Dizhi lixue xuebao (Feb 2024)
Characteristics of chlorites from the Haopinggou Ag–Au polymetallic deposit in the Xiong’ershan ore concentration area and its exploration implications
Abstract
Objective The Haopinggou Ag-Au polymetallic deposit is a typical intermediate-sulfidation epithermal deposit in the Xiong'ershan ore concentration area. Ag-Pb-Zn mineralization mainly occurs in steeply dipping veins and breccia matrix. The relationship between large-scale Pb-Zn mineralization and widely developed alteration minerals remains unclear. Methods In order to discuss chlorite's significance related to Pb-Zn mineralization, chlorite composition in the Haopinggou Ag-Au polymetallic deposit has been analyzed by field geological observation and electron microprobe analysis (EMPA) in this paper. Results Three types of chlorite were observed in the deposit, occurring in altered wall rocks (Type I), in(with) Pb-Zn sulfides (Type II), and in(with) the breccia matrix (Type III). All three types of chlorite are prochlorites and fall within the compositional range of Fe-rich chlorite, indicating that they could be formed in a partially reducing acidic environment. Fe2+ for Mg2+ is the primary substitution in chlorite lattice, suggesting a close association between chlorite formation and mafic wall rocks. Based on the corrected chlorite geothermometer, these chlorites formed under aluminum-saturated conditions in the medium to low-temperature range of 196-239℃. The temperatures of chlorites associated with mineralization (Types II and III) are higher than those in chlorites around quartz veins (Type I). It is believed that during the mineralization process, the hydrothermal fluids evolved from acidic to nearly neutral conditions as the temperature gradually decreased. The initial acidic environment facilitated interaction between water and rocks, promoting the dissolution of surrounding rocks and providing space for the further precipitation of metal sulfides. The evolution of ore fluid properties also corresponds to the deposition process of Ag-Pb-Zn. The genesis of chlorite in the deposit is well-correlated with the ore-forming and holds significant prospecting value. (1) Type I chlorites mainly develop on both sides of quartz veins, formed by the dissolution and metasomatism of basic wall rocks by ore-bearing hydrothermal fluids. Type I chlorite's Fe and Mg components are mostly derived from the wall rocks. Although this type does not contain mineralization, it can be used to trace veins. (2) Type III chlorites reflect the migration process of ore-bearing hydrothermal fluids carrying dissolved minerals (biotite/clinopyroxene), which precipitate with changes in the physicochemical environment. Type III chlorite's Fe and Mg components are mainly introduced by ore-bearing hydrothermal fluids. This type of chlorite fills the intergranular pore spaces between minerals and easily replaces minerals such as biotite and hornblende, exhibiting apparent mineral alteration features in hand specimens, which is beneficial for prospecting.(3) The formation mechanism of Type II chlorites includes the possibilities mentioned above. This type of chlorite is formed by complete dissolution and metasomatism of cement in ore-bearing hydrothermal fluids, forming fine-grained cryptocrystalline chlorite fillings in breccia rocks. Ore-bearing hydrothermal fluids and wall rocks both contribute Fe and Mg in the chlorites. Hand specimens of Type II chlorite are dark green, disseminated and filled in the matrix, making it easy to distinguish. The chemical characteristics of Type II chlorites are similar to those of chemical characteristics of granite-related deposits, implying the contribution of magmatic fluids to ore-forming fluids. Conclusion The Haopinggou Ag-Au polymetallic deposit contains three types of chlorites. Their chemical characteristics all reflect an acidic and reducing metallogenic environment. In cation exchange, the primary substitution is Fe for Mg, and other substitutions are insignificant. The effect of Fe/(Fe+Mg) must be eliminated in order to calculate the temperature of such deposits using chlorite geothermometers. The formation mechanism of the three chlorites is closely related to mafic wall rock, and their chemical properties suggest the involvement of magmatic fluids in ore-forming fluids. Significance These three types of chlorite are well-matched with intense Ag-Pb-Zn mineralization and can serve as key indicators for locating Pb-Zn veins.
Keywords
