Breccias and ores. Part 1: History, organization and petrography of breccias

Abstract

Although the term breccia is one of the oldest rock names still in use and proliferating in its application, a satisfactory definition remains problematic and an overall classification of breccias is non-existent. The names of breccia varieties (several hundred) please the heart rather than the computer, and there is little interdisciplinary correlation among the more than fifty specialized schools of breccia study. This contribution reviews briefly the history of breccia research and provides a list of the present study trends, complete with references, with emphasis on ore-associated breccias wherever possible. Conventional breccia definitions and their shortcomings are discussed and a case is made for the introduction of the petrographic category of “coarse fragmentites” of which the “typical” sedimentary breccias and conglomerates as well as volcanic, tectonic, hydrothermal, impact and other fragmental materials are members, whether designated or not as “breccia” in the conventional literature. This is followed by a proposed rational organization (classification) of coarse fragmentites and breccias based on the regime of formation and architecture (fabric, “Gefüge”). Two fundamental end-member sequences (a,b) and a transitional suite (c) can be recognized: (a) Disaggregation fragmentites (D-breccias), produced from a solid progenitor by increasing rate of fracturing, expansion, rotation, mixing and attrition. The transitional sequence is crackle→mosaic→rubble→mélange→milled breccia. (b) Aggregation fragmentites (A-breccias), generated by gathering, aggregation and accumulation of originally dispersed fragments at the site of deposition. These materials form a sequence ranging from tightly packed, fragment-supported matrix-free fragmentites through matrix-infilled, matrix-supported fragmentites to lonestone rocks. (c) In-transport fragmentites (T-breccias) formed by redeposition of an aggregate gathered in an earlier phase of development as an A-fragmentite, or by transportation and redeposition of an advanced member (i.e. rubble, mélange or milled fragmental aggregate) of a D-fragmentite. Examples of metallic mineralizations coeval with, or superimposed on the fragmentite sequences are reviewed.