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摘要
The large Huogeqi Cu-Pb-Zn deposit is located in the Langshan-Zhaertai area in the northern margin of the North China Block. This deposit is hosted in Proterozoic rift-related rocks, which were metamorphosed to amphibolite facies and mostly deformed by shearing. The major lithological units include mica schist, quartzite, carbonaceous shale, and marble. Some intercalated iron formations are distributed within the host rock sequences, and mainly composed of magnetite and Fe-rich amphibole. Cu-Pb-Zn mineralization of the Huogeqi deposit was controlled by shear zones in the mining field, and took place during uplift and retrograde metamorphism of the host rocks. The mineralization took place at pressure-temperature conditions that similar with those of greenschist facies metamorphism. Given that the host rocks were amphibolite facies metamorphosed, the Cu-Pb-Zn mineralization occurred at the retrograde stage of host rock metamorphism. This conclusion is evidenced by the fact that the peak metamorphic minerals ( e.g., almandine) in the host rocks were locally cutcross and replaced by Cu-Pb-Zn sulfides. The ore-fluid is CH4-rich and low-salinity, and of metamorphic origin. Although Cu-Pb-Zn orebodies show characteristics of epigenetic mineralization, sulfides of this deposit are poor in radiogenic lead and have Proterozoic two-stage model ages ( similar to 1000Ma), indicating that the ore-forming elements were originally derived from a crustal source in a syngenetic process during the development of the ore-hosting rift sequences. Furthermore, sulfides of the Huogeqi deposit are enriched in heavy sulfur isotopes, which is a characteristic of reduction of sulfates in sea water. To reveal the ages of hydrothermal Cu-Pb-Zn mineralization and host rock metamorphism, a Ar-39/Ar-40 geochronological study was carried out. Metamorphic Fe-rich amphibole is separated from iron formation of the host rock sequences, and it yields a Ar-39/Ar-40 plateau age of 271. 4 +/- 29. 5Ma. Given that the enclosure temperature of amphibole (ca. 500 similar to 600 degrees C) is close to the temperature of peak metamorphism (Ca. 600 similar to 650 degrees C), this Ar-39/Ar-40 age is interpreted as the time of peak metamorphism. The age of Cu-Pb-Zn mineralization is obtained by Ar-39/Ar-40 dating of syn-ore hydrothermal biotite separated from a hydrothermal Cu ore. It yields a Ar-39/ Ar-40 plateau age of 239. 8 +/- 3. 4Ma. Since the enclosure temperature of biotite ( ca. 300 degrees C) is close to the temperature of hydrothermal mineralization (334 +/- 420 degrees C), this age is interpreted as the time of Cu-Pb-Zn mineralization. The age of Cu-Pb-Zn mineralization is slightly younger than that of peak metamorphism, consistent well with the post-peak nature of Cu-Pb-Zn mineralization. Considering the thermal structures of active orogenic belts and the fact that metamorphic fluid is nearly absent during retrograde metamorphism, the metamorphic ore-fluid was deep-sourced and derived from devolatilization of underthrusting rock piles beneath the ore-hosting rocks. A two-stage genetic model of the Huogeqi deposit is proposed. In Proterozoic times, Cu-Pb-Zn pre-enrichment took place in a submarine exhalative ore-forming system, forming a large volume of rift rocks with high concentrations of ore-forming elements. During the Indosiniari metamorphism, the pre-enriched Cu, Pb, and Zn were remobilized during metamorphic devolatilization.
This ore-fluid injected into higher crustal levels and formed a shear zone-controlled Cu-Pb-Zn deposit in the overlying rock piles, which were undergoing retrograde metamorphism. The metamorphic fluid Cu-Pb-Zn mineralization of the Huogeqi deposit was a part of the large-scale metallogenic event in China during Indosinian times. This study provides a case that base-metal deposit was formed in a compressional tectonic setting together with magmatic fluid deposits and orogenic gold deposits. Accompanying the formation of the Huogeqi Cu-Pb-Zn deposit during Indosinian metamorphism, several large and giant orogenic gold deposits were simultaneously formed in the Langshan-Zhaertai and adjacent areas. These orogenic gold deposits share many similarities with the Huogeqi and they were formed in the same tectonic setting. To form Cu-Pb-Zn deposits during metamorphism, more restricted geological settings are required compared to orogenic gold deposits. In addition to the geological settings that required for the formation of orogenic gold deposits (e.g., metamorphism, shear deformation), two more necessities are required for the formation of large Cu-Pb-Zn deposits such as the Huogeqi. First, the source of the ore-fluids needs to be enriched in Cu, Pb, and Zn during the Proterozoic syngenetic process; and second, organic-rich lithological units are required for the formation of CH4-rich reduced metamorphic fluids, which would significantly enhance the dissolution of Cu.
- 出版日期2015-6
- 单位北京大学
