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.

Chemistry of gabbroic rocks of ODP Hole 118-735B

We present results of a microprobe investigation of fresh and least-deformed and metamorphosed gabbroic rocks from Leg 118, Hole 735B, drilled on the east side of the Atlantis II Fracture Zone, Southwest Indian Ridge. This rock collection comprises cumulates ranging from troctolites to olivine-gabbro and olivine-gabbronorite to ilmenite-rich ferrogabbros and ferrogabbronorites. As expected, the mineral chemistry is variable and considerably expands the usual oceanic reference spectrum. Olivine, plagioclase, and clinopyroxene are present in all the studied samples. Orthopyroxene and ilmenite, although not rare, are not ubiquitous. Olivine compositions range from Fo85 to Fo30, while plagioclase compositions vary from An70 to An27. Mg/(Mg + Fe2+) of clinopyroxene (mostly diopside to augite) varies from 0.88 to 0.54. Mg/(Mg + Fe2+) of orthopyroxene varies from 0.84 to 0.50. These minerals are not significantly zoned. All mineralogical data indicate that fractional crystallization is an important factor for the formation of cumulates. However, sharp contacts, interpreted as layering boundaries or intrusion margins, suggest polycyclic fractionation of several magma batches of limited volumes. Calculated compositions of magmas in equilibrium with the most magnesian mineral samples at the bottom of the hole represent fractionated liquids through separation of olivine, plagioclase, and clinopyroxene at moderate to low pressures (less than 9 kb). Crystallization of orthopyroxene and ilmenite occurs in the most differentiated liquids. Mixing of magmas having various compositions before entering the cumulate zone is another mechanism necessary to explain extremely differentiated iron-rich gabbros formed in this slow-spreading ridge environment.

Major and trace elements and minerals at DSDP Hole 79-544A

Fifty m of basement rocks underlying 185 m of Neogene and Mesozoic sediments were drilled seaward of the Mazagan Slope about 100 km west of Casablanca during Leg 79. These rocks are metagranites with mylonitic textures consisting dominantly of quartz, plagioclase, and potassium feldspar. Chemically, they are strongly peraluminous. This along with the absence of hornblende suggest that these rocks are similar to the S-type granites. Petrographic and chemical data suggest the possible existence of a former weathering surface on top of the Mazagan metagranite.