Middle Valley

Bent Hill Area

Structural Geology

Extensive stockwork mineralization was documented in Holes 856H, 1035F, and 1035H. Away from this stringer zone the vein density is typically very low. The transition between BHMS and the stringer zone is very sharp, except in Hole 1035F. We nave not documented a direct connection between igneous basement and the stockwork zone. With a few minor exceptions, all hydrothermal veins are millimeter-sized extension veins. Crosscutting veins and crack seal veins indicate multiple increments of cracking and mineral infilling. The veins have a strong tendency to be vertical although subhorizontal cracks are common. The intensity of veining implies a significant volume increase in the deeper part of the stringer zone, which could be induced by hydraulic fracturing. The quasi-absence of shear structures suggests that faults are absent in the BHMS deposit and that fluid flow is controlled by preexisting high permeability in sediments and by fluid-assisted extension veining. Coarse-grained sandstones act as major porous-flow conduits and also as major hydrothermal fluid collectors. This accounts for the lower density of veining in the coarse layers, characterized by mainly horizontal porous flow, and for the high subvertical vein density in the mudstone layers. Its also accounts for a greater sulfide enrichment in coarse layers both in lithified and non-lithified sediments. Basaltic sills are highly fractured and have induced fracturing in the surrounding sediments, thus they cannot be considered as a barrier for fluid flow.

Physical properties and the Deep Copper Zone

Magnetic susceptibility is consistently low between 40 and 120 mbsf in Hole 1035H. The deepest zone of high magnetic susceptibility occurs within the sulfide-impregnated sediments recovered from the bottoms of Holes 1035A and D at approximately 170 mbsf. The sediments are part of the laterally continuous Deep Copper Zone (DCZ), however no corresponding high magnetic susceptibility was found in the DCZ in Holes 856H, 1035G, 1035F, and 1035H. Another important feature is the subtle but significant low porosities (9% and 14%) and slightly higher bulk densities of two sandstone samples immediately below the DCZ (Hole 856H). There are also two samples from just above the DCZ which show low porosity, but these appear to be much more intensely mineralized than the sediments below the DCZ. Deeper in the section, the porosity increases gradually with depth. A similar pattern of increasing sediment porosity with depth immediately beneath the DCZ is also seen in Hole 1035G and possibly Holes 1035F and 1035H. The lower porosity beneath the DCZ may result from original sedimentation textures, early diagenesis, or alteration associated with sulfide mineralization.

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