Ancient evaporitic borate deposits are rare in the geological record, and the mechanisms governing their formation and long-term preservation remain poorly constrained. The Paleoproterozoic Liao–Ji orogenic belt of the North China Craton hosts several metamorphosed stratiform borate deposits that provide a unique opportunity to address these issues. Heavy δ11B (+ 7.5 to + 9.8‰) of borate minerals (suanite, ludwigite and szaibelyite) and δ34SV−CDT (+ 8.7 to + 16.1‰) of pyrrhotite demonstrate marine contributions, whereas whole-rock geochemistry, mineral chemistry, and negative εNd values (mostly between − 2 and − 7) reflect substantial crustal input. MORB-like iron isotopes (-0.049 to + 0.292‰) indicate a dominantly hydrothermal source of iron. These features point to boron concentration within an evaporitic basin at the continent margin that received mixed marine and terrigenous contributions. Phase modelling and Zr-in-titanite temperatures constrain peak P–T conditions of regional granulite-facies metamorphism of the Wengquangou deposit to 750–800 °C at 0.6–1.0 GPa. In situ LA–ICP–MS U–Pb dating of zircon defines two age populations at ca. 2.15 Ga and 1.87–1.84 Ga, corresponding to the ages of deposition and granulite-facies metamorphism, respectively. Apatite U–Pb ages of ca. 1.71–1.64 Ga record a later post-peak thermal or fluid-related overprint. Prograde dehydration reactions converted soluble hydrous borates into stable anhydrous assemblages, fundamentally modifying mineralogy while largely retaining primary isotopic characteristics. This high-grade metamorphic overprint played a key role in stabilizing the deposit against post-depositional dissolution. Our data show that ancient evaporitic borate deposits formed by similar processes as modern ones, involving the leaching and mobilization of B from volcanic and sedimentary rocks, transport of B into a continent-margin basin, and concentration of B by evaporation. The rare preservation of ancient borate deposits does not indicate that they rarely formed, but that they are generally not preserved as regions hosting major evaporitic borate deposits seldom undergo subsequent high-grade metamorphism.

In the past ten years,facing the national demand,based on the characteristics of magmatic sulfide metallogenies in China and our previous studies,the team for researching magmatic sulfide deposits at Institute of Geochemistry Chinese Academy of Sciences has carried out continuous and systematic studies on the frontier scientific issues in this field and achieved a series of innovative achievements which are mainly given below.The team proposed that the core connotation of the conduit model for magmatic sulfide mineralization is ″two deep processes,a shallow magma chamber within a focused plumbing system″,established a relatively completed model for the"explosive"/"short-lived"magmatic sulfide metallogenesis in orogenic belts,clarified significances of the subduction metasomatism and pyroxenite mantle for the magmatic sulfide mineralization in orogenic belts,analyzed factors driving the sulfide segregation from basaltic magma,and predicted the future prospecting targets in the Huangshan-Jingerquan metallogenetic belt,revealed the principle of oxygen fugacity controlling the geochemical behavior and occurrence state of platinum group elements,promoted the understanding of the physical processes for the migration and deposition of sulfide melts,and firstly identified the magma conduit branches of the Jinchuan giant Ni-Cu-PGE deposit and guided the deep-level exploration with achieved substantive results in the Jinchuan area.The aim of this paper is to inspire readers to think about the key research problems that need to be overcome and the shortcomings of the current researches based on the reviews and summaries of achievements of the research team.We hope this paper can be favorable for the communication and cooperation with other researchers and promoting the further development of researches on the magmatic sulfide metallogenesis.

The nature (e.g., whether or not depleted in platinum-group elements) and sulfide segregation history of the parental magmas of layered mafic-ultramafic intrusions in the Emeishan Large Igneous Province (ELIP), SW China, remain controversial, which limits our understanding of their mineralization potential and exploration of platinum-group elements (PGE). To address these issues, we investigate the PGE geochemistry of the Heigutian layered mafic-ultramafic intrusion, a representative Fe-Ti oxide-bearing intrusion located in the central part of the ELIP. It comprises two distinct petrographic zones separated by a sharp contact. The lower zone is composed of olivine clinopyroxenite, magnetite gabbro, and medium-grained gabbro, while the upper zone consists of finegrained gabbro. The olivine clinopyroxenite in the lower zone has a total PGE content of 28.8-31.7 ppb; the magnetite gabbro and medium-grained gabbro show much lower total PGE levels of 0.88-1.48 ppb and 0.09-0.18 ppb, respectively. The fine-grained gabbro in the upper zone exhibits a broader total PGE range of 2.01-32.27 ppb. The decoupling between Ru and Ir in the fine-grained gabbro, together with the negative Ru anomaly, suggests that the parental magma of the fine-grained gabbro did not undergo early-stage sulfide segregation but experienced significant removal of RuS2 before emplacement. Given the mantle-normalized patterns of PGE analogous to those of the Emeishan PGE-undepleted high-Ti basalts and mantle-like Cu/Pd ratios (4 x 103 to 7 x 104) in the fine-grained gabbro, it can be inferred that the parental magmas were undepleted in PGE. The positive correlations between PGE and TiO2, Ni, and Cr indicate in-situ sulfide immiscibility driven by fractional crystallization of olivine and Fe-Ti oxides during the formation of the lower zone. This sulfide immiscibility could potentially contribute to PGE mineralization in the lower part of the intrusion. Given the similarities in PGE features between the Heigutian intrusion and other layered mafic-ultramafic intrusions, there is substantial potential for exploring PGE mineralization within coeval Fe-Ti oxide-bearing layered intrusions in the ELIP.

The origin of chromitites in ophiolites remains a topic of debate. The Dazhuqu ophiolitic massif located in the central segment of the Yarlung-Zangbo ophiolite zone contains podiform chromitites. The massive chromitites exhibit significantly low Cr# (38-47) in chromite. In contrast, the disseminated chromitites have relatively wide range of Cr# (16-64) in chromite. The dunites can be classified into two group: low-Cr# chromite-bearing dunites (Cr# = 19-51) and high-Cr# chromite-bearing dunites (Cr# = 74-83). The chromite of harzburgites show similar Cr# range (27-55) to that of the disseminated chromitites and low-Cr# chromite-bearing dunites. The negative correlation between Ti and Cr# along with the positive correlation between Ni and Mg# in chromite suggests that the massive chromitites formed through the physical sorting and accumulation of chromite grains in the melt conduits. The compositional patterns of chromite in the disseminated chromitites and low-Cr# chromite-bearing dunites resemble those of the harzburgites, implying that their chromite may have inherited compositional signatures from the surrounding harzburgites. In contrast, the chromite in the high-Cr# chromite-bearing dunites exhibits compositional patterns associated with boninitic melts. The coexistence of the chromitites and dunites with lower Cr# of chromite and high-Cr# chromite-bearing dunites indicate that the Dazhuqu massif was once modified by both MORB-like and boninitic melts, causing highly variable Cr# of chromite. A model of subduction initiation accounts for the magmatic evolutionary history of the chromitites and mantle sequence of the Dazhuqu massif.

Rodingites, extremely calcium-rich but sodium- and silicon-poor rocks forming through fluid-rock reactions, are crucial for understanding the oceanic cycling of materials. However, the source of calcium in rodingites remains debated. Here, we investigate whole rock Ca and Sr isotopic compositions of samples collected across a continuous section of a rodingite dyke from the Xigaze ophiolite, SW Tibet. The rodingite dyke exhibits progressive metasomatism from core to margin, defining central, transitional, and marginal zones. The dyke displays a wide range of 544/40Ca values from 0.28 %o to 1.63 %o but limited variation in Sr isotope (87Sr/86Sr(i) = 0.70308-0.70438). The slightly metasomatized central rodingites exhibit lower 544/40Ca values (0.53 %o to 0.68 %o) than MORB, whereas the moderately metasomatized transitional rodingites display significantly lighter Ca isotope compositions (544/40Ca = 0.28 %o to 0.61 %o). The intensively metasomatized marginal rodingites show the highest 544/40Ca values (0.64 %o to 1.63 %o). The transverse profile of 544/40Ca values across the rodingite dyke reveals clear oscillatory zoning, suggesting multiple fluid sources for Ca during rodingitization. In the earlystage, the mafic dyke was metasomatized by a carbonate-bearing fluid characterized by low 544/40Ca values, which originated from the mixing of deserpentinization-related fluids with marine carbonates during subduction. Subsequently, the dyke experienced more intense metasomatism by a high-544/40Ca serpentinizing fluid, leading to additional Ca input and an elevation of 544/40Ca values. The gradual increase in 87Sr/86Sr(i) ratios from the central zone to the marginal zone indicates that the primary fluids were derived from seawater-triggered serpentinization. This study proposes that during the initial stages of subduction fluids released by serpentinite dehydration dissolved sedimentary carbonates on the subducted oceanic slab. As subduction progressed, extensive dehydration of the surrounding serpentinites produced high-544/40Ca serpentinizing fluids, which further elevated both 544/40Ca and 87Sr/86Sr(i) values of the dyke. The highly variable 544/40Ca values of rodingites from diverse tectonic settings support the model involving multiple fluid contributions with distinct Ca isotopic compositions during rodingitization.

The Taihe layered mafic intrusion, located in the central part of the Emeishan Large Igneous Province, hosts a giant Fe-Ti-V oxide deposit. However, the petrogenesis of the Fe-Ti-V oxide ore-bearing layered intrusion is still debated. The Taihe intrusion is classified into three lithologic zones based on mineral assemblages and textural characteristics: Lower Zone, Middle Zone, and Upper Zone. Six cyclic units (Cycles I-VI) are recognized within the Middle Zone based on the rhythmic variations of Fe-Ti oxide contents. The most economically important FeTi-V oxide ore layers are concentrated in the lower sections of these cycles, where magnetite clinopyroxenites are notably enriched in 5-12 vol% apatite. Petrographic and geochemical data indicate that these apatite grains were fresh and crystallized nearly simultaneously with Fe-Ti oxides and clinopyroxene, with no evidence of hydrothermal alteration. Apatite from the apatite-olivine gabbro and apatite-magnetite clinopyroxenite of Cycle II displays mantle-like 87Sr/86Sr(i) (0.7048-0.7050; average = 0.7049) and elevated delta 18O (6.78-8.76 %o; average = 7.73 %o). However, apatite separated from the apatite gabbro of Cycle IV exhibits mantle-like 87Sr/86Sr(i) ratios (0.7048-0.7052; average = 0.7049) and delta 18O values (5.55-6.79 %o; average = 6.09 %o). In contrast, apatite from the apatite-magnetite clinopyroxenite of Cycle VI shows slightly higher 87Sr/86Sr(i) (0.7050-0.7055; average = 0.7052) and notably higher delta 18O (7.99-9.38 %o; average = 8.50 %o). The decoupling of the apatite O-Sr isotopes in the Cycles II and VI indicates the contribution of recycled altered oceanic crust in the mantle source. The elevated delta 18O values of the apatite suggest interactions between subduction-related materials and the Emeishan plume, which resulted in the heavy oxygen isotopic signatures in the primitive magmas of the Taihe intrusion. The variability in apatite O-Sr isotopic compositions among cyclic units of the Taihe intrusion implies that the parental magmas originated from distinct melting domains that were heterogeneous due to the variable distribution and volume of altered oceanic crust components. The interaction of an upwelling mantle plume and the subcontinental lithospheric mantle that contains ancient recycled crustal materials may be a crucial factor in the formation of the giant Fe-Ti-V oxide deposit.

The Dahenglu copper-cobalt mine is classified as a large cobalt deposit in China, which has experienced complex sedimentary-metamorphic effects and has long been considered a typical sedimentary-metamorphic deposit. However, the late-stage hydrothermal mineralization may have been underestimated. In this study, we proved that the magmatic-hydrothermal fluid was involved in the mineralization process through detailed petrographic-mineralogical observations, in addition to metamorphic fluids mineralization. In-situ calcite strontium isotope, tourmaline boron isotope was analyzed. Boron isotope varies from the vein (ca.4%) to the matrix (ca.14%). Strontium isotope does not show differences between tourmaline and calcite, with the average of 1.0896 and 1. 0948, respectively. Both of which are slightly lower than that of whole rock. Although it is impossible to quantitatively estimate the contribution of magmatic-hydrothermal fluids to mineralization currently, this study shows that the hydrothermal fluid driven by the magmatic heat promotes the oxidation, activation, and migration of cobalt by in-situ calcite strontium isotope, tourmaline boron isotope, and trace elements techniques. Then the fluid was reduced and precipitated in the calcite-quartz veins, which was the most important way of mineralization during the magmatic hydrothermal ore-forming stage. However, the magmatic-hydrothermal fluid did not contribute significantly to mineralization in the aspect of cobalt source, and no evidence of marine evaporites participating in mineralization was found in this study. Therefore, the trace element and isotopic compositions of calcite and tourmaline are important indicator minerals for the reconstruction of hydrothermal mineralization.

Carbon-stable isotopes of diamonds provide clues regarding their growth processes. Thus, an accurate, efficient, and affordable method to determine the carbon isotope ratio is extremely urgent. Accurate and precise determination of the carbon isotope ratio with LA-MC-ICP-MS is limited by the high background intensity of 12C+ and instrumental settings. Hence, laser parameters were gradient-changed to investigate their influences on carbon isotope analysis. Besides, high-resolution spatial distributions of 12C+ and 40Ar3+ intensities coupled with the 13C/12C value in the ICP were investigated in detail to elucidate the ionization kinetics of 40Ar3+ and to determine the most stable zone for carbon isotope analysis in the ICP. Finally, two types of diamonds (a natural diamond (D-N-1) and synthetic diamond (D-HTHP)) were measured to verify their homogeneity and flexibility for in situ analysis of the carbon isotope on diamonds with LA-MC-ICP-MS. The signal-to-noise ratio (SINR) greatly affects precision, which could be significantly improved by optimizing laser parameters. The internal precision of in situ C isotope analysis is better than 0.2 parts per thousand (2SE) when the SINR is more than 4. The ionization efficiency of 40Ar3+ was found to be controlled by the catalysis of C ions and the thermodynamic parameters of the ICP. The most stable zone for carbon isotope analysis in the ICP was found to be located at approximate to 1.4 mm ahead of the 12C+ signal-maximum point. Hence, the precision of 13C/12C could be improved when the torch was retreated approximate to 1.4 mm from the maximum 12C+ intensity point axially. The natural and synthetic diamonds exhibited similar down-hole fractionation behaviors of the C isotope during laser ablation, indicating that they can be used to correct each other with LA-MC-ICP-MS. The accuracy of the carbon isotope was validated by comparing the data with those of nano-scale secondary ion mass spectrometry (NanoSIMS) and laser ablation-isotope ratio mass spectrometry (LA-IRMS). This study suggested that in situ LA-MC-ICP-MS is a rapid, precise and accurate way to measure the carbon isotope of diamonds. The interactive influence of the ionization process of nuclides leads to elemental and isotopic fractionations, which is one of the mechanisms of the matrix effect.

Podiform chromitites typically occur in harzburgites near the petrological Moho in ophiolite sections, and are petrologically and economically significant. Hydrous fluids may have an important role in the formation of podiform chromitites, although unambiguous evidence for the involvement of such fluids is rare. To examine whether hydrous fluids are involved in the formation of podiform chromitites, we measured the Ni, Co, Mn, and Zn contents of olivines and chromites in representative samples from the Luobusa ophiolite, southern Tibet, taking account of the different geochemical behaviors of these elements in melts and hydrous fluids. In the harzburgites, the olivine (Ol) has a limited compositional range (2605-2919 ppm Ni, 117-135 ppm Co, 736-1005 ppm Mn, and 28.9-35.7 ppm Zn), whereas the chromite (Chr) has a relatively variable composition (718-1239 ppm Ni, 364-493 ppm Co, 1314-2733 ppm Mn, and 1411-1900 ppm Zn). From the dunite lenses to the dunite envelopes, and to the chromitites, both olivine and chromite exhibit significant decreases in Co (Ol = 137 to 45 ppm; Chr = 542 to 168 ppm), Mn (Ol= 889 to 273 ppm; Chr = 2426 to 862 ppm), and Zn (Ol= 37.7 to 4.0 ppm; Chr = 1746 to 206 ppm) contents, and an increase in Ni contents (Ol= 3009 to 5946 ppm; Chr = 413 to 1462 ppm). These features cannot be fully explained by subsolidus re-equilibration, partial melting, fractional crystallization, and melt-rock reactions. Olivine relicts in chromite indicate the dissolution of pre-existing olivine in dunitic channels. The olivine dissolution may have been due to the water-rich nature of the parental melts of the chromitites. These observations and mass-balance calculations suggest that, during the formation of podiform chromitites, chromite crystallization was accompanied by olivine dissolution and exsolution of a hydrous fluid phase. This resulted in the preferential transfer of Ni into the melt and olivine relicts, and Co, Mn, and Zn into the fluid phase. As such, the chromite and olivine in the chromitites became Ni-rich and Co-, Mn-, and Zn-poor, which led to Ni-Co decoupling.

Although the mechanism of formation of podiform chromitite is debated, oxygen fugacity (fO(2)) is likely one of the controlling factors, although the detailed mechanisms are poorly understood. To provide insight into this issue, we determine the Fe3+/Sigma Fe ratio of chromite via Mossbauer spectrometry, and from this calculate the fO(2) of chromitites, dunite envelopes, dunite lenses, and their host peridotites in the Luobusa ophiolite in southern Tibet. The fO(2) (-1.49 to -0.52 log units relative to the fayalite-magnetite-quartz buffer) and mineral chemistry of the host peridotites are similar to those of both abyssal and forearc peridotites. Higher fO(2) values of the dunites and chromitites (-0.65 to -0.33, 0.69 to 1.13, and -0.46 to 0.64 log units for the dunite lenses, dunite envelopes, and chromitites, respectively) relative to those of host peridotites must have been inherited from their parental magmas. Both chromite compositions and the fO(2) of the dunites and chromitites are similar to those in boninite-like rocks. Importantly, there is a previously unrecognized but marked decrease in fO(2) from the dunite envelopes to the chromitites. This decrease is intimately related to the chromitite-forming processes and preferably explained as the result of the exsolution of oxidized fluids from high-fO(2) magmas during subduction initiation.

The tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity (fO(2)) (e.g., Ballhaus, 1993; Dare et al., 2009; Parkinson and Arculus, 1999). Here we present results of fO(2) of chromites determined by Mossbauer spectroscopy from both the Luobusha and Dazhuqu areas along Yarlung Zangbo suture zone, Southern Tibet.The fO(2) values (-1.02 similar to 0.04 log units against the FMQ buffer) and Cr# (22 similar to 54%) in chromites from lherzolites and harzburgites of both areas are similar to those of abyssal peridotites, indicating that they may be residues after partial melting at spreading centers. However, both dunite envelopes and chromitites from Luobusha have high fO(2) values (0.04 similar to 2.25 log units) and Cr# (73 similar to 84%), showing an affinity to boninitic melts, and thus form in a suprasubduction zone. Dazhuqu dunites show diverse fO(2) values (-0.22 similar to 2.19 log units) and Cr# (22 similar to 82%), indicating that they form in distinct settings. Chromitites and chromite dunites from Dazhuqu have low fO(2) values (-0.3 similar to 0.71 log units) and Cr# (16 similar to 63%), both of which are similar to those of MORB-like basalts, inferring that they form in an extensional setting. Both high-Cr and high-Al chromitites from other typical podiform chromite ore deposits, such as Kempirsai, Oman, and Albania ophiolites, also show high fO(2) values (e.g., Chashchukhin and Votyakov, 2009; Melcher et al., 1997; Quintiliani et al., 2006; Rollinson and Adetunji, 2015), while the distribution-limited small chromitites and chromite dunites from Dazhuqu exhibit low fO(2) values. The phenomenon infers that the suprasubduction zone is more beneficial to the formation of podiform chromitites.

西藏南部雅鲁藏布构造带分布有一系列蛇绿岩体.人们对这些蛇绿岩体的形成环境仍然存在较大的争议.雅鲁藏布构造带中段日喀则蛇绿岩路曲和大竹曲岩体镁铁质岩石的微量元素和Pb同位素特征指示其母岩浆起源于亏损地幔源区.这些镁铁质岩石的La/Sm和Sm/Yb比值显示其岩浆产生于尖晶石二辉橄榄岩地幔经过大约10％部分熔融作用.综合岩相学和全岩主量元素特征暗示这些镁铁质岩石形成于无水玄武质岩浆.而且这些镁铁质岩石的微量元素和REE元素配分模式均非常相似于N-MORB,除了弱Nb-Ta负异常.这些特征表明路曲和大竹曲岩体形成于大洋中脊环境.此外,路曲和大竹曲镁铁质岩石的Pb同位素结果指示其地幔源区与印度洋MORB地幔域具有相似的地球化学特征.这些镁铁质岩石N-MORB标准化微量元素模式显示弱Nb-Ta负异常可能是由于其地幔源区交代了古老的俯冲带物质.

Podiform chromitites crop out in ophiolitic harzburgites as pod-like bodies associated with dunite envelopes with various thickness. It is widely accepted that the change of melt compositions caused by melt-rock reaction, especially an increase in silica content, plays a crucial role in the generation of podiform chromitite (e.g., Arai and Yurimoto, 1994; Zhou et al., 1994). Due to the presence of ultrahigh pressure and highly reduced minerals, the genesis of some podiform chromitites was attributed to some deep processes (e.g., Arai, 2013; Yang et al., 2007). Although much progress has been achieved, the formation mechanism of podiform chromitites are still in dispute. Iron isotope may be a potential tool to give further insight to the issue, given that some high temperature processes, such as partial melting, metasomatism, magma differentiation and redox change, can result in measurable iron isotopic fractionation to different extent (e.g. Chen et al., 2014; Weyer and Ionov, 2007; Zhao et al., 2009). This study investigates the Fe isotope compositions of chromitites and chromite dunites from Dazhuqu and Luobusha ophiolites. For Dazhuqu chromite dunites, delta Fe-56 (relative to the standard, IRMM-014) values range from -0.02 parts per thousand to 0.11 parts per thousand in olivines and from 0.03 parts per thousand to 0.08 parts per thousand in chromites. Chromites in Dazhuqu chromitites show delta Fe-56 values varying from -0.03 parts per thousand to 0.02 parts per thousand. In nodular and densely disseminated chromitites from Luobusha, olivines have delta Fe-56 values of olivines and chromites are 0.09-0.35 parts per thousand and -0.15-0.08 parts per thousand, respectively. Chromites from Luobusha massive chromitites have delta Fe-56 values of 0.07-0.12 parts per thousand.Based on theorical calculations, chromites should be heavier than olivines in Fe isotope compositions Delta Fe-56(Ol-Chr) approximate to-0.08 parts per thousand at 1300 degrees C according to the ionic model (e.g., Macris et al., 2015; Sossi and O'Neill, 2017). However, most of our samples, except for two samples, have Delta Fe-56(Ol-Chr) values that are greater than zero, indicating a disequilibrium inter-mineral Fe isotopic fractionation. There is a positive correlation between Fo and delta Fe-56 (or Delta Fe-56(Ol-Chr)) of olivines but no positive correlation between Mg-# and delta Fe-56 (or Delta Fe-56(Ol-Chr)) of chromites. This phenomenon suggests that the Fe isotopic dis-equilibration may be caused by migrating melts in dunitic channels rather than by the sub-solidus Fe-Mg exchange (Xiao et al., 2016; Zhang et al., 2019). Additionally, the wide delta Fe-56 range of chromites is similar to those of the subduction-related basalts and boninites, inferring that their parental magmas form in the suprasubduction zone.

日喀则蛇绿岩位于雅鲁藏布构造带中段,其成因和构造环境仍存在较大争议.日喀则蛇绿岩下部为蛇纹石化地幔橄榄岩,壳幔过渡带缺失超镁铁质堆晶岩.少量辉长岩脉呈块状或韵律结构并侵入到地幔橄榄岩和辉绿岩中.辉绿岩呈席状岩床侵入到地幔橄榄岩之上,且少量辉绿岩脉侵入到下覆的地幔橄榄岩中.通过野外关系和地球化学研究,日喀则辉长岩可能并不是洋壳中岩浆房原位结晶堆积而成,而是深部位置岩浆囊经过不同程度分异演化形成富晶粥岩浆并向上侵入的结果.而席状辉绿岩床则是基性岩浆沿着构造薄弱面顺层侵入的结果.拆离断层可能导致了岩石圈地幔抬升和剥露,进而引起下覆软流圈地幔减压熔融和岩浆上侵.日喀则辉长-辉绿岩形成于慢速扩张脊较小规模的岩浆供应和不连续的岩浆侵入.

The geodynamic setting of the Xigaze ophiolite has long been debated. Structural and geochemical evidence suggest the Xigaze ophiolite was formed at a slow-spreading ridge (Nicolas et al., 1981; Liu et al., 2016). Based on incompatible element concentrations, the Xigaze ophiolite volcanics are consistent with the ubiquitous subduction signature in suprasubduction zone (Bedard et al., 2009; Hebert et al., 2012; Dai et al., 2013). It is noteworthy that the Xigaze ophiolite is different from the Geotimes and Lasail and Velly units from Oman ophiolite, respectively. The mafic rocks of the Xigaze ophiolite generally resemble typical N-MORB and Geotimes volcanics in composition except for slight depletions of Th and Nb (Fig.1a). Although the Xigaze rocks have similar Th and Nb concentrations to Lasail and Velly rocks, most incompatible elements in the Xigaze rocks are comparable to N-MORB. Petrography in gabbro of Xigaze ophiolite shows that euhedral plagioclases are enclosed by clinopyroxenes suggesting that these minerals have crystallized from an anhydrous magma (Sisson and Grove, 1993). Although the Xigaze volcanic rocks are slightly depleted in Th and Nb, they have MORB-like trace element characteristics implying that they are derived from an anhydrous MORB magma at spreading centre. Godard et al. (2006) suggested that the mantle source of the Oman ophiolite have element and isotopic characteristics similar to Indian Ocean MORB, where the mantle preserved some older slab materials. A negative Nb anomaly of Oman Geotimes volcanic rocks may be resulted from contamination of the slab materials via decompression melting of the convecting mantle. Moreover, the Xigaze rocks have 1.27-3.18 of (Th/Nb)N ratios similar with those of Geotimes volcanics ((Th/Nb)N =0.51-2.77) and lower * Corresponding author. E-mail:sheyuwei@cags.ac.cn than those of Lasail and Velly units ((Th/Nb)N =2.12-6.35). These features suggest that the Xigaze ophiolite may have formed at the spreading centre. *

The Xiarihamu mafic-ultramafic intrusion is situated in the East Kunlun Orogenic Belt (E-KOB), northern Tibetan Plateau, China, with Ni-Co sulfide mineralization mainly in orthopyroxenite and olivine orthopyroxenite. Wholerock and mineral compositions show cyclic variations in drill-hole intersections through the Xiarihamu mafic-ultramafic intrusion. Microbeam X-ray fluorescence (XRF) mapping shows reverse zoning (Cr-poor core, Cr-rich rim) and oscillatory zoning (Cr-poor core, Cr-rich mantle and Cr-poor rim) in orthopyroxene of olivine orthopyroxenite and orthopyroxenite. Zoned orthopyroxenes are euhedral or subhedral, and there are relatively sharp interfaces between zones with distinct Cr contents. Crystal-size distribution (CSD) curves of orthopyroxenes in olivine orthopyroxenite show concave-upward curvature at size larger than 3.0 mm, and larger orthopyroxenes commonly display zoning, implying a secondary recrystallization and grain growth of larger orthopyroxenes. Quantitative analyses show that Cr and enstatite contents of cores are relatively lower, but incompatible element (e.g., Ti, Zr, Ga and Sr) contents of cores are higher than rims in reverse zoned orthopyroxenes, and chemical profiles of unzoned orthopyroxenes resemble those of rims (but different from those of cores) in reverse zoned orthopyroxenes. Chemical profiles of orthopyroxenes indicate that Cr zoning is not due to the change in Cr partitioning behavior between orthopyroxene and silicate melt, which shows significant temperature and redox dependence. Textural feature and crystal-size distribution of orthopyroxenes suggest the formation of Cr-rich zones attests to the multi-stage growth in the process of magma replenishment, rather than a reactive liquid flow model or in-situ trapped liquid overgrowth. Orthopyroxene zoning, together with compositional variation and intrusive relationship between olivine orthopyroxenite and orthopyroxenite imply that the Xiarihamu mafic-ultramafic intrusion was formed by multiple magma replenishments with magma varying roughly from a more evolved to a more primitive composition.

It is widely accepted that the incorporation of external sulfur via crustal contamination is an important trigger for sulfide immiscibility that generates Ni-Cu-(PGE) sulfide mineralization, yet other controlling factors for sulfide immiscibility may also be present. The late Permian Panzhihua, Baima, Hongge, Xinjie and Taihe layered intrusions in the Emeishan Large Igneous Province (ELIP, SW China), are well-endowed with Fe-Ti oxide deposits, whereas their sulfide mineralization is mainly sub-economic. For example, the lower part of the Xinjie intrusion hosts a few thin PGE-rich ore layers, yet other ELIP layered intrusions do not contain any Ni-Cu sulfide mineralization and are PGE-depleted (0.01-1 ppb).Compared with the PGE-undepleted Emeishan high-Ti basalts that are genetically related to the intrusions, the extent of PGE depletion and elevated Cu/Pd ratios (up to 3.2 x 10(6)) of the Panzhihua, Baima, Taihe and Hongge intrusions suggest PGEdepletion in their parental magmas due to early-stage sulfide removal. Sr-Nd isotopic compositions of the Panzhihua, Baima and Taihe intrusions suggest crustal contamination was insignificant and sulfide saturation produced mainly by crustal sulfur input was unlikely. MELTS modeling shows that extensive fractionation of chromite, olivine and clinopyroxene in deep-seated magma chambers may have induced early-stage sulfide saturation of the primary magmas. The relatively high sulfide contents in the Fe-Ti oxide layers at Panzhihua, Baima, Hongge and Taihe indicate a close relationship between the second-stage sulfide immiscibility and extensive Fe-Ti oxide crystallization.Positive correlations between sulfur and total Fe2O3, V and TiO2 suggest that Fe-Ti oxide (magnetite and ilmenite) crystallization may have triggered the secondstage sulfide saturation via sharply lowering the Fe concentration and oxygen fugacity of the magmas. Moderate degree of crustal contamination for the Xinjie Fe-Ti oxide-barren rocks may have induced sulfide saturation and accumulation at the lower part of the intrusion. Our calculations indicate that the Xinjie PGE-rich rocks have high R-factors (1000-10000), which are ascribed to PGE-upgrading of the sulfides via reaction with new replenishments of PGE-undepleted magmas. A few Panzhihua, Baima and Taihe samples that contain higher PGE concentrations suggest that the early-stage sulfide droplets at depths were entrained in later magma pulses delivered to shallower magma chambers. The very high R-factors determined by mass balance calculation, implies a good potential for discovering more PGE mineralization in the deep-seated intrusions of the magma plumbing system.

Podiform chromitites are characteristically occurred in ophiolites (e.g., Thayer, 1964; Dickey, 1975). However, the metallogenic processes for podiform chromitites are still unclear. Early models involved fractional crystallization and crystal settling from picritic or basaltic melts in magma chambers (Dickey, 1975; Boudier and Coleman, 1981), but it was also proposed that podiform chromitites formed from partial melting and melt extraction in host mantle peridotites (Dick, 1977; Dick and Bullen, 1984). Recent studies by the majority of authors have suggested that melt‐rock interaction at the Moho transition zone may have played a key role in the formation of podiform chromitites (Zhou and Robinson, 1994; Zhou et al., 1996, 2005, 2014; Robinson, 2008; Page and Barnes, 2009; Uysal et al., 2009, 2012; González‐Jiménez et al., 2011, 2015). Based on the occurrence of some ultrahigh pressure minerals (e.g. diamond and coesite) in chromitites, it has been proposed recently that the formation of podiform chromitite is likely related to multiple processes inclusing mantle recycling (Yang et al., 2007; Yamamoto et al., 2013). Although geat progresses have been made towards understanding the genesis of podiform chromitites, some fundamental issues in remain unanswered. For examples, what are the major controls on the size of chromitites? And why some ophiolites contain large podiform chromitite bodies, whereas most ophiolitic massifs are essentially chromitite‐barren?The Yarlung‐Zangbo Ophiolite belt is one of the most famous ophiolite zone in the world. It contains fresh peridotites as well as different‐sided podiform chromitites. The Luobusha ophiolite in the eastern segment of the belt hosts the largest chromite deposit in China. In the central and western segments of belt the Dazhuqu and Dongbo ophiolitic massifs contain some small‐scale chromitite bodies. Such characteristics make the Yarlung‐Zangbo Ophiolites an ideal subject to investigate the major controls on the metallogenesis of podiform chromitites.The Luobusha chromitites are large lens and enclosed in dunite. In contrast, the Dazhuqu and Dongbo chromitites display generally as narrow dykes or irregular seams with dunite envelopes. The closely spatial association of the chromitites and dunite envelopes, together with their textural features, support a petrogenetic model that the chromitites from the Luobusha, Dazhuqu and Dongbo massifs form from reaction of melt with host peridotite. In terms of chemical composition of chromite, there are distinctive differences between those from the Luobusha and the Dazhuqu or the Dongbo. Chromite from the Luobusha chromitites has high Cr# (71–82), whereas Chromite in the Dazhuqu chromitites show relatively low Cr# (16–63), and chromite in the Dongbo chromitites includes low Cr# (11–47) and high Cr# (70–81) types. For the Dongbo and Dazhuqu massifs, linear trends of Cr# with MgO, FeOt, Ni, Ga, V and Sc in chromite from the chromitites and dunites of are similar to those of the host peridotites, suggesting that the melt‐rock reaction may provide major budget of Cr for the chromitites. The similar compositions at a given Cr# in chromite from these rocks also demonstrate that the chromitites may have been formed by in‐situ crystallization of chromite under low melt/rock ratio. In contrast, the Luobusha chromitites have different trends of compositions in chromite from that of the host peridotites, implying that the formation of the chromitite bodies requires a continual replenishment of Cr‐rich melts from deeper mantle. Fractionation and accumulation of chromite from a large volume of Cr‐rich melt may play an important role on the formation of the Luobusha chromitites. MORB‐normalized trace element patterns of chromite from the Luobusha chromitites suggest that it has been formed from Cr‐rich boninitic melt at surpra‐subduction zone (SSZ) setting. However, the Dongbo and Dazhuqu chromitites have formed originally from a MORB‐affinity melt at a mid‐ocean ridge (MOR) environment.In summary, the Luobusha chromitites crystallized from a Cr‐rich melt in a dynamic conduit, where fractional crystallization and crystal settling play a key role in formation of the large chromitites. In contrast, the small‐scale mineralizations of the Dongbo and Dazhuqu chromitite pods are formed from in situ produced melts. Podiform chromitites can be formed in MOR environment, whereas the higher Cr content in boninitic melt and assimilation of subducted slab materials at SSZ setting may benefit the formation of large chromite deposit.

1 研究目的(Objective) 西藏南部的雅鲁藏布构造带发育一系列中生代蛇绿岩,这些蛇绿岩体东西展布超过2500 km,可分为西段、中段和东段.大多数蛇绿岩体出露大规模地幔橄榄岩而镁铁质岩石出露较少,且地幔橄榄岩中赋存不同规模的豆荚状铬铁矿化,如东段的罗布莎蛇绿岩体赋存大型铬铁矿床,西段的普兰和东波岩体出露小规模铬铁矿化.然而,中段日喀则蛇绿岩一直较少有铬铁矿床(化)报道.笔者近期对日喀则蛇绿岩大竹曲岩体开展了野外考察,并在地幔橄榄岩中发现一定规模的铬铁矿化.铬铁矿是我国紧缺的战略性资源,该发现对日喀则蛇绿岩超基性岩体的铬铁矿含矿性评价具有重要意义.本文旨在报道大竹曲铬铁矿化的类型和分布特征,为在该地区开展进一步的豆荚状铬铁矿调查和研究提供基础资料.

The factors and processes that control trace-element partitioning among co-crystallizing cumulus minerals in layered intrusions have long been controversial. Here we address this issue using new laser ablation ICP-MS trace element data for magnetite, ilmenite, and clinopyroxene from the Panzhihua layered intrusion in the Emeishan large igneous province, SW China. The cumulus minerals display strong Ni, Co, and Cr depletions, indicative of parental magmas low in concentration of these elements probably due to prior sulfide removal and the fractionation of chromite or Cr-magnetite in a staging magma chamber at depth. Both magnetite and clinopyroxene show cyclical variations in some transition elements (e.g., Cr, V, and Ni) along the stratigraphic section. The average concentrations of these transition elements in magnetite are positively correlated with those in clinopyroxene, likely resulting from co-crystallization of magnetite and clinopyroxene. The incompatible element (e.g., Zr, Hf, and Nb) concentrations of the cumulus minerals from the Lower Zone are highly variable compared to those of the Middle and Upper Zones. These large variations in trace element compositions are attributed to a "trapped liquid shift" in the Lower Zone. Ilmenite crystals from the Panzhihua intrusion may have undergone extensive modification of transition elements during subsolidus re-equilibration with magnetite, leading to the decoupled variations of transition elements in ilmenite across the Lower Zone stratigraphy. Our study indicates that systematic trace element variations of the main cumulus mineral assemblage, rather than a single mineral, need to be considered to better constrain the magmatic differentiation and elemental fractionation of layered intrusions.