4 research outputs found
Lithology and mineralogy recognition from geochemical logging tool data using multivariate statistical analysis
The availability of a deep well that penetrates deep into the Ultra High Pressure (UHP) metamorphic rocks is unusual and consequently offers a unique chance to study the metamorphic rocks. One such borehole is located in the southern part of Donghai County in the Sulu UHP metamorphic belt of Eastern China, from the Chinese Continental Scientific Drilling Main hole. This study reports the results obtained from the analysis of oxide log data. A geochemical logging tool provides in situ, gamma ray spectroscopy measurements of major and trace elements in the borehole. Dry weight percent oxide concentration logs obtained for this study were SiO2, K2O, TiO2, H2O, CO2, Na2O, Fe2O3, FeO, CaO, MnO, MgO, P2O5 and Al2O3. Cross plot and Principal Component Analysis methods were applied for lithology characterization and mineralogy description respectively. Cross plot analysis allows lithological variations to be characterized. Principal Component Analysis shows that the oxide logs can be summarized by two components related to the feldspar and hydrous minerals. This study has shown that geochemical logging tool data is accurate and adequate to be tremendously useful in UHP metamorphic rocks analysis
Petrogenesis of A-type granites associated with Sn–Nb–Zn mineralization in Ririwai complex, north-Central Nigeria: Constraints from whole-rock Sm Nd and zircon Lu Hf isotope systematics
International audienceWe report a combined study of whole-rock major- and trace-element geochemistry, SmNd isotope composition, zircon U/Pb dating, and LuHf systematics of peralkaline and aluminous A-type granites from the Ririwai Ring Complex in north-central Nigeria. The Ririwai peralkaline and aluminous A-type granites are strongly ferroan, alkalic to alkali-calcic, and enriched in Hf, Zr, Ga, Rb, Y and REEs. They were emplaced between 176 ± 2.3 Ma and 169.6 ± 0.75 Ma. The peralkaline granites yield relatively higher ɛNd(t) (−2.3 to −1.2) and zircon ɛHf(t) values (−5.8 to −1.7) than the aluminous granites (ɛNd(t) = −3.6 to −3.3; zircon ɛHf(t) = −7.8 to −2.4). In addition, inherited zircons in the aluminous granites yield Pan-African (~590 Ma) ages and low ɛHf(t) values (−14.0). Taken together, these data suggest that the granites formed from extensive fractional crystallization of a transitional basaltic melt derived from an enriched OIB mantle source. The depletions of MgO, CaO, Ti2O, Sr and Ba in the granites indicate that Mg-rich olivine, calcic pyroxene, calcic amphibole, feldspars and FeTi oxides were the major fractionated phases during magma evolution. In addition, several types of evidence, e.g., moderately negative ɛNd(t) and ɛHf(t) values, and the presence of inherited zircons, imply that the parental melt was modified by assimilation of Pan-African upper crust into which the granites were emplaced. A transtensional regime generated prior to late Jurassic breakup of Gondwana, which led to reactivation of shear zones and opening of associated transcurrent faults, paved the way for emplacement of the A-type suite. The Sn–Nb–Zn mineralization of the Ririwai A-type suite was probably linked to complex magmatic evolutionary processes involving extensive fractional crystallization coupled with crustal assimilation and late-stage hydrothermal fluid activity
Pyrite Dissolution in the Cretaceous Yogou Formation of the Niger (Chad) Basin: Implications for Basin Evolution under a Rift Tectonic Setting
This is the first study of pyrite minerals in the entire
West and
Central African Rift System (WCARS). Several polished organic-rich
core samples from the Cretaceous Yogou Formation of the Niger (Chad)
Basin located in the WCARS were investigated for their pyrite content
using FE-SEM and SEM-EDS imaging techniques. An attempt was made to
classify the types and provenance of the pyrites and to highlight
the control of rift fractures on the oxidation and dissolution of
pyrites in the region. Three major types of pyrites are present in
the studied formation, including euhedral pyrite (EPy) crystals, pyrite framboids (FPy), and sunflower
pyrites (SPy). A statistical analysis of 307 FPy shows that the framboids are diagenetically formed with
an average diameter of 6.61 μm. SPy is present
in a relatively low amount compared to framboids. The pyrites underwent
a variety of diagenetic modifications, from mechanical compaction
to oxidation, dissolution, and recrystallization. Unoxidized pyrites
primarily contain Fe, S, and C, but oxidized pyrites also contain
O, Al, and Si. There is a strong correlation between the fractures
and the spatial distribution of the physicochemical alteration of
the pyrite in the study. Dissolution in relatively deep-buried samples
occurs mainly along fracture planes. The fractures provide a pathway
for oxidants and other metal elements to reach the pyrites. The pattern
of pyrite dissolution reflects the timing of fracture formation and
fracture activities as a purveyor or drainage for fluids in the organic-rich
samples investigated. The pyrites are associated intimately with organic
matter (OM); thus, the relationship between the fracture and the pyrites’
transformation is significant in the assessment of organic matter
preservation at deep-burial depth