Serpentine and chlorite as effective Ni-Cu sinks during weathering of the Aguablanca sulphide deposit (SW Spain). TEM evidence for metal-retention mechanisms in sheet silicates

摘要

Supergene alteration in the Aguablanca Ni-Cu-(PGE, platinum group elements) magmatic sulphide deposit (SW Spain) has formed distinctive soil profiles overlying the gabbroic host rocks. These profiles have subsurficial clayey horizons exceptionally enriched in Ni and Cu even in areas distant from the orebodies and the related gossan outcrops. A preliminary study of the mineralogy in these profiles including bulk analyses by electron microprobe (EMP) and energy dispersive X-ray spectrometry (EDS), showed that outstanding base-metal contents are retained by sheet silicates. Lizardite and clinochlore are the most remarkable Ni and Cu scavengers (up to 44 wt NiO and 9 wt CuO), with smectite, vermiculite and corrensite playing a subordinate role in this retention (up to 4 wt NiO and 3 wt CuO). These results suggest that Ni and Cu could be within the structure of these sheet silicates, but transmission electron microscopy (TEM) imaging has revealed that Ni, and especially Cu, also occur as discrete metallic particles. Notable differences are detected in the redistribution of base-metals among the major carriers after sulphides dissolution in supergene conditions. Copper is largely retained as native particles in sheet-silicates, particularly in chlorite, whereas Ni is largely bound to the structure, especially in serpentine. This reflects variable metal-retention mechanisms by sheet silicates at Aguablanca, related not only to the host phases but also to the low-temperature conditions in the deposit. Detailed analysis and imaging of the individual Ni-Cu-bearing phases were used to evaluate these retention processes.