Hydrothermal Exhalitive Sedimentary Siliceous Rocks and Their Relationship with Pb- Zn Mineralization in Xicheng Orefield, Gansu, China

Xicheng Pb-Zn Orefield is located in Huixian, Chengxian and Xihe counties, southeastern Gansu. The ore-bearing rock sequences consisting of marine clastic and carbonates rocks of middle Devonian. Two types of the strata-bound ore deposits have been divided based on the ore deposits feature: Changba and Bijiashan types. Especially Bijiashan type deposit, which's main host rocks are siliceous rocks. The siliceous rocks, distributing a little broader than ore bodies, are closely related to Pb- Zn mineralization and its thickness has a normal ratio with thickness of ore body. In section, the siliceous rocks, which are flatly contact with overlying phyllite and roughly contact with underlying carbonates, are bedded, bedded-like or lenticular and paralled or simultaneously folded with the adjoining rocks. The main types of the siliceous rocks are divided into brecciated siliceous rocks, dark-gyay or gray massive siliceous rocks, riband-band siliceous rocks and white vein quartzite. They have very obvious sedimentary charactes and coexist with hydrothermal sedimentary barite rocks except vein quartzite. Geochemical analysis show that the siliceous rocks possess characteristic geochemistry: (1) the content of silica and some significant ratios (SiO₂ = 82.97% , Fe₂O₃ / FeO<1, K₂O>Na₂O, A1₂O₃ = 28.41, SiO₂/ (K₂O+Na₂O) =143.05, SiO₂/ MgO=60.56, Fe+Mn/ Ti= 30.18) are quite similar to those in hydrothermal sedimentary siliceous rocks, but different from those in normal sedimentary siliceous rocks and volcanic siliceous roks; (2) the contents of trace elements such as Ag, As, Hg, Sb, Ge and Cd are higher than Clark value by 1-3 logarithmic scales, this feature is one of the indicators of hydrothermal sedimentary origin; (3) the results of REE analysis show that the REE models of siliceous rocks are HREE-rich type and negative cerium or europium anomalies, REE models of different types of siliceous rocks and Pb- Zn ore are quite similar. The total contents of REE in the siliceous rocks (Σ REE = 3.09-42.012ppm) is similar to those in Pb-Zn ore (Σ REE - 3.773-30.71ppm), and the character-istic parameters (Sm/Nd= 0.16-0.32, La/Yb= 6.62-22.14, δEu=0.42-1.15, δCe=0.49-0.98) are similar to those of hydrothermal exhalative sedimentary siliceous rocks from Fengtai and Dachang (China) orefields and so on, indicating both Pb-Zn ore and the siliceous rocks have the same material source, formation processes and environment. Through study of the characteristics of petrology and geochemistry of the siliceous rocks, the facts show that the siliceous rocks in Xicheng district are ocean floor hydrothermal exhalative sedimentary originated.