A preliminary study on exhalative mineralization in Permian basins, the southern segment of the Da Hinggan Mountains, China - Case studies of the Huanggang and Dajing deposits

The southern segment of the Da Hinggan Mountains is a well-known tin-polymetallic metallogenic belt of North China with Jurassic-Cretaceous volcanic-plutonic rocks widespread. Principally because of this, most of the deposits are regarded as epigenetic hydrothermal deposits in genetic connection with the Mesozoic magmatism. But nearly 90 % of the deposits occur in Permian strata, and show concordant stratiform mineralization with a spatial distribution constrained by sedimentary facies of the Permian strata, A close association between mineralization and Permian strata is recognizable. The Huanggang Fe-Sn deposit was regarded as a standard skarn-type deposit formed by magmatic hydrothermal solutions in connection with Mesozoic granites. But there are abundant fabrics indicating submarine hydrothermal exhalation both in magnetite ores and in skarns, including bedding/lamination, soft-deformation, synsedimentary brecciation, and colloform fabrics. The magnetite orebodies and skarn-bodies are predominantly concordant stratiform, and extend nearly 20 km along certain stratigraphic horizon, that is, the upper section of the Lower-Permian submarine volcanic rocks. The Mesozoic granitic rocks crosscut the magnetite and skarn zone. Instead of skarnization, they show strong greisenization associated with cassiterite-quartz veins, distinct from the magnetite skarn-ore with disseminated tin in the Permian rocks. The Dajing Sn-polymetallic deposit is generally regarded as subvolcanic-hydrothermal origin, principally because of the close spatial association between ores and some of the Mesozoic subvolcanic dikes (called rhyolitic porphyry). Detailed geological, fabric, petrographical and mineralogical study demonstrates that this very kind of subvolcanic rocks is actually a new type of exhalites (called 'siderite-sericite chert, according to its mineral assemblage), formed by hydrothermal sedimentation during the evolution of the Later-Permian lacustrine basin. There are, however, indeed some rhyolitic porphyry dikes that crosscut orebodies. The orebodies and their associated exhalite predate, and thus have no genetic relation, to the Mesozoic magmatic process. We thus conclude that subaqueous exhalative mineralization did occur during the basin evolution at the Permian time in the southern segment of the Da Hinggan Mountains, which is ignored and poorly understood, but might be as important as the hydrothermal mineralization connected with the Mesozoic magmatism.