John C. MenziesThe Vuggy Silica/Quartz textural atlas has been updated. Vuggy silica textures are formed by the dissolution of phenocrysts and clasts by hot and strongly acidic fluids that are generated during the cooling and crystallization of near surface intrusive bodies. Vuggy silica is often mineralized but is also a strong indicator of a very active hydrothermal system and epithermal Cu-Au-Ag and porphyry Cu-Au deposits at depth.
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The interaction of hot hydrothermal fluids with ground water creates strong acids which alter the host rocks, leaving behind a sponge-like formation of porous vuggy silica. Gold and copper-rich brines can then precipitate their metals within the permeable silica bodies. The shape of the deposits is determined by the distribution of silica, which can be widespread if the acids interact with a permeable rock unit like a volcanic breccia or tuff. If the host rocks have low natural permeability then the deposits may be restricted to or close to the structure providing access for fluids from depth.
The acidic fluids are progressively neutralized (buffered) by the host rock both vertically and away from the major fluid conduit from depth. The rocks in turn are altered by the fluids into progressively more stable minerals the further away from the fault. As a result, definable zones of alteration minerals are formed Typically the sequence is vuggy silica close to the main structure progressing through quartz-alunite to kaolinite-dickite, to illite and to chlorite rich rock further away. Alunite, kaolinite, dickite and illite are generally whitish to yellowish in color. The clay and sulphate alteration can cover hundreds of square kilometres.
We have compiled a collection of images of vuggy silica from high-sulphidation deposits around the world. These images are not well attributed and we will seek to redress this with your help. If you would like to contribute to this compendium with images from your deposits we would be most happy to provide full attribution and your co-operation will be greatly appreciated by your geoscience colleagues.