An Advanced Image Processing Technique for Backscatter-Electron Data by Scanning Electron Microscopy for Microscale Rock Exploration

Backscatter electron analysis from scanning electron microscopes(BSE-SEM)produces high-resolution image data of both rock samples and thin-sections,showing detailed structural and geochemical(mineralogical)information.This allows an in-depth exploration of the rock microstructures and the coupled chemical characteristics in the BSE-SEM image to be made using image processing techniques.Although image processing is a powerful tool for revealing the more subtle data“hidden”in a picture,it is not a commonly employed method in geoscientific microstructural analysis.Here,we briefly introduce the general principles of image processing,and further discuss its application in studying rock microstructures using BSE-SEM image data.

Metallogenetic model of Jiaodong-type gold deposits,eastern China

The genesis of giant gold provinces is an international scientific frontier,in which the source of a huge amount of gold and the drive for mineralization are key challenges.The mineralization intensity of the Jiaodong gold province in eastern China is a rare occasion in the world,because it owns gold reserves of~5500 tons within an area of less than 10,000 km^(2).The Jiaodong gold province formed in the superimposed domain of diverse tectonic regimes in an intracontinental setting.Paleoproterozoic regional peak metamorphism and Triassic continental collision are followed by the tectonic transition and gold mineralization at ca.120 Ma with time intervals of 1.9 billion years and about 100 million years,respectively.The Jiaodong gold deposits are different from orogenic and intrusion-related gold deposits in terms of the tectonic background,types of host rocks,and oreforming mechanism.These gold deposits show close spatial-temporal and genetic relationships to mafic igneous rocks,implying the derivation of ore-forming fluids from the metasomatic mantle domains.Mafic dykes in Jiaodong have negativeε_(Hf)(t)values of-29.9 to-9.1,Os content of 0.002-0.16 ppb,heavyδ^(18)O up to 8.23‰,and high initial^(187)Os/^(188)Os ratios of 0.1352-0.8858.These indicate that the lithospheric mantle was metasomatized by ancient crust-derived components.The lithospheric mantle in the western Jiaodong shows generally more enriched isotope features than that in the eastern part,which is explained to be an important reason for its huge gold resources.The mafic dykes show lighter Mg isotope characteristics(averageδ^(26)Mg of-0.33‰,n=50)and high Ca O content(overall greater than 6.5 wt%),indicating that the lithospheric mantle would also record the metasomatism by the carbonate rocks from the subducting oceanic slab.Under the background of the rollback of the subducting paleo-Pacific slab and the destruction of the North China Craton,partial melting of the lower crust would produce granitic magmas that led to the enrichment of gold in the residual crust.The syn-mineralization asthenosphere upwelling would promote the recycling of the lower crust and the partial melting of the metasomatic mantle domains.Basic magmas,produced by the partial melting,and the mantle itself would degas to form ore fluids.The ore fluids would further leach gold in the lower crust to increase its fertility.The auriferous fluids were transported to the middle to upper crust along the detachment and strike-slip faults.Water-rock interaction and fluid immiscibility,which occurred in and above the ductile-brittle transition zone to induce gold precipitation,formed the Jiaodong gold deposits.Given the unique geological features and genetic model of the Jiaodong gold deposits,they can be defined as“Jiaodong-type”gold deposits.