On Mesozoic Structure Deformation And Its Control Over The Gold-Lead-Zinc Mineralization In The Fengtai Ore Cluster In West Qinling, China

The Fengxian-Taibai (abbr. Fengtai) ore cluster in Shaanxi Provence is one of the most important polymetallic ore forming zone in the Western Qinling Orogen (WQO). There has been a lack of detailed studies on the structure style and ore-controlling mechanism in the Fengtai ore cluster, though it has been a common understanding that the gold, zinc-lead (-copper) deposits are controlled by the structures. And the granitoid plutons and dikes and their relationship with the tectonism and minerlization are also parts of the least studied aspects. In this paper, the structures (especially the ore-controling ones), the gold-lead-zinc deposits and granitoid plutons and dikes are taken as the research object. After detailed investigation of the Fengtai ore cluster and structural analysis, combined the ptrology, geochemistry and geochronological studies, conclusions are reached as follows:1. The NWW-trending and NE-trending strike-slipping faults, the NW-trending complex folds, SEE-or NWW-plunging B-type lineation and a series of other structures are developed in the Fengtai ore cluster. Inside the cluster, the occurrences of the B-type lineations and NE-trending joints show zoning regularity:B-type lineations plunge towards SEE direction and the NE-trending joints dip towards NWW-NW direction in the middle part of the cluster, and in the north and south sides, B-type lineations plunge towards NWW direction and the NE-trending joints dip towards SEE-SE direction. All the structures are associated with each other, constituting a strike-slip duplex under a primary stress orienting NE-SW direction, which was derived from the sinistral shear.2. The deformation in the Fengtai ore cluster took place during the Mesozoic period, especially from the middle-late Triassic to early Jurassic. The structural sequence in the area are reconstructed as following:〥eposition of the clastic sedimentary rocks formed bedding (So) during the Devonian period;︰nder the NE-SW compression(D1-1), the neoformation of sericite and muscovite are deformedwith an oriented arrangement, being phyllitic folication, which was parallel or subparallel to S0; and meanwhile, the NWW-trending folds, crenulation cleavage (S2) and lineations were formed;(3)then S2were folded to S3and the NWW-trending brittle-ductile shear zones were formed by NE-SW transpression(D1-2);④during the tectonic level changing from middle level to upper one(D1-3), the NWW-trending brittle faults were developed at the limbs of the folds by interlayer sliding, and the NWW-trending tension joints were developed near the core of the folds; and then NE-trending faults, jonits, and cleavages cut the NWW-trending structures;⑤the NNW-trending faults were formed at the late stage of the NE-SW compression;⑥after the main deformation period, a few tension fractures were developed under the vertical primary stress.3. Granitiods in the Fengtai ore cluster commonly belong to metaluminous, shoshonitic to high-K calc-alkaline series and and show similar characteristics to I-type granitic rocks. Geochemical data show that the magma might come from two different sources:there were garnet and no plagioclase left in one source and there were no garnet but plagioclase left in the other source. And the depth of the former might be larger than that of the latter. By integrating the regional tectonic and magmatic characteristics, we propose that the granitoid intrusions of early stage (±250Ma) are most likely developed during subduction, that of middle stage (230-220Ma) are formed during the continental collision, and the post-collisional granitoid rocks commonly constitute the late stage granitoid (220-190Ma).4. There exists mineralization zoning in the Fengtai ore cluster:deposits in the north part of the cluster are commonly gold and lead-zinc ones, while that in the south part are mainly lead-zinc and copper deposits. NWW-trending brittle-ductile shear zones caused the gold element to be remobilized and formed the gold bearing metamorphic fluid, and also provided the necessary driving force and channel for the migration of the fluid. The compressive and compressive-shear structures formed during the early stage, and the NE-trending brittle fractures (especially the NNE-trending joints) formed during the late stage act as host structures. The mian ore bodies of the lead-zinc deposits are controlled by anticlines and the Si-Ca interface, ore bodies developed in the limb part of the fold are controlled by the interlayer faults. Mineralization in the NE or NW trending fractures is very weak, and those structures commonly destroy the continuity of the lead-zinc ore bodies. Deformation and metamorphism caused the dehydration of the regional strata and the activation of the metallogenic materials, constituting the dynamic metamorphic hydrothermal fluid. Under the tectonic stress and temperature gradient driving, the metamorphic fluid and magmatic fluid migrated upward along the NWW-trending faults, and mixed to form the mixed fluid. In the brittle-ductile shear zone, the core of the anticlines and NE-trending joints, fluid’s losing its balance lead to the precipitation of the Au-Pb-Zn metallogenic materials, forming a series of deposits.5. More and more age data show that the time of deformation, metamorphism (tectonic metamorphism), magmatism and mineralization in the Fengtai ore cluster are all focused on190-230Ma, implying they are space-time coupling with each other, and are the products of the Qinling orogenic process in Indosinian. Structure deformation, magma and ore-bearing fluid promoted and influenced each other, and finally formed the current structure style and gold-lead-zinc-copper minerogenetic system.