Investigating the Whole-lithosphere Structure of a Mineral System——Pathways and Source of Ore-forming Fluids Imaged with Magnetotelluric Modeling

Whole-lithosphere structure has direct implications for both the genesis of minerals and the locations of mineral emplacement;thus knowledge of the deep structural framework of the lithosphere can advance understanding of the development and evolution of mineral systems.

Chemical and boron isotopic compositions of tourmaline in the Longtoushan hydrothermal gold deposit, Guangxi, China:implications for gold mineralization

The Longtoushan hydrothermal gold deposit is located in the southwestern region of the Dayaoshan Uplift.Tourmaline is widespread in the Longtoushan gold deposit and is mainly distributed in the rhyolite porphyry and associated cryptoexplosive breccia. The spatial distribution of tourmaline enrichment is similar to that of the gold orebody. Feldspar has been largely replaced by tourmaline in the rhyolite porphyry and cryptoexplosive breccia.Electron microprobe analysis revealed that tourmalines in the Longtoushan deposit belong mainly to the alkali group and partly to the X-vacant group; they mostly fell in the schorl-dravite series field. Two distinct sets of dominant substitutions were observed: MgFe_(-1) and Al□(NaR2+)-1,where R = Fe, Mg. In addition, minor substitutions include(CaMg)(NaAl)_(-1) and FeAl_(-1). The calculated d11 B value for the mineralizing fluids ranged from-12.8 to-9.7%,which is typical of S-type granites, and boron-enriched fluids predominantly derived from rhyolitic melt. Part of the tourmaline from the rhyolite porphyry crystallized during the magmatic-hydrothermal stage, whereas most tourmalines from the deposit formed in the post-magmatic hydrothermal stage. The tourmalines were deposited from a relatively reduced and acidic fluid system, and the gold predominantly precipitated during the post-magmatic hydrothermal stage in the Longtoushan deposit.

Trace element and sulfur isotope compositions of pyrite from the Tianqiao Zn–Pb–Ag deposit in Guizhou province,SW China:implication for the origin of ore-forming fluids

The Tianqiao Zn–Pb–Ag deposit in SW China,hosted by Devonian and Carboniferous limestone and clay rocks,is composed of sulfides such as sphalerite,galena,and pyrite.Pyrite is present in different paragenetic stages and can be divided into four types based on textures and mineral assemblages.Pyrite from the adjacent Shanshulin deposit(Py-SSL)is also used for comparison.Py1 shows framboid texture with grain diameter up to 1 mm and was commonly replaced by sphalerite.Py2 is characterized by overgrowth texture and displays inner oscillatory zoning.Py2 is associated with abundant sphalerite and galena.Py3 shows replacement relics textures where galena fills the fractures of pyrite.Py4 is a euhedral to subhedral crystal disseminated in dolomite and is characterized by deformation and fragmentation textures.Minor sphalerite and galena are associated with Py4.Py-SSL is subhedral and disseminated in dolomite,similar to Py4.Py1 was formed by a diagenetic or sedimentary process,whereas Py2 and Py3 were formed by multiple stages of ore fluids.Py4 and Py-SSL were formed at the carbonate-sulfide stage,but Py4 suffered from deformation after its formation.Py1,Py2,and Py3 are characterized by relative enrichment of Sb,Cu,and As,in contrast to Py4 and Py-SSL with higher Cr,W,Ge,Sn,Tl,Ni,and Ga contents.However,critical metals such as Ge,Ga,and In in pyrite are generally lower than10 ppm,which are not economically important.The trace element variation in Tianqiao pyrite with paragenesis results from fluid evolution in the Pb–Zn ore system and competition with co-precipitating minerals.Diagenetic and ore-forming hydrothermal fluids are responsible for the formation of different types of pyrite.Ore-related pyrite from the Tianqiao and Shanshulin deposits has Co and Ni distribution features similar to pyrite from sedimentary pyrite and submarine hydrothermal vents,different from those in volcanogenic massive sulfide,iron oxide-copper–gold,and porphyry Cu deposits,indicating their derivation of relatively low-temperature(<~250°C)hydrothermal fluids,similar to basin brines or seawater.,via fluid-rock interaction.This conclusion is also supported by the sulfur isotope composition of sulfides which are 13.0–13.5%,and 15.6–20.5%for Tianqiao and Shanshulin deposits,respectively.

GEOCHEMIC ALEVIDENCES OF METALLOGENIC MATERIALS′SOURCE IN XIONG′ERSHAN Au-Ag OREFIELD,HENAN

The H, O, S isotopic geochemical features and the REE geochemical parameters of some deposits in Xiong′ershan region are discussed, and the coherent various geological bodies are confirmed.