锆石氧逸度对相山地区火山岩型铀矿床成矿母岩判别的指示意义

我国华南广泛分布火山- 侵入杂岩体，其中个别岩体中伴有火山岩型铀矿床的产出，因此如何区分含铀矿与不含铀矿岩体成为矿床学界关注的重要课题。本文以相山地区含铀火山- 侵入岩以及周边不含铀矿侵入岩为例，开展锆石LA- ICP- MS U- Pb年龄和微量元素研究。结果显示含矿花岗斑岩的形成年龄为133. 7±1. 6 Ma（云际），含矿碎斑熔岩的形成年龄为132. 8±1. 5 Ma（邹家山），不含矿花岗斑岩的年龄为134. 9±1. 3 Ma（七琴）和133. 3±1. 3 Ma（桃溪），其在误差范围内基本一致。含矿与不含矿岩体的锆石Ce4+/Ce3+比值由高到低依次为：邹家山22. 12~68. 75（平均值为45. 61），云际19. 02~43. 48（平均值为27. 64），七琴7. 99~22. 03（平均值为15. 63），桃溪9. 70~22. 40（平均值为16. 19）。锆石Ti含量温度计结果显示含矿岩体锆石的结晶温度比不含矿岩体的锆石结晶温度相对更高。结合晶格应变模型和锆石Ti温度计，获得前者的绝对氧逸度亦高于后者。本研究发现相山地区含矿火山- 侵入岩体中锆石的Ce4+/Ce3+比值普遍大于22，笔者提出锆石氧逸度可以作为判别火山- 侵入岩体是否含矿的一个可能指标。此外，含矿与不含矿岩体中全岩铀含量的高低与氧逸度呈明显的正相关，我们推断含矿岩体具有较高的氧逸度可能指示了其岩体母岩浆相应地具有更高的铀含量。综合前人的研究资料，笔者发现含铀岩体的氧逸度明显低于斑岩型铜- 钼矿，但与花岗岩型钨矿以及火山岩型银- 铅- 锌矿类似。因此，氧逸度的相对高低对铜- 钼、钨- 钼、铀及银- 铅- 锌赋矿岩体的差异性成矿有明显制约作用，可能还间接反映了地幔物质贡献比例的多少。

Zircon oxygen fugacity as a tracer to distinguish the parent rock volcanic- hosted uranium mineralization in the Xiangshan area, South China

Volcanic- intrusive complexes are widely distributed in South China, and some of them are accompanied by volcanic- related uranium deposits. Therefore, distinguishing between ore- bearing and ore- free rocks has become an important hot topic in the field of ore deposits. This study focuses on examining the composition of trace elements in zircon using LA- ICP- MS, determining the U- Pb ages, and analyzing the characteristics of whole- rock trace elements in both ore- bearing volcanic- intrusive rocks and ore- free intrusive rocks in the Xiangshan area of South China. The results show that the ages of ore- bearing porphyroclastic lava (Zoujiashan) and granite porphyry (Yunji) are 132. 8±1. 5 Ma and 133. 7±1. 6 Ma, respectively. Correspondingly, the ages of the ore- free Qiqin and Taoxi granite porphyry are 134. 9±1. 3 Ma and 133. 3±1. 3 Ma, respectively. Those ages of the ore- bearing and ore- free plutons are basically synchronous within the error range. The ratios of zircon Ce4+/Ce3+ in the ore- bearing and ore- free rocks range from high to low as follows: Zoujiashan 22. 12~68. 75 (average 45. 61), Yunji 19. 02~43. 48 (average 27. 64), Qiqin 7. 99~22. 03 (average 15. 63), Taoxi 9. 70~22. 40 (average 16. 19). The results obtained from the zircon Ti content thermometer suggest that the zircon crystallization temperature of the ore- bearing rocks is relatively higher than that of the ore- free rocks. Furthermore, when considering the lattice strain model and zircon Ti thermometer, it can be inferred that the absolute oxygen fugacity of the former is also higher than that of the latter. This study indicates that the Ce4+/Ce3+ ratio of zircon in the ore- bearing volcanic- intrusive rock in the Xiangshan area generally exceeds 22. Therefore, we propose that the oxygen fugacity of zircon can be used as a potential indicator to determine whether the rock shows ore- bearing characteristics. Moreover, the uranium content of the whole rock in both the ore- bearing and ore- free rocks shows a significant positive correlation with the oxygen fugacity. This suggests that the high oxygen fugacity of the ore- bearing rocks may indicate a higher uranium content in the parent magma of these rocks. Based on previous research, it is suggested that the oxygen fugacity of uranium- bearing rocks is significantly lower than that of porphyry copper- molybdenum ore deposits, but similar to that of granite- type tungsten ore and volcanic rock- type silver- lead- zinc ore. Therefore, the level of oxygen fugacity serves as an important constraint on the differential mineralization types of Cu- Mo, W- Mo, U, and Ag- Pb- Zn ore- bearing rocks, which may indirectly reflect a substantial contribution from mantle materials.