湖南东南部湘东钨矿区老山坳断层性质的厘定及其对找矿的启示

摘要

Different viewpoints exist about the formation age, slip sense and displacement of the Laoshan’ao fault, the most important fault in the Xiangdong tungsten deposit, southeast Hunan province, China. Structural observations within the fault zone and nearby, comparison in mineral composition, structure and horst deformation between ore sets in the walls, and the type and distribution of alteration in the area reveal a post-metallogenic, normal nature of the fault. Considering both the north and south-south ore sets have a contact-parallel, banded structure, an alternation of black and white bands, we believe that they should have come from a single ore set before the fault formation. Accordingly, the Laoshan’ao fault would have a horizontal throw of 1.5–2.0 km. Both single and connected ore veins have aspect ratios of no less than 0.01. The ratios are much larger than the measurements made at the outcrop scale, implying the influence in aspect ratio of vein size. Based upon the above analysis, we suggest that future ore prospecting in the area should focus on the footwall for vein sets corresponding with the middle and south ore sets and in the hanging wall for a vein set corresponding with the north ore set.%老山坳断层是湖南东南部湘东钨矿区的主干断层,目前有关其活动时代、性质和位移量的认识还存在分歧。断层带内及附近的宏观构造变形观察、断层两盘矿脉成分、结构构造和围岩变形的比较、岩石蚀变的类型和空间分布特征等都表明该断层具有正断性质,形成于成矿之后。鉴于下盘北组脉与上盘南南组脉具有平行脉壁、黑白相间的条带结构,认为它们在老山坳断层形成前应该属于同一组脉,进而推断出该断层的水平断距达1.5~2.0 km。无论单一脉或连接脉,矿脉的平均宽长比都大于1.0×10–2,远高于前人在露头上测量得到的石英脉宽长比,意味着脉体宽长比与观察的脉体尺度有关。在上述分析的基础上,提出新的找矿思路：在断层下盘北组脉的北边寻找与中组脉和南组脉对应的矿脉,而在断层上盘南南组脉的深部寻找北组脉。