A Paleozoic mercury mineralisation event in South China:In situ U-Pb dating and chemical compositions of calcite from the Jianyan Hg deposit

The ages of hydrothermal Hg deposits are difficult to constrain because of the lack of suitable minerals for dating.The South China low-temperature metallogenic domain hosts numerous Hg deposits,including the Jianyan Hg deposit that is composed mainly of cinnabar and calcite.There are two stages of calcite in the deposit:syn-ore calcite(Cal-Ⅰ)and post-ore/barren calcite(Cal-Ⅱ).Cal-Ⅰis mainly euhedral-subhedral and fine-grained,has homogeneous grey luminescence,and is associated with cinnabar.Subhedral-anhedral Cal-Ⅱcrosscuts Cal-Ⅰand is relatively coarse-grained.The syn-ore Cal-Ⅰhas high U contents(0.1–1.3 ppm)and U/Pb ratios(up to 4.2),and is thus suitable for U-Pb dating.Using a laser ablation-inductively coupled plasma-mass spectrometer equipped with ion counters,we obtained a U-Pb age of 426.3±5.7 Ma(MSWD=1.5)for CalI.This age is interpreted to represent the timing of Hg mineralisation at Jianyan and is similar to ages of 440–400 Ma reported for many carbonate-hosted Pb-Zn and Ba-F deposits in South China.Based on the present results in combination with existing geochemical and geochronological data,we infer that these deposits belong to a Paleozoic Hg-Pb-Zn-Ba-F mineralisation system that was controlled by Caledonian tectonism.

Distinctive melt activity and chromite mineralization in Luobusa and Purang ophiolites, southern Tibet: constraints from trace element compositions of chromite and olivine

To investigate the factors controlling the mineralization in ophiolites we systematically compared the petrology and mineral compositions of the harzburgites/lherzolites, dunites and chromitites in the Luobusa and Purang ophiolites. Generally, the petrological features and trace element compositions of chromite and olivine in peridotite and chromitite are distinctly different between the two ophiolites.In Luobusa, boninitic melts are inferred to have interacted with the harzburgites and modified the distributions of some trace elements(e.g., Ni, Mn and V) in chromite and olivine. The subsequently formed dunites and chromitites experienced significant elemental exchange. In contrast, the Purang ophiolite contains a wider range of chromitite compositions and records diverse melt activities, such as the growth of relatively abundant secondary clinopyroxene. The metasomatic melts were enriched in Al and depleted in Si, Na and highly incompatible trace elements(e.g., Nb, Zr). Such melts resemble MORBlike melts proposed in the literature but are assumed to be more hydrous than typical MORB because of presence of hydrous minerals. The parental magmas of the Purang dunites and intermediate chromitites are inferred to be compositionally intermediate between boninitic and MORB-like melts. In addition,the more refractory nature of the Luobusa harzburgites facilitated a high Cr concentration gradient with the interacting melts, making it easier to increase Cr in the melts. Crystallization of clinopyroxene and amphibole in the Purang ophiolite accommodated significant amounts of Cr and water, respectively,and negatively affected Cr concentration and chromite crystallization. The concentration of chromite to form chromitites requires the presence of focused melt channels.

A new model for chromitite formation in ophiolites: Fluid immiscibility

Although the involvement of hydrous fluids has been widely invoked in formation of podiform chromitites in ophiolites, there is lack of natural evidence to signify the role and mechanism of fluids. In this study, a new model for the genesis of podiform chromitite is proposed on basis of revisits of comprehensive petrological, mineralogical and geochemical results of the well-preserved K?z?lda? ophiolite and the well-characterized Luobusa chromite deposit. In this model, ascending magmas intruding oceanic lithospheric mantle would presumably form a series of small magma chambers continuously connected by conduits. Tiny chromite nuclei would collect fluids dispersed in such magmas to form nascent droplets. They tend to float upward in the magma chamber and would be easily transported upward by flowing magmas. Chromite-rich droplets would be enlarged via coalescence of dispersed droplets during mingling and circulation in the magma chamber and/or transport in magma conduits. Crystallization of the chromite-rich liquid droplets would proceed from the margin of the droplet inward, leaving liquid entrapped within grains as precursor of mineral inclusions. With preferential upward transportation, immiscible chromite-rich liquids would coalesce to a large pool in a magma chamber. Large volumes of chromite would crystallize in situ, forming podiform chromitite and resulting in fluid enrichment in the chamber. The fluids would penetrate and compositionally modify ambient dunite and harzburgite, leading to significant fractionations of elemental and isotopic compositions between melts and fluids from which dunite and chromitite respectively formed. Therefore, fluid immiscibility during basaltic magma ascent plays a vital role in chromitite formation.

Granites: Origin and associated mineralization

As a dominant components in continental crust,granite is certainly the key target to investigate the crustal evolution, and more economically,to explore the mineral resources. With the improvement of our analytical approaches at dif- ferent scales,significant advances have been made in the origin of granites and associated mineralization in China. This is mainly focused on the following three aspects:(1)the geochemical differentiation of granitic melts during their production by partial melting and emplacement by fractional crystallization,(2)the growth and reworking of crustal rocks at convergent plate boundaries,and (3)the concentration of metallogenic elements and eventually ore deposition in the crust.