INTERPRETATION OF MICROSCOPIC ORE TEXTURES: AN UPDATE

The interpretation of origin of various mineral intergrowths is very complex, yet very important for understanding the history of ore deposit formation, as well as the exploration for mineral deposits.

Numerous names have been applied to ore textures, including such terms as disseminated, lamellar, basket-weave, vermicular, grain boundary segregations, colloform, flames, caries, etc. These textures most commonly develop from just four processes: 1) growth in open spaces, 2) replacement, 3) exsolution from solid solutions, and less commonly, 4) coprecipitation. According to Craig and Vaughan (1994), replacement is a secondary process that may result from dissolution and reprecipitation, oxidation, and/or solid state diffusion. Exsolution, however, occurs in the process of cooling, where one phase is expelled from another, commonly developing a characteristic pattern.

In more recent years, probably the most difficult texture to interpret, and one that has received the greatest attention, is “chalcopyrite disease”, first named by Jim Craig, who said, “That’s not a texture; that’s a disease” (Barton and Bethke, 1987), as well as Barton (1978), Wiggins and Craig (1980), and Bornikov, et al. (1991).

In this presentation, typical ore textures, including “chalcopyrite disease”, will be presented and briefly discussed from fifteen mineral deposits, including those from the Cu-Mo-Au deposit, Bingham Canyon, UT; magnetite-hematite ores at Iron Springs, UT; the Cu porphyry deposits at Mission, Safford, and Morenci, AZ; the colloform cassiterite, or wood tin occurrences in the Black Range, NM; chromite deposits at Stillwater, MT; Cu-Ni sulfides in the Duluth Complex, MN and Eagle, MI; vermicular textures of bornite-chalcocite intergrowths found at Salobo, Brazil; and dendritic sphalerite in the Pb-Zn ores of Red Dog, AK.