Microscopy of Copper Ore at the Creta Mine Southwestern Oklahoma

The primary copper in the Prewitt shale at the Creta mine in southwestern Oklahoma occurs as very fine grains of copper sulfides. The grains exhibit a variety of sizes and shapes, which are determined by the character of the organic matter and the grains of iron sulfide that they replace. Chalcocite replacements of numerous spores of the genus Triletes form elliptical to nearly spherical grains, which commonly are 100 to 200μ in diameter and usually have smooth exterior surfaces. Smaller, nearly spherical, copper sulfide grains are pseudomorphic after original iron sulfide granules. These are 20 to 40μ in diameter, have rough exterior surfaces, and consist mainly of grains of digenite and of lesser amounts of finely interspersed grains of chalcocite. Digenite grains and chalcocite grains of a similar size that possess six sides are less common; they are pseudomorphic after pyritohedral pyrite. This replacement of pyrite in the shape of an atoll and "sea island" textures attests to a replacement origin of iron sulfide by copper sulfide. Larger, irregularly shaped clumps of iron sulfide, up to 500μ across, are partly replaced by digenite and chalcocite. Very tiny, disseminated pyrite crystals of octahedral and cubic habit, mostly less than 10μ across, also appear to be partly replaced by copper sulfide.
Other grains of copper sulfide, which are irregularly shaped, and tabular, pseudohexagonal crystals of chalcocite appear to have crystallized directly from solution. The grains and the crystals are mostly less than 10μ in size and are abundantly disseminated throughout the host shale and mudstone. Discontinuous, thin, chalcocite veinlets and interstitial chalcocite fillings of silty lamellae account for lesser quantities of copper. Bornite and native silver are very minor constituents in some specimens of the primary ore.
The textures of the Creta ore sample indicate that the copper sulfides were deposited subsequent to the early diagenetic development of iron sulfide. The organic content is an important feature, one that led to the concentration of both iron sulfide and copper sulfides. The copper ore is a result of processes that affected the host sediment during late diagenesis and is not a product of magmatic processes.