Timing and duration of supergene mineralization at the Xinrong manganese deposit,western Guangdong Province,South China: cryptomelane <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating

Supergene Mn-oxide deposits are widely distributed in Guangxi, Guangdong, Yunnan, and Hunan Provinces, South China, accounting for 18% of the total Mn reserves in the country. Direct dating of supergene Mn enrichment, however, is lacking. In this paper, we present high-resolution ⁴⁰Ar/³⁹Ar ages of Mn oxides from the Xinrong Mn deposit, western Guangdong, to place numerical constraints on the timing and duration of supergene Mn enrichment. A total of ten cryptomelane samples, spanning a vertical extent of 67 m, were dated using the ⁴⁰Ar/³⁹Ar laser incremental heating technique, with seven samples yielding well-defined plateau or pseudo-plateau ages ranging from 23.48 ± 0.91 to 2.06 ± 0.05 Ma (2σ). One sample yields a staircase spectrum that does not reach a plateau; the spectrum, however, indicates the presence of two or more generations of Mn oxides in the sample, whose ages are best estimated at 22.34 ± 0.31 and 10.2 ± 0.86 Ma, respectively. The remaining two samples gave meaningless or uninterpretable results due to significant ³⁹Ar recoil and contamination by old phases. The ⁴⁰Ar/³⁹Ar data thus reveal a protracted history of weathering and supergene Mn enrichment that started at least in the end of the Oligocene or beginning of Miocene and extending into the latest Pliocene. Staircase-apparent age spectra, resulting from banded or botryoidal samples, yield an average growth rate of Mn oxides at 0.6–0.7 × 10⁻³ mm kyr⁻¹. The values indicate that a 1-mm grain of Mn oxides may host minerals precipitated during a time span of ca. 1.5 m.y., and accumulation of Mn oxides to form economic deposits under weathering environments may take millions of years. The distribution of weathering ages shows that the oldest Mn oxides occur on the top of the profile, whereas the youngest minerals are found at the bottom, suggesting downward propagation of weathering fronts. However, two samples located at the intermediate depths of the profile yield ages comparable with those occurring at the highest elevations. Such a complexity of age distribution is interpreted in terms of preferential penetration of Mn-rich weathering solutions along more permeable fault zones, or as a result of multi stages of dissolution and re-precipitation of Mn oxides. A synthesis of geochronological and geological data suggests that formation of the Xinrong deposit was a consequence of a combination of favorable lithological, climatic, and structural conditions. Because the climatic and structural conditions are similar among the provinces of South China during the Cenozoic, the geochronological results obtained at Xinrong may also have implications for the timing of supergene Mn enrichment throughout South China.     

Geochronological evidence for pervasive Miocene weathering,Minas Gerais,Brazil

⁴⁰Ar/³⁹Ar laser incremental‐heating analyses of 22 individual grains of supergene cryptomelane from three weathering pro?les, up to 400 km apart, in the Rio Doce valley and Barbacena regions at Minas Gerais, Brazil, show that the formation of weathering pro?les in these regions is contemporaneous, suggesting a strong weathering event in the Middle to Late Miocene (10–8 Ma). The preservation of these Miocene samples at or near the present surface suggests that either erosion rates have been very low in the region since the Miocene or that a much thicker weathering mantle was present in the region originally. Assuming a constant thickness of weathering pro?les in the region throughout the Tertiary, we may calculate weathering front propagation rates of 4–8 m Myr^?1 during the past 10 Ma. Copyright © 2004 John Wiley & Sons, Ltd.     

Origin of the Tongshankou porphyry–skarn Cu–Mo deposit,eastern Yangtze craton,Eastern China: geochronological,geochemical, and Sr–Nd–Hf isotopic constraints

The Tongshankou Cu–Mo deposit, located in the westernmost Daye district of the Late Mesozoic Metallogenic Belt along the Middle-Lower reaches of the Yangtze River, eastern China, consists mainly of porphyry and skarn ores hosted in the Tongshankou granodiorite and along the contact with the Lower Triassic marine carbonates, respectively. Sensitive high-resolution ion microprobe zircon U–Pb dating constrains the crystallization of the granodiorite at 140.6 ± 2.4 Ma (1σ). Six molybdenite samples from the porphyry ores yield Re–Os isochron age of 143.8 ± 2.6 Ma (2σ), while a phlogopite sample from the skarn ores yields an ⁴⁰Ar/³⁹Ar plateau age of 143.0 ± 0.3 Ma and an isochron age of 143.8 ± 0.8 Ma (2σ), indicating an earliest Cretaceous mineralization event. The Tongshankou granodiorite has geochemical features resembling slab-derived adakites, such as high Sr (740–1,300 ppm) and enrichment in light rare earth elements (REE), low Sc (<10 ppm), Y (<13.3 ppm), and depletion in heavy REE (<1.2 ppm Yb), and resultant high Sr/Y (60–92) and La/Yb (26–75) ratios. However, they differ from typical subduction-related adakites by high K, low MgO and Mg#, and radiogenic Sr–Nd–Hf isotopic compositions, with (⁸⁷Sr/⁸⁶Sr) _t  = 0.7062–0.7067, ɛ _Nd(t) = −4.37 to −4.63, (¹⁷⁶Hf/¹⁷⁷Hf) _t  = 0.282469–0.282590, and ɛ _Hf(t) = −3.3 to −7.6. The geochemical and isotopic data, coupled with geological analysis, indicate that the Tongshankou granodiorite was most likely generated by partial melting of enriched lithospheric mantle that was previously metasomitized by slab melts related to an ancient subduction system. Magmas derived from such a source could have acquired a high oxidation state, as indicated by the assemblage of quartz–magnetite–titanite–amphibole–Mg-rich biotite in the Tongshankou granodiorite and the compositions of magmatic biotite that fall in the field between the NiNiO and magnetite–hematite buffers in the Fe³⁺–Fe²⁺–Mg diagram. Sulfur would have been present as sulfates in such highly oxidized magmas, so that chalcophile elements Cu and Mo were retained as incompatible elements in the melt, contributing to subsequent mineralization. A compilation of existing data reveals that porphyry and porphyry-related Cu–Fe–Au–Mo mineralization from Daye and other districts of the Metallogenic Belt along the Middle-Lower reaches of the Yangtze River took place coevally in the Early Cretaceous and was related to an intracontinental extensional environment, distinctly different from the arc-compressive setting of the Cenozoic age that has been responsible for the emplacement of most porphyry Cu deposits of the Pacific Rim.     

Insights about the formation of a complex impact structure formed in basalt from numerical modeling: The Vista Alegre structure,southern Brazil

We present the outcomes of simulations of the formation of the Vista Alegre impact structure, Paraná Basin, Brazil. The target comprised a thick sequence of volcanic rocks of predominantly basaltic composition of the Serra Geral Formation that had been deposited on top of sedimentary rocks (sandstones) of the Pirambóia/Botucatu formations. The cratering process was modeled using the iSALE shock physics code. Our best‐fit model suggests that (1) the crater was originally ~10 km in size; (2) it was formed in ~115 s by a stony projectile of 1000 m in diameter, for an assumed impact velocity of 12 km s^?1; (3) target rocks underwent a peak pressure of ~20 GPa, in agreement with previous petrographic investigations of shock deformation. Furthermore, the model points out that the sedimentary strata below the layer of volcanic rocks were raised by ~650 meters at the central part of the crater, which resulted in the current partial exposure of the sandstones at the surface. The outcomes of our modeling suggest that parameters like cohesion and strength of the target rocks, after shock compression, determined the final morphology of the crater, especially the absence of a topographically prominent central peak. Finally, the results of the numerical modeling are roughly in agreement with gravity data over the structure, in particular with respect to the presence of the uplifted sedimentary strata, which are responsible for a low gravity signature at the center of the structure.     

Seawater temperature changes associated with the North Brazil current dynamics

This study used hydrographic data (CTD and ADCP) collected along the north Brazilian continental shelf and the adjacent oceanic area (the north Brazilian zone) and 13 years of monthly data of sea surface temperatures (SSTs) obtained from the Tropical Rainfall Measuring Mission satellite microwave images. In July and August 2001, the core of the North Brazil Current (NBC) with a velocity exceeding 1.2 m s^?1 in the upper 150 m was observed near the break of the north Brazilian continental shelf. The satellite and in situ SSTs from the same time indicate that relatively cold waters (<27.5 °C) extended throughout the NBC region where the vertical distribution of temperature displayed a rise of the 26 °C isotherm to near the surface on transects where the NBC was more intense. These observations indicated that the NBC plays an important role in temperature changes in the water column near and/or over the north Brazilian continental shelf margin even at times when, theoretically, the NBC may be expected to be less intense (April–May) and warm waters dominate the area. The distribution of accumulated temperature change reveals that the NBC may contribute to possible nutrient upwelling of the bottom boundary layer along the continental shelf to surficial areas and offshore following the NBC deflection or merely into the North Atlantic.     

Microbial fossilization in carbonate sediments: a result of the bacterial surface involvement in dolomite precipitation

ABSTRACT Recent dolomitic sediment samples from Lagoa Vermelha, Brazil, were examined microscopically to study the process of bacterial fossilization in carbonate sediments. Bacteria‐like bodies were intimately associated with carbonate mineral surfaces, and coatings on the former demonstrate the calcification of single bacterial cells. The bacterial fossilization process in Lagoa Vermelha sediments was simulated in the laboratory by cultivation of mixed and pure cultures of sulphate‐reducing bacteria, which were isolated from the Lagoa Vermelha sediments. These cultures produced carbonate minerals that were studied to provide insight into the initiation of the fossilization process. In mixed culture experiments, bacterial colonies became calcified, whereas in pure culture experiments, single bacterial cells were associated with dolomite surfaces. Dolomite nucleated exclusively in bacterial colonies, intimately associated with extracellular organic matter and bacterial cells. Electrophoretic mobility measurements of the bacterial cells in electrolyte solutions demonstrated the specific adsorption of Ca²⁺ and Mg²⁺ onto the cell surfaces, indicating the role of the bacterial surface in carbonate nucleation and bacterial fossilization. The affinity of the cells for Mg²⁺ was related to the capability of the strains to mediate dolomite formation. Combined with sulphate uptake, which dissociates the [MgSO₄]⁰ ion pair and increases the Mg²⁺ availability, the concentration of Mg²⁺ ions in the microenvironment around the cells, where the conditions are favourable for dolomite precipitation, may be the key to overcome the kinetic barrier to dolomite formation. These results demonstrate that bacterial fossilization is a consequence of the cell surface involvement in carbonate precipitation, implying that fossilized bacterial bodies can be used as a tool to recognize microbially mediated carbonates.     

Development of microbial carbonates in the Lower Cretaceous Codó Formation (north‐east Brazil): Implications for interpretation of microbialite facies associations and palaeoenvironmental conditions

The study of microbial carbonates has acquired new significance with the recognition that they retain valuable information related to biomineralization processes associated with microbial activity throughout geological time. Additionally, microbialites have a demonstrated economic potential to serve as excellent hydrocarbon reservoirs. The Lower Cretaceous Codó Formation, located in the Parnaiba Basin of north‐east Brazil, comprises a unique stratigraphic sequence of up to 20 m thick, well‐preserved carbonate microbialites. Deposited in a continental basin during the initial break up and separation of South America from Africa in the Early Cretaceous, this lacustrine carbonate sequence provides an excellent example to investigate the palaeoenvironmental conditions controlling microbialite facies development. Based on macroscopic and microscopic observations of outcrop and drill core samples, four microbialite facies (stromatolite, lamina, massive and spherulite) were defined and distinguished by textures and microbial fossil content. Changes in facies type are related to alternating palaeo‐water depths, as reflected by ⁸⁷Sr/⁸⁶Sr cycles resulting from fluctuations in the sources of meteoric water. Clumped isotope measurements of stromatolitic fabrics yield precipitation palaeo‐temperatures with an average value of 35°C. The δ¹⁸O values of bulk carbonate (?6·8 to ?1·5‰ Vienna Pee Dee Belemnite) imply precipitation from water with calculated δ¹⁸O values between ?1·6‰ and 1·8‰ Vienna Standard Mean Ocean Water, reflecting precipitation from variably modified meteoric waters. The δ¹³C values of bulk carbonate (?15·5 to ?7·2‰ Vienna Pee Dee Belemnite) indicate a significant input of carbon derived from aerobic or anaerobic respiration of organic matter. Combined, the data indicate that the evolution of the Codó Formation occurred in a closed lacustrine palaeoenvironment with alternating episodes of contracting and expanding lake levels, which led to the development of specific microbialite facies associations. The results provide new insights into palaeoenvironmental settings, biogenicity and early diagenetic processes involved in the formation of ancient carbonate microbialites and, by extension, improve the knowledge of the reservoir geology of correlative units in deep waters offshore Brazil.     

Geology and impact features of Riachão structure,northern Brazil

Riachão, located at S7°42′/W46°38′ in Maranhão State, northeastern Brazil, is a complex impact structure of about 4.1 km diameter, formed in Pennsylvanian to Permian sedimentary rocks of the Parnaíba Basin sequence. Although its impact origin was already proposed in the 1970s, information on its geology and shock features is still scarce in the literature. We present here the main geomorphological and geological characteristics of the Riachão impact structure obtained by integrated geophysical and remote sensing analysis, as well as geological field work and petrographic analysis. The identified lithostratigraphic units consist of different levels of the Pedra de Fogo Formation and, possibly, the Piauí Formation. Our petrographic analysis confirms the presence of shock‐diagnostic planar microdeformation structures in quartz grains of sandstone from the central uplift as evidence for an impact origin of the Riachão structure. The absence of crater‐filling impact breccias and melt rocks, shatter cones, as well as the restricted occurrence of microscopic shock effects, suggests that intense and relatively deep erosion has occurred since crater formation.     

Structural and geochronological constraints on the tectono-thermal evolution of the Danba domal terrane, eastern margin of the Tibetan plateau

The Songpan-Ganze terrane of the Tibetan plateau is underlain by Neoproterozoic crystalline basement rocks of the Yangtze block. These basement rocks are exposed as a series of extensional tectonic domes that form a nearly north–south trending extensional belt more than 1000 km long in the eastern margin of the Tibetan plateau. In the Danba area, detachment faults separate the basement core complexes (e.g., the Gezong and Gongcai complexes) from the Paleozoic strata which have been thinned or removed completely. The cover sequences have undergone upper greenschist to lower amphibolite facies metamorphism to form the Danba schist and are overlain by the Triassic Xikang Group, a thick flysch sequence. Both the basement rocks and the Paleozoic rocks have undergone multiple stages of deformation and thus provide an excellent opportunity to study the tectono-thermal evolution of the eastern margin of the Tibetan plateau. Two stages of deformation, corresponding to three generations of foliation (S₁, , and ), have been recognized on the basis of structural and microscopic observations. We selected amphibole and biotite separates associated with distinct generations of foliation for ⁴⁰Ar/³⁹Ar dating using laser microprobe incremental heating technique to place numerical constraints on the major tectono-thermal events within the Danba area. The geochronogical results reveal an earliest metamorphic event at 258.6 ± 0.5 Ma (S₁ biotite) and 263.6 ± 0.8 Ma (S₁ amphibole), coinciding temporally with the mantle plume that produced the voluminous Emeishan flood basalts. The second event was a progressive extensional deformation first occurred at 159–166 Ma ( amphibole) responsible for the earlier tectonic doming of the crystalline basement, and then the final tectono-thermal overprint recorded by foliation and metamorphism locally in the core complexes at 47–58 Ma for the Gezong complex and 64–81 Ma for the Gongcai complex. This major post-orogenic extensional event is believed to be a consequence of collision between the North China and South China blocks. The apparent discrepancy of the ⁴⁰Ar/³⁹Ar ages observed between localities suggests a slow cooling process associated with progressive uplift.