关键词: 下庄铀矿田, 断裂, 花岗岩原地重熔说

Abstract:

The Xiazhuang hydrothermal uranium orefield is located in the east part of IndosinianYanshan Guidong composite granite body, where developed three systems of NWW, NNE and NENEE faults. On the basis of the former research and our field and thin section investigations, the fault features, their active history and tectonic stress field since Indosinian tectonic movement are preliminarily revealed. Combining the explanation of insitu melting theory of granite, we have reached the following considerations: the NWW faults and NNE faults that were probably originated from conjugation shearing joints in the Late Jurassic had developed into two major fault systems in the orefield in the early Cretaceous. The former were extended so as to control the intrusion of diabase magma, and the latter were sheartensional so as to control coarse to microcrystallized quartz veins and various alterations. Nearly at the end of the early Cretaceous the NNE faults developed into ductile shear zones and then in the late Cretaceous changed to dextral tensional activities, which partly controlled intrusion of diabase magma. NENEE structures were mainly folds and reverse faults in the Jurassic and the NENEE faults changed to sinistral compressshear ones in the early Cretaceous and then changed to tensional ones in the late Cretaceous, which controlled the formation of the red bed basin. In transitional period from the early to late Cretaceous, regional compression stress changed from NWSE to NESW. The Guidong granite body experienced multimelting in IndosinianYanshan tectonic movement, provided very good conditions for reactivation, migration and enrichment of uranium. Two mineralization stages of uranium in the early and late Cretaceous are considered to be related to two remelting events of granite magma layers. When melting interface (MI) moved upwards and granite magma layer melted, faults cut shallowly into remelting layer and became “gateways” for magma hydrothermal fluid. However, when MI moved downwards and granite magma layer crystallized, faults could cut deeply down into the lower part of remelting layer and became “gateways” for basic magma. Because of more intensive tension, NWW faults were major “gateways” for basic magma. NENEE faults were relatively smaller ones, often acted as “host structure”. Whereas NNE faults as relatively larger ones were tensional in the early to late Cretaceous so as to become major “gateways” for uprising of uranium bearing fluid and became important “gateway structure”. When upward migration of the oreforming fluid met diabase dyke, the uranium was reduced so that was enriched. When entering into NENEE faults, the uranium bearing fluid was “jammed” and concentrated. The two locations were all favorable places for mineralization of hydrothermal uranium. Layer granite+“uranium mineralization horizon”+faults are thought to be guiding principles for finding deeper and hidden uranium mines in the Xiazhuang orefield.

Key words:  Xiazhuang uranium orefield, fault, remelting theory for granite formation