Geochronology and geochemistry of the Dianfang gold deposit, western Henan Province, central China: Implications for mineral exploration

Abstract

The Dianfang gold deposit, located in the Xiong’ershan–Waifangshan region within the southern margin of the North China Craton, consists of eastern and western ore blocks, which are hosted in the Dianfang breccia pipe and Paleoproterozoic volcanic rocks, respectively. The eastern ore block, which occurs within the southern and hanging wall margins of the Dianfang breccia pipe and in fractures in the rhyolites of the Jidanping Formation, consists of rhyolite-hosted and breccia-hosted disseminated gold mineralization structurally controlled by ENE-striking faults. The western ore block contains gold-bearing quartz-sulfide veins and minor disseminated rhyolite-hosted gold mineralization that occurs in the annular fractures around the Hougou breccia pipe. The two ore blocks have long been researched and prospected as two different genetic types. Detailed fieldwork and microscopic observations reveal that the eastern and western ore blocks contain the same hydrothermal alteration and mineral assemblages, which include K-feldspar, quartz, sericite, chlorite, epidote, carbonate, pyrite, galena, sphalerite, marcasite and chalcopyrite, as well as minor magnetite, anatase, cervelleite, matildite, and bismuthinite. Gold occurs as electrum inclusions in pyrite or along microfractures in sulfides and quartz. Zircon U–Pb geochronology of the granite porphyry around the Hougou breccia pipe constrains the timing of breccia pipe formation to 145.9 Ma which is consistent with the formation age of the Dianfang breccia pipe (145.8 Ma), indicating that the two breccia pipes formed coevally at the end of the late Jurassic, rather than during the Paleoproterozoic, as was suggested by previous studies. The hydrothermal sulfides from the main orebody yield a Rb–Sr isochron age of 119.5 ± 1.8 Ma, suggesting that the eastern and western ore blocks formed coevally at the end of the early Cretaceous. In situ mineral trace element data, as well as sulfur and lead isotope data suggest that the eastern and western ore blocks were formed by a common ore-forming fluid that evolved from a magmatic-hydrothermal system that was genetically related to a hidden late Mesozoic granite. Pre-existing fractures and breccia then served as ideal conduits for fluid flow, accumulation, and gold deposition. Consequently, we propose that the ENE-striking faults developed in and around the breccia pipes and the annular brittle faults around the breccia pipes are good targets for future gold exploration at the Dianfang gold deposit. Based on these results and data from other gold deposits of similar ages around the margins of the North China Craton, we argue that the gold deposits in Xiong’ershan region formed under conditions of lithospheric extension from the late Jurassic to the early Cretaceous, which resulted from lithospheric thinning due to the westward subduction of the Pacific Plate.