The Gem-mineralogy Characteristics Of Garnet In The Nanminghe Skarn Iron Deposit In Wuan,Hebei

Nanminghe iron deposit is one of the representative skarn-type deposits in the Wu'an,Hebei.Garnet is one of the common skarn minerals.In this paper,Nanminghe garnet was systematically analyzed by means of Infrared spectroscopy,Raman spectroscopy,X-ray powder diffraction,electron probe,and LA-ICP-MS,and the forming process,physical and chemical conditions and genesis of garnet were explored.The garnet occurs as granular aggregates or veins,brown-dark brown and mostly euhedral-subhedral,and generally shows a combination form bounded by dodecahedral faces and trapezohedron faces.The primary mineral assemblage in the skarn is garnetdiopside-magnetite-quartz-calcite-pyrite.rhombic growth pits and mounds can be seen on the rhombic dodecahedron face,and the growth mechanism of the garnet is estimated to be layer growth based on the crystal patterns.The results of Infrared spectroscopy and Raman spectroscopy indicate that Nanminghe garnets were grossular-andradite solid solutions,mainly andradite.The infrared spectrum moves to the low-frequency region with the increase of iron content,and the frequency is negatively correlated with the cell parameter.Optical anomalies such as speckle extinction and fan extinction re common under the polarized light microscope,which may be caused by Al-Fe ordering on the octahedral position and strain.Magnified observation can also see the oscillatory zonation caused by boiling.The content of Al and Fe in oscillatory-zoned rim changes continuously,and the composition of the core is relatively uniform.The above features may indicate that garnet was in a stable environment at the early stage of growth and during the oscillating growth environment at a later stage.According to the microscopic observation and composition characteristics,Nanminghe garnet can be divided into three stages: stage I garnet is isotropic and subhedral-xenomorphic,occurring as granular aggregates,near andradite end member,very obvious Eu positive anomaly;stage II garnet is euhedral-subhedral,with common oscillatory zonation and abnormal extinction,grossular-andradite solid solutions with a weak Eu positive or negative anomaly;stage ? garnet is isotropic,occurring as veins,cut through early euhedral garnet,grossular-andradite solid solutions with obvious Eu positive anomaly.There was little difference in the total amount of rare earth of garnet at each stage,and partition between LREE/HREE and Ce anomaly was higher in the early stage than in the late stage.Regarding the rare earth distribution model,the Nanminghe garnet is generally in the right-dipping mode enriched in LREE and depleted in HREE,which may be mainly controlled by adsorption.It is speculated that the rare earth elements may mainly come from magmatic hydrothermal fluids,and chloride complexes play a certain role in the material transportation.Nanminghe garnet may be formed under infiltration metasomatism,with a high water-rock ratio,and a high oxygen fugacity in the formation environment.It is speculated that garnet is the cause of hydrothermal fluid and provides conditions for the later magnetite precipitation.