Abstract

Martite–goethite (M–G) ores are characterised by the pseudomorphic textural replacement of gangue phases (carbonate, silicate and quartz) in the primary banded iron formation (BIF) by fine-grained goethite. This differentiates them petrographically from rocks enriched in Fe as a result of lateritic weathering and leaching of gangue. A preliminary petrographic study of martite–goethite mineralisation from the Grand Central deposit at BHP’s South Flank in the central Hamersley Province reveals that pseudomorphic textures are abundantly preserved. The mineralisation occurs in the Marra Mamba Iron Formation. Magnetite is converted to martite, and the quartz–silicate–carbonate bands are pseudomorphed by goethite. Of the gangue phases, carbonate and silicate minerals are more susceptible to replacement than is quartz. The replacement of quartz by goethite does not always continue to completion, and relict quartz grains are typically preserved on the lower margin and down-dip extremity of the body of mineralised BIF. The mineralisation appears to take place in three stages. The first stage involves replacement of magnetite by hematite (martitisation) and of the carbonate–silicate–quartz gangue by very fine-grained goethite that pseudomorphs and thus preserves the original texture of the non Fe-oxide portion of the BIF protolith. This ‘Stage 1’ goethite generation appears to be ochreous. Micro-porosity development, most likely developing through dissolution of any remaining quartz that has escaped replacement by goethite, although possibly through the dissolution of goethite after quartz, forms the second stage. The third and final stage is characterised by infill of the micro-porosity to varying degrees by a second generation of coarser-grained goethite that typically rims pores, giving them a comb-textured, drusy appearance. This ‘Stage 3’ goethite generation appears to be brown goethite. This tripartite paragenesis has not been explicitly recognised in previous petrographic studies of M–G mineralisation. Subsequent lateritic weathering affects material in the vadose zone above the pre-mining water-table, introducing additional complexity to the mineralisation. Further dissolution takes place, resulting in meso-scale porosity that is clearly visible to the naked eye. Voids are rimmed by a colloform intergrowth of goethite and hydrohematite. This ‘Stage 4’ goethite is likely to be vitreous goethite. Previous studies have shown that the three different types of goethite (ochreous, brown and vitreous) have distinctly different chemical and physical characteristics, and behave differently during downstream processing of Fe ore, so it is important to understand their genesis and distribution in time and space.

1. KEY POINTS

2. The process of M-G mineralisation involves a tripartite paragenesis: Stage 1—replacement of magnetite by hematite (martitisation) and of the carbonate–silicate–quartz gangue by very fine-grained goethite that pseudomorphs the original texture, Stage 2—dissolution of remaining gangue phases (principally quartz), and Stage 3—infill of the micro-porosity to varying degrees by a second generation of coarser-grained goethite.

3. Overprinting effects of weathering form Stage 4—further dissolution and reprecipitation of Fe as colloform goethite-(hydro)hematite layers lining vugs.

4. Goethites formed during the three different stages are likely to have different chemical and physical properties and this has implications for geometallurgical and materials handling behaviour during processing.

摘要

马铁矿-针铁矿 (M-G) 矿石的特征在于细粒针铁矿在原生带状铁地层 (BIF) 中的脉石相（碳酸盐、硅酸盐和石英）的假晶质结构替代。这在岩相学上将它们与由于红土风化和脉石浸出而富含铁的岩石区分开来。对位于哈默斯利省中部必和必拓南侧的 Grand Central 矿床的马铁矿-针铁矿矿化的初步岩相学研究表明，假晶质结构被大量保存。矿化发生在马拉曼巴铁组中。磁铁矿转化为马铁矿，石英-硅酸盐-碳酸盐带被针铁矿假晶化。在脉石相中，碳酸盐和硅酸盐矿物比石英更容易被置换。针铁矿对石英的替代并不总能持续完成，而残留的石英颗粒通常保存在矿化 BIF 体的下缘和下倾末端。矿化似乎分三个阶段进行。第一阶段包括用赤铁矿（马氏体化）替代磁铁矿，用非常细粒的针铁矿替代碳酸盐-硅酸盐-石英脉石，这种针铁矿假晶，从而保留了 BIF 原岩的非铁氧化物部分的原始质地。这个“第一阶段”针铁矿一代似乎是赭色的。微孔隙发育，最有可能通过任何未被针铁矿替代的剩余石英溶解而形成，尽管可能通过石英之后针铁矿的溶解，形成第二阶段。第三个也是最后一个阶段的特点是微孔在不同程度上被第二代粗粒针铁矿填充，这些针铁矿通常环绕孔隙，使它们具有梳状纹理、毛刺状外观。这个“第 3 阶段”针铁矿一代似乎是棕色针铁矿。这种三方共生作用在以前的 M-G 矿化岩相研究中没有得到明确认识。随后的红土风化影响了开采前地下水位以上包气带中的物质，给矿化带来了额外的复杂性。发生进一步溶解，产生肉眼清晰可见的中尺度孔隙。空洞的边缘是针铁矿和水赤铁矿的胶体共生物。这种“第 4 阶段”针铁矿很可能是玻璃质针铁矿。

1. 关键点

2. MG 矿化过程涉及三重共生过程：第 1 阶段——用赤铁矿替代磁铁矿（马氏体化），用非常细粒的针铁矿替代碳酸盐-硅酸盐-石英脉石，假晶原始结构，第 2 阶段-剩余脉石相的溶解（主要是石英）和第 3 阶段——第二代粗粒针铁矿不同程度地填充微孔。

3. 风化形式第 4 阶段的叠印效应——铁进一步溶解和再沉淀为胶状针铁矿-（水合）赤铁矿层衬砌孔洞。

4. 在三个不同阶段形成的针铁矿可能具有不同的化学和物理特性，这对加工过程中的地质冶金和材料处理行为有影响。