Study Of Mineralogy And Mechanism Of Water-rock Interaction Of Volcanogenic Massive Sulfide Deposit In Brothers Submarine Volcanos Caldera,New Zealand

This dissertation aims to study the structure and texture in both macro and micro scale using microscope,scan electron microscopy(SEM)and transmission electron microscopy(TEM).The X-ray diffractometry(XRD)and Ultra Violet-visible diffuse reflectance spectroscopy were used to analysis mineral composition of core samples.Based on the qualitative analysis of mineral phase in the core samples,we calculated the mineral content using a TOPAS Full pattern fitting software which is based on the Rietveld techniques.The chemical composition of the core samples of entire U1530A drilling hole sequence were tested as well at the same time by IODP scientists on cruise.The lithology of Northwest caldera of Brothers volcano is mainly composed by dacite,which has undergone different degrees of alteration,and resulted in the forming of different structure and alteration textures in the vertical direction.The shallow core rocks which are volcanic clastic rocks have been altered weakly to moderately and showed relatively obvious volcanic structure.The lithology has transformed to into altered volcanic sedimentary rocks and tuff gradually which show fine-grained or relative uniform structure with the increase of depth.And the deepest drill core samples are assigned to intensely altered volcanic rocks which show none volcanic texture.XRD results suggest that the main minerals in study area rocks are cristobalite,quartz,pyrite,illite,chlorite,pyrophyllite,anhydrite,montmorillonite,diaspore,and rutile,vivianite and zunyite which are in very low content but have much more important significance.Quantitative calculations of samples show a significant variation in vertical,and the bulk rock geochemical data shows similar variation as well.Many altered crystals by pseudomorphic replacement reaction are found in SEM indicate that the phenocryst minerals,such as plagioclase etc.,were metasomatized in the process of water-rock reaction,and eventually transformed into clay minerals such as illite,chlorite and pyrophyllite etc.The quartz+anhydrite±pyrite±natro-alunite fine veins are widespread in stockwork,this phenomenon indicates the chemical composition of later stage fluid is both silica-rich and sulfur-rich.The main mineralogical changes of the core rocks in the study area include:(1)the homogeneity and polymorphism of SiO₂.SiO₂ appears in the form of cristobalite/Opal-C to quartz/silica with the increase of drilling depth.(2)Changes of clay minerals.The clay mineral assemblage changed from chlorite+illite to pyrophyllite+illite gradually,representing the temperature and p H of water-rock reaction become higher.(3)Changes of sulfate minerals.Anhydrite is common and universally distributed in the rock,and alunite is observed in some samples,which represents high sulfur acid fluids interact with rock.The comprehensive analysis results show that the main types of alteration in the study area are determined as following:(1)silicification;(2)chloritization;(3)argillation alteration;(4)pyritization;(5)Sulfurization.Then,according to the mineral assemblages of rock,we have established the alteration rock sequences in the hydrothermal active regions of the fraternal subvolcanic volcanoes based on both clay mineral and silica assemblage:(1)the weak to moderate alteration with the present of Opal-C+quartz+illite+chlorite±montmorillonite±vivianite assemblage,which represents the epithermal alteration at low temperature;(2)quartz+chlorite+illite,moderate to intensive alteration,silica is mainly quartz;(3)pyrophyllite+quartz+illite±diaspore±zunyite,intense alteration,which stands for a high temperature alteration.