Hematite/maghemite trace element geochemistry in base metal exploration

Fe-oxide micro-concretions are examined as a possible sampling medium in geochemical exploration for base metal mineralisation. Samples were extracted from loose, partly transported regolith overlying the Hillside Fe-oxide–Cu–Au deposit (Rex Minerals Ltd., South Australia) along two traverses across the deposit, using a rare earth magnet. Fe-oxide minerals in the magnetic separates are maghemite (25–35 wt.%) and hematite (15–40 wt.%). Results of the chemical analysis are compared to the bulk regolith composition. Combined content of Cu + Pb + Zn + Ni of the magnetic mineral separates of up to 1300 ppm is compared to 150 ppm in the bulk sample. Electron Microprobe and Laser Ablation ICP-MS analyses of the Fe-oxide phase in the petrographically complex Fe-oxide rich micro-concretions revealed base metal contents of > 3000 ppm Ni + Cu + Zn (average 490 ppm) indicating the selective accumulation of base metals in this phase. A Manhattan distance cluster analysis was performed on the bulk regolith and magnetic separate samples to determine geochemical affinities of elements within the sample types. Based on this a base metal index (Cu + Pb + Zn + Ni) was generated and used to identify positions of base metal anomalies. The results clearly reproduce the established anomaly and demonstrate the viability of using magnetically separated mineral fractions of regolith as a geochemical sampling medium.

► Magnetic separates (maghemite, hematite) have up to 103 ppm base metal content.
► Fe-oxide aggregates show textures of precipitation, replacement and brecciation.
► Cluster analysis shows affiliation of litho-, chalco- and siderophile elements.
► The Hillside Cu–Au anomaly is well represented in the magnetic separates.