Characteristics of ore geology and fluid inclusion of the Yindonggou silver deposit, Neixiang County, Henan Province: Implication for metallogenic type

Abstract

The Yindonggou silver (poly-metals) deposit, a fault-controlled lode deposit in Neixiang County, Henan province, occurs in the Erlangping Terrane, northeast Qinling Orogen. The ore is classified into quartz-vein and alteration rock types. The wall rock alterations are dominated by silicification, sericitization, carbonation, and chloritization as well as pyritization and polymetallic sulfidization. The main silver minerals are freibergite, argentite, silver, etc. The hydrothermal ore-forming process includes: Early, Middle and Late stages, characterized by pyrite and/or arsenopyrite-bearing quartz-vein, polymetallic sulfides stockworks and carbonate quartz veinlets, respectively. Quartz and carbonate contain lots of fluid inclusions that are recognized as three compositional types, i. e. CO2-rich or pure CO2, CO2 bearing solution and solution. The deposit was formed by a mesothermal (270°C - 370°C), low-density and dilute (<11. 4%) fluid-system conditions. The fluid-system was rich in CO2 , in the early stage, boiled in middle stage, and characteristic of water solution in late stage. The capture pressure of fluid inclusions decreased from 280 MPa-320 MPa in early stage to 90 MPa-92 MPa in late stage, the metallogenic depths became slightly shallower and the fluid-pressure changed from lithostatic to hydrostatic as the system turned open. This indicates that metallogenesis occurred in a tectonic setting changed from compression to extension. The notations above show that the ore-forming fluid-system and geological characteristics of the Yindonggou deposit are similar to those of the orogenic-type gold deposits as summarized by previous studies, which suggests that it is a typical orogenic-type silver deposit. Considering the tectonic evolution of the host terrane, we suggest the deposit was formed in continental collision regime and the CMF model can be used to interpret its origin.