The Geological Feature Of Deposit And Ore-controlling Factors Of Nanhong Agate From Meigu,Liangshan

Nanhong agate is aphanitic mineral aggregates, which belongs to a kind of agate,in recent years it is one of new favorite in jewelry collection market. Because of the Nanhong agate has the unique bright red color and oil luster which is different from the common agate, it is very popular in jewelry collector, it has very high market value currently. Since 2009, the Nanhong agate deposit has been found in Meigu County, Liangshan Yi Autonomous Prefecture, Sichuan province, both in academia and collectors, it has reached a climax to study the Nanhong agate. Combined with the previous researches and mine field results on Nanhong agate, this paper described Nanhong agate market conditions, distributions and classification. Analyzed Nanhong agate structures and chemical compositions by polarizing microscope, electron probe data. XRF and Raman microscope were used to analyze geological characteristics of Nanhong agate deposit, mineral chemical composition, structure and ore controlling factors. Combined mining geological conditions and historical tectonics, anal yses were made to discuss the environment and mineralization genesis of Nanhong agate.Our country has a long history in appreciating and playing with Nanhong agate, However, with the limits of mining technology, in Qianlong period of Qing dynasty, it has been announced resource depletion, since then, Nanhong agate gradually withdrew from the line of sight of people. In 2009, it gradually discovered in Meigu county, Liangshan Yi Autonomous Prefecture, Sichuan province, the Meigu Nanhong agate is not only more red than others, but also high yield and high quality, become the object of domestic and foreign players who chase it. Also has became the darling of the collectors and the economic value continue rising.There are some results found in this paper. First, NanHong agate is deposited in the bottom of Xuan Wei Group and the top of Emeishan Basalt Group, and in the wing of Huangmaogeng Anticline and the core of Youmuhe Anticline in Meigu. Second, based on the petrography and EPMA analysis, we find that The content of SiO₂ is tested as 95% and more by electron microprobe data, other contents are less implying that main mineral component of Nanhong agate is quartz, its components is pure and impurities are less. Inspected Nanhong agate by microscope,it shows cryptocrystalline texture, which can be classified into fibrous structure and granular structure, visible morphology is fibers which directional arrangement are according to the growth ring or long fibers, short fibers and fine granular.Due to the limitations of electron microprobe analysis, we have done the XRF test. According to the result of the XRF test, except the major chemical composition?SiO₂?, there are also Fe₂O₃, Al₂O₃, MnO, Cr₂O₃, CaO, MgO, TiO₂ and other chemical compositions. It shows that the concentration of red and saturation is positively correlation with the content of Fe₂O₃, indicating that the main coloring element is Fe, the formation of red color is related to the leach and transformation of Fe-Si rich magmatic hydrotherm from basalt in the eruption cycle under the oxidizing condition.Combining with the field survey and analysis of the Nanhong agate ore and surrounding rock, and also link to the geological data and geological evolution of the background, we can conclude that Nanhong agate deposit belongs to the hydrothermal deposit, concerned with basaltic magma, in the middle of late Permian to late. The ore-forming process is divided into initial mineralization and secondary mineralization. Initial mineralization is formed by magmatic hydrothermal crystallizing in basalt stomata, at high temperature oxygen-rich environment in which Fe²⁺ ions turn into Fe³⁺ ions, so that the hematite particles could form in Nanhong agate, and these particles make the agate look red, in the other word, the particles forming process is consistent with the ore-forming process. After weather-worn of basalt, Nanhong agate gravels transport and sediment in the bottom of volcanic clastic rock of Xuanwei Group, which forms secondary deposit. The outcrop area of Emeishan Basalt Group and Xuanwei Group which are host rocks is large in Meigu area. The uncomfortable interface in the two sets of rock which has the feature of ancient weather-worn is an good place for ore-forming and is easy to mining. Near the top interface of basalt eruptible cycle is easy to mineralizing. It can be used for further prospecting.