A METACOLLOIDAL DENDRITE-RICH BI-NI-CO QUARTZ VEIN, POZOBLANCO, SPAIN

A Bi-Ni-Co vein in the contact zone of a granite near Pozoblanco, Cord oba, Spain, shows a zoned structure with mm-thin quartz wall zones, up to 20 cm thick Bi-rich quartz zones, and a few cm thick quartz-bismut h transition zones grading into a central quartz vug zone. The Bi-rich zones show textures indicating a metacolloidal origin from a silica g el enriched in adsorbed Bi: (1) globular and chain aggregates of fine- granular allotriomorphic quartz represent morphological relics of a gl obulated silica gel; (2) quartz rosettes, chain rosettes and stringer- combs radiating from the quartz globules and chains represent crystall ised silica gel matrix; (3) crack fillings of quartz, gersdorffite and platy bismuth dendrites represent precipitates of metal-saturated dil ute hydrosols filling shrinkage cracks in the desiccating and crystall ising gel; (4) quartz spherocrystals in rosettes and combs are clouded with colloidal particles inherited from the silica gel; (5) unsupport ed wall rock inclusions represent inclusions in a viscous silica gel p recursor covering fissure walls. The thin wall zones are quartz fillin gs of cracks between shrinking gel and walls. The transition and vug z ones are recrystallised colloidal silica crusts precipitated by fissur e-filling rest-solutions. Tree-like bismuth-gersdorffite dendrites in the transition zones are precipitates of metal-rich solutions infiltra ting from the Bi-rich zones into the recrystallising silica crusts. Th e pre-concentration of metals in silica gels explains the dendrite for mation without assuming abnormally high metal concentrations in transp orting solutions or long range diffusion processes.