Geochemical characteristics of chlorites in the Guanfang Cu deposit in the western Yunnan, China and their significances

Chlorite is a common silicate mineral in hydrothermal deposits and has been widely applied for the inversion of metallogenic physicochemical conditions. The Guanfang copper deposit is located in the northern section of the Yunxian-Jinghong volcanic arc zone in the western Yunnan. It is a typical copper deposit occurred in basalt. There is substantial exploration potential due to the excellent mineralization condition. The highly developed chloritization of this ore deposit is closely related to the Cu mineralization. Based on field geological surveys, this study employs in-situ micro-beam analytical techniques including Electron Probe Microanalysis (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) to analyze compositions of chlorites from the Guanfang copper deposit, in order to reveal the physicochemical conditions during the formation of chlorite and their significances. Research findings indicate that the chlorites in the Guanfang copper deposit can be clssified into two types. The The Chl-I chlorite occurred within basalt is generally and paragenetically associated with metal minerals such as pyrite, chalcopyrite, and bornite. The Chl-II chlorite occurred in diorite is generally and paragenetically associated with metal minerals such as chalcopyrite and galena. In the Guanfang copper deposit, there are pycnochlorite and ripidolite which belong to the Al-rich dioctahedral and Mg-rich trioctahedral types of chlorite, respectively. The random substitution of Fe2+ and Mg2+, and Tschermak substitution are important mechanisms of substitution for chlorites. Results of the chlorite geothermometer indicates that chlorites were formed under medium to low temperature conditions (141 ℃ to 266 ℃), with the crystallization temperatures ranging from 141 ℃ to 219 ℃ (average 162 ℃) for the Chl-I and from 177 ℃ to 266 ℃ (average 206 ℃) for the Chl-II. There is obvious fifference between crystallization temperatures of the Chl-I and those of the Chl-II. The lgfO2 and lgfS2 values of chlorites vary from –53.56 to –46.03, and from 14.97 to –6.01, respectively, indicating that they were formed under the environmental condition of low oxygen fugacity and relatively high sulfur fugacity. From the Chl-I to the Chl-II, their crystallization temperatures and contents of trace elements (V, Sc, Cr, Co) are gradually increased, reflecting that these two types of chlorites were not products of the one same mineralization period. Thus, it is suggested that there should be two stages of copper mineralization in the Guanfang copper deposit.