DRILL HOLE AND SAMPLE SELECTION

One can characterize the evolution of the fluid regime and the structural characteristics of sulfide mound genesis through the study of hydrothermal vein textures in a massive sulfide deposit. This paper describes the hydrothermal system as evident from the infilling, replacement, and vein textures of the core recovered from three holes: Holes 856H, 1035F, and 1035H (Fig. F3). These are the three deepest holes drilled in the Bent Hill area that penetrated and recovered the underlying sulfide feeder system. Hole 856H extends to a depth of 500 m and was drilled through the center of the BHMS. Hole 1035F was drilled ~100 m south of the BHMS and extends to a depth of ~230 m. Hole 1035H, located at the center of ODP Mound, extends to a depth of ~245 m.

Twenty-five samples were selected for this study from the massive sulfide and sulfide feeder zones of Holes 856H, 1035F, and 1035H to evaluate the formation of veins and impregnation zones that feed the sulfide system. Sulfide impregnated samples exhibit a range of mineralization that includes bedding-parallel infilling as well as densely packed sulfide disseminations. Sulfide veined samples exhibit single veins, vein networks, and vein networks with sulfide impregnation zones. Those samples with crosscutting vein networks and sulfide impregnation zones were especially relevant to this study to constrain the relationship between horizontal and vertical fluid flow. Samples of veins and impregnation textures were selected throughout the core. However, where possible, multiple samples were collected within a 1- to 2-m interval of the core to evaluate textures generated within the same lithology and stress regime. Although this sampling scheme does not guarantee the same stress conditions, the likelihood of similar physical conditions is increased. This becomes important when creating a structural model based on limited exposure in drilled core.

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