| Jinding Cenozoic Pb-Zn deposit in western Yunnan Province is a super-large Pb-Zn deposit. At present, the ore-forming material source and the metallogenic mechanism are still controversial, it is difficult to explain the process of abnormal enrichment of ore-forming materials in Jinding deposit. Through regional metallogenic dynamics research, some researchers found that the Cenozoic polymetallic deposits of Western Yunnan Province are restricted to the extensive development of Cenozoic mantle-derived alkali-rich magma and fluid activities in a coherent way. Based on the previous researches and the metallogenic theory on trans-magma and mantle fluids, we present the evolution of mantle fluids and accompanied mantle-crust interaction and mineralization as the main standpoints, to explain the special ore-forming process of Jinding deposit. Main conclusions are as follows:(1) There is a sort of substance, black and opaque in the optical microscope, intergranual(or in cements) of rock or ore in Jingding deposit. The substance is cryptocrystalline under the optical microscope and is super microcrystalline under the electron microscope. The crystal grain size of the substance is less than 10μm, 2~6μm normally. The minerals of the substance are composed of sphalerite, galena, smithsonite, siderite, chalcopyrite, pyrite, quartz, calcite, dolomite, K-feldspar, anglesite, ferric sulfate, etc, including sulfides, sulfates, carbonate and silicate minerals in addition to quartz. Our study suggest that the substance was formed by melt in rapid solidification process of immiscible sulfide, silicate and carbonate fluids. With many metallogenic elements, mantle fluids could escape from the alkali-rich magmas in open environment, continually extracted metallogenic elements throughout their journey, produced the interaction with crustal materials/fluids in the process of entering into surrounding strata rocks and mineralization. Mantle fluids could give rise to mantle-crust mixture and the superimposition of ore materials. The substance is microscopic trace material of the effects of mantle fluids in the rocks or ores. The fluids, which resulted in the formation of the substance, may be the main material source and power source of Jingding deposit, they caused abnormal enrichment of metallogenic materials.(2) The characteristics of carbon-oxygen isotopes indicate that carbon partly derived from the degassing of upper mantle fluids, and the another one partly derived from organic compound or marine carbonate. The characteristics of strontium isotope composition indicate that the strontium in ore-forming fluids may derive from the mixing of mantle materials and crustal materials. The statistical analysis of sulfur isotope composition indicates that the sulfur came from a variety of sources, including the mantle. Moreover, the lead isotope values are variety, which fall into the field of upper mantle, lower and upper crust Pb isotope. In summary, we suggest that the ore-forming fluids origin from the enriched mantle. The transition characteristics of carbon-oxygen isotope, strontium isotope and lead isotope compositions from mantle to crust are caused by the crustal contamination when ore-forming fluids through the crust. The silicon isotopic compositions of the ore, wall-rock and silicification veins are similar(δ30Si =-0.9‰ ~-0.5‰), indicating the ore-forming fluids origin from the enriched mantle. The silicon-rich mineralizing fluids could escape from the alkali-rich magma and entered into the surrounding strata of different distance, triggered and formed the long-distance epithermal system. This metallogenic process must be accompanied by the transformation of mantle fluids to hydrothermal fluids and the mantle-crust interaction and the superimposition of the different sources of mineral.(3) The chemical type of the ore-forming fluids is SO4-Cl-Na-Ca-K, Na+ is the main cation, secondly are Ca2+and K+, with the Mg2+. SO42+ and Cl- are the main anion, F- comes second. Ore-forming fluids contain organic components and display characteristics of weak acid. Carbon dioxide is one of the components in the inclusions. The shock temperature of fluid inclusions is in the range of 100~364℃, while the homogenization temperature is in the range of 54~498℃, indicating that the thermal fluids once had relatively high temperature. The ore-forming fluids as a whole have the characteristic of low temperature(The shock temperature is 299.3℃ in average, while the homogenization temperature is 162.1℃ in average), middle-low salinity(1.73~21.05%), high-density(0.951~0.987g/cm3). The main existence form of lead and zinc could be chloride complexes or sulfate or sulfite. In the process of mineralization, with the decrease of temperature, pressure, salinity, ?O2, ?CO2, ?S2 and rise of p H, the ore-forming fluids evolved from reducing properties to weak reducing properties. There are sulfate formation and widely distributed organic compound in the deposit. There were the ancient oil and gas reservoirs in the deposit at one time. There may be three kinds of different precipitation mechanism in ore-forming elements: fluid mixing mechanism, sulfate reduction mechanism and sulfur reduction mechanism.(4) The compositions of hydrogen and oxygen isotopes indicate that ore-forming fluids were mixed with the magmatic water, basin hot brine and paleo meteoric water. The values of 3He/4He(0.03 to 0.82Ra) in fluid inclusions fall into crust and mantle box. The values of 4He/40Ar(0.009~4.197, 1.718 on average) are similar to the mantle. The 20Ne/22 Ne values of the samples range from 10.57 to 10.93(except PMP-01, 17.82), tend to mantle fluids value. 21Ne/22 Ne values vary between 0.03 and 0.05, close to mantle. The majorities of 129Xe/130 Xe value of the samples are significantly higher than the atmospheric value, indicating the fluids origin from mantle. In summary, all of the features indicate that the presence of mantle fluids in Jinding deposit during the mineralization process, and the ore-forming mantle fluids played a role in the process of mineralization and interacted with crustal rocks and fluids. Such case could lead to strong mixing between crustal materials and mantle materials.(5) The ores and wall-rocks display similar REE patterns. All the samples are enriched in LREE relative to HREE and have large variation in REE contents. The REE contents are gradually decrease from carbon rich shale clastic rocks â†' sandstone â†' limestone breccia rock type. For homologous type of rock, the REE contents display a decreasing trend from fresh rockâ†'near ore wall-rock to ore. The REE contents display a decreasing trend from sparse disseminatedâ†'disseminatedâ†'heavily disseminated to massive ore for the ore. These features indicate that the ore-forming process was accompanied by a separation of REE elements. Moreover, the analyzed samples show variable Ce or Eu anomaly from negative to positive, indicating the diversity of the metallogenic fluids. The regular changes of ΣREE, δEu and LaN/YbN values from ore may be caused by mantle fluids rich in CO2. The complexity of the distribution pattern of trace elements in Jinding deposit indicates the diversity of the ore-forming materials/fluids and the complexity of mineralization process. The complex metallogenic characteristics of the deep source, high-low temperature, fluid properties and mineralization types of diversity, hint the metallogenic process subject to the ore-bearing mantle-derived fluids and its resulting mantle-crust mixture and the superimposition of multi-source minerals.(6) This comprehensive study indicates that Jinding deposit is the production of the abnormal enrichment process of metallogenic materials of different sources(e.g. mantle and crust source). Under the conditions of many geologic factors combined, Jinding deposit came into being by both the actions of mantle fluids and crustal fluids, which had underwent a long term of complex evolvement. The large scale development of mantle-derived alkaline magmatic-fluid activity in western Yunnan in Cenozoic is a good factor for the formation of Jinding deposit. The contaminations between alkaline magmas, mantle fluids with wall-rocks lead the mineral constantly activated and migrated from deep to shallow crust. The crust-mantle mixture and the multi-mineral superimposed mineralization caused by the mantle fluids action with its evolving properties are the main factors of the deposit formation. The Lanping basin may have experienced the syngenetic, gushing, and organic mineral enrichment events before the action of mantle-derived fluids. And they formed a series of large scale lead-zinc ore- body along Bijiang fault before the formation of Jinding nappe fault. Before the final positioning of ore bodies, the dome already existed. The mantle-derived deep fluids with recharge system excreted along the Bijiang Faultâ†'the nappe faultâ†'the dome, finally from the faults or cracks on the top of the dome. The accumulation of oil and gas occurred in the dome. The two mechanisms hereinbefore were the key of the renewed positioning of the ore bodies. The mantle fluids with metal and reductive sulfur to mineralize directly were the mainly way of mineralization at the early stage of excretion along the faults or cracks on the top of the dome. With the excretion down, the mineralization tended to be stable and formed the crust-mantle mixing fluids under the action of crust-mantle mixture process induced by fluids upwelling. The pre-formed ore body in the Bijiang Fault and nappe fault(the fluids channel) had been actived and carried by the ore-bearing crust-mantle mixing fluids to the nappe fault near the top of the dome and then mineralized repeatedly. The process of mantle fluids action as well as the process of the mantle fluids evolved to hydrothermal fluids. The mantle fluids action and its evolution caused the curst-mantle mixture and superimposed mineralization of multi-source minerals. Finally they led to the abnormal enrichment of metallogenic materials and the formation of Jinding super-large Pb-Zn deposit. |
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