E. Roedder
1968
<p><span>"</span>Colloform<span>" </span>ores<span> have generally been considered to have been deposited as </span>colloidal<span> sulfide gels, and even transported as </span>colloidal<span> "sols." However, studies of doubly polished plates of "</span>colloform<span>" </span>sphalerite<span>-wurtzite assemblages from various deposits reveal crystal growth features that cannot have been formed by crystallization from gels, and indicate that most, and perhaps all, grew directly as minute druses of continuously euhedral crystals projecting into an </span>ore<span> fluid. Each of the many textural criteria proposed for recognizing </span>colloidal<span> deposition is shown to be invalid, ambiguous, or inapplicable to these samples, and perhaps also to most other "</span>colloform<span>" mineral samples.Four conclusions pertinent to </span>ore<span> research are derived from this study: (1) Primary fluid inclusions </span>in<span> "</span>colloform<span>" samples are believed to represent the original </span>ore<span> fluid, not merely a residual fluid from the crystallization of a gel. (2) Although euhedral crystals may possibly grow directly from a sol, several features make a noncolloidal (true solution) </span>ore<span> fluid more probable. (3) Maintenance of the large number of crystal nuclei responsible for the "</span>colloform<span>" </span>texture<span> is attributed to relatively high supersaturation, and hence relatively high nucleation and growth rates, for the temperatures involved. (4) Remarkably uniform, regular compositional microbands, traversing many crystals </span>in<span> samples from the east Tennessee, Aachen, and particularly the Pine Point deposits, are tentatively interpreted as annual "varves." No actual growth rates have been determined, but each "varve" consists of a dark and a light band, outlining sharply euhedral former crystal growth patterns and suggesting an annual change </span>in<span> the </span>ore<span> fluid due to dilution with surface waters of varying volume or chemistry (e.g., oxygen or organic content).</span></p>
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10.2113/gsecongeo.63.5.451
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Society of Economic Geologists
The non-colloidal origin of 'colloform' textures in sphalerite ores
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