The results of research in the use of Au grain morphological and compositional properties applied in primary Au ore exploration are presented here. Two different and independent topics are discussed: (1) morphological characteristics of Au grains from active stream sediments for use as a distance-to-source indicator; (2) compositional signature of Au grains from various deposit types for use as a discrimination tool for source type and present deposit erosion level determination. The purpose of this study is to improve and integrate these two approaches as an exploration tool for Andean covered areas. Au grain morphology for over 1500 nuggets recovered from 60 active stream sediment samples in the Coastal Cordillera of Central Chile shows morphological variations (general shape, outline, surface, primary crystal imprints, associated minerals, flatness index) characteristic of three distance ranges (0–50 m; 50–300 m; >300 m) from source. Comparison with results from other similar studies of Au morphology characteristics in different climatic and/or sedimentological environments (arid, semi-arid, wet, lateritic, fluvial, fluvio-glacial and glacial) resulted in the determination of the recommended parameters (outline, surface, associated minerals, flatness index) to be used as distance-to-source indicator, independent of the climatic and/or sedimentological environment. Au grain morphological characteristics may assist in location of target but are not indicators of source type. Study of Au composition via electron microprobe analysis of Au grain cores from epithermal, Au-rich porphyry and Au-rich porphyry Cu systems indicated Au–Ag–Cu contents to be the best discrimination tool for these different types of Au-bearing deposits. In addition, such analysis of grains recovered at different vertical levels from the Cerro Casale Au-rich porphyry provides evidence that the Au compositional signature for a single type of deposit can also aid in the determination of vertical position. This may provide an estimate of the current level of erosion and remaining potential of the source. Some limitations of the proposed techniques are: (1) Au liberated from rock fragments already distant from source would be common in cordilleran and glacial environments, although this would be a detectable feature; (2) these techniques are applicable only for coarse-Au sources; (3) estimate of erosion level of liberated Au is limited to the case here presented.
ABSTRACT. The Liquiñe-Ofqui fault zone (LOFZ) in the Patagonian Andes is an active major transpressional intra-arc fault system along which Quaternary faulting and volcanism develop. Subaerial and submarine geomorphologic and structural characterization of latest Pleistocene-Holocene faults and monogenetic volcanoes allows us to assess geological cartography of active faults and the kinematic model for recent tectonics during postglacial times, since 12,000 cal. years BP. This allows increasing the basic geological knowledge necessary for determining the seismic hazard associated with cortical structures in the Aysén region in southern Chile. Fault cartography and field observations suggest dominant dextral-reverse strike slip along north-south and locally NNW-striking faults, dextral-normal strike slip along NE to NNE-striking faults, and sinistral strike slip along east-west faults. This kinematics is consistent with regional SW-NE shortening in the context of a major transpressional fault zone. Holocene and even historic monogenetic and sub-aquatic volcanism occurred in this tectonic setting in a close spatial relationship and probably favored by the activity and local architecture of faults. Submarine fault scarps and deformed sediments observed at the bottom of the Aysén Fjord were associated with the destructive April 2007 Mw6.2 earthquake located along the LOFZ. Our observations show that this earthquake occurred along dextral 15-20 km long N-S structure named Punta Cola Fault (PCF). This fault system is located some kilometres to the east of the main N-S Río Cuervo Fault (RCF). Most of the epicentres of the seismic swarm during 2007 were located along or in between both structures. The study area is a transference zone between N-S regional branches of the LOFZ. The cartography of fault segments proposed here together with geophysical and geologic data suggest that large earthquakes Mw6.2-6.5 can be typically expected along most of the active faults. Besides, seismic hazard assessment should also consider the possibility of earthquake magnitude in the order of 7.1 along the main fault systems like the RCF. RESUMEN.Ruptura sísmica submarina, tectónica y volcanismo activo a lo largo de la Falla Liquiñe-Ofqui e implicancias para el peligro sísmico en los Andes patagónicos. La Zona de Falla Liquiñe-Ofqui es un sistema de intraarco activo y paralelo al contacto entre las placas tectónicas de Nazca y Sudamérica, a lo largo del cual el fallamiento y el volcanismo cuaternarios están estrechamente asociados. A partir de observaciones geomorfológicas y estructurales subaéreas, la prospección subacuática de fallas con evidencia de actividad pleistocena tardía-holocena y la caracterización de volcanes monogenéticos, se propone una cartografía y un modelo cinemático para la tectónica reciente, con énfasis en los últimos 12.000 años, asociada a fallas activas en el área del Fiordo Aysén. Esto permite incrementar sustancialmente el conocimiento geológico básico necesario para la determinación del peligro sísmico asocia...
No abstract
Oruro, located in the Bolivian Altiplano, has been subjected to intense mining and smelting activities since Colonial times (17th Century), yet the current geochemical composition of sediments and trace element behavior is practically unknown. A collection of 91 sediment samples retrieved from five sedimentary cores from Lake Uru Uru (Altiplano) and Cala Cala Lagoon (Eastern Cordillera) subjected to a total digestion technique, with a compilation of a pre-existing database of trace element concentrations in soils and lacustrine sediments obtained from the Oruro Pilot Project (PPO), facilitated the proposal of geochemical backgrounds and a present-time baseline for As, Cd, Cu, Pb, Sb and Zn in sediments from this area. Results obtained by statistics and geographical information system (GIS) analyses showed that the natural geochemical backgrounds of As and Sb are significantly enhanced in comparison to the Upper Continental Crust (UCC) concentrations, world background levels, and those of industrial sites and historical mining sites. The use of a local enrichment factor (EF) normalized by the mean concentration within the Cala Cala lagoon (CCLAC) demonstrated that using UCC concentrations to calculate EFs (EF UCC ) is inadequate for this highly mineralized environment and therefore is not supported. Regarding metals and metalloids, the strong multiplicity of sources in this environment makes it difficult to discriminate between natural and anthropogenic inputs into this endorheic drainage basin, although it is suggested that surficial soils are probably impacted by airborne particulates dispersed from the Vinto (Sb-Sn) smelter, while Lake Uru Uru is influenced by mining activities, particularly drainage waste of the San José and Huanuni mines. As a final contribution, a geochemical background and a present-time baseline for Bolivian highlands sediments are provided, which will be helpful for the improvement of environmental legislation and for the future interpretation of geochemistry data in contamination and/or pollution studies in the altiplanic area.
The Atacama Desert, the driest of its kind on Earth, hosts a number of unique geological and geochemical features that make it unlike any other environment on the planet. Considering its location on the western border of South America, between 17 and 28 °S, its climate has been characterized as arid to hyperarid for at least the past 10 million years. Notably dry climatic conditions of the Atacama Desert have been related to uplift of the Andes and are believed to have played an important role in the development of the most distinctive features of this desert, including: (i) nitrates and iodine deposits in the Central Depression, (ii) secondary enrichment in porphyry copper deposits in the Precordillera, (iii) Li enrichment in salt flats of the Altiplano, and (iv) life in extreme habitats. The geology and physiography of the Atacama Desert have been largely shaped by the convergent margin present since the Mesozoic era. The geochemistry of surface materials is related to rock geochemistry (Co, Cr, Fe, Mn, V, and Zn), salt flats, and evaporite compositions in endorheic basins (As, B, and Li), in addition to anthropogenic activities (Cu, Mo, and Pb). The composition of surface water is highly variable, nonetheless in general it presents a circumneutral pH with higher conductivity and total dissolved solids in brines. Major water constituents, with the exception of HCO, are generally related to the increase of salinity, and despite the fact that trace elements are not well-documented, surface waters of the Atacama Desert are enriched in As, B, and Li when compared to the average respective concentrations in rivers worldwide.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.