美国福特-诺克斯金矿床产出环境、地质特征和形成作用

美国阿拉斯加州中东部福特—诺克斯(Fort Knox)金矿床是北极圈周缘规模最大的超大型金矿床之一。对该矿床的形成背景、矿体特征、形成模式的研究表明,该矿床主要产于前寒武纪—早古生代绿片岩与白垩纪—古近纪花岗质侵入岩接触带上,受构造-岩浆活动控制明显,地表出露面积为1100×600m2,并向深部逐渐变小。与其他与侵入岩有关的金矿床不同的是,该矿床热液蚀变作用强烈,与金矿化具密切空间分布关系的热液蚀变主要为钾长石化、钠长石化、绢云母化和青磐岩化。同位素年代学证据表明,其成矿时代为晚白垩世—古近纪(92.5±0.2Ma),是多期次岩浆作用的产物。依据同位素特征、成矿流体、围岩蚀变等特征可以判定,成矿物质主要来源于花岗质熔浆,成矿作用与氧化型中酸性侵入体侵入作用及其相关流体活动有关,属于典型的与侵入岩有关的金矿床。

Production environment, geological characteristics and formation of the Fort Knox gold deposit, USA

Located in east-central part of Alaska, USA, the Fort Knox gold deposit is one of the largest superlarge gold deposits in Arctic circle. The study of its geological setting, orebody features and ore genesis indicates that the deposit was mainly formed in the contact zone of Precambrian-early Paleozioc greenschist intruded by Cretaceous-Paleogene granitic rocks and prominently controlled by tectonomagmatic activity. It has an area of 1100×600m2 at surface but is gradually reduced with the depth. Unlike other intrusion-related gold deposits, the hydrothermal alteration of the deposit is well developed. Gold mineralization has close relationship with K-feldspar, albite, sericite and propylitization. Isotope chronologic evidence suggests that the metallogenic epoch is late Cretaceous-Paleogene (92.5±0.2Ma), suggesting a product of multi-stage magmatism. Based on the characteristics of isotopic data, ore-forming fluid and wall rock alteration, the authors have reached the conclusion that the ore-forming materials were mainly derived from granitic magma, the metallogenic process was related to the oxidized intermediate-acid intrusions and fluid activities, and hence the deposit should belong to the intrusion-related gold deposit.