右江盆地酸性脉岩继承锆石成因及地质意义

Origin and geological implication of the inherited zircon from felsic dykes,Youjiang basin,China

卡林型金矿与岩浆作用的关系一直以来是矿床学家关注的热点。右江盆地发育众多卡林型金矿床，但由于区内岩浆岩出露十分有限，岩浆活动对金成矿有无贡献目前还无定论。火成岩中的继承锆石常常可以用来指示早期的、隐伏的岩浆活动。桂西北地区晚白垩世巴马、料屯和下巴哈石英斑岩脉中含有大量的继承锆石。其中，料屯和下巴哈两处脉岩分别与料屯和明山金矿在空间上紧密相关。脉岩中锆石的LA．ICP．MS和SIMS原位U-Pb定年表明，其主要集中于130—140Ma、242Ma左右、400．450Ma、700～1000Ma和1700～1800Ma等五个年龄段。关于较年轻的两个时间段，来自料屯和下巴哈石英斑岩脉的继承锆石分别获得两组谐和年龄，即136．3±3．9Ma（2σ）和242．3±1．7Ma（2σ）、128．2±2．3Ma（2σ）和243．1±3．6Ma（2σ）。这些脉岩侵入的最年轻地层为中三叠统百逢组，因而前三叠纪继承锆石可能捕获自地层，而晚于三叠纪的130．140Ma锆石则应来自于深部隐伏岩体。关于242Ma左右锆石，尽管其与围岩地层百逢组1～2段近于同期，但其晶形较差且多呈不规则碎片状，暗示其更可能源自百逢组中的沉积碎屑锆石。结合区域地质背景，认为该期锆石所代表的岩浆事件可能与古特提斯洋闭合有关。以上130～140Ma和约242Ma左右的两组年龄，与初步确定的右江盆地卡林型金矿的两次成矿的时代基本一致，据此推断金矿化与岩浆作用可能有一定成因联系。

The relationship between the Carlin-type gold deposits with magmatism is still debatable. Numerous Carlin-type gold deposits occur in the Youjiang basin, Southwest China. Due to the limited exposures of magmatic rocks within this region, it is unknown if there is genetic connections between magmatism and gold mineralization. The inherited zircons found in igneous rocks could give insight into the older or hidden magmatic events which are poorly exposed at the surface. The Bama, Liaotun, and Xiababa quartz porphyry dykes host plenty of inherited zircons, among which the Liaotun and Xiabaha dykes are spatially present within the Liaotun and Mingshan gold deposits, respectively. Five main age clusters at 130 - 140Ma, ca.242Ma, 400 - 450Ma, 700 - 1000Ma, and 1700- 1800Ma were recognized in the inherited zircons by both LA-ICP-MS and SIMS U-Pb dating. The two younger groups of inherited zircons from the Liaotun and Xiabaha dykes give concordant U-Pb ages of 136. 3 ±3.9Ma （20.）, 242. 3± 1.7Ma （20.） and 128. 2 ±2. 3Ma （2σ）, 243.1 ±3.6Ma （2σ）, respectively. All the three dykes intruded the strata including the Middle Triassic Baifeng Formation （Member 1 and 2 ; 247 - 244Ma） and older sequences. Therefore, those zircons with ages older than the Middle Triassic should be probably captured from the host sedimentary rocks. Conversely, those at 130 -140Ma might be derived from the hidden magmatic intrusions. For the zircons at ca. 242Ma, though coeval to the first and second members of the Baifeng Formation, they usually occur as anhedral fractions, indicating they might be inherited from the detrital zircons hosted by the sandstone and sihstone of the Baifeng Formation. Combined with the regional magmatism and geological setting, we propose that the ages of Triassic zircon forming events could represent the magmatic pulse related to the closure of the Paleo-Tethys Ocean. The zircon ages of 130 - 140Ma and ca. 242Ma are generally consistent with the timing of gold mineralization in the Youjiang basin. Thus, we prefer that there might be genetic links between magmatism and the formation of Carlin-type gold deposits in the Youjiang basin.