Ore-related breccias in volcanoplutonic arcs

Abstract

An overview of breccias related to a variety of base metal, precious metal, and lithophile element deposits in volcanoplutonic arcs permits definition of six possible mechanisms for subsurface brecciation.1. Release of magmatic-hydrothermal fluids from high-level hydrous magma chambers during second boiling and subsequent decompression generates a spectrum of breccia types in which fragments may suffer collapse and/or ascent. Single or multiple intrusion-related breccia pipes and pre- and intermineral breccias in porphyry copper deposits provide widespread examples.2. Magmatic heating and expansion of meteoric pore fluids may lead to brecciation, commonly of late or postmineral age and including pebble dikes, in porphyry-type and related deposits. Magmatic heating of rocks saturated with seawater may cause submarine hydrothermal eruptions late in the emplacement histories of Kuroko-type massive sulfide deposits; many of the resultant breccias underwent limited sedimentary transport. Overpressuring of heated fluids beneath semipermeable, partly self-sealed cap rocks may lead to brecciation and subaerial hydrothermal eruptions in shallow epithermal precious metal settings; magmatic heating or tectonic disturbance may have triggered brecciation.3. Interaction of cool ground waters with subsurface magma can generate phreatomagmatic explosions. Postmineral phreatomagmatic diatremes associated with porphyry systems and premineral diatremes with epithermal precious (+ or - base) metal deposits were generated in this manner; these attained the palcosurface to produce pyroclastic base surge and fall deposits that accumulated as tuff rings around maar craters.4. Magmatic-hydrothermal brecciation may lead to disruption of rocks through to the paleosurface, decompression, and fragmentation and eruption of the top part of an underlying magma chamber. Pre- and postmineral magmatic diatremes of this sort are inferred to accompany a few porphyry-type and other base and precious metal systems; they were manifested at the palcosurface by accumulations of pyroclastic fall and flow deposits.5. Breccias may result from mechanical disruption of wall rocks during subsurface movement of magma. Any intrusion-related deposit may include such intrusion breccias.6. Tectonic breccias resulting from fault displacement may accompany any type of ore deposit.A continuum exists between many of these breccia types and it is difficult to identify unique criteria for their unambiguous distinction.