New stratigraphic,geochronological, and structural data from the southern Guanajuato Mining District,México: implications for the caldera hypothesis

Abstract

The Cenozoic stratigraphy of the southern Guanajuato Mining District (GMD) was established 40 years ago. The existence of a caldera structure that produced the Cenozoic volcanic cover was postulated and the world-class silver ore deposit of the Oligocene age has been closely related to magmatism. In this context, we present a new geological map of the southern GMD, U–Pb and Ar–Ar ages of the volcanic units, and structural data for the Cenozoic faults. Our results document that the volcanic centre was active between ca. 33.5 Ma and ca. 31.3 Ma, coeval with NW–SE normal faulting. We propose that the Bufa, Calderones, and Cedro formations are stratigraphic units directly related to the volcanic centre. Although the younger Chichíndaro Rhyolite scarcely crops out within the study area, it appears to be more extensive outside of the study area, forming part of the rhyolitic volcanism of the Mesa Central of Mexico. In the study area, the Chichíndaro Rhyolite buries major faults, demonstrating that it was emplaced after the peak of faulting. The two main structures are the El Cubo and Veta Madre grabens; also there are several faulted and brecciated zones where silver–gold mineralization was emplaced. The extension direction changed from NE to NW producing normal faulting, reactivating older structures and allowing dike intrusion. The extensional phase continued to be active throughout the Oligocene. The age of the volcanic event and a new K–Ar age of the Veta Madre vein of 29.8 ± 0.8 Ma (K–Ar in adularia) indicate that the hydrothermal event began immediately after the emplacement of the Cedro Formation. The emplacement of the Chichíndaro Rhyolite allowed hydrothermal activity to be active for two million years or more.     

Imaging the Kristineberg mining area with two perpendicular magnetotelluric profiles in the Skellefte Ore District,northern Sweden

New magnetotelluric (MT) data from two perpendicular profiles in the Kristineberg area, northern Sweden, were analysed and modelled. In the Skellefte Ore District, the Kristineberg volcanic hosted massive sulphide (VHMS) deposit mine is one of the largest and deepest (1250 m). Seventeen broadband magnetotelluric stations were installed along two existing seismic reflection lines. The profiles were 6 and 12 km long with 500 m and 1 km site spacing, respectively. The obtained MT transfer functions in the period range of 0.0015–200 s are of fairly good quality. Detailed strike and dimensionality analysis reveal consistent but period dependent, strike directions, indicating a change in the geoelectrical strike with depth. From the two‐dimensional inversion of the determinant of the impedance tensor, two stable conductivity models with good data fit were obtained. The addition of seismic reflection information from the co‐located survey, improved the data fit of one of them. Extensive sensitivity analyses helped to delineate the well resolved regions of the models and to determine the position of pronounced boundaries. The results are in good agreement with previous studies, especially regarding the presence of a deep conductor interpreted as a structural basement to the district. They also reveal with more detail the configuration of the main geological units of the Skellefte Ore District, especially of the ore bearing volcanic rocks and the embedded alteration zones.     

Geochemical characterization of the mining district of Linares (Jaen,Spain) by means of XRF and ICP-AES

Linares (Jaen, Spain) has been subjected to an intense mining and metallurgic activity during 2500 years. Basically, lead and silver have been extracted from galena; as well as copper and zinc. Different studies have been conducted to evaluate trace element concentrations to obtain relationships with anthropogenic and geologic factors. The analytical results of these studies have been obtained with ICP-AES. This analytical technique is relatively expensive and inaccessible in zones with scarce economic means. In this study, XRF analyses have been conducted and the obtained results are presented, as well as a comparison with the previously obtained ICP results. In both cases, a cluster analysis has been made to try to identify the same relations in the target area. 5 groups have been identified, mainly related with lithology. Only 10 of the 122 grid squares (each one of 1 km²) are classified in different groups if the cluster analysis is conducted with XRF results or ICP results. ICP classifies better where these grid squares are located, mostly, in contact zones of different lithologies.     

江苏省矿业权评估系统的设计与实现

本文通过对江苏省矿业权评估管理业务的分析与需求梳理,提出了电子政务框架下的矿业权评估系统的构建模式。基于电子政务平台构建矿业权评估系统,快速满足了矿业权评估管理业务的应用需要,转变了传统工作模式,拓展了电子政务建设内容,加快了依法行政的进程。     

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非洲矿产资源非常丰富,其中铬、钴、锰、铝土矿、铂族金属、金、金刚石等矿产资源储量居世界重要位置。非洲矿产资源开发潜力巨大,受到国际矿业投资者的关注。本文针对非洲的矿产资源、矿业开发、矿业政策、投资环境等进行了较全面的介绍和分析,供有意赴非洲投资矿业者参考。     

加拿大育空地区矿业投资环境

本文简要讨论了加拿大育空地区的矿业投资环境,包括基本地质特征、矿业经济、矿业管理体制、主要矿业管理机构、主要矿业法律与矿产使用权设置、主要矿业税费、主要矿业投资政策、管理政策和环境政策等。     

2013年全球矿业政策与管理形势回顾

2013年,全球矿业经济继续下行,但全球矿业政策和管理的改革仍按既定方向和节奏进行.近30个国家计划通过、正在通过或已经通过修改矿法,对本国的矿业投资环境进行调整.非传统矿业国也借助矿产资源的新发现,完善矿产资源管理制度,希望推动经济新的发展.许多国家为增加本国的矿产资源收益,纷纷提高资源税费.在放宽外资投资矿业的准入条件的同时,对矿产开采可能造成的环境影响也更加重视.     

沂水县矿业用地状况探析

矿业用地相对复杂,既有露天开采,又有地下开采;既有矿区用地,又有配套通道及生活设施用地;既有永久性建筑,又有临时性建筑。以沂水县为例,对当前矿业用地现状及存在的问题进行简要分析,并就如何规范管理矿业用地促进地方经济发展提出建议和对策。     

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