Research On Metallization Of The Bolong Copper Deposit In Tibet, China

The Bolong copper deposit, newly discovered the most porphyry copper deposit in the central Tibet, is located in the Southern Qiangtang depression belt of southern edge of South Qiangtang Block and the Duolong ore concentration area which is the most important in the Foye-Duobuza Mesozoic-Cenozoic iron and copper metallogenic sub belt in the western part of northern margin of Bangongco-Nujiang metallogenic belt. The ore district is located in the north west of County, which was came within the jurisdiction of Wuma County, Gaize County, Ngari Prefecture.The ore body has been occured with hidden or semi hidden, and the morphology is a plate body, tilt to the south. The ore body is hosted in granodiorite porphyry and surrounding rock of the First Segment Lower Jurassic Quse Formation(J1q1) etamorphic sandstone. The granodiorite porphyry is the ore-forming parent rock.Ore structure mainly are veinlet-disseminated and disseminated structures, Secondly, are massive, massive vein and colloidal structure. Ore textures is mainly three kinds of types which are metasomatic texture, crystal structure, solid solution separated structure and so on, and the metasomatism and crystal structure is the main structure of ore types. The mineral composition of ore body is more complex, which are mainly pyrite and chalcopyrite, and secondary are the magnetic pyrite, bornite, molybdenite, stibnite, magnetite, hematite, malachite, azurite, covellite, hematite and limonite and so on. The gangue minerals are mainly quartz, feldspar, sericite, biotite and anhydrite, and secondary are Chlorite, calcite, epidote, gypsum, kaolin and rutile et al..By means of electron probe test, Fe, the main component of chalcopyrite are Fe,Cu and S, which is similar to the theoretical value. The crystal chemical formula for chalcopyrite by Calculation is Fe1.011Cu1.003S2Fe1.031Cu1.034S2. And the crystal chemical formula for Pyrite by Calculation is Fe0.991S2Fe1.032S2. According to the electron probe analysis of test data,The genesis of the Bolong copper deposit of pyrite mainly are hydrothermal, Which was also affected by the volcano function and may also have a certain relationship with sedimentation. The average Cu content of ore is 0.47%, associated with Ag is 1.27 g/t and Au is 0.22 g/t. In addition to the main elements of the Cu, the elements Au and Ag also can be comprehensive utilization.The alteration zone between the wall rock and ore-related porphyry body can be divided into the potassic alteration, intermediate argillic and propylitic alteration zone from the ore-bearing porphyry center outwards. However, the phyllic alteration is not well developed, and quartz–sericite veins occur only locally.The formation of the Bolong copper deposit can be divided into three mineralization periods which are late magmatic, hydrothermal and supergene period. Which of the most important mineralization period is hydrothermal mineralization period can be divided into three stages of mineralization of magnetite-molybdenite stage, chalcopyrite-pyrite stage and pyrite-gypsum stage.The litman index(σ) of mineralization granodiorite porphyry ranged from 0.64 to 2.35, which is the strong or strong calc alkaline rock, and alkalinity(AR) ranged from 1.52 to 2.82, is lower alkalinity. The differentiation index(DI) ranged from 67.28 to 77.81, consolidation index(SI) values ranged from 8.51 to 15.94, differentiation of the rocks is complete. Peraluminous index A/CNK is greater than 1.1(from1.15 to 2.29),which is peraluminous characteristics. The whole rock with low calcium and high potassium, belong to shoshonitic and high-K calc alkaline series.The Carbon isotopic composition of the ore bearing porphyries is negative, and the range is-4.4‰ to 0.1‰, the average value of-1.27‰±1.7‰, and oxygen isotopic composition varies from 12.7‰ to 21.2‰, with the average of 17.8‰± 3.13‰, and which is the carbon isotope characteristics of magmatic rock of porphyry. While the distribution of sulfur isotope data is narrow, 34 S values are low, which variation range of-5.3‰ ~-3.1‰, the average value is-4.06‰±0.88‰.The value of(87Sr/86Sr)i of mineralization granodiorite porphyry ranged from 0.70759 to 0.70983, with an average of 0.70852±0.00086, is a typical copper gold porphyry initial Sr isotopic values, and is effected by the upper crust component.(143Nd/144Nd)i value from 0.51209 to 0.51230, with an average of 0.51218, and the range of εNd(t) is-7.714774--3.572778, with average-5.905932, which two stage model ages(TDM2) ranged from 1.21 to 1.55 Ga, which suggesting that the Mesoproterozoic basement participated in magmatic processes.The lead isotope ratio of granodiorite porphyry samples is relatively low, and 208Pb/204 Pb is 36.861- 37.804, 207Pb/204 Pb is 15.485- 15.556, 206Pb/204 Pb is 17.068-17.820, the ratio of range is very small, which shows that the diagenetic material is the characteristics of mantle derived components. Hf isotopic composition of zircon of the mineralization porphyry is uniform and the values of εHf(t) ranged from-1.28 to 10.46, the two stage model age was significantly greater than the formation age of zircon, so that source materials persisted longer in the earth’s crust.The magma source area of the Bolong porphyry is wedge mantle by fluid metasomatism which formded by dehydration in the subduction of oceanic crust. The mineralization granodiorite porphyry is most likely formed in the island arc tectonic environment, and may be associated with continental arc environment more closely, which was formed in a continental margin arc environment of high maturity.The fluid inclusions of the Bolong ore district are mainly three categories, which mainly is gas-liquid-solid three-phase inclusions fluid inclusions including halite and sulfur, followed by gas-liquid two-phase inclusions, as well as a small amount of gas inclusions and fluid inclusions. The variation range of homogenization temperature of inclusion is 232 to 549 ℃, with an average of 396±70 ℃, so that the deposit belongs to middle-high temperature deposits. With the mineralization the metallogenic temperature decreased which gradually reduced from 467 ℃ to 332 ℃, the pressure is decreased as while reduced from 107.44×105 Pa to 16.57×105 Pa, which shows shallow or super shallow mineralization environment.The characteristics of ore-forming fluid is high salinity, while with the mineralization salinity has decreased gradually from 43.59% Na Cleqv to 32.59% Na Cleqv. The characteristic of ore-forming fluid is higher density, which with the mineralization gradually increased from 1.0636 g/cm3 to 1.1281 g/cm3, and the p H value(5.149â†'5.454) and Eh(0.009â†'0.016) also gradually increased, but the 2lgOf(-26.714â†'-29.118), 2 lgCOf(1.851â†'1.242) and 2 lgSf(-3.284â†'-4.138) gradually decreased.The gas composition of ore-forming fluids is based on H2 O, as CO2, CH4, C2H6, N2, H2 S, a small amount of Ar gas, and individual samples containing O2. The liquid phase composition is mainly SO42-, followed by Na+, K+ and Cl-, which show that the characteristics of fluid is high salinity together with appearance of salt crystal body, and water chemical type of fluid is mainly type SO42--K+-Na+. The quartz inclusions hydrogen isotopes composed(δDV-SMOW) of ore deposit is-91.5‰--72.2‰, with the average of-78.9‰,OH18 2dO oxygen isotopic composition of water of quartz inclusions by calculate was 5.4‰- 9.9‰ with the average of 6.6‰.The diagenetic age of the bearing porphyries was(124.5±1.1)-(121.41± 0.61) Ma by the zircon LA-ICP-MS U-Pb method, which show that the granodiorite porphyry was formed in the middle early cretaceous. According to the test data of porphyry Rb and Sr isotope we obtain Rb-Sr isochron age is(115.5±9.6) Ma. And according to the Rb and Sr isotopic compositions of pyrite we obtain the Rb-Sr isochron age which is(103.6±5.6) Ma(Sri=0.70912±0.00013), whose upper limit(109.2 Ma) may representatived the age of the mineralization tends to the end. Based on previous data we determined the time limit of the Bolong deposit mineralization was 119.4- 109.2 Ma.The total sulfur isotopic composition(δ34S∑S) is-2.7‰, which is the characteristics of mantle sulfur, so that sulfur may come from deep source magma. Pb isotopic characteristics of pyrite with normal Pb isotope, so that it is the orogenic belt lead formed in the island arc and mixed source in the crust and mantle also. According to the results of correlation analysis we regards that the source of Cu, Au in the deposits may be related to the granodiorite porphyry. As well as, the granodiorite porphyry and pyrite with similar to value of(87Sr/86Sr)i, which were the crust mantle mixing Sr isotopic characteristics, so that the source of ore-forming materials is from the crust mantle mixed source and granodiorite porphyry are closely linked, ore-forming materials may be derived from the magmatic system. The ore fabric is typical characteristics is that formed with magmatism and pneumatolytichydrothermal mineralization, which indicated that the ore-forming materials are mainly derived from magmatic system.It is considered that the origin of ore-forming is closed to magma system and may be derived from the crust mantle mixed source.The characteristics of the fluid inclusions gas-liquid phase components in the deposit showed that is from mixing of meteoric water and magmatic water and dominated by meteoric water. As well as water of the H and O isotopic compositions of quartz inclusions show that the ore-forming fluid may mainly derived from magmatic water, but also was mixed with atmospheric precipitation. The rare gas isotopes show that the source of ore-forming fluid is the crust-mantle mixed source and the crustal fluid as the main body.According to the comprehensive analysis and research we have established of the genetic model of the Bolong copper deposit, as well as we have analyzed the mineralization process of the deposits.