SAND INJECTION AND LIQUEFACTION STRUCTURES IN THE JINDING Zn-Pb DEPOSIT, YUNNAN, CHINA: INDICATORS OF AN OVERPRESSURED FLUID SYSTEM AND IMPLICATIONS FOR MINERALIZATION

摘要

The Jinding deposit is a large Zn-Pb deposit hosted in coarse siliciclastic strata of Early Cretaceous and Paleocene ages in the Lanping-Simao basin, western Yunnan, China. The deposit, characterized by fine-grained sphalerite and galena disseminated in sandstones of the Cretaceous Jingxing Formation (K_1j) and sandstones and limestone breccias of the Paleocene Yunlong Formation (E_1y), was generally considered as one formed from basinal fluids passively controlled by sedimentary lithofacies and structures. This paper reports sand injection and liquefaction structures occurring as millimeter- to centimeter-wide mineralized sand veins and irregular bodies of light-colored sandstones cutting red-colored sedimentary rocks, and relates them to episodic release of overpressured fluids. The proposed model for the flow of ore-forming fluids differs from previous models in that the fluid flow did not passively follow preexisting pore space and was actively creating new porosity and permeability in the conduits. This implies that the development and distribution of fluid over-pressures in the basin may have exerted a control on mineralization, in addition to lithofacies and structures, and the sand injection and liquefaction structures as described in this paper may be used as indicators of favorable conditions for mineralization.