Weathering of PGE sulfides and Pt–Fe alloys in the Freetown Layered Complex, Sierra Leone
Weathering of PGE sulfides and Pt–Fe alloys in the Freetown Layered Complex, Sierra Leone
复制标题
塞拉利昂弗里敦层状复合体中 PGE 硫化物和 Pt-Fe 合金的风化
作者：
J. Bowles;S. Suárez;H. Prichard;P. Fisher
来自弗里敦层状复合体地平线B的新鲜和风化的岩石和腐岩含有铂族矿物（PGM）。新鲜岩石中的PGM直径为1 ~ 7 μm，包括铜榴石（PtS）、异铁铂（Pt3Fe）、次四铁铂（PtFe）、土拉铁矿（Pt2FeCu）、含os的laurite （RuS2）和其他贱金属硫化物（BMS）的PGM。风化岩石中PGM含量较少，而含Cu -（±Pd）的无序Pt-Fe合金比例较高。腐石中含有较少的、较小的（1 ~ 3 μm）有序PtFe和无序PtFe3。随着风化过程的进行，Pt-Fe合金的富铁性逐渐增强。在风化过程中出现了Pt-Fe氧化物。与原生PGM和cooperite （Pd <3%）相关的铜硫化物被破坏，以提供在某些无序Pt-Fe合金中发现的少量Cu和Pd。含铂和含钯腐辉岩保留了原始的岩石结构，并在大约2米深的地方环绕着显示逐渐风化的残余岩石（岩心）。地下水穿过腐岩，将Pt和Pd（可能还有Au）溶液从斜坡上带到了未矿化岩石上的腐岩中。Pt/Pd比值的变化表明金属的移动是独立的。深部腐岩中钯的含量略高于岩心，说明深部腐岩中钯有一定的保留。铂和钯在上层腐岩中的富集程度低于深层腐岩，表明地表浸出。在一个有机和微生物丰富的环境中发生了长时间的变化，这可能有助于PGE的浸出和运输。
Fresh and weathered rocks and saprolite from Horizon B of the Freetown Layered Complex contain platinum-group minerals (PGM). The PGM in the fresh rocks are 1–7 μm across, including cooperite (PtS), isoferroplatinum (Pt3Fe), minor tetraferroplatinum (PtFe), tulameenite (Pt2FeCu), Os-bearing laurite (RuS2), and other base metal-sulfide (BMS)-bearing PGM. The weathered rocks contain fewer of those PGM but a high proportion of disordered Cu–(±Pd)-bearing Pt–Fe alloys. The saprolite hosts scarce, smaller (1–3 μm) ordered PtFe and disordered PtFe3. The Pt–Fe alloys became increasingly Fe rich as weathering proceeded. Pt–Fe oxides appeared during weathering. Copper sulfides associated with the primary PGM and cooperite (with <3% Pd) were destroyed to provide the minor Cu and Pd found in some of the disordered Pt–Fe alloys. Platinum- and Pd-bearing saprolites have retained the original rock fabric and, to a depth of about 2 m, surround residual rocks that show progressive weathering (corestones). Ground water passing through the saprolite has transported Pt and Pd (and probably Au) in solution down slope into saprolite over unmineralized rocks. Transport is marked by changes in the Pt/Pd ratio indicating that the metals have moved independently. Palladium is present in marginally higher concentrations in the deeper saprolite than in the corestones suggesting some retention of Pd in the deeper saprolite. Platinum and Pd are less concentrated in the upper saprolite than the deeper saprolite indicating surface leaching. Alteration occurred over a long period in an organic and microbial rich environment that may have contributed to the leaching and transport of PGE.