Gemological Characteristics And The Genesis Of Red-Yellow Agate

Agate is a variety of silica aggregates mainly consisting of quartz,which is characterized by the banded structure,often occurs in the pores or cavities of igneous rocks and usually shows colorless-light colors.However,some brightly red-yellow agates appear in the domestic market in recent years,of which Nanhong agate from Baoshan and Zhanguohong agate from Xuanhua are typical examples.The two varieties of agate are popular in the market but are not well recognized.Nevertheless,the study on their gemological genesis is meaningful to the quality appraisal,market regulating and deposit prospecting.Therefore,this thesis has done some research on their charateristics of gemology,rock magnetism and geochemistry,comparing the matter inheritance between the two agates and their host rocks,discussing their genesis and its relationship to gemological characteristics.Related conclusions in the thesis can provide reference for the genesis analysis and deposit prospecting of red-yellow agate with similar characteristics.Nanhong agate from Baoshan and Zhanguohong agate from Xuanhua are mainly composed of?-quartz,and there is minor moganite in the latter.As to the crystallinity index of quartz,the former(mostly 5?6)is usually higher than the latter(mostly 2?4).The red color of both agates is caused by hematite while the red and orange colors of the latter are respectively due to goethite and the mixture of goethite and hematite.The rock magnetic characteristics of both agate can show obvious discrepancy when the variety and content of color-causing minerals change:for Zhanguohong agate from Xuanhua,samples with different colors and higher color-causing mineral content show differences in hysteresis loop,IRM curve and coercive components;from ?-T curve,there may exsist some regulation among the variety and content of color-causing minerals and the difference value of the susceptibility of unit mass agate samples before and after the heating experiment up to 700?.However,Nanhong agate from Baoshan with lower color-causing mineral content only reflects the diamagnetic charateristics of ?-quartz.By analyzing the results of geochemical data,the host rock of Nanhong agate from Baoshan belongs to the medium-K calc-alkaline peraluminous basalt while that of Zhanguohong agate from Xuanhua is alkaline peraluminous trachyte,both of which has the geochemical characteristics of island arc magmatic rocks.The similarity of the normalized trace elements and REE patterns between Zhanguohong agate from Xuanhua and its host rock indicates the substances of such agate are greatly affected by the rock.In contrast,the normalized patterns of Nanhong agate from Baoshan are distinct from those of its host rock,suggesting the source of its substances is more complicated.Combined with the results of temperature measuring experiment,hydrogen and oxygen isotope composition and the component analysis of fluid inclusions,the two agates are formed by fluid which is rich in both Si and Fe and contains Cl-,CO32-or HCO3-,filling the pores or cavities of host rocks in multiple periods.The mineralizing fluid is considered to be derived from the mixture of metamorphic water and geothermal water,and shows a tendency to be a kind of slightly alkaline fluid with medium-low temperature(<300?),low sanility(9 wt.%?16 wt.%)and the density between 0.85 g/cm3 to 1.10g/cm3 during the last formation period of agate.During this period,the forming fluid of Nanhong agate from Baoshan has lower ore-forming temperature but higher density and salinity than those of Zhanguohong agate from Xuanhua.Besides,the ion composition of the former is more complicated than the latter.In the end,the difference of forming environment of the two agates and its influence on their gemological characteristics are discussed,and the indicators for deposit exploration are preliminarily summarized.