Breccia Pipes, Products of Exsolved Vapor from Magmas

Abstract

A fundamental question any proposed model of breccia pipe formation must answer is how the void, necessary to accommodate the breccia, was created. Once a void of sufficient size is produced, the major aspects of breccia pipes (fragment character and distribution, sheet fracture boundaries, and dimensions) follow naturally and in analogy to the collapse of underground workings.We propose that the prebreccia void was formed by magmatic water which exsolved as the pluton rose to shallower depths and lower pressures and was trapped for a time beneath the cooled rind in the apical region of the pluton. Eventual piercement of the rind by this hydrous bubble would lead to: 1) a drop in P (sub H 2 O) in the bubble, 2) pinching of the lower portion of the vapor void by viscous magma, 3) crystallization of magma as P (sub H 2 O) drops, 4) buckling of void walls and onset of stope caving, 5) continued stope cave filling of void, and 6) invasion of the breccia pipe by ground water not in chemical equilibrium with fragments. Mass balance calculations indicate the proposed void-generating mechanism is feasible for the Santa Rita, New Mexico, porphyry system. The porphyry stock mass, 1.6 X 10 13 kg, could have exsolved 9.6 X 10 11 kg of H 2 O during its cooling history. At 800 degrees C and 0.5 kb, V (sub H 2 O) = 8.7 X 10 9 m 3 , the exsolved water volume is more than adequate to account for the prebreccia void volume, V void [asymp] 5.5 X 10 7 m 3 , of the two pipes associated with the Santa Rita stock.