Characteristics, Protolith Restoration And Petrogenesis For Metamorphic Rocks Of Longbeixi Formation In Dingjiashan Lead-zinc Mining Area, Youxi County, Fujian Province

The metamorphic rock series in middle-late proterozoic Longbeixi group is the major host strata of Dingjiashan lead-zinc deposit in Youxi county, Fujian province. The lithology of the host strata comprises mainly quartz-mica schist, marble, garnet-epidote-diopside series rocks. Based on geological occurrence, metamorphic mineral composition and their formation sequence, residual mineral and fabric, chemical typomorphic characteristics of representative minerals, geochemical characteristics of main and trace elements, the research results are summarized as follows. First, there were two episodes of metamorphism in the study area, which occurred in Caledonian and Yanshanian periods respectively. Second, the garnet epidote diopside rock in upper Longbeixi group recovered to be calcareous mudstone, calcareous siltstone or marl, and suffered green-schist regional metamorphism in Caledonian period and then strong skarnized caused by the contact metamorphism with granitic magma in Yanshanian period. Third, quartz-mica schist in middle Longbeixi group is the products of Caledonian regional metamorphism from silty mudstone with original rhythmic layered sedments. There existed general silicification or other hydrothermal alteration by Yanshanian magmatism, expecially strong in contact zone around dike and structural fracture zone. Fourth, The marble, dolomitic marble and impurity composition-banded marble in the upper and middle Longbeixi group, are considered to be derived form a terrigenous sedimentary sequence with carbonate and calcium-ferruginous fine clastic materials. they experienced recrystallization caused by Caledonian regional metamorphism, but were not transformed obviously because of undeveloped structural fissure and poor chemical mobility during the process of large-scale magmatism in Yanshanian. Fifth, useful components and the early-formed skarn are the products of the evolution process of magmatic hydrothermal fluids, the master mineralization epoch of the deposit was Yanshanian period, and it belongs to skarn type deposit.