REE Geochemistry of Fluorite from the Maoniuping REE Deposit,Sichuan Province,China:Implications for the Source of Ore-forming Fluids

Fluorite is one of the main gangue minerals in the Maoniuping REE deposit, Sichuan Province, China. Fluorite with different colors occurs not only within various orebodies, but also in wallrocks of the orefield. Based on REE geochemistry, fluorite in the orefleld can be classified as the LREE-rich, LREE-flat and LREE-depleted types. The three types of fluorite formed at different stages from the same hydrothermal fluid source, with the LREE-rich fluorite forming at the relatively early stage, the LREE-flat fluorite in the middle, and the LREE-depleted fluorite at the latest stage. Various lines of evidence demonstrate that the variation of the REE contents of fluorite shows no relation to the color. The mineralization of the Maouiuping REE deposit is associated spatially and temporally with carbonatite-syenite magmatism and the ore-forming fluids are mainly derived from carbonatite and syenite melts.

Lead isotopic composition and lead source of the Tongchanghe basalt-type native copper-chalcocite deposit in Ninglang , western Yunnan , China

The Tongchanghe native copper-chalcocite deposit at Ninglang occurs in low-Ti basalts of western Yunnan, and the mode of fault-filling & metasomatism metallogenesis indicates that this deposit is of late-stage hydrothermal origin. This makes it more complicated to define the source of ore-forming materials. This paper introduces the Pb isotope data of Himalayan alkali-rich porphyries, regional Early-Middle Proterozoic metamorphic rock basement and various types of rocks of the mining district in western Yunnan with an attempt to constrain the origin of the Tongchanghe native copper-chalcocite deposit at Ninglang. The results showed that the ores are relatively homogeneous in Pb isotopic composition, implying a simple ore-forming material source. The three sets of Pb isotopic ratios in the Himalayan alkali-rich porphyries are all higher than those of the ores; the regional basement metamorphic rocks show a wide range of variations in Pb isotopic ratio, quite different from the isotopic composition of ore lead; the Pb isotopic composition of the Triassic sedimentary rocks and mudstone and siltstone interbeds in the Late Permian Heinishao Formation (corresponding to the forth cycle of basaltic eruption) in the mining district has the characteristics of radiogenic lead and is significantly different from the isotopic composition of ore lead; like the ores, the Emeishan basalts in the mining district and those regionally distributed possess the same Pb isotopic composition, showing a complete overlap with respect to their distribution range. From the above, the possibilities can be ruled out that the ore-forming materials of the Tongchanghe deposit were derived from the basement, a variety of Himalayan magmatic activities, etc. It is thereby defined that the ore-forming materials were derived largely from the Emeishan basalts. From the data available it is deduced that the native cupper-chalcocite-type metallogenesis that occurred in the Emeishan basalt-distributed area has the same metal source as the Tongchanghe deposit.

^(40)Ar-^(39)Ar dating of lamprophyre dykes in the Baimazhai nickel deposit,Yunnan Province,China,and its geological significance

The Baimazhai nickel deposit,Yunnan Province,China,is located in the southern part of the Sanjiang (Tri-river) alkali-rich intrusive rock belt (Sanjiang ARIR). In this paper was conducted 40Ar-39Ar dating of two phlogopites in lamprophyres which are,as dikes,widely distributed in the orefield,and two plateau ages were ac-quired,i.e.,32.46±0.62 Ma and 32.01±0.60 Ma,respectively (averaging 32.23±0.60 Ma). The ages are obviously younger than those of the ore-hosted complex and mineralization of the Baimazhai nickel deposit. In combination with the characteristics,it is indicated that lamprophyres in the orefield and those in the Sanjiang ARIR are similar in tectonic setting,mineral assemblage and geochemistry. It is considered that lamprophyres in the orefield are the im-portant component of the Sanjiang ARIR,and the lamprophyres and ore-hosted complex in the orefield represent the products of two times of different magmatic activity from different mantle sources. On the other hand,the age of lamprophyres in the orefield is older than that of the strike-slip shearing of the Ailaoshan-Honghe fault belt,sug-gesting that the strike-slip shearing of the Ailaoshan-Honghe fault belt is not the factor leading to magmatic activity of lamprophyres in the orefield,while it is more possible that magmatic activity of the Sanjiang ARIR promoted strike-slip shearing of the fault belt.

Experimental study on liquid immiscibility of lamprophyre-sulfide melt at high temperature and high pressure and its geological significance

With lamprophyre and pyrite from the Laowangzhai gold deposit, Yunnan Province, China, as starting materials, and at pressures from 1.5 to 3.0 GPa and temperatures from 1160 to 1560℃ , an experimental study was carried out on the liquid immiscibility of lamprophyre-sulfide melt at high temperature and ultra-high pressure in the DS-29A cubic 3600T/6-type high pressure apparatus. Results showed that the liquid immiscibility of lampro-pyre-sulfide melt in the magmatic system would happen during the early magmatic evolution (high temperature and high pressure conditions) and was controlled by temperature and pressure. The sulfide melt which was separated from the lamprophyric melt would make directional movement in the temperature and pressure field and assemble in the low-temperature and low-pressure region. Because the density of SM is higher than that of the lamprophyric melt, the former would gather together at the bottom of the magma chamber and there would exist a striking boundary between the two melts. On the other hand, the results also suggested that there would be little possibility for lampro-phyric magma to carry massive gold, and lamprophyres can't provide many of oreforming materials (Au) in the processes of gold mineralization.

Os isotopic compositions of a cobalt-rich ferromanganese crust profile in Central Pacific

187Os/188Os ratios in all sub-layers of the Co-rich ferromanganese crust MP5D17 from Line Seamount in Central Pacific have been systematically determined. The 187Os/188Os values in this crust are highly variable, ranging from 0.4262 to 1.0155. From the bottom to the top, the os-mium isotopic ratios of the crust display a regular variation trend, which correlates well with its changes in structure and texture in the vertical profile. In comparison with the evolu-tion curve of Os isotopic composition in seawater over the past 80 Ma, a similar evolution tendency can be found with respect to the Os isotopic composition of the crust in this study. It is preliminarily concluded that the crust MP5D17 began to grow at about 72 Ma, and underwent three growth hiatuses, corresponding to the geological ages of 64, 53 and 37 Ma, respectively.