Typomorphic mineralogical characteristics of pyrrhotite in Jiama Cu polymetallic deposit, Tibet, and its geological significance

The Jiama deposit is located in the eastern part of Gangdise metallogenic belt, and is one of the largest copper polymetallic deposits in Tibet. Pyrrhotite is one of the most common metallic minerals of the Jiama deposit.Its typomorphic characteristics not only reflect its formation environment but also indicate its formation mechanism and deposit genesis. In this paper, pyrrhotite samples from different lithologies were collected, and the morphology, composition and structure of pyrrhotite were analyzed by means of mineralogy, X-ray diffraction and electron microprobe analysis. The study shows that pyrrhotite of the Jiama deposit is mainly distributed in the skarn and hornfels, which are far away from the center of the porphyry intrusion. The powder X-ray diffraction curves and cell parameters of pyrrhotite show that the pyrrhotite in the skarn is mainly high-temperature hexagonal pyrrhotite. The pyrrhotite in the hornfels is a mixture like associated body of high-temperature hexagonal pyrrhotite and low-temperature monoclinic pyrrhotite, with the monoclinic pyrrhotite being dominant. The results of electron microprobe analysis of pyrrhotite in skarn and hornfels show that the content of Fe in pyrrhotite of skarn is 60.09%~60.71%, averaging 60.38%, and the content of S is 38.18%~38.69%, averaging 38.35%, with the corresponding chemical formula being Fe₈S₉~Fe₁₀S₁₁. At the same time, the content of Fe in pyrrhotite of hornfels is 59.05%~59.57%, averaging 59.10%, and the content of S is 39.28%~39.95%, averaging 39.59%, with the corresponding chemical formula being Fe₅S₆~Fe₇S₈. Based on the above mineralogical characteristics, the author hold that the precipitation mechanism of pyrrhotite in the deposit is as follows: the hot magma surged and interacted with the carbonate and clasolite formation, and the addition of atmospheric water caused the ore-forming fluid to quickly cool down in the hornfels, forming associated body of high-temperature hexagonal pyrrhotite and low temperature monoclinic pyrrhotite. At the same time, a large number of ore-bearing hydrothermal fluids formed and filled in favorable ore-forming space (mainly interlayer fracture zone) for precipitation and mineralization, forming skarn orebodies; the fluid then experienced a slow cooling in the skarn ore segment to form a high temperature hexagonal pyrrhotite. Based on geological characteristics of the deposit and geochemical characteristics of related elements, it is concluded that the Jiama deposit type is of the porphyry-skarn type.