Abstract:

The chert, with simple component and high stability, is widely distributed in orogenic area. It’s stable in macroscopic traces, however, such traces showing geological evolution and diagenic metamorphism of large scale orogeny can be found by the microfabric study, as temperature, pressure and fluids. The ore-hosting chert from Bafangshan-Erlihe Pb-Zn ore deposit is studied by optical microscope, RAMAN, XRD and SEM-EDS. It is indicated that the chert was of obvious alteration during the orogeny, metallogenesis and recrystallization. Under microscope, the chert is composed of cryptomereminicrystal quartz and carbonate minerals(calcite, dolomite and etc. ) and is altered by different stresses. Tiny fissures and veins result from extensional stress, while the tiny fold and oriented arrange of the minerals indicate the effect of compressive stress. Moreover, the effect of compressive stress is also confirmed by the shortening of crystallographic axis for quartz of XRD analysis and oriented carbonate minerals under SEM. In the RAMAN analysis, the crystallinity of quartz grain decrease from the center to the edge, while the crystallinity increase sharply at the edge between quartz and carbonate veins, indicating that the upper fluids accelerating the recrystallization of the quartz grain. Under SEM, the metal sulphides distributed as linear and disseminated structures, probable are caused by the alteration of temperature, stress and fluids. Under a certain temperature and pressure, the chert are deformed, producing oriented mineral (carbonate minerals), which provide transportation and deposition space for the ore-forming fluids.

Key words: Qinling, orogenic belt, Pb-Zn ore deposit, microfabric, chert