Serpentinization Alteration Of Ultrabasic Rocks And Evolution Of Layered Silicate Minerals

Mangya metamorphic ultrabasic rock alteration evolution deposit in Qinghai Province is the largest chrysotile asbestos deposit in China.The deposit is located at the junction of the northwest margin of Qaidam Basin and the south margin of middle Altun Mountain in Qinghai Province.Large and medium chrysotile asbestos deposits in China are mainly distributed in several ophiolite belts.Among them,the ophiolite belt in the Al Ti N mountain is a famous chrysotile ore-forming belt.Mangya chrysotile deposit is produced in the layered ultramafic rocks at the lower part of ophiolite formation.The protolith of the metamorphic ultrabasic rock mass is a mg rich ultrabasic rock mainly composed of clinopyroxene olivine and clinopyroxene olivine.Due to self metamorphism and later hydrothermal alteration,the ultrabasic rock mass has been completely altered into serpentine,and some serpentine rocks have been further altered into talc magnesite schist.The ore body is mainly composed of strongly altered serpentine and talc magnesite.In this paper,the Mangya chrysotile asbestos deposit in Qinghai Province is taken as the research object.On the basis of previous studies on the genesis of typical ultrabasic rock alteration deposit and the transformation of layered silicate minerals,the alteration evolution process of ultrabasic rock in the deposit is discussed,The transformation mechanism of island silicate mineral forsterite and 1:1 type layered trans silicate mineral serpentine,and 1:1 type layered trans silicate mineral serpentine and 2:1 type layered trans silicate mineral talc were studied.The kinetics and chemical thermodynamics of the transformation process were discussed(1)On the basis of field geological survey,the petrology,mineralogy and petrochemistry characteristics were studied by means of microscopic comprehensive identification,whole rock chemical composition analysis and electron probe analysis,and the alteration evolution process was summarized.(2)The serpentinization of olivine was simulated by controlling temperature,time,p H and reaction system through hydrothermal experiments.The hydrothermal products were characterized by XRD,SEM,HRTEM,XRF and FTIR.(3)In alkaline condition,serpentine reacts with active Si O2 and transforms into talc by controlling temperature,time and p H.the crystal chemical mechanism and crystal structure evolution of serpentine are discussed.(4)The thermodynamic conditions of forsterite to serpentine and serpentine to talc were studied by controlling temperature and pressure.Based on the above research results,the main innovative conclusions are as follows:(1)The serpentine of the metamorphic ultrabasic rock mass is characterized by serpentine(rhyolite,chrysotile,chrysotile),magnetite,magnesite,talc,brucite and chromite.The residual minerals are orthopyroxene,clinopyroxene and chromite.The main minerals of talc magnesite schist are magnesite,talc,serpentine and magnetite,Quartz veins can be seen locally;The metamorphic ultrabasic rocks in this area have recorded all stages of hydration,sliding and carbonation of peridotites.The evolution of ultrabasic rocks can be divided into two stages:(1)serpentinization of olivine and pyroxene minerals and chloritization of serpentine((2)carbonation of serpentine,talc and brucite by Ca and CO2 rich fluids.Serpentinization and other metamorphic alteration promoted the chemical activity of Si,Mg and Fe elements,enriched and migrated elements,and played a catalytic role in the formation and evolution of secondary minerals.The metasomatic process of multi-stage hydrothermal fluids with different compositions clearly shows the evolutionary sequence of rhyolite,chrysotile and chrysotile,as well as the formation process and typomorphic characteristics of talc,brucite,chromite and magnetite.(2)The solid products of forsterite are mainly chrysotile,rhyolite and brucite.The results show that serpentine and chrysotile are flaky and leaf like under SEM,and their diameters are positively correlated with the initial particle size.The larger the particle size is,the larger the serpentine diameter is.The transformation pathway is that active Si O2 in solution adheres to the surface of the destroyed peridotite to form serpentine to structure layer,which is mainly reflected in the transformation of olivine(001)lattice to 0.73 nm serpentine layer in HRTEM lattice image.(3)The transformation of serpentine to talc is due to the incorporation of Si O2 in the solution into the tetrahedral sheet of to type structure layer in the solid state to form tot type structure.It is also found that the tetrahedral sheet with to type structure layer is inverted and the octahedral sheet dissolves to form tot type structure layer.Serpentinization of olivine and sliding of serpentine are transformed into new minerals by solid phase recrystallization.(4)In the temperature and pressure range of 400 k and 4.9 * 106 kpa,with the increase of reaction temperature and pressure,the conversion rate of forsterite to serpentine and serpentine to talc increases.Beyond this range,the forward reaction is limited.The alteration evolution of ultrabasic rocks is a late hydrothermal alteration under low temperature and low pressure.When the original rocks are subjected to different hydrothermal processes,olivine and other minerals are often transformed into serpentine group minerals with different shapes in solid-liquid coexistence state.Serpentine can form serpentine talc paragenesis or associated association by stages through the action of sliding.Moreover,the metamorphism and evolution of the layered silicate minerals are mostly in the state of solid-liquid coexistence,which is of great significance to reveal the reorganization mode of crystal structure,the construction mode and the migration behavior of chemical elements in the process of mineral transformation.