An uneven erosion surface separates calcareous sedimentary rocks above and vesicular andesite porphyry below at the Guayacan mine of central Chile. Channel-like depressions filled with sedimentary rock flank elongate, stratiform bodies of disseminated copper ore which impregnate the vesicular crests of andesite lava flows. The "channels" lie parallel to and mark the margins of tongues of lava. Erosional features such as scour surfaces, conglomerate and crossbedding mark some of the channels. Other channels show no evidence of erosion, suggesting that some, at least, represent depressions or spaces that were not filled during extrusion of the lava. The flows strike east. Their crests are vesicular and impregnated with copper sulfide minerals, mainly bornite, to depths as much as 3 m below the contact. Bornite fills vesicles and fractures, impregnates the andesite ground mass, and replaces andesine phenocrysts, mainly along cleavage planes. Chalcopyrite is mixed with bornite in the upper, more vesicular part of the flows and occurs with fine-grained pyrite as disseminations and replacing fossils in the base of the overlying sedimentary rock. The channel axes also strike roughly east and, in most places, are underlain by barren ground. These mappable features, when recognized, are valuable in determining the size, shape, and orientation of adjacent ore bodies and, thereby, assist in the evaluation, exploration and mining of these stratiform copper deposits.
