Studying On Coloring Element And Metallogenic Evironment For Nanhong Agate From Liangshan Prefecture,Sichuan Province

Nanhong agate is aphanitic mineral aggregates, which belongs to a kind of agate. Natural output red gem is rare in nature, gem like Nanhong agate which has bright red color, but also has oily sheen can be described as unique in the jade market. This uniqueness makes Nanhong agate become collectors’ chasing target, and the best choice for collections preservation. 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, infrared spectrometer and electron probe data. XRF and Raman microscope were used to analyze coloring elements and coloration mineral of Nanhong agate. Combined mining geological conditions and historical tectonics, analyses were made to discuss the environment and mineralization genesis of Nanhong agate.Recent years, Nanhong agate fames in the market, prices are rising, demands of original stones are exceeding the supply, and good material is rare. Nanhong agate has two main producing areas, Baoshan, Yunnan and Liangshan, Sichuan. Yield in Baoshan, Yunnan is little and Nanhong agate is dissected, while the output of Liangshan, Sichuan is relatively intact. Liangshan Nanhong agate are divided by color into persimmon red, rose red, cherry red, red and white, floating ice, wrapping and other categories.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 single polarizing microscope it is colorless, under orthogonal polarizing microscope, the Nanhong agate 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, could barely see inclusions. By infrared spectroscopy test, Nanhong agate shows Si-O adsorption in the range of 2000~2800 cm-1, at 3800 cm-1 the Si-O showed double adsorption, the results show the structure of Nanhong agate is the transition structure from aphanitic chalcedony to phanerocrystalline quartz.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. Fe2O3 is in an amount of 0.6256%~1.2602%, MnO content is 0.545%~0.1384%, Cr2O3 is 0~0.168%. As Fe2O3 content increases, Nanhong agate red color is more concentrated, indicating that the main coloring element is Fe, coloration composition is Fe2O3. The contents of Mn2+ and Cr3+ also have impact on Nanhong agate color, it shows preliminary that MnO content is higher, the red color of Nanhong agate is rosier. The higher Cr2O3 content is, the brighter of Nanhong agate color is. Part of the Nanhong agate of better transparency, the visible color is an aggregate of red fine particles. By micro-Raman spectroscopy test, the spectrum displayed in the 290 cm-1、610 cm-1、1320cm-1 have obvious peak which could prove that the particles are hematite. Therefore, the main coloring element Fe exist in the form of hematite. Nanhong agate color is caused by impurity, the distribution of hematite particles are resulted from the Nanhong agate color. Under the microscope find that the distribution of hematite particles in Nanhong agate is three forms, first form distributes along with growth rings as microcrystal-microscopic crypto-crystal particles. The second shows as crypto-crystal particles distributed as disseminations, diffusions and nebular shapes. The third one is spread according to the original growth zoning, showing fine vein along the fracture. All these imply that the first one is native coloration, second and third are secondary cause color, the reason of color cause have relationship with mineralization environment.According to the field investigation and analyses of original and surrounding rock of Nanhong agate, it implies that Nanhong agate ore deposit is magmatic hydrothermal. The age of deposit formation is late Permian, which source is hydrotherm produced during basaltic magma erupt. Ore-forming process is divided into primary and secondary mineralization generation. Primary 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; Since the repeat eruption of magma lead to changes in mineralization environment, which make the formation of Nanhong agate ring layered, and high temperature and pressure make it fibrous. Secondary mineralization occurres mainly in the late Permian and early Triassic, due to the weathering denudation, Nanhong agate mainly occur in volcaniclastic rock, its secondary deposit occur in basal conglomerate at the bottom of Xuanwei group and in eluvium at the bottom of Dongchuan group. The ones that have direct relationship with the mineralization are the volcaniclastic rock and the partial basic tuff. Tufaceous volcaniclastic rock between the Permian Emeishan basalt group and Xuanwei group directly control over the output and distribution of Nanhong agate mineral.