Geology, Mineralization, and Fluid Inclusion Characteristics of the Kensu W-Mo Skarn and Mo-W-Cu-Au Alkalic Porphyry Deposit, Tien Shan, Kyrgyzstan

摘要

The Kensu deposit is located within a large metallogenic belt of W-Mo, Cu-Mo, W-Au, Au, and Pb-Zn deposits along the Late Paleozoic active continental margin of Tien Shan. The deposit is related to a small Late Carboniferous to Early Permian multiphase gabbro-monzonite(syenite)-granite pluton representing an alkaline potassic (shoshonitic) suite. The igneous rocks are related to a deep-rooted magmatic source but the pluton emplacement corresponds to shallow (“porphyry”) levels. The deposit represents an example of a complex W-rich alkalic magmatic-hydrothermal system that complements the typical mineral deposits related to shoshonitic igneous suites. It contains large bodies of W-Moskarn of the oxidized type, with abundant andradite garnet, capolite, K-Na feldspars, and Fe oxides (magnetite, hematite). The skarns are overprinted and surrounded by extensive halos of stockwork-disseminated (porphyry-style) Mo, W-Mo, W-Cu, Pb-Zn, and Au-W mineralization with propylitic (chlorite-amphiboleepidote–dominated) and phyllic (quartz-sericite-carbonate-sulfide) alteration assemblages. Consistent with the oxidized nature of the major mineral assemblages, the deposit may represent the “oxidized lithophile” W-Mo-Cu-Au metallogenic type. The hydrothermal stages alternate with magmatic phases, and fluid inclusion data show the predominance of high-temperature (～550°–450℃), high-pressure (1,200–700 bars) and high-salinity (60–45 wt % NaCl equiv)magmatic-hydrothermal fluid at prograde and early retrograde skarn stages, with its possible direct exsolution from crystallizing magma. The general trend of cooling and dilution was complicated by the influx of highertemperature and higher-salinity fluid that correlates to the intrusion of quartz monzonite, and the initial stages of its degassing and cooling. This more advanced phase of magmatic differentiation likely corresponded to the enrichment of residual melt and related magmatic fluids in W and Mo. Intense fluid boiling and phase separation triggered the deposition of molybdoscheelite in skarns that formed at T = >625°–500℃ and P = 510–300 bars and continued to a nonboiling fluid at T = >420°–370℃ and P = 1,150–750 bars. Mineralization in propylitic alteration zones was formed by highly to moderately saline nonboiling fluids. Late quartz-sericite-carbonate-sulfide stage involved both high to low salinity aqueous fluids and CO2-rich fluids, under decreasing temperature (from >400℃ to 200°–300℃) and varying pressure (from ~1,200 bars to 750–600 bars).