Two stages of immiscible liquid separation in the formation of Panzhihua-type Fe-Ti-V oxide deposits,SW China

Magmatic oxide deposits in the～260 Ma Emeishan Large Igneous Province（ELIP）,SW China and northern Vietnam,are important sources of Fe,Ti and V.Some giant magmatic Fe-Ti-V oxide deposits, such as the Panzhihua,Hongge,and Baima deposits,are well described in the literature and are hosted in layered mafic-ultramafic intrusions in the Panxi region,the central ELIP.The same type of ELIP- related deposits also occur far to the south and include the Anyi deposit,about 130 km south of Panzhihua,and the Mianhuadi deposit in the Red River fault zone.The Anyi deposit is relatively small but is similarly hosted in a layered mafic intrusion.The Mianhuadi deposit has a zircon U-Pb age of～260 Ma and is thus contemporaneous with the ELIP.This deposit was variably metamorphosed during the Indosinian orogeny and Red River faulting.Compositionally,magnetite of the Mianhuadi deposit contains smaller amounts of Ti and V than that of the other deposits,possibly attributable to the later metamorphism.The distribution of the oxide ore deposits is not related to the domal structure of the ELIP.One major feature of all the oxide deposits in the ELIP is the spatial association of oxide-bearing gabbroic intrusions,syenitic plutons and high-Ti flood basalts.Thus,we propose that magmas from a mantle plume were emplaced into a shallow magma chamber where they were evolved into a field of liquid immiscibility to form two silicate liquids,one with an extremely Fe-Ti-rich gabbroic composition and the other syenitic.An immiscible Fe-Ti-（P） oxide melt may then separate from the mafic magmas to form oxide deposits.The parental magmas from which these deposits formed were likely Fe-Ti-rich picritic in composition and were derived from enriched asthenospheric mantle at a greater depth than the magmas that produced sulfide-bearing intrusions of the ELIP.

Petrology and geochemistry at the Lower zone-Middle zone transition of the Panzhihua intrusion,SW China:Implications for differentiation and oxide ore genesis

A sequence of gabbros showing isotropic,layered and fine-grained textures is exposed in the Nalaqing mine at the southern tip of the～260 Ma Panzhihua intrusion,SW China.The field relations,structure,texture and mineralogy of the rocks indicate that the sequence represents the transition between the Lower zone and Middle zone of the intrusion.Isotropic gabbros characteristic of the Lower zone pass upward to layered gabbros of the Middle zone through a～5 m-thick microgabbro sheet,within and close to which small-scaled, concordant Fe-Ti oxide ore horizons are identified.Strong fractionation between HFSE and REE in a subset of samples is ascribed to cumulus titanomagnetite into which HFSE are preferentially incorporated over REE,as reflected in the parallel relations between Nb/La,Hf/Sm and Ti/Ti＊.Both the isotropic and layered gabbros display cumulate textures and have similar mineral compositions（Mg# of clinopyroxene =～76-79 and An59-61）,isotopic compositions[（87Sr/86Sr）i = 0.7044-0.7045 andεNd（t） = ＋2.4 to ＋3.9]and trapped liquid contents inferred from Zr abundance（～17-34 ppm）.However,there are substantial variations in elemental abundances（V,Cr and PGE） and ratios（Ti/V,La/Yb,Ba/Y and Cu/Pd） between the two types of gabbros,features that cannot be explained by cumulate formation from a common magma in a closed system.The microgabbros generally resemble high-Ti Emeishan basalts in major element compositions,but their low trace element abundances indicate some lost of residual liquid is inevitable despite rapid nucleation and cooling.Combined with available data and observations,we propose a model involving in-situ crystallization,followed by magma recharge and closed-system fractionation to explain the formation of texturally distinctive gabbros at Nalaqing and the evolution of the lower part of the Panzhihua intrusion.

Provenance and tectonic setting of the Triassic Yidun Group, the Yidun Terrane, Tibet

The Yidun Group extends from the Shangri-La region to the south and the Changtai region to the north,and is an important component of the Triassic Yidun arc in the eastern Tibetan plateau.It is composed of the Lieyi,Qugasi,Tumugou and Lanashan Formations from the base upward.Both the Lieyi and Lanashan Formations consist dominantly of black or gray slate and sandstone,whereas the Qugasi and Tumugou Formations have variable amounts of mafic to felsic volcanic rocks and turfs accompanied with gray slate and sandstone.Sandstone from the Yidun Group has variable CIA values from 55 to 76,indicative of mild to moderate weathering condition for the source rocks.All the sandstones define a general weathering trend nearly parallel to the A-CN boundary in the A-CN-K triangular diagram,implying limited effect of diagenetic and post-depositional K-metasomatism.Dominant detrital quartz and feldspar grains of the sandstones suggest predominantly felsic sources.Relatively high Y/Ni and low Cr/V ratios of sandstones from the Yidun Group indicate more contribution from felsic than mafic sources.Similarly,the Yidun sandstones have Co/Th and La/Sc ratios generally similar to upper continental crust （UCC） and cluster between UCC and felsic sources,indicating felsic rocks as primary sources.Granodiorite represents the average chemical composition of sources as evaluated by extending the predicted weathering trend back to the feldspar join in A-CN-K diagram.Prominently high Zr/Sc ratio or Hf concentration and Paleoproterozoic Nd modal ages （1.94-2.21 Ga）point to input of recycling components derived from old sedimentary source in a relatively stable tectonic setting.

风化壳型稀土和钪矿床成矿过程的研究进展和展望

稀土和钪是世界各国竞相争夺的战略矿产资源.世界上90%以上的重稀土来自中国南方及邻区的风化壳型稀土矿床,而全球各地也有越来越多风化壳型钪矿床被发现,提供了潜在可用的钪资源.因此,风化壳型矿床的特点和成矿过程亟需更多的研究以助于圈定有利的地质地貌环境,开展稀土和钪的勘查找矿工作.前人研究表明,风化壳型稀土和钪矿床分别主要源于长英质岩浆岩和基性-超基性岩的风化,矿体成分对母岩有很强的继承性,暗示成矿母岩在成岩过程中已经发生稀土和钪的预富集作用.风化过程中,稀土和钪分别主要吸附在黏土矿物和铁氢氧化物中.这些元素的活化-迁移与地下水的活动和它们在次生矿物中的吸附和解吸密切相关,并最终导致这些元素富集成矿.表生地质过程对风化壳型矿床的成矿和保存有重要的制约,强弱侵蚀间歇交替对厚大矿体的发展有利,而风化作用与剥蚀作用之间存在的某种平衡亦可能对这类矿床的形成有重要影响.未来仍需更多研究关注成矿母岩中元素的预富集机制、矿体形成及保存有利的表生条件、元素赋存状态等问题.此外,可见-近红外光谱技术在找矿上的应用也是未来有关研究的一个重要方向.