Geochronology, geochemistry and mineralogy of ore-bearing and ore-barren intermediate-acid intrusive rocks from the Jiurui ore district, Jiangxi Province and their geological implications
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摘要: 江西九瑞矿集区主要发育有斑岩型、矽卡岩型及块状硫化物型三种类型的铜多金属矿床，其中斑岩型与矽卡岩型为典型的岩浆热液矿床，块状硫化物型矿床产在五通组砂岩与黄龙组白云岩界面之中，但多数也均位于侵入体附近，通常认为岩浆热液对层状黄铁矿体的叠加作用对其成矿起了重要作用。区内同时发育有大量与成矿岩体岩性相似的未成矿侵入岩，长期以来，一直未能找出成矿侵入岩的判别标志和侵入岩成矿与否的原因。本文对区内岩性为花岗闪长斑岩、石英闪长玢岩的成矿与未成矿侵入岩进行了锆石U-Pb定年、岩石主量元素分析以及造岩矿物电子探针测试工作，结果显示，九瑞地区岩浆活动的时代相对集中，成矿侵入岩的年龄分布于138.2±1.8Ma至148.0±1.0Ma之间，未成矿侵入岩的年龄分布于139.0±1.3Ma至149.6±3.0Ma之间，成矿与未成矿侵入岩的侵位时代基本相同，年龄在误差范围内一致。二者的SiO2以及其它9种主量元素含量的变化范围都非常接近，或有很大重叠，表明成矿与未成矿侵入岩的主要岩石化学成分基本相同。通过造岩矿物成份计算了岩浆开始固结的温度和侵位压力，成矿侵入岩的平均温度769℃与未成矿侵入岩的平均温度723℃在误差范围内大体一致，但成矿侵入岩的温度变化范围（580~915℃）明显大于未成矿侵入岩（656~796℃）。侵位压力方面，未成矿侵入岩的压力值（平均5.7kbar）显著大于成矿侵入岩（平均1.4kbar）。因此我们认为，成矿与未成矿侵入岩相比，前者经历了更为显著的减压过程，该过程也许对形成岩浆热液矿床非常重要，可能是产生独立热液相的一个主要途径，并且能够在岩体内及围岩中造成大量裂隙，为热液循环提供通道。但是，数据同时显示，并非经历了显著减压过程的侵入岩都能够成矿，最终成矿与否是受到不同阶段多种因素联合控制的，因此显著减压并不是成矿的充要条件，而更可能是前提条件。由已获得的未成矿侵入岩数据来推测，在九瑞地区，侵位压力大于4kbar的中酸性侵入岩，成矿潜力可能较小。由黑云母成份估算的氧逸度显示，成矿侵入岩的氧逸度变化较大并且覆盖了未成矿侵入岩的氧逸度范围，但是未成矿侵入岩的氧逸度均没有超过HM（Fe3O4-Fe2O3）缓冲线，而成矿侵入岩中有一半左右的数据点高于该缓冲线，表明较高的氧逸度对成矿更为有利，在九瑞地区，氧逸度高于HM（Fe3O4-Fe2O3）缓冲线的侵入岩，其成矿潜力可能更大。Abstract: The major types of copper polymetallic ore deposit in Jiurui ore district include porphyry, skarn and massive sulfide types. Porphyry and skarn types are typical magmatic-hydrothermal deposits, and the massive sulfide deposits of Jiurui area occurred along the strata near the sandstone and dolomite boundary but also located near intrusive rocks, which shows that magmatic-hydrothermal fluids may have also played a crucial role in the mineralization processes. In Jiurui area, petrographic characters of ore-barren intrusive rocks are almost the same as ore-bearing samples, and parameters to identify between ore-bearing and ore-barren intrusive rocks as well as how these parameters affect mineralization are still unknown. In order to solve above problems, a detailed study on geochronology, major elemental geochemistry and mineral chemistry was performed for the ore-bearing and ore-barren quartz diorite-porphyrite and granodiorite porphyry from the Jiurui ore district, Jiangxi Province. Zircon U-Pb dating results demonstrate that extensive magmatism was coeval and intensive in this area, with emplacement ages of 138.2±1.8Ma to 148.0±1.0Ma for the ore-bearing intrusive rocks, and 139.0±1.3Ma to 149.6±3.0Ma for the ore-barren intrusive rocks, which are consistent within the error range with each other. The variation ranges of SiO2 and other 9 major oxides are quite similar between ore-bearing and ore-barren intrusive rocks. The average temperatures of ore-bearing and ore-barren intrusive rocks are 769℃ and 723℃ respectively, which are also not much difference. However, the variation range of the ore-bearing intrusive rocks (580~915℃) is obviously greater than the ore-barren ones (656~796℃). It is remarkable that the average pressure of ore-barren intrusive rocks (5.7kbar) is evidently greater than the ore-bearing samples (1.4kbar). Therefore, the ore-bearing intrusive rocks from the Jiurui area should have experienced a more distinctive pressure decreasing process, which is quite important to form magmatic aqueous phase and brittle failure for hydrothermal circulation. However, decreasing pressure process of an intrusive rock is not the sufficient condition for mineralization, but a necessary condition. Based on the data from Jiurui area, the mineralization potential is weak when the pressure of intrusive rock exceeds 4kbar. The fO2 of the ore-bearing intrusive rocks has a wider variation range than the ore-barren samples and half of the data are plotted above the HM buffer line, but the fO2 data of the ore-barren samples are all plotted below the HM buffer line, which means that high fO2 (higher than HM buffer line) is beneficial to mineralization in Jiurui ore district.
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