (1 to 5 wt % NaCl equiv) in th ese early veins are minor, and homogenize between 130 degrees and 285 degre es C. Fluid inclusions in quartz-molybdenite ribbon veins with sericite-bea ring alteration assemblages are dominated by moderate-salinity, liquid-rich (type 1) inclusions, with minor type 2 fluid inclusions. Type 1 fluid incl usions of ribbon veins homogenize between 360 degrees and 400 degrees C. Fl uid inclusions in postore cal cite veins are of only type 2 fluid inclusion s, which homogenize at 209 degrees C. Hydrothermal fluids recorded by type I and type 3 fluid inclusions in early veins were trapped under lithostatic to hydrostatic conditions between 0.3 and less than or equal to 2.0 kbar, and 360 degrees and 560 degrees C. Postore fluids recorded by type 2 fluid inclusions were trapped under conditions less than or equal to 0.5 kbar, an d between 190 degrees and 300 degrees C. Quartz stockwork and ribbon Veins possess delta(18)O values of 8.4 +/- 0.2 (n = 9) and 8.4 +/- 0.6 (n = 13), respectively. Hydrothermal K feldspar and biotite from K feldspar alteration assemblages possess delta(18)O values o f 6.8 +/- 0.4 (n = 7) and 3.5 +/- 0.8 (n = 8), respectively. Oxygen isotope geothermometry of quartz-biotite and quartz-K feldspar pairs from K feldsp ar alteration assemblages yield temperatures between 200 degrees and 490 de grees C, which is similar to the trapping temperatures of hydrothermal flui ds determined from fluid inclusion studies associated with molybdenite mine ralization, the development of kaolinite, and calcite veins. The oxygen iso tope temperatures of the quartz-biotite and quartz-K feldspar pairs suggest that K feldspar and biotite either record the approximate O-18 composition of hydrothermal fluids associated with K feldspar alteration or have under gone O-18 exchange with late-stage hydrothermal fluids. Hydrogen isotope co mposition of quartz stockwork and ribbon veins fluid inclusion waters range between -105 and -113 per mil. Solute chemistry studies of fluid inclusion waters indicate that ore formin g fluids from Endako have low Br/Cl and Br/Na ratios, and high I/Cl and I/B r ratios in comparison to Porgera (epithermal), Babine Lake (porphyry Cu), and St. Austell, Capitan Pluton (vein) deposits associated with magmatic pr ocesses. Na/K ratios of fluid inclusion waters yield temperatures (308 degr ees to 429 degrees C) similar to those determined from type 1 and type 3 fl uid inclusions and stable isotope thermometry. Results from fluid inclusion and solute chemistry studies indicate the invo lvement of hydrothermal fluids exsolved from a crystallizing melt in the fo rmation of the Endako molybdenum deposit. However, oxygen and hydrogen isot ope values deviate from the generally accepted magmatic compositions, which suggests the early involvement of meteoric water in the ore-forming fluids and ore genesis.