西藏拿顿铜(金)矿床岩矿相学、流体包裹体和地球化学特征与成矿作用研究

The Study of Facieology-Mineragraphy,Fluid Inclusions,and Geochemical Characteristics and Mineralization in Nadun Cu(Au) Deposit,Tibet

拿顿矿床位于多龙矿集区西南部,地处西藏成矿潜力巨大的班公湖-怒江成矿带上,目前该矿床的勘查研究程度非常低,仅在地表圈定出一个铜金矿体,具有很大的找矿前景。本文对拿顿矿床的岩矿相学、流体包裹体及地球化学特征进行了研究。矿床中主要蚀变矿物有高岭土、绢云母、绿帘石、方解石、白云石等,并存在银金矿、辰砂、方铅矿、闪锌矿等特征金属矿物组合。斑晶与胶结物中的流体包裹体均一温度集中在222~360℃之间,主要成分以H2O为主,含有少量CO_2,斑晶中包裹体的盐度为一组高盐度值(18.35%~26.7%NaCleqv.)和一组低盐度值(0.18%~12.85%NaCleqv.),胶结物中包裹体盐度值较低,主要分布在1.6%~12.5%NaCleqv.区间内,是在中低温、低盐度、氧化环境中形成,成矿流体受大气降水的影响较为明显。花岗闪长斑岩中SiO2含量为55.9%~65.67%,平均值为60.52%,Na_2O+K_2O含量为3.54%~5.563%,平均值为4.83%,属于高钾钙碱性系列至钾玄岩系列。A/CNK值为1.63~2.93,平均值为2.6,反映原岩为过铝质岩石。花岗闪长斑岩中稀土总量ΣREE变化范围为29.09×10^(-6)~98.73×10^(-6),整体上具轻微铕负异常(δEu=0.7~1.12),不具铈异常(δCe=0.84~0.99);岩石富集大离子亲石元素(LILE:Rb、K),亏损Nb、Ta、P、Ti等高场强元素(HFSE),具岛弧火山岩的典型特征。矿床脉状黄铁矿中Co/Ni比值为0.05~8.523,平均值为4.219,为热液成因,脉状黄铁矿多与石英脉共生,为后期热液作用阶段的产物。矿床中黄铁矿δ34S值的分布范围为-4.5‰~0.7‰,平均值为-1.95‰,黄铜矿的δ34S值为-7‰^-3.3‰,平均值为-5.43‰,硫同位素δ34S值偏小,且为负值,指示矿床中的硫受到了还原硫的影响。矿床中赋矿岩体为花岗闪长斑岩,发育银金矿、辰砂等特征矿物组合,流体包裹体具有斑岩型铜(金)矿床的中高温流体特征,据此判断矿床成因类型为低硫型浅成低温热液矿床叠加斑岩型铜(金)矿床。根据因子分析,矿床最早期的成岩作用形成早期的中酸性岩体;矿床的成矿作用主要为热液铜金矿化作用,随后发生铅锌矿化、磁铁矿化和钾化等作用,在成矿后期有含碳酸盐热液的侵入。

Nadun deposit is located in the southwest of the Duolong ore-cluster district in the Bangong Co- Nujiang River metallogenic belt of Tibet, which has huge potential of mineralization. But its research and exploration degree is very low and only one Au-bearing body has been delineated. Therefore, this deposit is of great outlook for prospecting. This study carried out research on facieology and mineragraphy, fluid inclusions and geochemical characteristics of the Nadun deposit. Principal altered minerals in the Nadun deposit include kaoline, sericite, epidote, calcite and dolomite, etc., in company with typical metal mineral assemblage, such as electrum, cinnabar, galena and sphalerite. Homogenization temperatures of fluid inclusions in phenocrysts and cements distribute mainly between 222℃ and 360℃. The fluid inclusions are dominated by H2O, with minor CO2. Salinities of fluid inclusions in phenocrysts can be divided into two groups, high values （18.35% ~26.7% NaCleqv. ）and low values （0.18% ~ 12.85% NaCleqv. ）. While the salinity of fluid inclusions in cements is low and ranges from 1.6% to 12.5% NaCleqv. These results suggest that fluid inclusions formed in the environment of medium-low temperature, low salinity and oxidization and ore-forming fluids of the Nadun deposit were distinctly affected by atmospheric precipitation. The SiO2 content of granodiorite porphyry is in the range of 55.9~65.67% （averaging 60. 52%） and the Na2O＋K2O values of samples are in the range of 3. 54~5. 563 percent （averaging 4. 83%）, indicating that rocks belong to peraluminous rocks of high-potassium calc-alkaline series to shoshonite series. A/CNK values are in the range of 1.63 ~ 2. 93, and the average is 2. 6, reflecting that protolith is peraluminous rocks. Granodiorite porphyry has a total REEs of 29.09 × 10 ^-8-98. 73 ×10^-6 , with slightly negative Eu anomalies（δEu =0. 7~1.12）and without Ce anomaly（δCe=0.84~0. 99）. The rocks are characterized by enrichment of large ion lithophile elements （Rb and K） and depletion of high field strength elements （Nb, Ta, P and Ti）, suggesting typical features of island arc volcanics. Ratios of Co/Ni in veined pyrites range from 0.05 to 8. 523 with an average of 4. 219, and this indicates that the veined pyrites are of hydrothermal genesis. Paragenetie relationship between veined pyrites and quartz breccia vein suggests that they are the product of late hydrothermal mineralization. 8：34 S values of pyrites are in the range of -- 0.45 ~0.07 percent, and the average is --0. 195 percent; δ34S values of chalcopyrites are in the range of --0.74--0.33 percent, and the average is --0. 543 percent; relatively low δ34S values with part being negative show that sulfurs in the deposits were affected by reducing sulfur. Main ore-bearing rock of the Nadun deposit is granodiorite porphyry in which characteristic minerals （electrum and cinnabar） have been found in the deposit. Fluid inclusions are characterized by medium-high temperature fluids of porphyry copper （gold） deposits. Therefore, the genetic type of the deposit belongs to porphyry copper （gold） deposit superimposed by low- sulfur epithermal deposit. The factor analysis shows that the earliest diagenesis resulted in the formation of intermediate felsic rock bodies. The main mineralization of the deposit should be hydrothermal copper- gold mineralization, followed by lead-zinc mineralization, magnetitization and potassic alteration, with involvement of carbonatic fluid in the late stage of mineralization.