The geology of the Morro Velho gold deposit in the Archean Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil

The Morro Velho gold deposit, Quadrilátero Ferrífero region, Minas Gerais, Brazil, is hosted by rocks at the base of the Archean Rio das Velhas greenstone belt. The deposit occurs within a thick carbonaceous phyllite package, containing intercalations of felsic and intermediate volcaniclastic rocks and dolomites. Considering the temporal and spatial association of the deposit with the Rio das Velhas orogeny, and location in close proximity to a major NNW-trending fault zone, it can be classified as an orogenic gold deposit. Hydrothermal activity was characterized by intense enrichment in alteration zones of carbonates, sulfides, chlorite, white mica±biotite, albite and quartz, as described in other Archean lode-type gold ores. Two types of ore occur in the deposit: dark gray quartz veins and sulfide-rich gold orebodies. The sulfide-rich orebodies range from disseminated concentrations of sulfide minerals to massive sulfide bodies. The sulfide assemblage comprises (by volume), on average, 74% pyrrhotite, 17% arsenopyrite, 8% pyrite and 1% chalcopyrite. The orebodies have a long axis parallel to the local stretching lineation, with continuity down the plunge of fold axis for at least 4.8 km. The group of rocks hosting the Morro Velho gold mineralization is locally referred to as lapa seca. These were isoclinally folded and metamorphosed prior to gold mineralization. The lapa seca and the orebodies it hosts are distributed in five main tight folds related to F1 (the best examples are the X, Main and South orebodies, in level 25), which are disrupted by NE- to E-striking shear zones. Textural features indicate that the sulfide mineralization postdated regional peak metamorphism, and that the massive sulfide ore has subsequently been neither metamorphosed nor deformed. Lead isotope ratios indicate a model age of 2.82 ± 0.05 Ga for both sulfide and gold mineralization. The lapa seca are interpreted as the results of a pre-gold alteration process and may be divided into carbonatic, micaceous and quartzose types. The carbonatic lapa seca is subdivided into gray and brown subtypes. Non-mineralized, gray carbonatic lapa seca forms the hanging wall to the orebodies, and is interpreted as the product of extreme CO₂ metasomatism during hydrothermal alteration. This dolomitic lapa seca ranges in composition from relatively pure limestone and dolomite to silty limestone and dolomite. The brown carbonatic and micaceous lapa secas are the host rocks to gold. These units are interpreted to correspond to the sheared and hydrothermal products of metamorphosed volcaniclastic and/or volcanic rocks of varying composition from dacitic to andesitic, forming various types of schists and phyllites. The high-grade, massive sulfide orebodies occur at the base of the gray carbonatic lapa seca. Both disseminated mineralization and quartz veins are hosted by micaceous lapa seca. The data are consistent with a model of epigenetic mineralization for the lapa seca, from a hydrothermal fluid derived in part from the Archean basement or older crust material.     

滇东南芦柴冲大型银多金属矿床的海底喷流成矿作用沉积旋回划分

芦柴冲矿床位于滇东南砚山县境内,属南岭地区西南端－－滇东南早泥盆世的层控矿带。经野外调研和室内系统研究,发现矿床为典型非火山成因的、严格受沉积相（台盆相区）控制的海底喷流沉积成因矿床。矿床中发育的南北向同生扩张断裂是重要的控盆和控矿构造,是海底喷流成矿的含矿流体运移的通道,它具周期性的振荡运动的特点,导致海底喷流成矿具周期性的沉积旋回。     

Focal mechanisms of small earthquakes in the southeastern Brazilian shield: a test of stress models of the South American plate

Focal mechanisms of small earthquakes with magnitudes of about 3 in the SE Brazilian shield are calculated using S / P amplitude ratios. Low attenuation ( Q _p from 400 to 800) in the shield upper-crustal layers allowed sharp S arrivals to be recorded up to distances of 100 km. Besides P -wave polarities, SH -wave first motions were also used to constrain the nodal-plane orientations. Normal and reverse faulting mechanisms with strike-slip components were found. The inversion of four mechanisms to estimate the stress tensor indicated a strike-slip stress regime with roughly E–W-orientated σ ₁ and N–S σ ₃. Both the orientations and the shape factor ( φ =0.7) of the inverted stress are in excellent agreement with theoretical predictions for that part of Brazil from the driving-force model of Coblentz & Richardson (1996) . Good agreement with the nature of the stress, as well as its orientation, was also found for the model of Meijer (1995) . Both of these theoretical models include spreading stresses along the continent/ocean lithospheric transition. Because the earthquakes are more than 300 km from the continental shelf they should not be affected by the local flexural forces caused by sediment load in the marginal basins. The agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses.     

Chemostratigraphy of the Tamengo Formation (Corumbá Group,Brazil): A contribution to the calibration of the Ediacaran carbon-isotope curve

The Corumbá Group, cropping out in the southern Paraguay Belt in Brazil, is one of the most complete Ediacaran sedimentary archives of palaeogeographic, climatic, biogeochemical and biotic evolution in southwestern Gondwana. The unit hosts a rich fossil record, including acritarchs, vendotaenids (Vendotaenia, Eoholynia), soft-bodied metazoans (Corumbella) and skeletal fossils (Cloudina, Titanotheca). The Tamengo Formation, made up mainly of limestones and marls, provides a rich bio- and chemostratigraphic record. Several outcrops, formerly assigned to the Cuiabá Group, are here included in the Tamengo Formation on the basis of lithological and chemostratigraphical criteria. High-resolution carbon isotopic analyses are reported for the Tamengo Formation, showing (from base to top): (1) a positive δ¹³C excursion to +4‰ PDB above post-glacial negative values, (2) a negative excursion to −3.5‰ associated with a marked regression and subsequent transgression, (3) a positive excursion to +5.5‰, and (4) a plateau characterized by δ¹³C around +3‰. A U-Pb SHRIMP zircon age of an ash bed interbedded in the upper part of the δ¹³C positive plateau yielded 543 ± 3 Ma, which is considered as the depositional age ( Babinski et al., 2008a). The positive plateau in the upper Tamengo Formation and the preceding positive excursion are ubiquitous features in several successions worldwide, including the Nama Group (Namibia), the Dengying Formation (South China) and the Nafun and Ara groups (Oman). This plateau is constrained between 542 and 551 Ma, thus consistent with the age of the upper Tamengo Formation. The negative excursion of the lower Tamengo Formation may be correlated to the Shuram–Wonoka negative anomaly, although δ¹³C values do not fall beyond −3.5‰ in the Brazilian sections. Sedimentary breccias occur just beneath this negative excursion in the lower Tamengo Formation. One possible interpretation of the origin of these breccias is a glacioeustatic sea-level fall, but a tectonic interpretation cannot be completely ruled out.     

松辽盆地东南隆起区白垩系嫩江组一段沉积相、旋回及其与松科1井的对比

对松辽盆地东南隆起区嫩江组剖面进行厘米级刻画,分析其为一套深湖相与滨浅湖交互相沉积。在滨浅湖交互相沉积段,水体频繁波动,且波动幅度大。根据颜色、岩性将嫩江组一段地层划分为34个米级旋回,Fischer图解分析将米级旋回归并为11个五级旋回和2个四级旋回。剖面中发育的介形虫灰岩、白云岩结核和火山灰层在松科1井嫩江组一段连续取心中都有响应;剖面和井的Fischer图解对比发现,第15个旋回开始到第25个旋回结束,松辽盆地沉积中心向西北方向迁移,且可能伴随着盆地东缘的抬升运动。     

Assessing changing vulnerability to dengue in northeastern Brazil using a water-associated disease index approach

Water management practices and access to safe water supplies have major implications for human health. While a range of assessments has been developed to assess water vulnerability, limited work has extended these concepts to health and wellbeing. Water-associated disease cycles are characterized by complex linkages between social and ecological determinants, thus conceptualizing vulnerability in the context of health offers a useful framework for analysis. This paper applies a water associated disease index (WADI) as a tool to deepen understanding of changing vulnerability to dengue, comparing conditions in 2000 and 2010 in Pernambuco, Northeast Brazil. Multi-dimensional data were integrated into indicators of exposure and susceptibility using the WADI approach, including water access, land cover, climate, and solid waste collection, and outputs were validated and visualized in map form. The findings illustrate heterogeneous patterns of vulnerability to dengue in the region, and highlight trends of seasonal and long-term changes. Highest vulnerability was observed in densely populated Recife and the surrounding coastal region in both time periods, with climate conditions creating seasonal trends in exposure to dengue. While more remote areas in the semi-arid Sertão showed low vulnerability overall, increases were observed in some areas between 2000 and 2010 due to land use intensification and growing population densities. These findings suggest that interventions should consider the dynamic nature of social and ecological factors that contribute to health outcomes and address current as well as future populations vulnerable to dengue transmission. This vulnerability mapping approach can be applied to other water-associated diseases impacted by global environmental change to highlight priority areas for further investigation and contribute towards improving interventions.     

湘东南—赣西地区加里东期沉积演化特征

通过对研究区加里东期沉积物的野外地质调查、剖面横向对比和岩相古地理编图,发现研究区加里东期主要发育五种沉积相类型,分别为碳酸盐岩陆棚相、碎屑岩—碳酸盐岩混积陆棚相、冰海相、欠补偿型硅质岩—页岩盆地相和补偿型浊积岩盆地相。剖面横向对比显示,研究区加里东期沉积物由西向东为连续相变沉积,横向上各种沉积相指状交叉、相互过渡;岩相古地理研究表明,加里东期湘东南—赣西地区主要发育硅质岩—页岩盆地相和浊积岩盆地相两大相带,从南华纪—奥陶纪时期两大相带的发育规模略有差异,在两大相带的结合部往往发育两种沉积相的交叉过渡带。笔者认为该区沉积相的过渡性质对探讨华南洋加里东期属性的问题有重要意义。     

Stratigraphical framework of basaltic lavas in Torres Syncline main valley,southern Parana-Etendeka Volcanic Province

The Paraná-Etendeka Volcanic Province records the volcanism of the Early Cretaceous that precedes the fragmentation of the South-Gondwana supercontinent. Traditionally, investigations of these rocks prioritized the acquisition of geochemical and isotopic data, considering the volcanic stack as a monotonous succession of tabular flows. Torres Syncline is a tectonic structure located in southern Brazil and where the Parana-Etendeka basalts are well preserved. This work provides a detailed analysis of lithofacies and facies architecture, integrated to petrographic and geochemical data. We identified seven distinct lithofacies grouped into four facies associations related to different flow morphologies. The basaltic lava flows in the area can be divided into two contrasting units: Unit I - pahoehoe flow fields; and Unit II - simple rubbly flows. The first unit is build up by innumerous pahoehoe lava flows that cover the sandstones of Botucatu Formation. These flows occur as sheet pahoehoe, compound pahoehoe, and ponded lavas morphologies. Compound lavas are olivine-phyric basalts with intergranular pyroxenes. In ponded lavas and cores of sheet flows coarse plagioclase-phyric basalts are common. The first pahoehoe lavas are more primitive with higher contents of MgO. The emplacement of compound pahoehoe flows is related to low volume eruptions, while sheet lavas were emplaced during sustained eruptions. In contrast, Unit II is formed by thick simple rubbly lavas, characterized by a massive core and a brecciated/rubbly top. Petrographically these flows are characterized by plagioclase-phyric to aphyric basalts with high density of plagioclase crystals in the matrix. Chemically they are more differentiated lavas, and the emplacement is related to sustained high effusion rate eruptions. Both units are low TiO₂ and have geochemical characteristics of Gramado magma type. The Torres Syncline main valley has a similar evolution when compared to other Large Igneous Provinces, with compound flows at the base and simple flows in the upper portions. The detailed field work allied with petrography and geochemical data are extremely important to identify heterogeneities inside the volcanic pile and allows the construction of a detailed lithostratigraphical framework.     

Two paleoecological histories spanning the period of human settlement in southeastern Brazil

The absence of human occupation sites in southeastern Brazil during the mid Holocene has been referred to as the 'Archaic Gap' (8970–1940 cal. a BP) and is predicted to have resulted from increased aridity. A ca. 14 000 cal. a pollen history from two well‐dated lake sediment cores located within the archeological district of Lagoa Santa, in the State of Minas Gerais, southeastern Brazil, was used to test this hypothesis. Our analyses indicated that the present cerrado and tropical semi‐deciduous forest mosaic persisted throughout the mid Holocene, until ca. 5500 cal. a BP, and the Lagoa Santa region did not experience especially dry conditions during the Holocene period. The early Holocene pollen spectra contained an assemblage of cold‐adapted taxa such as Podocarpus, Myrsine and Araucaria, co‐occurring with taxa from cerrado, e.g. Caryocar. A replacement of cold taxa by the modern cerrado–semi‐deciduous forest vegetation took place progressively, but appears to have been completed by the mid Holocene. No evidence of sustained drought was found in sedimentation or forest composition, nor any prolonged dry event in the study region. Holocene dryness as an explanation for the abandonment of Lagoa Santa region is not supported by the palynological analyses conducted in this study. Rather it is suggested that unpredictable climate may have underlain that abandonment. Copyright © 2012 John Wiley & Sons, Ltd.     

东南亚地震活动与b 值的时空分布特征

东南亚地区是“21世纪海上丝绸之路”（以下简称“海洋丝路”）的重要组成部分,该区历史上曾发生十余次巨大地震,地震及其次生地质灾害是威胁东南亚地区经济社会发展和国际合作的主要自然灾害。系统梳理该区地震活动的时空分布特征及评估未来灾害风险格局,对于推进“一带一路”倡议实施及区域经济社会可持续发展具有重要意义。文章基于东南亚地区1900年以来M≥5地震的时空分布统计分析和地震b值计算,揭示出该区的地震活动在时间上表现出活跃期与平静期交替变化的特征;空间上表现出明显的聚集效应,成丛性强且主要集中在5个地震统计区内,其中印尼—马来多岛弧盆系地震区和菲律宾群岛地震区的地震活动最为活跃。总体而言,东南亚5个地震区的b值偏低,在0.42~0.91之间。该区内的地震b值也存在时空差异,受大地震事件、俯冲带年龄、活动断裂带和震源深度等众多因素影响,但主控因素在不同区域有所不同。地震b值时空变化特征对区域地震活动预测具有启示作用。上述认识为推进“海洋丝路”工程建设和“一带一路”防灾减灾对策提供了科学支撑。