TEXTURES FORMED BY DEPOSITION IN OPEN SPACE
- Minerals growing in vugs and open veins are characterized by well developed crystal faces, crystals exhibiting growth zoning, and colloform or zoned monominerallic bands.
- All these features result from unobstructed growth of minerals in fluid filled voids.
- Banding results due to a change in the physico-chemical environment of mineralization with time.
- Deposition from hydrothermal solutions in open fissures can result in Comb Structures and in Symmetrically and Rhythmically Crustified Veins.
- Movement along such a vein may cause brecciation resulting in Breccia Ore.
- All these textures may develop on scales ranging from macroscopic to microscopic.
- These textures are exemplified by Cu-Pb-Zn (Ag) vein deposits.
- Pyrite in these veins is generally found in the form of isolated cubes, octahedra or pyritohedra, or as aggregates of interfering crystals along the fracture walls.
- Sphalerite may occur as honey-yellow to black crystals or radiating colloform aggregates, which often contain a well developed growth banding or zoning. (This structure is clearly visible in doubly polished thin sections).
- Darker sphalerite colour generally indicates higher iron content.
- In open voids, galena - which in other deposits is found in anhedral intergranular aggregates, occurrs as well formed cubes, octahedral and skeletal crystals.
- Episodic precipitation, sometimes with intervening periods of leaching, often leaves hopper-like crystals of galena.
- Unobstructed growth of chalcopyrite, tetrahedrite and 'ruby-silvers' (polybasite-pearcite) commonly result in the formation of euhedral crystals.
- Sequential deposition from cobalt- and nickel-bearing solutions results in the development of concentrically zoned pyrite-bravoite crystals, which often reveal changing crystal morphology (cube, octahedron, pyritohedron) during growth.
- Sequential deposition from metal- and sulfur-bearing fluids circulating through the intergranular pore spaces in sediments may leave sediment grains coated with sulfides.
- Iron and manganese oxides and hydroxides often form in open fractures as a result of meteoric water circulation.
- These minerals (viz., goethite, lepidocrocite, pyrolusite, cryptomelane) may form concentric overgrowths inwards from vein walls or complex masses of fibrous (brush-like) crystals, radiating from multiple growth-sites along the open fractures.
- "Colloform" textures have often been cited as evidence for initial formation by colloidal deposition.
- It has also been demonstrated that many colloform sphalerites in Pb-Zn ores grow as tiny fibrous crystals projecting into a supersaturated ore fluid.
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