The giant Bayan Obo REE-Nb-Fe deposit,China:Controversy and ore genesis

Bayan Obo ore deposit is the largest rare-earth element（REE） resource,and the second largest niobium（Nb） resource in the world.Due to the complicated element/mineral compositions and involving several geological events,the REE enrichment mechanism and genesis of this giant deposit still remains intense debated.The deposit is hosted in the massive dolomite,and nearly one hundred carbonatite dykes occur in the vicinity of the deposit.The carbonatite dykes can be divided into three types from early to late：dolomite,co-existing dolomite-calcite and calcite type,corresponding to different evolutionary stages of carbonatite magmatism based on the REE and trace element data.The latter always has higher REE content.The origin of the ore-hosting dolomite at Bayan Obo has been addressed in various models,ranging from a normal sedimentary carbonate rocks to volcano-sedimentary sequence,and a large carbonatitic intrusion.More geochemical evidences show that the coarse-grained dolomite represents a Mesoproterozoic carbonatite pluton and the fine-grained dolomite resulted from the extensive REE mineralization and modification of the coarse-grained variety.The ore bodies,distributed along an E-W striking belt,occur as large lenses and underwent more intense fluoritization and fenitization.The first episode mineralization is characterized by disseminated mineralization in the dolomite.The second or main-episode is banded and/or massive mineralization,cut by the third episode consisting of aegirinerich veins.Various dating methods gave different mineralization ages at Bayan Obo,resulting in long and hot debates.Compilation of available data suggests that the mineralization is rather variable with two peaks at～1400 and 440 Ma.The early mineralization peak closes in time to the intrusion of the carbonatite dykes.A significant thermal event at ca.440 Ma resulted in the formation of late-stage veins with coarse crystals of REE minerals.Fluids involving in the REE-Nb-Fe mineralization at Bayan Obo might be REE-F-C02-NaCI-H20 system.The presence of REE-carbonates as an abundant solid in the ores shows that the original ore-forming fluids are very rich in REE,and therefore,have the potential to produce economic REE ores at Bayan Obo.the Bayan Obo deposit is a product of mantle-derived carbonatitic magmatism at ca.1400 Ma,which was likely related to the breakup of Columbia.Some remobilization of REE occurred due to subduction of the Palaeo-Asian oceanic plate during the Silurian,forming weak vein-like mineralization.

Structural Networks Constraints on Alteration and Mineralization Processes in the Jiaojia Gold Deposit, Jiaodong Peninsula, China

Structural network studies could give appropriate opportunities to understanding structural/hydrothermal events,transportation of ore-forming fluids and water/rock interaction process.Four structural deformation/hydrothermal events have been identified in the Jiaojia fault zone according to microtexture and deformation of quartz and feldspars.Plagioclase experienced ductile deformation period with bended polysynthetic twin stripes(>450℃)in the early stage,followed by K-feldspar alteration period with ductile-brittle deformation and subgrain rotation recrystallization of quartz(380-450℃).Then,sericitization period occurred extensive ductile-brittle deformation(350-420℃)and extensive subgrain rotation recrystallization with a little bulging recrystallization in quartz.In the last,gold precipitation-related pyrite-sericite-quartz alteration was dominated by brittle deformation(300-380℃)and total bulging recrystallization of quartz.From the K-feldspar alteration zone and sericitization zone to pyrite-sericitequartz alteration zone,fractal dimension values of dynamically recrystallized quartz grains increase from 1.07 and 1.24 to 1.32,the calculated paleo strain rate values of dynamically recrystallized quartz range from 10^-10^.7(380℃)-10^-9.6(450℃)and 10^-9.3(350℃)-10^-8.2(420℃)to 10^-9.5(300℃)-10^-8.0(380℃),and the paleo differential stress values increase from 36.9 and 39.3,to 121.3 MPa.The increase of fractal dimension values and decrease of grain size from pyrite-sericite-quartz alteration zone and sericitization zone to K-feldspar alteration zone decreased average water/rock ratio values,which could lead to different acidity and redox conditions of ore-forming fluids and mineralization differences.Two kinds of orecontrolling fractures have been distinguished which include the gentle dip types(18°-50°)with NW(315°-355°)and SW(180°-235°)dip hosting No.Ⅰorebodies and the steep dip types(74°-90°)with NE(45°-85°)and SE(95°-165°)dip hosting No.Ⅲorebodies.These faults/fractures crosscut altered Linglong granite of footwall of the Jiaojia fault zone as rhombohedrons that promoted the connection between fractures in the K-feldspar alteration zone and fluid flow passages near the main fault face.Research results indicate No.Ⅰand No.Ⅲorebodies should be derived from the same mineralization event and belong to different orebody types in different mineralization sites under the same structural networks.

Late Early-Cretaceous Magma Mixing in the Langqi Island, Fujian Province, China: Evidences from Petrology, Geochemistry and Zircon Geochronology

The mafic enclaves from Mesozoic intermediate-acid magmatic rocks,widely developed along Fujian coast,are considered to be the results of large-scale crust-mantle interaction and magma mixing.In this paper,petrography,mineralogy,and geochemistry of granites and mafic microgranular enclaves(MMEs)in Langqi Island are studied to provide new information for tracing crust-mantle interaction.The zircon U-Pb dating results show that the Langqi rocks were formed at^101 Ma,which are metaluminous,enriched in silica and high-K calc-alkaline I-type granites.The enclaves have a typical magmatic structure,which is characterized by magma mixing between high-temperature basic magma and low-temperature acidic magma through injecting.The enclaves and host granites show a tendency to mixed major and trace elements,displaying a clear-cut contact relationship,which is indicative of coeval magmatism.The genesis of Langqi rocks is related to the extensional setting caused by the subduction of Paleo-Pacific Plate,and they are the results of mixing of subduction-related metasomatized mantle-derived mafic and induced crustal-melted granitic magma originating from partial melting of the crustal material.