Cretaceous to Paleogene polyphase faulting and paleostress inversions in the Taoshan-Zhuguangshan uranium metallogenic belt, South China Block

Abstract

Polyphase faulting and fault reactivation are liable to occur in hydrothermal ore fields. However, the relationship between fault reactivation and mineralization remains ambiguous, particularly in the NE-SW-striking Taoshan-Zhuguangshan uranium metallogenic belt, South China Block. Field investigations, core observations, and paleostress inversions were carried out for the Lujing and Gulonggang hydrothermal ore fields. Field investigations and previous exploration profiles show that the main ore-controlling structures are the NE-SW-striking high-angle faults. The maximum density of poles shows that these faults dip mainly to SE at high angles (70–88°). Kinematic indicators and plunges of slickenlines indicate that these NE-SW-striking high-angle faults have undergone polyphase faulting and fault reactivation, including left-lateral strike-slip shearing, normal dip-slip motion, and right-lateral strike-slip shearing. Dip angles of faults (n = 667) and quartz veinlets (n = 407) in sub-vertical drill cores vary mainly from 30 to 65°and 60 to 70°for the Lujing ore field and from 20 to 50°and 25 to 85°for the Gulonggang ore field, respectively. The difference suggests that hydrothermal fluids migrated preferentially along the high-angle secondary fractures of the main faults. Paleostress inversions of fault-slip data (n = 375) revealed six stages of paleostress regimes, including 1) the early to middle Early Cretaceous extensional regime (subhorizontal NW-SE-trending σ₃), 2) the late Early Cretaceous strike-slip regime (subhorizontal NNW-SSE-trending σ₁), 3) the latest Early Cretaceous to early Late Cretaceous extensional regime (subhorizontal NW-SE-trending σ₃), 4) the latest Cretaceous strike-slip regime (subhorizontal WNW-SEE-trending σ₁), 5) the early Eocene strike-slip regime (subhorizontal NE-SW-trending σ₁), and 6) the middle to late Eocene extensional regime (subhorizontal NE-SW-trending σ₃). Field investigations, core observations, and previous metallogenic ages show that uranium mineralization in the Taoshan-Zhuguangshan uranium metallogenic belt occurred mainly along NE-SW-striking high-angle normal faults under the latest Early Cretaceous to early Late Cretaceous NW-SE extension. These high-angle normal faults were not new-born but reactivated from left-lateral strike-slip faults formed in the late Early Cretaceous strike-slip regime. NE-SW-striking polyphase faulting and fault reactivation during the Cretaceous were triggered by the subduction of the Paleo-Pacific Plate underneath the South China Block.