Ore-forming Response to Syndepositional Submarine Volcanism in Langshan-Zhaertai Mesoproterozoic SEDEX Ore Belt, Inner Monsolia, China

摘要

Ore-forming process of the Dongshengmiao, Huogeqi, Tanyaokou and Jiasbengpan deposits in Langshan-Zhaertai Mesoproterozoic SEDEX metallogenic belt is closely related to the syndepositional volcanic activities based on the following facts: (1) The Sm-Nd isochron age of the basic volcanic rocks varies from 1 491 Ma to 1 824 Ma (more than or close to the model age of the lead isotope of all sulfide minerals) in these deposits, with ε(Nd, t) = (3.48 - 6.40) ± 0.80, whose REE composition is enriched in LREE and depleted in HREE, indicating that these volcanic rocks were derived from the mantle or lower crust. (2) The REE composition of some Pb-Zn-Py-ores is also enriched in LREE and depleted in HREE. The chondrite-normalized REE patterns are similar to those of the basic-volcanic rocks. (3) In the lead isotope composition diagram of Doe and Zartman, most of sphalerite, galena, pyrite, pyrrhotite and chalcopyrite are plotted on both sides of the line for the mantle or between the lines for the mantle and lower crust. (4) Cobalt content of some pyrites is much higher than their nickel content (w(Co)/w(Ni ) = 11.91 - 12.19). (5) Some volcanic blocks and debrises have been picked out from some pyrite and pyrohotite ores. (6) All Zn-Pb-Cu-Fe sulfide orebodies in these deposits occur in the strata overlying the metamorphic volcanic rocks in the only second ore-bearing formation. The Jiashengpan deposit lacks in syndepositional volcanic rocks in the host succession, only Pb and Zn occur without Cu, but the Dongshengmiao, Tanyaokou and Huogeqi deposits with syndepositonal volcanic rocks in the host succession contain Cu, indicating the relatively high ore-forming temperatures, besides Pb and Zn. The syndepositional volcanic eruption directly supplied some ore-forming metals and resulted in the secular geothermal anomaly favorahle for the circulation of the submarine convective hydrothermal sys- tem, and in the precipitation of deep mineralizing fluids exhaling into the anoxide basins along the syndepositional fault system in the Langshan-Zhaertai rift.