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Abstract
Archean Banded Iron Formation (BIF) hosts high-grade (>55% Fe) iron ore at the Matthew Ridge prospect, in the Jack Hills greenstone belt, of the Narryer terrane, Yilgarn Craton. The ca 3000 Ma (SHRIMP U–Pb on zircons) Algoma-type BIF contains magnetite–hematite ore zones that are a product of successive overprinting hydrothermal alteration events. Rock types in the prospect area include Archean gneiss, metasedimentary rocks and BIF that are interlayered with dolerite. All rocks record peak amphibolite metamorphic mineral assemblages that are variably replaced by greenschist facies metamorphic minerals. Hypogene iron ore formed owing to: (1) the replacement of primary silica-rich bands in the quartz–magnetite BIF by Stage 1 hypogene magnesite and magnetite alteration; and (2) subsequent removal of Stage 1 magnesite to concentrate residual Stage 1 magnetite. These magnetite-rich ore bodies define <50 m-long by 20 m-wide lenses that trend NE and coincide with the hinge zones of Z-shaped, tight F1 folds, which are most likely parasitic folds to the regional NE-trending, steeply NE-plunging anticline in the Jack Hills greenstone belt. Magnetite-rich ore zones are enriched in Fe and depleted in SiO2, with only minor changes in other major oxides and trace elements, compared with least-altered BIF. Magnetite-rich ore zones are locally cut by shear zone-hosted, talc–magnetite hydrothermal alteration overprinted by talc–microplaty hematite alteration. The shear zones and both talc mineral assemblages formed during a second deformation event that was characterised by NNE–SSE shortening. The third deformation event resulted in at least two generations of NE- to NW-trending extensional veins and strike-slip faults that locally cut the talc-rich shear zones. Supergene goethite–hematite replaces magnetite-rich ore bodies within 80 m of the present surface. The strong structural control and distinct hydrothermal alteration assemblages of the Matthew Ridge prospect are the best exploration indices for high-grade magnetite-rich ore in the region.
Yilgarn克拉通Narryer地块的杰克山绿岩带内的太古代条带状铁岩组（ BIF）含有高品位（ > 55 ％ Fe）的铁矿石，其地点是Matthew Ridge 矿场。在约3000 Ma（U-Pb锆石SHRIMP年龄法）Algoma型BIF含有磁铁矿-赤铁矿矿石带，是连续的叠加热液蚀变事件的产物。矿场区的岩石类型包括太古宙片麻岩，变质沉积岩和与辉绿岩夹层的BIF。所有的岩石纪录着顶级斜长角闪变质矿物组合，在不同程度上由变绿片岩相变质矿物质替换。深成铁矿形成的原因是：（ 1 ）石英 - 磁铁矿BIF中的初级富硅带由第一阶段深成菱镁矿和磁铁矿蚀变所替代;（ 2 ）第一阶段菱镁矿的移去以集中第一阶段的剩余磁铁矿。这些富磁铁矿的矿石体是<50米长20米宽的透镜体， 东北走向，并与Z形的F1紧密皱褶铰合带重合，这些皱褶最有可能是Jack Hills绿岩带内的区域性北东走向北东陡倾背斜的寄生褶皱。富磁铁矿的矿石带富集铁，但缺乏二氧化硅，与改变最小的BIF相比，其他主要氧化物和微量元素只有轻微的变化。富磁铁矿的矿石带局部由剪切带中的被滑石-微板赤铁矿蚀变所叠加的滑石-磁铁矿热液蚀变切割。该剪切带和两个滑石矿物组合在第二次变形事件过程中形成，变形事件的特点是北北东-南南西向缩短。第三次变形事件造成至少两代东北-北西向的伸展岩脉和走滑断层，它们局部切割富滑石剪切带。表生针铁矿-赤铁矿在表面80米处取代富磁铁矿矿体。Matthew Ridge矿场的强力结构性控制和独特的热液蚀变组合是该地区高品位富磁铁矿矿石的最佳勘查指数。
ACKNOWLEDGEMENTS
This project was funded by Sinosteel Midwest Corporation Ltd. Peter Stockman and Dennis Kruger are thanked for their support and logistical assistance in the field. Additional thanks to Thomas Angerer and Iris Sonntag and the two anonymous reviewers for their constructive comments and overall improvement of the text.