甘肃文县阳山金矿田地质特征及控矿地质因素分析

Geological Features and Ore-Controlling Factors of the Yangshan Gold Field in Wen County,Gansu Province

甘肃文县阳山金矿田是中国最大的金矿床,最近发现于西秦岭的勉略缝合带附近,它是由赋存于泥盆系三河口群地层中的受文县弧形构造带控制的类卡林型金矿系统。对阳山矿田安坝矿段钻探和坑探工程资料的统计表明,所有金矿体均受安昌河-观音坝断裂系统(含主干断裂和次级断裂)控制,多数金矿体定位于断裂构造与泥质、粉砂质千枚岩的复合部位,少数定位于花岗斑岩脉及其接触带,说明印支期花岗岩浆活动并不是前人认为的主导金矿化的地质条件。结合区域地质分析,本文认为三河口群及其下伏碧口群等地层在早侏罗世造山事件为阳山金成矿系统提供了成矿流体和物质,三河口群泥质、粉砂质千枚岩是成矿流体聚集和成矿金属卸载的有利空间。沿安昌河-观音坝断裂系统侵位的矿前花岗斑岩脉,尽管数量较少,但也是成矿流体聚集、圈蔽和成矿的有利场所。更为重要的是,花岗斑岩脉与地层之间的能干性具有显著差异,使接触带在后期碰撞造山过程中演化为韧-脆性变形带,进而成为十分理想的成矿流体循环、聚集和金矿化空间。

The Yangshan gold-field, Wen County, Gansu Province, is the largest gold system in China. It has been recently discovered near the Mian-Lue geosuture zone in the western Qinling Orogen. As a Carlin-like gold-system, the deposit is located in the Wenxian arc-shaped tectonic zone and hosted in the Sanhekou Group of Devonian. Statistics of data from drillings and tunnels in the Anba exploration district of the Yangshan gold-field shows that all the orebodies are controlled by the Anchanghe-Guanyinba fault system comprising of the backbones and subsidiary brit- tle-ductile shear zones or fractures. The majority of orebodies locate in fractured pelitic and silty phyllites, with a minority in the pre-ore Indosinian granite-porphyry dykes or their contact zones, suggesting that the Indosinian granite magmatism was not, as interpreted in previous studies, the dominant factor to cause gold mineralization. Combining with regional geological study, we argue that the Sanhekou Group and its underlying strata such as the Bikou Group provided ore-fluids and metallogenic materials for the Yangshan gold-system during the Early Jurassic orogen- esis, while the pelitic and silty phyllites of the Sanhekou Group provided favorable loci for ore-fluid accumulation and ore-metal discharge. The pre-ore granite-porphyry dykes emplaced along the Anchanghe-Guanyinba fault-system. Despite of their paucity, they could also focus and trap ore-fluids, and host gold orebodies. Moreover, due to great difference in strength between the phyllites and the porphyry dykes, most of the porphyry dyke-phyllite contact zones became brittle-ductile shear zones during collisional orogenesis and favorably accommodated ore-fluid circulating, focusing and gold mineralization.