Characteristic and genesis of dolostone reservoirs around the Proterozoic/Cambrian boundary in the Upper Yangtze block for Mississippi valley-type Zn-Pb ores: A review

Abstract

Reservoirs as the direct targets for hydrocarbon exploration represent the permeable and porous rocks. In this sense, reservoirs provide the crucial control for Mississippi Valley-type (MVT) Zn-Pb ore formation, but they have received less attentions in the research field of MVT deposits. Based on reprocessing published data and the field and petrographic observations, this study reviews the characteristics and genesis of the reservoirs for MVT ores in the Dengying Formation and the early Cambrian dolostone units around the Proterozoic/Cambrian boundary in the Upper Yangtze block. The reservoir horizons consist of breccias and coarse-grained carbonates at the top of a shallowing-upward sedimentary cycle mainly in the tidal flat facies and subordinately in the platform margin mound-shoal facies. Bird’s eye and zebra textures are common in the reservoir horizons and the pseudomorphs after evaporite minerals can be found in the breccias. Moreover, the breccia bodies have a large lateral extension up to kilometers level, sharp boundaries with the enclosing rocks, and are absent of carbonate dissolution-related internal sediments, where the clasts are angular and show a downward collapse from their primitive stratigraphic positions. These suggest that the reservoir dolostones deposited in a sabkha environment and the breccias mainly resulted from evaporite dissolution. Multistage dolomitization and silicification intensively superimposed on the reservoir horizons and might take advantage of pre-existing interclast, intergranular, and dissolution pores. The related intercrystalline pores provide the predominant spaces for the Zn and Pb sulfide precipitation. Pressure dissolution- and structural deformation-related factures provide minor reservoir spaces. Prospecting MVT Zn-Pb ores in the Proterozoic/Cambrian boundary dolostone units in the Upper Yangtze block should give priority to the sabkha environment of dolostone horizons with post-depositional brecciation and dolomitization/silicification at the top of a shallowing-upward sedimentary cycle in the tidal flat facies and in the platform margin mound-shoal facies.