Middle Valley

Bent Hill Area


The lithostratigraphic characterization of the BHMS, initiated during Leg 139 and continued during Leg 169, is one of the most remarkable achievements of the Ocean Drilling Program. Eight holes were drilled in the vicinity of the BHMS to assess the thickness and lateral extent of the massive sulfide and to determine the nature of the hydrothermal feeder zone in the sediments and basalt underlying the deposit. BHMS is the result of a complex interaction between hemipelagic and turbiditic sedimentation, igneous activity, and hydrothermal circulation. The deposit includes iron- and zinc-rich massive and semi-massive sulfides, a well developed feeder zone characterized by crosscutting copper-rich veins and sulfide impregnation of sediment, and a deep stratiform zone of rich copper mineralization that may be developed into an important conduit for lateral fluid flow. Hydrothermal alteration provides a record of past and present fluid flow and is controlled by small variations in lithology and large-scale variations in sedimentary facies. A second massive sulfide mound was drilled 350 m south of the BHMS. The ODPMS mound is significantly enriched in Zn relative to the BHMS. It is not clear if it is correlative with BHMS or a product of a spatially and temporally separate hydrothermal system. However, it appears that both mounds are underlain at present by identical hydrothermal fluids.

The major lithologic units recognized in the Bent Hill area are summarized below and are shown on vertical sections constrained by the drilling. Eight lithostratigraphic units have been recognized as follows:

€ Unit I: hemipelagic sediment (Holocene and upper Pleistocene)

€ Unit II: Interbedded turbidites and hemipelagic sediments (Pleistocene)

€ Unit III: Clastic sulfides

€ Unit IV: Clastic sulfate-chimney debris

€ Unit V: Massive and semi-massive sulfides

€ Unit VI: Sulfide feeder zone and mineralized sediments

€ Unit VII: Basaltic sills

€ Unit VIII: Basaltic flows

Hole 856H

Hole 856H can be considered as a reference section where the major lithologies were drilled (Fig. 7). From top to bottom they include successively: massive sulfide (0-103.6 mbsf), a sulfide feeder zone (103.6-210.6 mbsf), interbedded turbidite and pelagic sediments (210.6- 431.7 mbsf), a 39.4 m interval of basaltic sills and sediment, and 28.9 m of basaltic flows.

Most of the massive sulfides were drilled during Leg 139. During Leg 169, pyrrhotite- and pyrite-rich massive sulfides were drilled from the lower part of the mound between 98.8 and 103.6 mbsf (Unit V).

The underlying feeder zone is divided into three subunits. The uppermost unit is sulfide-veined siltstone and mudstone (Unit VIa, 103.6-152.9 mbsf) where the major sulfides are isocubanite, chalcopyrite, and pyrrhotite occurring in vertical veins. Below this interval (152.9 to 201 mbsf-Unit VIb), sediments have only minor disseminated and vein controlled sulfide. From 210.6 to 249.0 mbsf (Unit VIc) is a sulfide- banded sandstone, highly enriched in copper relative to the overlying mineralization, which was informally called the Deep Copper Zone (DCZ). The contact between the DCZ and the underlying sediment is extremely sharp. The next unit (Unit II, 210.6-431.7 mbsf) is non-mineralized to slightly mineralized turbidites. Based on hydrothermal alteration and color, three intervals have been defined. Subunit IIa is relatively unaltered sediment occurring in the upper (210.6-249.0 mbsf) and the lower part (3245.5-431.7 mbsf) of the unit and consists of gray fine sandstone. Between these two intervals is a greenish gray siltstone and mudstone (Unit IIb, 249.0-345.5 mbsf) with abundant chlorite. The next unit (Unit VII, 431.7-471.3 mbsf) is the basaltic sill complex where igneous rocks are variably altered and crosscut by veins containing quartz, chalcopyrite, pyrrhotite, sphalerite, calcite, and epidote. These sills are intercalated with indurated sediment. The underlying basaltic flows (Unit VIII, 471.1-500 mbsf) are similar in mineralogy and alteration to the sill unit.

Holes 1035A and 1035G

Holes 1035A and 1035G were drilled to 170.8 and 208.5 mbsf at 75 m and 65 m west of the top of the sulfide mound. Two types of mineralization were recovered. A sequence of surficial bedded clastic sulfide and sulfide sand interbedded with turbidites was intersected between 0 and 1.40 m. These sulfides consist of pyrrhotite, pyrite, sphalerite, and Cu-sulfides. A deeper massive sulfide zone was drilled between 55 and 61 mbsf in Hole 1035A and between 44.4 and 73.6 mbsf in Hole 1035G. In both holes the massive sulfide consists of recrystallized vuggy pyrite. Minor carbonates, anhydrite, and barite partially infill the vugs. Chalcopyrite and sphalerite are less than 5% by volume. The massive sulfides are underlain by hemipelagic silty claystone interbedded with fine sand turbidite and silty clay. Soft sediment deformation is pervasive in these sediments. Sulfides (<1%) consisting of sphalerite, pyrrhotite, and chalcopyrite occur as disseminated grains, pore filling, and rare subvertical veins. The alteration is moderate and authigenic anhydrite and pyrite are common as veins, nodules, and disseminated crystals. Compared to the central part of the mound this lateral extension is depleted in high temperature minerals such as chalcopyrite, isocubanite, pyrrhotite, and sphalerite. Highly silicified sediment that prevented penetration with the XCB was encountered at approximately 170 mbsf. Between 169.80 and 180 mbsf semi-massive pyrite and copper/iron sulfide with very minor pyrrhotite occur as impregnations in hydrothermally altered siltstone. Altered siltstone and sandstone with disseminated pyrite and pyrrhotite were recovered from the deepest part of the section (179.5-208.5 mbsf). These sediments are altered to clay, chlorite, epidote, and quartz, and sulfides (up to 5%) occur as subvertical veins and bedding parallel impregnation.

Hole 1035D

Hole 1035D was drilled to 173.9 mbsf at 75 m east of the mound. As for the western part of the mound, the surface mineralization consists of clastic sulfides occurring in several intervals between 0 and 12.3 mbsf. The entire 35.6 m of the turbidite sediment sequence is similar to that of Hole 1035A. Deeper in the hole, massive sulfide, sulfide-veined claystone, and semi-massive sulfide was sampled between 40.7 and 77.4 mbsf. Vuggy massive pyrite containing veins, patches, and disseminations of chalcopyrite, sphalerite, magnetite, and anhydrite, is the dominant sulfide type in the upper part of the core (40.6-67.8 mbsf). The lower part of the section consists of massive pyrrhotite intercalated with altered mudstone, cut and impregnated by pyrrhotite and pyrite. The massive sulfides from this hole are enriched in sphalerite relative to most of the sulfide recovered from Hole 856H. Under the massive sulfide is a feeder zone consisting of massive to semi-massive pyrrhotite-pyrite-magnetite with minor anhydrite and sulfide-impregnated mudstone (66.50 to 77.40 mbsf). Between 115.80 and 135 mbsf the sulfides are less abundant. Between 135.00 and 168.50 mbsf a section of siltstone with disseminated pyrite and pyrrhotite is pervasively altered to chlorite and clay. The lowermost core consists of highly fractured and intensely silicified silty claystone with disseminated sulfides that was too hard to penetrate with the XCB.

Holes 1035B and 1035C

Hole 1035B is located 51 m to the south of Hole 856H, just off the southern edge of the BHMS mound. The mud line piston core encountered a hard layer near the seafloor and was severely bent, requiring a pipe trip. Hole 1035C was started at the same site by using a camera survey to place the bit on seafloor where massive sulfide talus was separated by flat sedimented areas, and was cored with the XCB system. Coring between 0 and 44 mbsf recovered massive sulfide, but the hole was abandoned due to poor conditions, so the thickness of massive sulfide on the south flank of BHMS mound remains unconstrained.

Hole 1035F

This hole was drilled at the base of the BHMS mound 60 m south of Hole 856H. The surface of the sulfide deposit (0-14.5 mbsf) is characterized by clastic pyrrhotite and vuggy pyrite. The clasts include gossanous iron-oxide fragments. In the subjacent cores, massive sulfide was recovered between 14.5 and 89.9 mbsf. This section includes massive to semi-massive pyrrhotite and pyrite with altered sediment (14.5-22.5 mbsf); vuggy massive pyrite with minor chalcopyrite, anhydrite, and sphalerite (22.5-77 mbsf), and massive to semi-massive, fine-grained pyrrhotite and pyrite with white clay-altered mudstone (77-89.9 mbsf). The pyrrhotitic sulfides are similar to massive sulfides from the upper and central part of Holes 856H and 856G, recovered on Leg 139. Between 89.9 and 99.7 mbsf, at the contact between sulfide and sediment, sulfide-banded siltstones and sandstones are present. They are mineralized with pyrite, pyrrhotite, and sphalerite which form irregular bedding-parallel bands and veins. Stringer zone sulfide mineralization was cored between 99.7 and 176.8 mbsf. Three zones are identified in the stringer mineralization. The upper zone (99.7-119 mbsf) comprises siltstone and sandstone in which pyrrhotite and chalcopyrite form an anastomosing network. In the next zone (119-157.5 mbsf) sulfide occurs both as veins and as disseminated minerals in sandstone and siltstones. The base of the stringer zone is sulfide-banded siltstone and sandstone (157.5-176.8 mbsf) where pyrite, pyrrhotite, and sphalerite form irregular bedding-parallel bands in altered sediments that are silicified and chloritized. The lowermost unit is altered sandstone and claystone (176.8-224.8 mbsf). Hydrothermally altered, interbedded greenish gray sandstone and medium gray silty claystone are pervasively silicified. Disseminated epidote, chlorite, and minor pyrite are observed.

Hole 1035E

Hole 1035E was located 100 m south of Hole 856H to test the southern extent of clastic sulfide shed from the BHMS mound, and to examine the possible continuity of the BHMS mound with the morphologically younger and hydrothermally active ODPMS mound that occurs further south. The sediments recovered (0.0-46.19 mbsf) are interbedded turbidites and hemipelagic mud with silty clay interbedded with clastic sulfide sand near the surface (4.10 to 9.58 mbsf). Coring problems prevented testing the possible subsurface connection between the two sulfide mounds.

Hole 1035H

Hole 1035 H was drilled on a relatively flat bench near the southern peak of the ODPMS mound, which is located about 350 m south of the BHMS mound. The only known active venting in the Bent Hill area prior to Leg 169 is located 50 m away at the north end of this mound and consists of a single anhydrite chimney issuing fluid at 265°C. Hole 1035H recovered 238 m of a relatively complex and extremely interesting sequence with three massive sulfide zones interbedded with feeder zones and weakly mineralized sediments (Fig. 8). Sulfide-veined and impregnated sediment generally stratigraphically underlies the massive to semi-massive sulfide zones. These zones generally grade into nonmineralized sediment. The massive sulfide consists of coarse-grained pyrite variably infilled and replaced by sphalerite (5%-40%), magnetite (up to 30%), clay minerals, minor chalcopyrite, and traces of galena. Massive sulfide from this mound contains significantly more sphalerite than material recovered from the BHMS deposit.

The uppermost core (0-8.8 mbsf) recovered clastic vuggy pyrite and sphalerite and hydrothermally altered claystone. Between 8.8 and 30 mbsf is a unit of massive sulfide consisting of angular, variably sized clasts composed of pyrite, marcasite, and sphalerite with minor chalcopyrite and isocubanite in a matrix of finer grained clastic sulfides. Magnetite and hematite are present in variable amounts. The major non-sulfide phases are dolomite and ankerite. Underlying this massive sulfide is a sulfide feeder zone where fine sandstone, siltstone, and silty claystone are impregnated and cut by thin veins of pyrrhotite, pyrite, sphalerite, chalcopyrite, and anhydrite (30-55.2 mbsf). The feeder zone grades into weakly mineralized fine sandstone and claystone (55.2-74.6 mbsf). The next unit in the section is massive sulfide composed of pyrite, pyrrhotite, and sphalerite (74.6-84.2 mbsf) followed by a second feeder zone where siltstone and fine sandstone are veined and impregnated with pyrrhotite and chalcopyrite (80-123 mbsf). Pyrrhotite is the dominant sulfide mineral at the base of this unit. Sedimentary rocks are locally brecciated and hydrothermally altered to chlorite. Between 103.7 and 113.4 mbsf within this unit is a chloritized fine sandstone with minor chalcopyrite and pyrrhotite. Starting in the underlying section, an interval of base-metal-rich massive sulfide was intersected (123-142.3 mbsf) consisting of compact to vuggy black sphalerite (40% to 70% of the sulfides) with pyrite, magnetite, and talc or clay infilling voids. Copper sulfides are a minor component in this interval. This sulfide type has not been recognized in the BHMS. At the base of this massive sulfide zone, a short interval of highly altered and mineralized rocks rich in amphibole and epidote was recovered. Deeper in the section is a relatively complex feeder zone (142.3-190.3 mbsf) where three zones of massive to semi-massive sulfide (between 150 and 154 mbsf, 162 and 163 mbsf, and 180.7 and 182.3 mbsf) occur within sulfide-veined sediment. Silty claystone, siltstone, and sandstone are impregnated with Cu-sulfides and hydrothermally altered to chlorite. The semi-massive to massive sulfide with altered sediment contains up to 80% Cu-sulfide and has partly replaced and silicified both planar laminae and cross-bedding in the original sediment. This assemblage looks very similar to the banded Cu-Fe sulfide found in Hole 856H. Between 190.3 and 219.1 mbsf the sediments are less intensely mineralized. Interbedded claystone, siltstone, and sandstone are partly silicified and altered to chlorite and contain veinlets and impregnation of Cu-Fe sulfides. The last section was cored in hemipelagic and turbiditic sediment (219.1-247.9 mbsf). Interbedded claystone, siltstone, and sandstone are partly silicified, weakly chloritized, and contain anhydrite molds. Minor disseminated pyrite occurs locally in this interval.

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