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A metacolloidal dendrite-rich Bi-Ni-Co quartz vein, Pozoblanco, Spain

Published online by Cambridge University Press:  05 July 2018

I. S. Oen  and
C. Kieft
I. S. Oen
Affiliation:
Instituut voor Aardwetenschappen De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
C. Kieft
Affiliation:
Instituut voor Aardwetenschappen De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
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Abstract

A Bi-Ni-Co vein in the contact zone of a granite near Pozoblanco, Córdoba, 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-bismuth transition zones grading into a central quartz vug zone. The Bi-rich zones show textures indicating a metacolloidal origin from a silica gel enriched in adsorbed Bi: (1) globular and chain aggregates of fine-granular allotriomorphic quartz represent morphological relics of a globulated silica gel; (2) quartz rosettes, chain rosettes and stringer-combs radiating from the quartz globules and chains represent crystallised silica gel matrix; (3) crack fillings of quartz, gersdorffite and platy bismuth dendrites represent precipitates of metal-saturated dilute hydrosols filling shrinkage cracks in the desiccating and crystallising gel; (4) quartz spherocrystals in rosettes and combs are clouded with colloidal particles inherited from the silica gel; (5) unsupported wall rock inclusions represent inclusions in a viscous silica gel precursor covering fissure walls. The thin wall zones are quartz fillings of cracks between shrinking gel and walls. The transition and vug zones are recrystallised colloidal silica crusts precipitated by fissure-filling rest-solutions. Tree-like bismuth-gersdorffite dendrites in the transition zones are precipitates of metal-rich solutions infiltrating from the Bi-rich zones into the recrystallising silica crusts. The pre-concentration of metals in silica gels explains the dendrite formation without assuming abnormally high metal concentrations in transporting solutions or long range diffusion processes.

Keywords

metacolloidsdendritesBi-Ni-Co veinsSpain
Type
Mineralogy
Information
Mineralogical Magazine , Volume 57 , Issue 387 , June 1993 , pp. 241 - 247
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1993

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