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

中文翻译：

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使用井下图像和岩石物理数据，克马德克弧兄弟火山的岩石物理相和渗透率推断

国际海洋探索计划 (IODP) 第 376 次探险期间在克马德克弧兄弟火山收集的井下数据和岩心提供了前所未有的现场视图，显示活跃形成的火山成因块状硫化物 (VMS) 矿床中的火山相和流体通道。兄弟火山是一个海底破火山口，具有广泛的海底热液蚀变。井下数据在两个孔中收集：位于火山口西北的 U1530A 孔和位于上锥体的 U1528D 孔。U1530A 孔中微电阻率图像的纹理分析提供了连续的图像相记录，极大地改进了基于零星和部分 (18%) 岩心的发现恢复。在海底（mbsf）以下90米至214米之间，具有局部图案变化的非均质图像相与岩心解释的火山碎屑相一致。在 232 至 445 mbsf 之间，由于叠印蚀变，岩心中无法识别火山相，除了 5 个变化较小的连贯熔岩流区间外。基于相当恒定的岩石物理数据、Vp-孔隙度关系以及微电阻率图像上存在 5 至 6 个相干图像相间隔，我们提出在 232 mbsf 以下的岩心和微电阻率图像上观察到的明显火山碎屑纹理主要是熔岩流。从火山碎屑岩到主要熔岩流相的变化发生在过渡带（214-232 mbsf），在此所有岩石物理性质逐渐发生变化。在 U1528D 孔中，岩心和岩石物理数据显示，从深部连贯熔岩流到较浅的、主要是火山碎屑序列的转变类似，约为 270 mbsf。在 U1530A 孔中降至 232 mbsf，在 U1528D 孔中降至 360 mbsf，整体一阶向下下降孔隙度被解释为是由压实和蚀变强度增加引起的。火山相和裂缝对岩石物理性质具有二级局部控制作用。在 U1530A 洞 232 mbsf 以下，推断长期的热液活动减少了拟议的熔岩流主导的岩石包内的局部岩石物理性质变化。U1528D 孔的井下流体温度较高，而 U1530A 孔的井下流体温度适中。渗透区显示出两个孔中的构造（U1530A 孔中的推断断层）和岩性控制的混合。一些低渗透率层和/或岩性界面可能将流体横向集中在较高渗透率层中，这可能充当富金属流体的陷阱，形成层控块状硫化物和沉积物。基质的渗透性可能太低而无法传导液体，但为储存超盐水提供了完美的条件。在位于海底活跃喷口附近的 U1530A 洞中，相对较低的流体温度和中等温度的蚀变叠印表明兄弟火山存在较高的时空变化。