CHEMICAL BRECCIATION, AN UNRECOGNIZED MECHANISM FOR BRECCIA FORMATION

The occurrence of finely brecciated pyrite in a chalcedony infilled vug in the Providencia lead-zinc deposits, Mexico, has led the writer to investigate the mechanisms by which the breccias associated with some ore deposits were formed. Details of the occurrence and texture of the breccias associated with the Tri-State lead-zinc deposits indicate that none of the presently accepted modes of breccia formation is adequate to explain them. The restriction of the breccia to areas of introduced jasperold, and the spatial relationships of the fragments broken from chert nodules and lenses essentially precludes external forces as a means of breccia formation. Brecciation is thought to be related to expansive cracking of the chert during emplacement of the jasperoid. It is suggested that a similar mechanism of brecciation could have produced the crackle breccias associated with the East Tennessee zinc deposits, and can be invoked to explain some puzzling breccias associated with the Atacocha lead-zinc deposits in the Peruvian Andes. The crackle breccias associated with many porphyry copper deposits may also be an example of this proposed chemical brecciation. Although the exact mechanism by which the rocks are broken is not understood, what appears to be a modern day example of this phenomenon has been well documented by engineers. This is known as cement-aggregate reaction or alkali reactivity, and results in expansion and cracking of concrete. It is significant that aggregate material containing fine-grained silica or dclcmite causes this problem, and in the breccias under consideration the material interstitial between breccia fragments or the breccia fragments themselves are either fine-grained silica or dolomite. © 1969 Society of Economic Geologists, Inc.