Magmatic hydrothermal overprinting in the Mesoproterozoic Dongshengmiao deposit, Inner Mongolia: geological and fluid inclusion evidences

摘要

The Dongshengmiao Pb-Zn deposit located in the Mesoproterozoic aulacogen in a passive continental margin in the northwest margin of the North-China Craton is widely considered to be a untypical SEDEX deposit. Recently, new types of mineralization such as chalcopyrite veins and re-crystallized sphalerite ores with visible hydrothermal alteration have been found in the deposit at depth. In this paper we report the decrepitation temperatures of fluid inclusions in chalcopyrite, sphalerite and quartz from these new types of ores. The decrepitation temperatures of fluid inclusions in chalcopyrite (4 samples) , sphalerite (2 samples) and quartz (5 samples) are 303 ～ 456℃ , 97 ～ 497℃ , 146 ～ 350℃ and 350 ～ 556℃, respectively. The decrepitation temperatures of fluid inclusions in the vein-type chalcopyrite are similar to the decrepitation temperatures of fluid inclusions in chalcopyrite from the Hercynian Oubulage porphyry Cu-Au deposit (313 ～ 514℃) and the Chehugou porphyry Cu-Mo deposit (277 ～ 485℃), supporting our interpretation that the Dongshengmiao deposit was overprinted by magmatic hydrothermal mineralization. The decrepitation temperatures of fluid inclusions in re-crystallized sphalerite from the Dongshengmiao deposit are characterized by two peaks, 97 ～ 358℃ and 358 ～ 497℃. The decrepitation temperatures of fluid inclusions in quartz in chalcopyrite veins from the Dongshengmiao deposit are also characterized by two peaks, 146 ～ 350℃ and 350 ～ 556℃. The lower and higher temperature peaks in both cases are considered to represent two separate mineralization events, original SEDEX mineralization and magmatic hydrothermal overprinting, respectively. The higher decrepitation temperatures of fluid inclusions in quartz and sphalerite from the Dongshengmiao deposit are similar to the decrepitation temperatures (340 ～ 526℃ ) of fluid inclusions in sphalerite from the Baiyinnuoer skarn-type Pb-Zn deposit in the region. Replacement of pyrite by sphalerite and overgrowth of chalcopyrite on pyrite in the Dongshengmiao support our interpretation that the original SEDEX mineralization was overprinted by magmatic hydrothermal activity in the deposit. Our results suggest that there may be separate porphyry and skarn-type deposits related to Hercynian magmatism and associated hydrothermal activities in the Langshan area, which are potential exploration targets in the future.