西澳皮尔巴拉地区鲸背山铁矿床地质特征与形成规律

Geological Characteristics and Metallogeny of the Mount Whaleback Iron Deposit in Pilbara Region,Western Australia

澳大利亚是全球铁矿石的生产大国,也是我国铁矿石的主要进口国。其铁矿主要产于西澳洲皮尔巴拉克拉通的哈默斯利省,与前寒武纪的条带状含铁建造(BIF)有关。对位于皮尔巴拉南部纽曼镇附近的世界上最大的露天铁矿山——鲸背山(Mt Whaleback)铁矿的形成背景、矿体特征及成因进行了总结。矿体产于哈默斯利群布洛克曼含铁建造中,矿体东部走向北东,西部走向近东西,总长度5 000余米。矿体在地表出露最宽处达600余米,一般200m;矿体向下延深最深达500m。主要矿石矿物为微板状赤铁矿和假象赤铁矿,还有少量针铁矿,基本不含磁铁矿。矿石基本没有磁性。围岩蚀变作用主要有:去硅化、高岭土化、赤铁矿化和碳酸盐化。元素地球化学、氧同位素和围岩蚀变等研究表明,鲸背山铁矿床属于热液改造型铁矿,其形成过程主要受原始矿源层BIF、正断层和热液流体等控制,使得BIF中的脉石矿物迁出,铁不断得到富集,从而形成高品位的铁矿床。由于西澳哈默斯利省面积大,含BIF的地层分布范围广,地质工作程度较低,因此该区隐伏铁矿找矿潜力依然较大。

As a major iron ore production country in the world, Australia is a main country exporting the iron ores to China. The iron ores in Australia associated with the Precambrian banded iron formation （BIF） predominantly occur in Pilbara craton of Western Australia. The Mount Whaleback iron deposit, the lar- gest open-pit iron mine in the world, is typical in Pilbara region. Its geological setting, ore-body features and ore-genesis were summerized in this paper. The ore-bodies occur within the Brockman Iron Formation of Hamersley Group, extend NE and EW in eastern and western part of the ore district, respectively, with alengthof5 000 m, width of ca. 200m （up to 600 m in the surface）, and the depth up to 500 m. The major ore minerals are microplaty hematite, martite, with minor goethite and the ore is not magnetic due to sparce magnetite. Desilication, kaolinization, hematitization and carbonation are the main forms of wall rock alteration. According to the studies on the element geochemistry, oxygen isotope and wall rock alteration, the Mount Whaleback iron deposit is a type of hydrothermal reworking, and the ore-forming process was controlled by the original iron-bearing strata （i. e. BIF）, normal fault and hydrothermal fluids, which leached the gangue minerals out from the BIF, and made the iron enriched in the ore gradually, and finally formed a high grade iron ore deposit. Due to the large area of the Hamersley Province with widespreaded BIF-bearing strata, the potentials to find the concealed iron bodies is still promising in Pilbara region because of the moderate level of the geological work.