Depositional Age And Genesis Of The Host Strata Of The Shuangjianzishan Ag-Pb-Zn Deposit: Implications For The Late Carboniferous Magmatic-Hydrothermal Activities And Tectonic Evolution Of The Eastern Central Asia Orogenic Belt

The Shuangjianzishan Ag-Pb-Zn deposit is hosted by an over 2030-m-thick succession of Late Carboniferous slate-dominated deep-water sediments. Three tuff layers interbedded with slates yield zircon U-Pb ages of 314.4 +/- 2.5 Ma, 315.9 +/- 3.6 Ma, and 317.9 +/- 2.3 Ma. Using Monte-Carlo simulations, we constrain the maximum time span between the bottom tuff and the top tuff to no longer than 6.9 Myr at a 2 sigma (95.3%) confidence level. On the basis of this time span, the long-term accumulation rate of the Shuangjianzishan slates is 294 m/Myr. This accumulation rate is faster than any subduction-related accumulation rates by at least 30%; it is similar to that of fine deposits in a rift setting. The variably low delta O-18 of the zircons from the interbedded tuffs also suggest that a high-temperature hydrothermal alteration occurred during the generation of the low delta O-18 parental magma. Magmatic-hydrothermal events related to a rifting event are the most likely mechanism that caused the low delta O-18 signature. The chemical compositions of the slates suggest that their source rocks were very immature, probably similar to contemporary intermediate to acidic volcanic rocks. On the basis of down-well logging data, the dipping angle data are plotted with the stratigraphic depth of each stratum. The stable dipping angle of the slates also supports a sedimentation processes in an active rifting environment.