Petrophysical Facies and Inferences on Permeability at Brothers Volcano, Kermadec Arc, Using Downhole Images and Petrophysical Data

Abstract Downhole data and cores collected during International Ocean Discovery Program (IODP) Expedition 376 at Brothers volcano, Kermadec arc, provide unprecedented, in situ views of volcanic facies fluid pathways an actively forming volcanogenic massive sulfide (VMS) ore deposit. volcano is a submarine caldera with extensive sea floor hydrothermal alteration. were two holes: Hole U1530A the NW Caldera U1528D Upper Cone. Textural analysis microresistivity images provides continuous image record that greatly improves findings based upon sporadic partial (18%) core recovery. Between 90 214 meters below (mbsf), heterogeneous local pattern variations consistent volcaniclastic interpreted from cores. 232 445 mbsf, was not recognizable because overprinting alteration, apart five intervals coherent lava flows less altered. Based on fairly constant petrophysical data, Vp-porosity relationship, presence to six image, we propose apparent textures observed beneath mbsf are dominantly flows. The change dominant flow occurs over transition zone (214–232 mbsf) where all properties gradually change. In U1528D, show similar deep shallower, largely sequences ~270 mbsf. Down 360 overall first-order downward decrease porosity be caused by compaction increased alteration intensity. Volcanic fractures exert second-order control properties. Beneath U1530A, prolonged activity inferred have diminished property within proposed flow-dominated rock package. High downhole temperatures contrast moderate U1530A. Permeable zones mix structural (inferred fault U1530A) lithological controls both holes. Some low-permeability layers and/or interfaces possibly focus fluids laterally higher-permeability layers, which may act as trap for metal-rich form stratabound sulfides deposits. Matrix likely too low permeability conduct but perfect conditions storage super saline brines. located near active vents floor, relatively temperature overprint demonstrate high spatial temporal volcano. implications new stratigraphy here include better understanding following: 1) eruption sequences, 2) volcanoes, 3) formation deposits (and near) floor.