Chemical composition of phosphorites, Fe-Mn nodules and crusts, and basalts from the West Pacific

The monograph gives the first systematic description of ore-bearing guyots from the West Pacific. It is mostly based on data obtained in numerous expeditions of Russian vessels during 1984-1992. Ore deposits located on upper parts of all slopes and tops of the guyots include phosphorites associated with cobalt- and platinum-rich ferromanganese crusts. Location, origin and prospecting of mineral deposits are discussed on the base of new data on metallogenic factors (geodynamics, tectonics, magmatism, sedimentation and morphostructures).

Petrology of mafic and ultramafic intrusions from the Portneuf-Mauricie Domain, Grenville Province, Canada

The Portneuf-Mauricie Domain (PMD), located in the south-central part of the Grenville Province, comprises several mafic and ultramafic intrusions hosting Ni-Cu ± platinum-group element (PGE) prospects and a former small mining operation (Lac Édouard mine). These meter- to kilometer-scale, sulfide-bearing intrusions display diverse forms, such as layered and tabular bodies with no particular internal structure, and zoned plutons. They were injected ~ 1.40 Ga into a mature oceanic arc, before and during accretion of the arc to the Laurentian margin. The pressure-temperature conditions of the magmas at the beginning of their emplacement were 3 kbar and 1319-1200 °C (according to the petrologic modeling results from this study). The PMD mineralized intrusions are interpreted to represent former magma chambers or magma conduits in the roots of the oceanic arc. The parent magmas of the mineralized intrusions resulted mainly from the partial melting of a mantle source composed of spinel-bearing lherzolite. Petrologic modeling and the occurrence of primary amphibole in the plutonic rocks indicate that these parent melts were basaltic and hydrous. In addition, fractional crystallization modeling and Mg/Fe ratios suggest that most of the intrusions may have formed from evolved magmas, with Mg# = 60, resulting from the fractionation of more primitive magmas (primary magmas, with Mg# = 68). Petrologic modeling demonstrates that 30% fractional crystallization resulted in the primitive to evolved characteristics of the studied intrusive rocks (as indicated by the crystallization sequences and mineral chemistry). Exceptions are the Réservoir Blanc, Boivin, and Rochette West parent magmas, which may have undergone more extensive fractional crystallization, since these intrusions contain pyroxenes that are more iron rich and have lower Mg numbers than pyroxenes in the other PMD intrusions. The PMD mafic and ultramafic intrusions were intruded into an island arc located offshore from the Laurentian continent. Thus, their presence confirms the existence of a well-developed magmatic network (responsible of the fractionation processes) beneath the Proterozoic arc, which resulted in the wide range of compositions observed in the various plutons.

Geochemistry of selected minerals at DSDP Leg 65 Holes

The majority of the basalts drilled on Leg 65 in the Gulf of California are aphyric to sparsely phyric massive flows ranging in average thickness between 5 meters in the upper part of the sections in Holes 483 and 483B, where they are interlayered with sediment, and 14 meters in Hole 485A, where interlayered sediments constitute more than half of the section. Massive flows interlayered with pillows are generally less than 4 meters thick. The pillow lavas recovered are more phyric (up to 15 modal%) and contain two to three generations of plagioclase and olivine ± clinopyroxene. Plagioclase generally exceeds 60% of any given phenocryst assemblage. Resorbed olivine, clinopyroxene, and plagioclase megacrysts may reflect a high-pressure stage, the phenocrysts crystallizing in the main magma chamber and the skeletal microphenocrysts in dikes. Precise measurements of length/width ratios of different phenocryst types and compositions show low aspect ratios and large crystal volumes for early crystals and high ratios and low volumes for late crystals grown under strong undercooling conditions. The minerals examined show wide ranges in composition: in particular, plagioclase ranges from An92 to An36; clinopyroxene ranges from Ca41Mg51Fe8 in the cores of phenocrysts to Ca40**36 Mg45**49Fe15**20 in the groundmass; and olivine ranges from Fo86 to Fo81. The wide range in mineral compositions, together with evidence of disequilibrium based on textures and comparisons of glass and mineral compositions, indicate complex crystallization histories involving both polybaric crystal fractionation and magma mixing.

Chemical composition of upper Cenozoic at DSDP Leg 70 Holes

The hydrothermal deposits that we analyzed from Leg 70 are composed of ferruginous green clays and fragments of manganese-hydroxide crust. Data from X-ray diffraction, IR-spectroscopy, electron diffraction, and chemical analyses indicate that the hydrothermal green clays are composed of disordered mixed-layer phases of celadonite-nontronite. Electron diffraction shows that the parameters of the unit cells and the degree of three-dimensional ordering of mixed-layer phases with 80% celadonite interlayers are very close to Fe-micas of polymorphic modification IM-celadonite. In some sections, there is a tendency for the number of celadonite layers to increase with depth. The manganese-hydroxide crust fragments are predominantly composed of todorokite (buserite). An essential feature of hydrothermal accumulation is the sharp separation of Fe and Mn. Ba/Ti and Ba/Sr ratios are typical indicators of hydrothermal deposits. Sediments composing the hydrothermal mounds were deposited from moderately heated waters, which had extracted the components from solid basalts in environments where there were considerable gradients of temperature, eH, and pH. The main masses of Fe and Mn were deposited in the late Pleistocene. Postsedimentary alteration of deposited hydrothermal sediments led to their slight recrystallization and, in the green clays, to celadonitization. Further, factor analysis (by Varentsov) of chemical components from these hydrothermal deposits revealed paragenetic assemblages. Green clays corresponding to a definite factor assemblage were formed during the main stage of hydrothermal mineral formation. Manganese hydroxide and associated components were largely accumulated during an early stage and at the end of the main stage.