Geochemistry and origin of tin–polymetallic sulfide deposits hosted by the Devonian black shale series near Dachang, Guangxi, China

Abstract

Two tin–polymetallic sulfide deposits hosted by the Devonian black shale series of the Guangxi Zhuang Autonomous region occur in the Dachang ore field. The Upper Devonian (Frasnian Epoch) black shale series contains the giant Tongkeng-Changpo deposit, and the Middle Devonian (Givetian Epoch) reef limestone contains the giant Longtoushan deposit. Intrusive rocks occur at Lame and the western part of Dachang. Ores occur as stratified bodies, large veins, stockworks and zones of veinlets. Ores are massive, banded, laminated and disseminated. The ores contain more than 180 different minerals, the most important of which are cassiterite, sulfides and sulfosalts. Primary laminated sedimentary sulfide ores occur at Tongkeng. The mean ratio of SiO 2/Al 2O 3 for shale–marl is 3.5, but values averaging 56 and 67, respectively, for ore and chert suggest a biogenic or hydrothermal source for silica. Ores and country rocks are rich in Sb, Pb, Zn, Sn and Cu; granites are rich in Sb, W, Zn and Sn. Dispersed elements, such as Te, Cd, Se and In, are abundant in ores, country rocks and granites, implying contributions of sedimentary and intrusive activities to the ore formation. Mean Y/Ho values for the black shale series (28) and granite (31) are similar to most shales and most volcanic rocks; the Y/Ho value for reef limestone is 47, comparable to sea water. Devonian black shale series are slightly LREE-enriched on chondrite-normalized plots and show small negative Eu anomalies; mean δEu (0.56), δCe (0.9) and Ce anom (−0.07) values suggest a shallow-water deposition near the redox boundary. The δ 34S values of sulfides from various deposits of Dachang vary with the δ 34S values for Lame ranging from −2‰ to +2‰, and values for Tongkeng-Changpo ranging from −4.9‰ to +2.6‰. The highly negative δ 34S values (−9.5‰ to −11.3‰ ) of Huile and Kengma sulfides suggest the important role of biogenic sulfur. The mean δ 13C value of organic matter of Devonian black shale series is −24.6‰. The δ 30Si NBS-28 values of Upper Devonian black chert are 0.2–0.3‰, indicating a sedimentary origin. The Dachang black chert has been dated at 124±2 Ma; that of Luofu section at 387±18 Ma, implying that the Rb/Sr isotope of Dachang chert was reset during Yanshan Movement. These results suggest that the Dachang tin–polymetallic sulfide deposits exemplify strongly remobilized sedimentary mineralization.