Hydrothermal Alteration and Fluid Chemistry of the Endako Porphyry Molybdenum Deposit, British Columbia

摘要

Hydrothermal alteration and fluid chemistry dataof the early Cretaceous Endako porphyry molybdenum deposit,British Columbia, provide new information on the hydrothermalfluids associated with low-fluorine molybdenite mineralization.Molybdenite mineralization and hydrothermal alteration occuras early quartz +- molydenite stockwork veins with K feldspar-bearing selvages and paragenetically later quartz-molybdeniteribbon veins with sericite-bearing selvages. Late hydrothermalalteration is associated with the development of kaolinite andpostore (Tertiary age) calcite veins.Fluid inclusions in early-formed quartz +- molybdenitestockwork veins with K feldspar-bearing alteration assemblagesare dominated by moderate-salinity (5 to 15 wt percent NaClequiv), liquid-rich (type 1) and rare high-salinity (30 to 45 wtpercent NaCl equiv), halite-bearing (type 3) fluid inclusions.Type 1 and type 3 fluid inclusions in early veins homogenizebetween 390 deg and 430 deg C and 375 deg and 420 deg C,respectively. Secondary fluid inclusions (type 2) of low salinity(1 to 5 wt percent NaCl equiv) in these early veins are minor,and homogenize between 130 deg and 285 deg C. Fluidinclusions in quartz-molybdenite ribbon veins with sericite-bearing alteration assemblages are dominated by moderate-salinity, liquid-rich (type 1) inclusions, with minor type 2 fluidinclusions. Type 1 fluid inclusions of ribbon veins homogenizebetween 360 deg and 400 deg C. Fluid inclusions in postore cal-cite veins are of only type 2 fluid inclusions, which homogenizeat 209 deg C. Hydrothermal fluids recorded by type 1 and type 3fluid inclusions in early veins were trapped under lithostatic tohydrostatic conditions between 0.3 and less than or equal to 2.0kbar, and 360 deg and 560 deg C. Postore fluids recorded bytype 2 fluid inclusions were trapped under conditions less thanor equal to 0.5 kbar, and between 190 deg and 300 deg C.Quartz stockwork and ribbon veins possess #delta#~(18)Ovalues of 8.4 +- 0.2 (n valence 9) and 8.4 +- 0.6 (n valence 13),respectively. Hydrothermal K feldspar and biotite from Kfeldspar alteration assemblages possess #delta#~(18)O values of6.8 +- 0.4 (n valence 7) and 3.5 +- 0.8 (n valence 8),respectively. Oxygen isotope geothermometry of quartz-biotiteand quartz-K feldspar pairs from K feldspar alterationassemblages yield temperatures between 200 deg and 490 deg C,which is similar to the trapping temperatures of hydrothermalfluids determined from fluid inclusion studies associated withmolybdenite mineralization, the development of kaolinite, andcalcite veins. The oxygen isotope temperatures of the quartz-biotite and quartz-K feldspar pairs suggest that K feldspar andbiotite either record the approximate ~(18)O composition ofhydrothermal fluids associated with K feldspar alteration orhave undergone ~(18)O exchange with late-stage hydrothermalfluids. Hydrogen isotope composition of quartz stockwork andribbon veins fluid inclusion waters range between -105 and -173 per mil.Solute chemistry studies of fluid inclusion waters indicate thatore-forming fluids from Endako have low Br/Cl and Br/Naratios, and high I/Cl and 1/Br ratios in comparison to Porgera(epithermal), Babine Lake (porphyry Cu), and St. Austell,Capitan Pluton (vein) deposits associated with magmaticprocesses. Na/K ratios of fluid inclusion waters yieldtemperatures (308 deg to 429 deg C) similar to those determinedfrom type 1 and type 3 fluid inclusions and stable isotope thermometry.Results from fluid inclusion and solute chemistry studiesindicate the involvement of hydrothermal fluids exsolved from acrystallizing melt in the formation of the Endako molybdenumdeposit. However, oxygen and hydrogen isotope values deviatefrom the generally accepted magmatic compositions, whichsuggests the early involvement of meteoric water in the ore-forming fluids and ore genesis.