The non-colloidal origin of 'colloform' textures in sphalerite ores

Colloform ores have generally been considered to have been deposited as colloidal sulfide gels, and even transported as colloidal sols. However, studies of doubly polished plates of colloform sphalerite-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 ore fluid. Each of the many textural criteria proposed for recognizing colloidal deposition is shown to be invalid, ambiguous, or inapplicable to these samples, and perhaps also to most other colloform mineral samples.Four conclusions pertinent to ore research are derived from this study: (1) Primary fluid inclusions in colloform samples are believed to represent the original ore 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) ore fluid more probable. (3) Maintenance of the large number of crystal nuclei responsible for the colloform texture 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 in 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 in the ore fluid due to dilution with surface waters of varying volume or chemistry (e.g., oxygen or organic content).