Flows with 'Cu-type' characteristics at ODP Hole 104-642E (Table 1)

Native Cu occurs in amygdules, fractures and groundmass of tholeiites from Ocean Drilling Program Site 642 on the Vøring Plateau. Similar occurrences have been reported in other tholeiites of the early Tertiary North Atlantic Volcanic Province drilled at Deep Sea Drilling Project Sites 342 on the Vøring Plateau and 553 on the Rockall Plateau. The flows containing the native Cu have distinctive alteration patterns characterized by the combination of reddened flow tops, distinctive pastel coloration of the upper parts of the flows, relative abundance of celadonite, and the presence of native Cu. These associations suggest that subaerial weathering and subsequent seawater-basalt interaction are related to the occurrence of native Cu. An additional factor may be the increase in compatibility of Cu in silicates and Fe- Ti oxides that may accompany sub-solidus oxidation of basaltic flows. Native Cu occurrences in Site 642 tholeiites have some striking similarities to the large native Cu deposits in the Precambrian basalts of the Keweenaw Peninsula, Michigan, that are suggestive of similar mineralization processes.

(Table 1) Mineralogy of veins in basalts at DSDP Hole 61-462A

Numerous veins are present in basalts recovered from Hole 462A, Leg 61 of the Deep Sea Drilling Project. Three mineral assemblages are recognized and stratigraphically controlled. These assemblages are (1) a zeolite-bearing, quartz-poor assemblage which occurs from Core 44 to the bottom of the hole and contains smectite, clinoptilolite, calcite, pyrite, ± chabazite, ± analcime, ± quartz, ± apophyllite, ± talc (?); (2) a quartz-rich, pyrite-bearing assemblage, found between Cores 19 and 29, which contains smectite, calcite, quartz, and pyrite; and (3) a quartz-rich, celadonite-bearing assemblage which occurs from Cores 14 through 17 and contains smectite, calcite, quartz, celadonite, and Fe oxide. These data are interpreted to represent two episodes of vein mineral formation with an oxidative overprint on the more recent. The first episode followed the outpourings of basaltic lavas onto the sea floor. Zeolite-bearing veins were formed at elevated temperatures under low PCO2 while the thermal gradient was high and before a cover of calcareous sediments had formed. The second mineralization episode followed injection of basalt and microdiabase sills into a thick layer of sediments, and produced all the vein minerals now occurring between Cores 14 and 29. These veins formed at lower temperature and higher PCO2 than zeolite-bearing veins. The presence of pyrite indicates a nonoxidative environment. After the initial formation of these veins, oxygenated seawater diffused through the sedimentary cover and oxidized the pyrite and smectite, forming celadonite and Fe oxides.

Hydrochemical and geothermic data on the Caribbean-Mexican region

Results of detailed geophysical, geological and gas- and hydrochemical research in the Caribbean-Mexican Basin and the Western Atlantic obtained during Cruise 4 of R/V Akademik Nikolaj Strakhov are published in the book. Distribution of the thermal field in different tectonic structures of the region is shown. Places of submarine hydrothermal vent discharge in tectonically active structures are described. They are confirmed by geothermal, geological and hydrochemical data. Based on lithofacies analysis of modern sediments installed their Specificity of different genetic types, facies and macrofacies of recent sediments in different geomorphological zones of the sea floor is shown. For description of hydrogeochemical situation of modern sedimentation and primary diagenesis the water column and interstitial sediment waters have been studied.

Minerals, native metals, and intermetallides in bottom sediments of the Markov Deep, Mid-Atlantic Ridge

Bottom sediments of the Markov Deep contain rather large (>0.1 mm) grains of native minerals and intermetallides of noble and nonferrous metals that can be concentrated in placers. Intermetallides of Pt and Fe are likely to be derivates of the gold-hematite-barite assemblage that forms at late (low-depth) stages of hydrothermal massive sulfide formation. Mineral association of native forms of lead, tin, and copper with Zn-bearing copper may be related to hydrothermal transformation of ultrabasic and basic rocks accompanied by massive sulfide copper mineralization. The association of these minerals of native elements in bottom sediments can also serve as a prospecting guide for sulfide mineralization both at the Sierra Leone site, in particular, and on the seafloor, in general.

Chemical composition of hydrothermal sulfide minerals from the Rainbow, Logachev-1, and Logachev-2 hydrothermal fields

Composition of ore minerals in MAR sulflde occurrences related to ultramaflc rocks was studied using methods of mineragraphy, electron microscopy, microprobe analysis, and X-ray analysis. Objects are located at various levels of maturity of sulflde mounds owing to differences in age, duration and degree of activity of the following hydrothermal systems: generally inactive Logatchev-1 field (up to 66.5 ka old), inactive Logatchev-2 field (3.9 ka), and generally active Rainbow field (up to 23 ka). Relative to MAR submarine ore occurrences in the basalt substrate, mineralization in the hydrothermal fields mentioned above is characterized by high contents of Au, Cd, Co, and Ni, along with presence of accessory minerals of Co and Ni. The studied mounds differ in quantitative ratios of major minerals and structural-textural features of ores that suggest their transformation. Ores in the Logatchev-1 field are characterized by the highest Cu content and development of a wide range of multistage contrast exsolution structures of isocubanite and bornite. In the Logatchev-2 field, sphalerite-chalcopyrite and gold-arsenic exsolution structures are present, but isocubanite exsolution structures are less diverse and contrast. The Rainbow field is marked by presence of homogenous isocubanite and the subordinate development of exsolution structures. The authors have identified four new phases in the Cu-Fe-S system. Phases X and Y (close to chalcopyrite and isocubanite, respectively) make up lamellae among isocubanite exsolution products in the Logatchev-1 and Logatchev-2 fields. Phase Y includes homogenous zones in zonal chimneys of the Rainbow field. Phases A and B formed in the orange bornite domain at low-temperature alteration of chalcopyrite in the Logatchev-1 field. Mineral assemblages of the Cu-S system are most abundant and diverse in the Logatchev-1 field, but their development is minimal in the Logatchev-2 field where mainly Cu-poor sulfides of the geerite-covellite series have been identified. Specific features of mineral assemblages mentioned above reflect the maturity grade of sulfide mounds and can serve as indicators of maturity.

Isotopic composition of oxygen, carbon, and sulfur in interstitial water and cores at DSDP Leg 59 Holes

Comprehensive isotopic studies based on data from the Deep Sea Drilling Project have elucidated numerous details of the low- and high-temperature mechanisms of interaction between water and rocks of ocean crustal seismic Layers 1 and 2. These isotopic studies have also identified climatic changes during the Meso-Cenozoic history of oceans. Data on the abundance and isotopic composition of sulfur in the sedimentary layer as well as in rocks of the volcanic basement are more fragmentary than are oxygen and carbon data. In this chapter we specifically concentrate upon isotopic data related to specific features of the mechanisms of low-temperature interaction of water with sedimentary and volcanogenic rocks. The Leg 59 data provide a good opportunity for such lithologic and isotopic studies, because almost 600 meters of basalt flows and sills interbedded with tuffs and volcaniclastic breccias were cored during the drilling of Hole 448A. Moreover, rocks supposedly exposed to hydrothermal alteration play an important role at the deepest horizons of that mass. Sulfur isotopic studies of the character of possible biogenic processes of sulfate reduction in sediments are another focus, as well as the nature and origin of sulfide mineralization in Layer-2 rocks of remnant island arcs. Finally, oxygen and carbon istopic analyses of biogenic carbonates in the cores also enabled us to investigate the effects of changing climatic conditions during the Cenozoic. These results are compared with previous data from adjacent regions of the Pacific Ocean. Thus this chapter describes results of isotopic analyses of: oxygen and sulfur of interstitial water; oxygen and carbon of sedimentary carbonates and of calcite intercalations and inclusions in tuffs and volcaniclastic breccias interbedded with basalt flows; and sulfur of sulfides in these rocks.

Composition of sulfides from the Sierra Leone Fracture Zone (Central Atlantic Ocean)

An analysis of data on the location of hydrothermal fields, seismicity, and satellite altimetry evidences that in mid-ocean ridges with low spreading rate hydrothermal fields tend to be grouped in areas with generally low seismic activity and at intersections of discontinuities and rift zones. Based on this assumption, the Sierra Leone Fracture Zone was studied in 2000 during Cruise 22 of R/V Akademik Nikolaj Strakhov. A study of gabbrodolerite and dolerite showed that sulfide ore minerals in them were formed both by hydrothermal and magmatic processes. An analysis of melt inclusions demonstrated that magmatic complexes formed from a high-temperature (1210-1255°C) low-potassium melt of the N-MORB type. Investigations of fluid inclusions revealed that gabbro and dolerite formed under influence of an active hydrothermal system at temperature 205-226°C. Thus, the Sierra Leone Fracture Zone is considered to be perspective for a discovery of a new hydrothermal field.