Ore genesis and fluid evolution of the Kaladawan South Zn-Pb-Cu ore field, eastern Altyn Mountains (NW China): Evidence from fluid inclusions, H-O isotopes and geochronology

摘要

The newly-discovered Kaladawan South Zn-Pb-Cu orefield in eastern Altyn Mountains is composed of five Zn-Pb(-Cu) deposits, with the largest one containing 414.4 thousand tonnes (kt) Zn, 265.2 kt Pb and 149.2 kt Cu. Stratiform/lenticular orebodies are hosted by the Ordovician volcanic-sedimentary sequences subjected to greenschist-facies metamorphism. Two major ore-forming stages with significant cross-cutting relationship are identified at Kaladawan South: Stage (I) of the earlier massive, banded, disseminated barite-rich sulfide ores, and Stage (II) of the later polymetallic sulfide-quartz veins with economically less importance. Fluid inclusions (FIs) locked in quartz of Stage I massive sulfide ores are homogenized completely at 226-354 degrees C with calculated seawater-like salinities of 2.7-4.2 wt% NaCl equiv. These characteristics of ore-forming fluids coupled with ore textures of typical VMS deposits, are interpreted to be originated from the modified seawater by sea-floor volcanic-hydrothermal activities. In compared, the Stage II FIs yielded lower homogenization temperatures ranging 251 degrees C-357 degrees C with higher salinities of 0.8-14.4 wt% NaCl equiv. Laser Raman analyses identified large amounts of CO2 +/- (CH4-N-2) in the hydrothermal fluids of Stage II. The delta(OHO)-O-18-O-2 values (0.5-5.7 parts per thousand) and delta D-H2O values of (-68.1 parts per thousand to -32.3 parts per thousand) of Stage II quartz are plotted in the range of metamorphic fluids in the D-O isotopic diagram, which indicating that the CO2-rich and mesothermal fluids are metamorphic-derived. The phlogopite paragenetic in the Stage II polymetallic vein yielded a Ar-40/Ar-39 plateau age of 435.4 +/- 3.9 Ma, similar to the age of the metamorphic muscovite (451.0 +/- 4.1 Ma) from the ore-hosting schist. Based on the ore deposit geology, mineral assemblages and fluid geochemistry, the Kaladawan South Zn-Pb-Cu mineralization is attributed to be an Ordovician VMS-type deposit overprinted by the Silurian metamorphic fluids.