Isotope geochronology study of the granitoid intrusions in the Changshanhao gold deposit and its geological implications
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摘要:
长山壕金矿床是近年来在内蒙古中西段找到的一特大型金矿床。尽管金矿化在中元古界白云鄂博群变质沉积岩内呈层状、似层状和透镜状产出，但是与各类花岗岩类侵入岩具有密切空间分布关系。本次研究采用LA-ICP-MS方法分别对有关花岗岩类侵入岩进行了系统年代学研究，花岗斑岩和二长花岗斑岩样品的锆石U-Pb年龄值分别为290.9±2.8Ma (MSWD=1.4)和287.5±1.9Ma (MSWD=2.4)；2件黑云母花岗岩样品的年龄值分别为267.9±1.2Ma (MSWD=0.95)和274.0±2.3Ma (MSWD=1.4)。根据上述同位素年龄数据，同时结合金矿区野外地质调查和岩(矿)相学证据，可以认为，长山壕金矿区花岗斑岩和二长花岗斑岩及相关金矿床的形成时间为早二叠世早期，并且遭受到早二叠世晚期-中二叠世早期构造-岩浆活动的叠加改造，矿区切穿含金矿脉黑云母花岗岩体的存在就是很好的例证。强烈的中酸性岩浆作用为金矿床的形成提供了动力、热力和物质来源，初步研究结果表明，长山壕金矿床是海西期构造-岩浆作用及相关流体活动的产物，属于与侵入岩有关的中温热液脉型金矿床。
Abstract:The Changshanhao gold deposit is super large gold deposit found in the western part of Inner Mongolia in recent years. Although gold mineralization occurs in the metamorphic rocks of the Proterozoic Bayan Obo Group as stratoid layer, stratiform and lenticular, the gold mineralization has a close spatial relationship with various types of granitoid intrusions. The paper has systematically studied the chronology of granitoid intrusions with LA-ICP-MS. The zircon grains separated from the altered granite porphyry and monzonitic granite porphyry samples have given average LA-ICP-MS U-Pb ages of 290.9±2.8Ma, with MSWD value of 1.4 and 287.5±1.9Ma, with MSWD value of 2.4 respectively. Meanwhile, the zircon grains separated from the two fresh biotite granite samples have given average LA-ICP-MS U-Pb ages of 267.9±1.2Ma, with MSWD value of 0.95 and 274.0±2.3Ma, with MSWD value of 1.4 respectively. Based on the zircon LA-ICP-MS U-Pb age data mentioned above and combined with other geological evidences, it has been suggested that the granite porphyry, the monzonitic granite porphyry and their related gold deposit were formed in the early period of Early Permian, and which suffered the superposition of the tectonic-magmatic activity between the late period of Early Permian and the early period of Middle Permian. The presence of the biotite granite cutting through the gold-bearing veins is a good example. The granitoid intrusions were probably the products of Hercynian tectonic-magmatic activities, and played an important role during the ore-forming processes of the Changshanhao gold deposit. Preliminary results show that the Changshanhao gold deposit is a product of Hercynian tectonic-magmatic and fluid-related activities, and belongs to mesothermal vein-type gold deposit related to intrusive rocks.
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Key words:
- Gold deposit /
- Zircon LA-ICP-MS dating /
- Isotope chronology /
- Granitoid intrusions /
- Changshanhao /
- Inner Mongolia
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图 1
长山壕及邻区代表性金属矿床分布简图(据聂凤军等，2010修改)
Figure 1.
Simplified geological map showing the regional distribution of representative metallic deposits occurring in the changshanhao and its neighbouring area(after Nie et al.，2010)
图 2
长山壕金矿床地质简图(据聂凤军等，2010修改)
Figure 2.
Sketch geological map of the Changshanhao gold deposit(after Nie et al.，2010)
图 3
长山壕金矿区侵入岩的SiO2-K2O 图解
Figure 3.
SiO2 versus K2O diagram for granitoid intrusions in the Changshanhao gold deposit
图 4
长山壕金矿区花岗岩类侵入岩的代表性锆石阴极发光图像、LA-ICP-MS分析点位及206Pb/238U视年龄
Figure 4.
CL images，localities of the points for LA-ICP-MC measurements and the 206Pb/238U apparent ages of typical zircons from granitoid intrusions in the Changshanhao gold deposit
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