The Genesis Of The Xiaozaohuo Nephrite Deposit In Eastern Kunlun Orogenic Belt, Qinghai Province

The Eastern Kunlun Orogenic Belt is divided into North, Central and South Zones by North Kunlun and Central Kunlun suture zones. The Xiaozaohuo nephrite deposit is located in the western segment of the Eastern Kunlun Orogenic Belt and also in the North Zone adjacent to the southern margin of the Qaidam basin, with the Xinjiang Hetian nephrite located in the same mineralization belt. Up to now, most studies of Kunlun nephrites are focused on gemmological characteristics such as resilience, hardness, color and structure, while few on ore deposit geology. Here we present an integrated study of mineralogy, gemology, petrology, ore deposit geology and element and isotope geochemistry for the Xiaozaohuo nephrite deposit. The results not only provide important information about the ore-forming age, but also the nature and evolution of the ore-forming fluids, which all have implications for the genesis of the nephrite deposit. We summarize the main conclusions as follows:(1) The Xiaozaohuo nephrite has a refractive index of 1.62, a specific gravity of 2.91-2.94 and mohs hardness of more than 5.5. It exhibits fibroblastic texture under polarizing microscope. X-ray diffraction reveals several diffraction peaks at 8.38 A, 3.26 A,3.11 A and 2.69 A, which are similar to those for the mineral tremolite from International Standard Database. Two infrared absorption peaks (3700～3400 cm-1 and 1100～400 cm-1) of the nephrite are related to M-OH and Si-O stretching vibrations respectively. The tremolite crystals from the nephrite display a pillar shape, are weakly directionally arranged and are mainly composed of Ca, Mg, Fe and Si as revealed by SEM-EDS.(2) There are two nephrite ore bodies in the Xiaozaohuo mining area, which occur as bedded and veined shape within the contact zone between the dolomitic marble of the Jinshuikou Group and the Caledonian intrusion consisting of biotite adamellite. The nephrite ores are mainly green nephrite consisting of tremolite of more than 95% and exhibit fibrous interlocking texture, felt-like texture, irregular vein texture, inequigranular texture or metasomatic texture and massive structure, vein-type structure or comb structure. They are mainly composed of SiO2,CaO and MgO, have relatively high content of Cr and Ni and are enriched in MREE and HREE. The tremolites in the nephrite have uniform trace element contents, and are depleted in LREE and enriched in MREE and HREE. However, their REE element contents are relatively lower than those of tremolites in tremolite marble.(3) Two types of fluid inclusions were recognized from the late stage quartz in light of phase proportions at room and subzero temperatures:liquid rich two-phase inclusions and gas rich two-phase inclusions. The homogenization temperature of the fluid inclusions mainly ranges from 170℃ to 200℃, and the salinity varies from 5.0% to 8%. Combined with H-O isotopic data of previous and our studies, the ore-forming fluid may be a mixture of magmatic water and metamorphic water, and meteoric water may have participated in mineralization during the late stage. It’s worth noting that hafnium isotopes should be used with caution to trace the nature of the nephrite-forming fluid as the element Hf in it has been redistributed between hydrothermal zircon and co-crystallizing tremolite.(4) The biotite adamellite in the mining area was emplaced in Early Devonian (-416 Ma), while the dioritic porphyrite in Late Permian (253.5±1.4 Ma). Zircon Hf isotopes reveal a similar source for both two magmatic rocks, i.e., a juvenile Neoproterozoic crust source. Compilation of existing geological and geochronological data suggests that the biotite adamellite may be produced in post-collisional stage of the Proto-Tethys Orogeny, while the dioritic porphyrite formed in an arc environment induced by oceanic plate subduction during Permian to Early Triassic.(5) The main zircon group (Group â… ) in the nephrite sample, exhibits patchy zoning, planar zoning or no obvious zoning. Although zircons of this group yielded a U-Pb age of 416.4±1.5 Ma resembling that for the biotite adamellite, they possess distinct REE and Hf isotopic characteristics from those of the latter, suggesting they are hydrothermal zircons precipitated from aqueous fluid responsible for the nephrite formation. Thus the age of 416.4±1.5 Ma is considered as the timing of nephrite formation, which is also consistent with metamorphic age of the dolomitic marble. Penecontemporaneous magmatism, metamorphism and mineralization (ca.416 Ma) reported here reveal a close relationship between magmatic intrusion and nephrite formation. Sources of ore-forming materials include the dolomitic marble and biotite adamellite. The former provided sufficient Ca and Mg for tremolite precipitation, while the later provided necessary SiO2.(6) The process of Xiaozaohuo nephrite deposit formation is divided into three stages, that is, the depositional stage of dolomite â†' the regional metamorphic stage of dolomiteâ†' the mineralization stage which is divided into three sub-stages. In the key nephrite formation sub-stage, mineralization is mainly characterized by hydrothermal metasomatism and filling. The most probable ore forming process can be described as follow:magmatic fluids (may mixed with metamorphic fluids) rich in Si4+ and H2O extracted Mg and Ca from the dolomitic marble, then the ore-forming fluids migrated in layers’ fissures or structural fracture zones and finally precipitated to form nephrite.