Multiple Episodes of Brecciation and Mineralization Associated with an Epizonal Granite Porphyry, True Hill, Southwestern New Brunswick

Exploration & Mining Geology, Vol. 3, No. 1, 1994

DAVID R. LENTZ, Department of Geology, University of New Brunswick, Fredericton, New Brunswick, Canada

At True Hill in southwestern New Brunswick, low-grade porphyry-style Bi-Sn-Mo greisen mineralization is associated with the cupola of an epizonal granite porphyry (granite) that also exhibits three episodes of brecciation. The first episode of brecciation is an in situ, 8 m to 10 m wide transitional zone of net-veined to coarse-grained "blocky" endogranitic mesobreccia that is chlori-tized and greisenized and hosts weak disseminated mineralization. In a zone closer to the host rock-granite contact, angular fine- to coarse-grained endogranitic mesobreccia with greisenized matrix is superimposed on the earlier transitional breccia. The latest episode is fault-controlled and consists of quartz- and chalcedony-hosted subangular to rounded fine-grained "pebble" mesobreccias that crosscut and brecciate the other breccia types, as well as the adjacent altered metasedimentary country rocks. The first two episodes of brecciation are magmatic-hydrothermal in origin related to hydrofrac-turing of the granite carapace and are accompanied by greisenization and mineralization, whereas the post-mineralization, silica-hosted, "pebble" mesobreccias related to hydromagmatic, low-temperature fluid circulation and/or boiling along faults near the contact. These magmatic-hydrothermal and hydromagmatic mesobreccias are similar to those associated with W-Mo-Bi porphyry-greisen mineralization at the Mount Pleasant deposit.