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1.
《Ore Geology Reviews》2007,30(3-4):260-286
Geographic Information Systems (GISs) are very useful tools for managing, checking, and organizing spatial information–from many sources and of many types–in thematic layers. Processing of these data enables exploration-oriented GISs to produce potential and predictive maps for a given commodity, which constitute documents of real use in decision-making. Integration of all information in a single reference system enables a better understanding of the parameters controlling a region's metallogeny, in terms of both time and space. But what scale should be used for developing a mineral exploration GIS? Should preference be given to systems with high spatial resolutions (scale < 1:500,000), or to more general systems with scales of around 1:1,500,000 or 1:2,000,000? Will the gain be worthwhile relative to the additional work generated by compilation at a higher scale? In order to make greater use of previous predictive studies performed on gold-rich epithermal and porphyry systems at the scale of the entire Andes, an expert-guided data-driven approach is now applied to a regional-scale GIS of NW Argentina, between the Puna and the Sierrras Pampeanas, where known deposits like Bajo de la Alumbrera, Agua Rica, and others, account for a metal potential of over 10 Mt Cu and 750 t Au. In developing this new predictive map, three criteria that were likely to be connected to the mineralizing event were selected and quantified: (i) lithostratigraphy, because of its role as a favourable environment for the development of mineralization, based on its physico-chemical properties; (ii) lithostratigraphic contacts, based on the rheological properties of the formations in contact; and (iii) the orientation of structural discontinuities, which channel source magmas and encourage the circulation of hydrothermal fluids. Assigning a score enables classification of the favourabilities calculated for each of the criteria considered. This approach is employed here to check and standardize the statistical results obtained by methods such as Weight of Evidence Modelling or an algebraic approach. For each criterion, four classes were distinguished: very favourable: score = 3; favourable: score = 2; slightly favourable: score = 1; and unfavourable: score = 0. The predictive map is obtained by adding the scores for the three favourable criteria defined above.The regional-scale work identified 20 anomalous envelopes with cumulative scores greater than 5. They correspond to mining areas that are active (e.g., Bajo de la Alumbrera), under development (e.g., Agua Rica), or abandoned (e.g., La Mejicana), or to new areas (e.g., the Vicuña Pampa Volcanic Complex). Structural analysis of the region, integrating the orientation of the favourable envelopes, suggests that the mineralizing fluids were emplaced under extensional conditions, sub-parallel to the principal directions of shortening: (i) WNW–ESE, found along the southern edge of the Puna; and (ii) E–W, seen in the Sierra de Famatina. It appears that a regional-scale information system is a tool that is well suited to the definition of areas for mineral prospecting, and to the study and confirmation of metallotects usable for mineral exploration.Comparison with work conducted on the basis of a 1:2,000,000 geological compilation shows that the principal mining districts can indeed be found at continental scale. On the other hand, the lack of detail inherent at a scale of 1:2,000,000 may lead to inaccuracies, in particular fictitious favourabilities assigned to formations that are genetically unrelated to the mineralization, but that contain, for example, small Tertiary intrusive bodies that cannot be recorded at this scale. This comparison therefore shows that the use of a continental-scale GIS is effective, and well suited to the definition of prospective areas at a strategic level.  相似文献   

2.
Important copper-gold (± molybdenum) porphyry deposits occur in the northwest region of Argentina, part of the Central Andes. This paper provides new isotope information on two of these deposits, Bajo de la Alumbrera and Agua Rica, the latter having an epithermal overprint event. The two deposits are genetically associated with the Miocene Farallón Negro Volcanic Complex. Whole rock and sulfide samples were analyzed for Pb, Sr and Nd isotopes to characterize the sources of magma and mineralization of both deposits. Sr and εNd data made it possible to divide the samples into three distinct groups. Most samples are slightly differentiated, have OIB (Ocean Island Basalts) isotope characteristics, and show some Nd fractionation. Lead isotope also shows distinct groups with an enrichment trend in radiogenic lead. In both deposits, the sulfides are more radiogenic than the host rocks, but are clearly related to them because all plot on a single trend. A crustal contribution to the deposits is suggested on the basis of lead isotope signatures. The similarity on the Pb isotope signatures suggests the same magmatic fluid source, although at Agua Rica the crustal component is more evident than at Bajo de la Alumbrera, possibly because of the assimilation of Paleozoic country rocks at Agua Rica.  相似文献   

3.
Application of multiple chronometers (including U–Pb and 40Ar/39Ar geochronology and zircon and apatite (U–Th)/He thermochronology) to porphyry intrusions at the Bajo de la Alumbrera porphyry copper–gold deposit, Argentina, reveals a complex history of reheating that spans millions of years. Previous U–Pb geochronology, combined with our new 40Ar/39Ar data, shows that the multiple porphyritic intrusions at Bajo de la Alumbrera were emplaced during two episodes, the first at about 8.0 Ma (P2 and associated porphyries) and the second about a million years later (Early and Late P3 porphyries). Complex overprinting alteration events have obscured the earliest hydrothermal history of the deposit. By contrast, 40Ar/39Ar data reveal the close temporal relationship of ore-bearing potassic alteration assemblages (7.12 ± 0.13 Ma; biotite) to the emplacement of the P3 intrusions. Consistent with low closure temperatures, younger ages have been determined for associated hydrothermal alkali feldspar (6.82 ± 0.05 Ma and 6.64 ± 0.09 Ma). The temperature-sensitive Ar data also record an unexpected prolonged cooling history (to below 200°C) extending to 5.9 Ma. Our data suggest that the Bajo de la Alumbrera system underwent protracted cooling, after the collapse of the main hydrothermal system, or that one or more low-temperature (~100–200°C) reheating events occurred after emplacement of the porphyritic intrusions at Bajo de la Alumbrera. These have been constrained in part by our new 40Ar/39Ar data (including multidomain diffusion modeling) and (U–Th)/He ages. Single-grain (U–Th)/He ages (n = 5) for phenocrystic zircon from P2 and P3 intrusive phases bracket these thermal events to between 6.9 (youngest crystallization of intrusion) and 5.1 Ma. Multidomain modeling of alkali feldspar data (from both igneous and hydrothermal crystals) is consistent with the deposit cooling rapidly from magmatic temperatures to below about 300°C, with a more protracted history down to 150°C. We conclude that the late-stage low-temperature (150 to 200°C) thermal anomaly localized at Bajo de la Alumbrera resulted from radiation of heat and/or fluids sourced from deeper-seated magma bodies, emplaced beneath the deposit. To produce the observed thermal longevity of the porphyry system, magma bodies underlying the Bajo de la Alumbrera deposit must have been repeatedly replenished by new magma batches. Without replenishment, crystallization of the source magma will occur, and heat release will stop, leading to rapid cooling (in less than ten thousand years). The influx of deep-seated magma may have caused the development of late low-temperature hydrothermal alteration assemblages at Bajo de la Alumbrera, at the same time that mineralization formed at Agua Rica, some 25 km away. All available chronologic data for the Bajo de la Alumbrera deposit suggest that the hydrothermal system was active episodically over at least a three-million and possibly up to a four-million-year period. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

4.
ELA-ICP-MS U–Pb zircon geochronology has been used to show that the porphyritic intrusions related to the formation of the Bajo de la Alumbrera porphyry Cu–Au deposit, NW Argentina, are cogenetic with stratigraphically well-constrained volcanic and volcaniclastic rocks of the Late Miocene Farallón Negro Volcanic Complex. Zircon geochronology for intrusions in this deposit and the host volcanic sequence show that multiple mineralized porphyries were emplaced in a volcanic complex that developed over 1.5 million years. Volcanism occurred in a multi-vent volcanic complex in a siliciclastic intermontane basin. The complex evolved from early mafic-intermediate effusive phases to a later silicic explosive phase associated with mafic intrusions. Zircons from the basal mafic-intermediate lavas have ages that range from 8.46±0.14 to 7.94±0.27 Ma. Regionally extensive silicic explosive volcanism occurred at ~8.0 Ma (8.05±0.13 and 7.96±0.11 Ma), which is co-temporal with intrusion of the earliest mineralized porphyries at Bajo de la Alumbrera (8.02±0.14 and 7.98±0.14 Ma). Regional uplift and erosion followed during which the magmatic-hydrothermal system was probably unroofed. Shortly thereafter, dacitic lava domes were extruded (7.95±0.17 Ma) and rhyolitic diatremes (7.79±0.13 Ma) deposited thick tuff blankets across the region. Emplacement of large intermediate composition stocks occurred at 7.37±0.22 Ma, shortly before renewed magmatism occurred at Bajo de la Alumbrera (7.10±0.07 Ma). The latest porphyry intrusive event is temporally associated with new ore-bearing magmatic-hydrothermal fluids. Other dacitic intrusions are associated with subeconomic deposits that formed synchronously with the mineralized porphyries at Bajo de la Alumbrera. However, their emplacement continued (from 7.10± 0.06 to 6.93±0.07 Ma) after the final intrusion at Bajo de al Alumbrera. Regional volcanism had ceased by 6.8 Ma (6.92±0.07 Ma). The brief history of the volcanic complex hosting the Bajo de la Alumbrera Cu–Au deposit differs from that of other Andean provinces hosting porphyry deposits. For example, at the El Salvador porphyry copper district in Chile, magmatism related to Cu mineralization was episodic in regional igneous activity that occurred over tens of millions of years. Bajo de la Alumbrera resulted from the superposition of multiple porphyry-related hydrothermal systems, temporally separated by a million years. It appears that the metal budget in porphyry ore deposits is not simply a function of their longevity and/or the superposition of multiple porphyry systems. Nor is it a function of the duration of the associated cycle of magmatism. Instead, the timing of processes operating in the parental magma body is the controlling factor in the formation of a fertile porphyry-related ore system.Electronic Supplementary Material Electronic supplementary material to this paper can be obtained by using the Springer Link server located at Editorial handling: N. White  相似文献   

5.
The relationship between long-lived deep crustal lineaments and the locations of magmatic centers and associated mineral deposits has been investigated in the Puna region of northwestern Argentina, through the analysis of regional aeromagnetic surveys, Landsat images, and geological information. The good exposure and excellent preservation of basement and supracrustal geology in this region makes it particularly suitable for such a study. At a regional scale, several contrasting magnetic domains are recognized, which correlate with crustal geology. Two basement domains are separated by a NNE-trending boundary, which is believed to correlate with a Paleozoic suture zone between the Pampia (to the southeast) and Arequipa–Antofalla terranes (to the northwest). Locally overlying these basement terranes is the Cenozoic magmatic domain, which is best developed in the N–S-trending volcanic arc at the western edge of the Puna (the Cordillera Occidental). In addition, four southeast-trending volcanic zones extend for several hundred kilometers across the Puna. Many important mineral deposits and areas of hydrothermal alteration are associated with these volcanic breakouts, and we have selected three such areas for more detailed study: Bajo de la Alumbrera (Argentina's largest porphyry copper deposit), Cerro Galán (the largest ignimbrite caldera in Argentina, with associated hydrothermal alteration zones), and El Queva (a historic polymetallic district located within a major volcanic range). A comparison of lineament maps generated from aeromagnetic and Landsat TM images reveals broad correlation between these different remote sensing techniques, which respectively highlight subsurface magnetic and surface geological features. In addition, the locations of magmatic and hydrothermal centers can be related to the interpreted structural framework, and are seen to occur near the intersections of major lineament zones. It is suggested that in three dimensions, such intersection zones form trans-lithospheric columns of low strength and high permeability during transpressional or transtensional tectonic strain, and may thereby serve as conduits for magma ascent to the shallow crust. Pooling of large volumes of deeply derived magma in shallow crustal magma chambers may then result in voluminous devolatilization and the formation of hydrothermal mineral deposits. It is important to note that in this model, structural intersections serve as facilitators for magma ascent and volatile exsolution, but do not in themselves cause this process—other factors such as magma supply rate and tectonic stress are essential primary ingredients, and local magmatic and volcanic processes affect the ultimate potential for ore formation. Nevertheless, we suggest that lineament analysis provides a valuable framework for guiding the early stages of mineral exploration; other regional and local geological considerations must then be applied to identify priority targets within this framework.  相似文献   

6.
A high-resolution ion-microprobe (SHRIMP) U–Pb zircon age from a tuff layer intercalated in the ammonoid bearing sedimentary succession of the Neuquén Basin in Argentina provides a robust geochronologic date to add to the absolute ages and to improve the relative chronology of the Early Cretaceous Hauterivian stage. The tuff layer appears interbedded between shales of the upper member (Agua de la Mula) of the Agrio Formation within the Spitidiscus riccardii ammonoid zone (base of the Late Hauterivian) yielding a date of 132.5 ± 1.3 Ma. This date confirms and supports an accurate correlation between the ammonoid biostratigraphy of the Neuquén Basin with the Western Mediterranean Province of the Tethys during the Early Cretaceous and matches with the most recently published time scale. It also casts doubts on the validity of K–Ar ages on glauconite-grains recently reported from the Lower Cretaceous of the Vocontian Basin of France.  相似文献   

7.
Summary Bajo de la Alumbrera forms an example of a series of world class copper-gold deposits that are hosted by potassic igneous rocks. The Bajo de la Alumbrera porphyry copper-gold deposit is situated in the Catamarca Province of Northwest-Argentina and belongs to the ten largest copper mines in the world; it is also one of the largest gold producers in South America.The deposit is characterized by an alteration-zoning pattern with a central potassic core which grades outwards into a zone with propylitic alteration and an intermediate annular phyllic overprint. Porphyry copper-gold mineralization consists mainly of disseminated and/or vein-related chalcopyrite within the central potassic alteration zone which also hosts the highest gold grades (up to 2 ppm). Mineralization is genetically related to co-magmatic hypabyssal dacite domes with shoshonitic compositions which intruded the high-K calc-alkaline andesitic to latitic lava flows, breccia tuffs, and agglomerates of the Upper Miocene Farallon Negro Formation. The Farallon Negro Formation is interpreted as the remnants of a major stratovolcano some 16 km in diameter, with volcanism being controlled by sinistral pull-apart tectonics along a major NW-trending lineament.The dacitic rocks associated with the porphyry copper-gold mineralization have porphyritic textures with phenocrysts of alkali feldspar, plagioclase, biotite, and very rare amphibole in a fine-grained feldspathic groundmass. The abundance of hydrous minerals, such as biotite phenocrysts and apatite microphenocrysts, is consistent with the volatile-rich nature of the melts. The rocks underwent moderate degrees of clinopyroxene-biotite-âpatite-titanite fractionation. Several samples are overprinted by weak to moderate potassic or carbonate-sericite alteration. Geochemically, the dacites are characterized by high LILE (e.g. K20 up to 7.16 wt%, and Rb, Sr, and Ba up to 145 ppm, 445 ppm, and 610 ppm, respectively), moderate LREE (La < 73 ppm and Ce < 140 ppm), and low HFSE (TiO2 < 0.92 wt%, Zr < 142 ppm, Hf < 4.6) which are typical for subduction-related potassic igneous rocks from a mature continental-arc setting. The rocks have evolved compositions with relatively low mg# < 53. Their high Ba/Nb and high Rb/Nb ratios (up to 87 and 31, respectively) clearly distinguish them from rocks derived from within-plate settings.The rocks from Bajo de la Alumbrera contain average ratios of Fe2O3/(FeO+Fe2O3) as high as 0.49 reflecting the oxidized nature of their parental melts.
Die Shoshonit Porphyry Cu-Au Assoziation von Bajo de is Alumbrera, Catamarca Provinz, Argentinien
Zusammenfassung Bajo de la Alumbrera ist ein Beispiel für eine bedeutende Gold-Kupfer Lagerstätte in Kalium-reichen Magmatiten. Die Kupfer-Gold Lagerstätte Bajo de la Alumbrera gehört zum sogenannten Porphyry-Typ und befindet sich in der Catamarca Provinz im Nordwesten von Argentinien. Bajo de la Alumbrera zählt zu den zehn größten Kupferminen der Welt und ist einer der größten Goldproduzenten in Südamerika.Die Lagerstätte zeichnet sich aus durch eine klassische Zonierung der Alterationszonen, beginnend mit der zentral aufgeschlossenen `potassischen' Alterationszone (sekundärer Biotit-Magnetit-Orthoklas), die weiträumig von propylitischer Alteration (sekundärer Epidot-Chlorit) umgeben wird. Beide Alterationstypen werden von einer spätgenetischen phyllischen Alteration (sekundäre Tonminerale-Serizit-Pyrit) umgewandelt. Die Kupfer-Gold-Vererzung besteht überwiegend aus Kupferkies, der entweder feinverteilt in den Silikaten oder in Quarzadern innerhalb der potassischen Alterationszone vorkommt. Diese Zone beherbergt neben den höchsten Kupfergehalten auch die höchsten Gold-Gehalte von bis zu 2 ppm. Die Vererzung steht in direktem genetischen Zusammenhang mit subvulkanischen Dazit-Domen mit außergewöhnlich hohen Kalium-Gehalten, die die kalkalkalischen Andesite, Latite, Tuffbrekzien und Agglomerate der Ober-Miozänen Farallon Negro Formation intrudierten. Die Farallon Negro Formation stellt die erodierten Reste eines Stratovulkans mit schätzungsweise 16 Kilometern Durchmesser dar. Der Vulkanismus ereignete sich innerhalb eines Pull-Apart Beckens, entlang eines großen Nordwest-streichenden Lineaments.Die dazitischen Muttergesteine der Kupfer-Gold-Vererzung haben porphyrische Texturen mit Phänokristallen bestehend aus Kalifeldspat, Plagioklas, Biotit, und, sehr selten, Amphibol in einer fein- bis mikrokristallinen Grundmasse. Die Häufigkeit von Mineralen wie Biotit und Apatit reflektiert die hohen Gehalte an volatilen Phasen in der Schmelze. Die mineralogische Zusammensetzung der Gesteine wurde von einer moderaten Klinopyroxen-Biotit-Apatit-Titanit Fraktionierung mitgeprägt. Einige der untersuchten Dazite wurden von einer schwachen bis moderaten Biotit-Magnetit-Orthoklas oder Karbonat-Serizit-Alteration überprägt.Die chemische Zusammensetzung der Dazite zeichnet sich aus durch hohe Gehalte an LILE (z. B. K20 bis zu 7.16 Gew%, Rb, Sr, und Ba bis zu 145 ppm, 445 ppm, und 610 ppm), moderate Gehalte an LREE (z.B. La < 73 ppm, und Ce < 140 ppm), und geringe Gehalte an HFSE (z.B. TiO2 < 0.92 Gew%, Zr < 142 ppm, und Hf < 4.6 ppm), die typisch sind für Kalium-reiche Magmatite von kontinentalen Subduktionszonen. Die untersuchten Gesteine sind relativ fraktioniert und besitzen geringe mg# < 53. Ihre hohen Ba/Nb und Rb/Nb Verhältnisse sind typisch für Kalium-reiche Magmatite aus Subduktionszonen und unterscheiden sie eindeutig von Gesteinen aus within-plate settings.Die hohe Oxidationsstufe der Mutterschmelze reflektiert sich in den hohen Fe2O3/ (FeO + Fe2O3) Verhältnissen der untersuchten Gesteine von 0.49.

With 7 Figures  相似文献   

8.
The Cerro Durazno Pluton belongs to a suite of Paleozoic granitoid intrusions in NW-Argentina, that are central for understanding the tectonic setting of the western margin of Gondwana in Ordovician and Silurian times. The pluton and its host rocks were tectonically overprinted by metamorphic mineral shape fabrics formed under middle greenschist-facies metamorphic conditions and associated with the nearby Agua Rosada Shear Zone. Kinematic analysis of the shear zone based on the geometric relationship between individual segments of the shear plane and principal axes of mineral fabric ellipsoids indicates reverse-sense of shear with a minor component of left-lateral displacement. This is compatible with the kinematics of other ductile deformation zones in this area, collectively forming a network, which accomplished orogen-parallel extension in addition to vertical thickening. Using the Rb–Sr isotopic system, an undeformed pegmatite dike of the Cerro Durazno Pluton was dated at 455.8 ± 3.6 Ma and mineral fabrics of the Agua Rosada Shear Zone formed at middle greenschist-facies metamorphism gave deformation ages of 437.0 ± 3.8 Ma and 428.4 ± 4.5 Ma. Thus, tectonic overprint at low metamorphic grade occurred about 20–30 Ma after terminal magmatism in the Cerro Durazno area. Our data from the Cerro Durazno area and regional considerations suggest that the western margin of Gondwana was characterized by orogen-parallel extension in addition to crustal thickening as well as episodes of magmatism and ductile deformation that varied greatly in time and space.  相似文献   

9.
This paper is prepared within the frameworks of IGCP Project 470 and the associated BRGM scientific project “Africa 1999–2004” to accompany the 1:4,000,000 scale map “Geology and major ore deposits of Central Africa, presented at the 20th Colloquium of African Geology in Orleans in June 2004. It incorporates geological and metallogenic data from eight countries in Central Africa (Angola, Cameroon, Chad, Central African Republic, Congo Brazzaville, Democratic Republic of Congo (DRC), Equatorial Guinea and Zambia). The map is a harmonised and geo-referenced preliminary map, based on a GIS at 1:2,000,000 scale, and focusses on the spatial and temporal distribution of selected major deposits.  相似文献   

10.
The Central Ebro River Basin (NE Spain) is the most northern area of truly semi-arid Mediterranean climate in Europe and prehistoric human occupation there has been strongly influenced by this extreme environmental condition. Modern climate conditions single out this region due to the harsh environment, characterised by the highest absolute summer temperatures of the Ebro River Basin. The Bajo Aragón region (SE Ebro River Basin) was intensively populated during the Early Holocene (9400-8200 cal yr BP) but the settlements were abandoned abruptly at around 8200 cal yr BP. We propose that this “archaeological silence” was caused by the regional impact of the global abrupt 8.2 ka cold event. Available regional paleoclimate archives demonstrate the existence of an aridity crisis then that interrupted the humid Early Holocene. That environmental crisis would have forced hunter-gatherer groups from the Bajo Aragón to migrate to regions with more favourable conditions (i.e. more humid mountainous areas) and only return in the Neolithic. Coherently, archaeological sites persist during this crisis in the nearby Iberian Range (Maestrazgo) and the North Ebro River area (Pre-Pyrenean mountains and along the northwestern Ebro Basin).  相似文献   

11.
Previous prospectivity modelling for epithermal Au–Ag deposits in the Deseado Massif, southern Argentina, provided regional-scale prospectivity maps that were of limited help in guiding exploration activities within districts or smaller areas, because of their low level of detail. Because several districts in the Deseado Massif still need to be explored, prospectivity maps produced with higher detail would be more helpful for exploration in this region.We mapped prospectivity for low- and intermediate-sulfidation epithermal deposits (LISEDs) in the Deseado Massif at both regional and district scales, producing two different prospectivity models, one at regional scale and the other at district-scale. The models were obtained from two datasets of geological evidence layers by the weights-of-evidence (WofE) method. We used more deposits than in previous studies, and we applied the leave-one-out cross validation (LOOCV) method, which allowed using all deposits for training and validating the models. To ensure statistical robustness, the regional and district-scale models were selected amongst six combinations of geological evidence layers based on results from conditional independence tests.The regional-scale model (1000 m spatial resolution), was generated with readily available data, including a lithological layer with limited detail and accuracy, a clay alteration layer derived from a Landsat 5/7 band ratio, and a map of proximity to regional-scale structures. The district-scale model (100 m spatial resolution) was generated from evidence layers that were more detailed, accurate and diverse than the regional-scale layers. They were also more cumbersome to process and combine to cover large areas. The evidence layers included clay alteration and silica abundance derived from ASTER data, and a map of lineament densities. The use of these evidence layers was restricted to areas of favourable lithologies, which were derived from a geological map of higher detail and accuracy than the one used for the regional-scale prospectivity mapping.The two prospectivity models were compared and their suitability for prediction of the prospectivity in the district-scale area was determined. During the modelling process, the spatial association of the different types of evidence and the mineral deposits were calculated. Based on these results the relative importance of the different evidence layers could be determined. It could be inferred which type of geological evidence could potentially improve the modelling results by additional investigation and better representation.We conclude that prospectivity mapping for LISEDs at regional and district-scales were successfully carried out by using WofE and LOOCV methods. Our regional-scale prospectivity model was better than previous prospectivity models of the Deseado Massif. Our district-scale prospectivity model showed to be more effective, reliable and useful than the regional-scale model for mapping at district level. This resulted from the use of higher resolution evidential layers, higher detail and accuracy of the geological maps, and the application of ASTER data instead of Landsat ETM + data. District-scale prospectivity mapping could be further improved by: a) a more accurate determination of the age of mineralization relative to that of lithological units in the districts; b) more accurate and detailed mapping of the favourable units than what is currently available; c) a better understanding of the relationships between LISEDs and the geological evidence used in this research, in particular the relationship with hydrothermal clay alteration, and the method of detection of the clay minerals; and d) inclusion of other data layers, such as geochemistry and geophysics, that have not been used in this study.  相似文献   

12.
New mapping in the northern part of the Paleozoic Acatlán Complex (Patlanoaya area) records several ductile shear zones and brittle faults with normal kinematics (previously thought to be thrusts). These movement zones separate a variety of units that pass structurally upwards from: (i) blueschist-eclogitic metamorphic rocks (Piaxtla Suite) and mylonitic megacrystic granites (Columpio del Diablo granite ≡ Ordovician granites elsewhere in the complex); (ii) a gently E-dipping, listric, normal shear zone with top to the east kinematic indicators that formed under upper greenschist to lower amphibolite conditions; (iii) the Middle–Late Ordovician Las Minas quartzite (upper greenschist facies psammites with minor interbedded pelites intruded by mafic dikes and a leucogranite dike from the Columpio del Diablo granite) unconformably overlain by the Otate meta-arenite (lower greenschist facies psammites and pelites): roughly temporal equivalents are the Middle–Late Ordovician Mal Paso and Ojo de Agua units (interbedded metasandstone and slate, and metapelite and mafic minor intrusions, respectively) — some of these units are intruded by the massive, 461 ± 2 Ma, Palo Liso megacrystic granite: decussate, contact metamorphic muscovite yielded a 40Ar/39Ar plateau age of 440 ± 4 Ma; (iv) a steeply-moderately, E-dipping normal fault; (v) latest Devonian–Middle Permian sedimentary rocks (Patlanoaya Group: here elevated from formation status). The upward decrease in metamorphic grade is paralleled by a decrease in the number of penetrative fabrics, which varies from (i) three in the Piaxtla Suite, through (ii) two in the Las Minas unit (E-trending sheath folds deformed by NE-trending, subhorizontal folds with top to the southeast asymmetry, both associated with a solution cleavage), (iii) one in the Otate, Mal Paso, and Ojo de Agua units (steeply SE-dipping, NE–SW plunging, open-close folds), to (iv) none in the Patlanoaya Group. 40Ar/39Ar analyses of muscovite from the earliest cleavage in the Las Minas unit yielded a plateau age of 347 ± 3 Ma and show low temperature ages of  260 Ma. Post-dating all of these structures and the Patlanoaya Group are NE-plunging, subvertical folds and kink bands. An E–W, vertical normal fault juxtaposes the low-grade rocks against the Anacahuite amphibolite that is cut by megacrystic granite sheets, both of which were deformed by two penetrative fabrics. Amphibole from this unit has yielded a 40Ar/39Ar plateau age of 299 ± 6 Ma, which records cooling through  490 °C and is probably related to a Permo-Carboniferous reheating event during exhumation. The extensional deformation is inferred to have started in the latest Devonian ( 360 Ma) during deposition of the basal Patlanoaya Group, lasting through the rapid exhumation of the Piaxtla Suite at  350–340 Ma synchronous with cleavage development in the Las Minas unit, deposition of the Patlanoaya Group with active fault-related exhumation suggested by Mississippian and Early Permian conglomerates ( 340 and 300 Ma, respectively), and continuing at least into the Middle Permian (≡ 260 Ma muscovite ages). The continuity of Mid-Continent Mississippian fauna from the USA to southern Mexico suggests that this extensional deformation occurred on the western margin of Pangea after closure of the Rheic Ocean.  相似文献   

13.
This work presents a geoscientific map and database for geology, mineral and energy resources of Ethiopia in a digital form at a scale of 1:2,000,000, compiled from several sources. The final result of the work has been recorded on CD-ROM in GIS format so that the map and the database could be available to users on a personal computer.Metallic resources (precious, rare, base and ferrous–ferroalloy metals) are widely related to the metamorphic meta-volcano-sedimentary belts and associated intrusives belonging to various terranes of the Arabian–Nubian Shield, accreted during the East and West Gondwana collision (Neoproterozoic, 900–500 Ma).Industrial minerals and rock resources occur in more diversified geological environments, including the Proterozoic basement rocks, the Late Paleozoic to Mesozoic sediments and recent (Cenozoic) volcanics and associated sediments.Energy resources (oil, coal, geothermal resources) are restricted to Phanerozoic basin sediments and Cenozoic volcanism and rifting areas.  相似文献   

14.
Abstract. At the Bajo de la Alumbrera porphyry Cu-Au deposit, NW Argentina, several key textural elements preserve evidence for volatile separation. Interconnected miarolitic cavities, while being studied extensively in granites, have now been recognized in intrusions related to porphyry Cu mineralization. Pods of saccharoidal quartz are connected by narrow, anastomosing zones of graphic quartz-alkali feldspar intergrowths and ragged biotite (with lesser apatite and magnetite). Their connectivity can be as much as 15 cm; however, more commonly, the interconnected miarolitic cavities are approximately 1 to 2 mm across and 5 cm or less, long. Features such comb-quartz layered textures and magmatic-hydrothermal veins (P veins), combined with aqueous fluid phase equilibria from fluid inclusions, better constrain physical models of exsolution. We interpret these textures in the context of vapour phase formation, coalescence and accumulation in an evolving silicic magma. Recognition of textures, such as the interconnected miarolitic cavities reported here, may provide a simple exploration tool for porphyry Cu deposits, helping explorers to recognize evidence for a potentially fertile intrusions.  相似文献   

15.
New field, petrological, geochemical, and geochronological data (U–Pb and Sm–Nd) for Ordovician rock units in the southeastern Puna, NW Argentina, indicate two lithostratigraphic units at the eastern–northeastern border of salar Centenario: (1) a bimodal volcanosedimentary sequence affected by low- to medium-grade metamorphism, comprising metasediments associated with basic and felsic metavolcanic rocks, dated 485 ± 5 Ma, and (2) a plutonic unit composed of syenogranites to quartz-rich leucogranites with U–Pb zircon ages between 462 ± 7 and 475 ± 5 Ma. Felsic metavolcanic and plutonic rocks are peraluminous and show similar geochemical differentiation trends. They also have similar Sm–Nd isotopic compositions (TDM model ages of 1.54–1.78 Ga; εNd(T) values ranging from −3.2 to −7.5) that suggest a common origin and derivation of the original magmas from older (Meso-Paleoproterozoic?) continental crust. Mafic rocks show εNd(T) ranging from +2.3 to +2.5, indicating a depleted mantle source. The data presented here, combined with those in the literature, suggest Ordovician magmatism mainly recycles preexisting crust with minor additions of juvenile mantle-derived material.  相似文献   

16.
In the northwest of the Sierras Pampeanas of Córdoba (Central Argentina), in the Tuclame area, rocks called ‘banded schists’ are recognized. They are known since 120 years ago and are one of the most important lithologies of the metamorphic complex in this region. The compositional banding is the most conspicuous structural mesoscopic feature, composed of quartz-rich and mica-rich layers. It is a tectonic banding produced by pressure solution during a compressive event. P–T conditions of 557 ± 25 °C and 3.9 ± 1 kb were obtained for the main metamorphic event. A detailed field checking allowed recognition of the banded schists as decimetric or centimetric xenoliths isolated within the regional migmatites and the anatectic granitoids and as kilometric-scale belts within Sierras de Córdoba and San Luis. The authors have also identified banded schists in the Sierras de Aconquija, Ambato, Ancasti and Guasayán. Other workers recognized them in the Puna, Cumbres Calchaquíes, Sierras de Quilmes and Fiambalá, among the most well known outcrops. The banded schists have systematic petrological features and a distinctive mesoscopic structure that allow their identification and correlation with the other outcrops, which are arranged as a huge belt 2000 km long and 150 km wide, between 64°00′–66°30′W and 25°00′–41°34′S. In this work, all these rocks are proposed to be integrated into the Puncoviscana Basin, since field evidence indicated that the banded schists transitionally pass by transposition to phyllitic rocks typical of this metamorphosed basin, which would cover a region of about 300,000 km2. At present, there is no accurate geochronology available for the metamorphic and deformation events proposed in this work for the Tuclame banded schists. However, considering the regional geological evidence, the great spread of the petrostructural process forming these rocks, the transition between the Puncoviscana Formation and the banded schists, and the earlier idea that the Puncoviscana Formation is the shallowest equivalent of deeper structural levels in the Sierras Pampeanas, we favor for the moment the hypothesis that the banded schists could be part of the oldest evolution of the Pampean orogeny (early Pampean stage) and could represent different structural levels of the same orogen, probably a late Precambrian–early Palaeozoic orogen. The events of migmatization and emplacement of anatectic granitoids could represent a late Pampean stage of early Palaeozoic age. Thus, the Pampean orogeny could have lasted around 30–40 Ma (570–530 Ma).  相似文献   

17.
The Granada ignimbrite, an Upper Miocene volcanic unit from the northern Puna, previously has been interpreted as an extensive ignimbrite (>2300 km2) associated with eruptions from the Vilama caldera (trap-door event). On the basis of new data, we revise its correlation and redefine the unit as a compound, high aspect ratio ignimbrite, erupted at approximately 9.8 Ma. Calculated volumes (100 km3) are only moderate in comparison with other large volume (>1000 km3) ignimbrites that erupted approximately 2–6 m.y. later in the region (e.g. Vilama, Panizos, Atana). Six new volcanic units are recognized from sequences previously correlated with Granada (only one sourced from the same center). Consequently, the area ascribed to the Granada ignimbrite is substantially reduced (630 km2), and links to the Vilama caldera are not supported. Transport directions suggest the volcanic source for the Granada ignimbrite corresponds to vents buried under younger (7.9–5 Ma) volcanic rocks of the Abra Granada volcanic complex. Episodes of caldera collapse at some stage of eruption are likely, though their nature and timing cannot be defined from available data. The eruption of the Granada ignimbrite marks the onset of a phase of large volume (caldera-sourced) volcanism in the northern Puna.  相似文献   

18.
Silicate and sulfide melt inclusions from the andesitic Farallón Negro Volcanic Complex in NW Argentina were analyzed by laser ablation ICPMS to track the behavior of Cu and Au during magma evolution, and to identify the processes in the source of fluids responsible for porphyry-Cu-Au mineralization at the 600 Mt Bajo de la Alumbrera deposit. The combination of silicate and sulfide melt inclusion data with previously published geological and geochemical information indicates that the source of ore metals and water was a mantle-derived mafic magma that contained approximately 6 wt.% H2O and 200 ppm Cu. This magma and a rhyodacitic magma mixed in an upper-crustal magma chamber, feeding the volcanic systems and associated subvolcanic intrusions over 2.6 million years. Generation of the ore fluid from this magma occurred towards the end of this protracted evolution and probably involved six important steps: (1) Generation of a sulfide melt upon magma mixing in some parts of the magma chamber. (2) Partitioning of Cu and Au into the sulfide melt (enrichment factor of 10,000 for Cu) leading to Cu and Au concentrations of several wt.% or ppm, respectively. (3) A change in the tectonic regime from local extension to compression at the end of protracted volcanism. (4) Intrusion of a dacitic magma stock from the upper part of the layered magma chamber. (5) Volatile exsolution and resorption of the sulfide melt from the lower and more mafic parts of the magma chamber, generating a fluid with a Cu/Au ratio equal to that of the precursor sulfide. (6) Focused fluid transport and precipitation of the two metals in the porphyry, yielding an ore body containing Au and Cu in the proportions dictated by the magmatic fluid source. The Cu/S ratio in the sulfide melt inclusions requires that approximately 4,000 ppm sulfur is extracted from the andesitic magma upon mixing. This exceeds the solubility of sulfide or sulfate in either of the silicate melts and implies an additional source for S. The extra sulfur could be added in the form of anhydrite phenocrysts present in the rhyodacitic magma. It appears, thus, that unusually sulfur-rich, not Cu-rich magmas are the key to the formation of porphyry-type ore deposits. Our observations imply that dacitic intrusions hosting the porphyry–Cu–Au mineralization are not representative of the magma from which the ore-fluid exsolved. The source of the ore fluid is the underlying more mafic magma, and unaltered andesitic dikes emplaced immediately after ore formation are more likely to represent the magma from which the fluids were generated. At Alumbrera, these andesitic dikes carry relicts of the sulfide melt as inclusions in amphibole. Sulfide inclusions in similar dykes of other, less explored magmatic complexes may be used to predict the Au/Cu ratio of potential ore-forming fluids and the expected metal ratio in any undiscovered porphyry deposit.Editorial handling: B. Lehmann  相似文献   

19.
During the last glaciation, most of the British Isles and the surrounding continental shelf were covered by the British–Irish Ice Sheet (BIIS). An earlier compilation from the existing literature (BRITICE version 1) assembled the relevant glacial geomorphological evidence into a freely available GIS geodatabase and map (Clark et al. 2004: Boreas 33, 359). New high‐resolution digital elevation models, of the land and seabed, have become available casting the glacial landform record of the British Isles in a new light and highlighting the shortcomings of the V.1 BRITICE compilation. Here we present a wholesale revision of the evidence, onshore and offshore, to produce BRITICE version 2, which now also includes Ireland. All published geomorphological evidence pertinent to the behaviour of the ice sheet is included, up to the census date of December 2015. The revised GIS database contains over 170 000 geospatially referenced and attributed elements – an eightfold increase in information from the previous version. The compiled data include: drumlins, ribbed moraine, crag‐and‐tails, mega‐scale glacial lineations, glacially streamlined bedrock (grooves, roches moutonnées, whalebacks), glacial erratics, eskers, meltwater channels (subglacial, lateral, proglacial and tunnel valleys), moraines, trimlines, cirques, trough‐mouth fans and evidence defining ice‐dammed lakes. The increased volume of features necessitates different map/database products with varying levels of data generalization, namely: (i) an unfiltered GIS database containing all mapping; (ii) a filtered GIS database, resolving data conflicts and with edits to improve geo‐locational accuracy (available as GIS data and PDF maps); and (iii) a cartographically generalized map to provide an overview of the distribution and types of features at the ice‐sheet scale that can be printed at A0 paper size at a 1:1 250 000 scale. All GIS data, the maps (as PDFs) and a bibliography of all published sources are available for download from: https://www.sheffield.ac.uk/geography/staff/clark_chris/britice .  相似文献   

20.
The Upper Miocene Cerro Morado Andesites constitutes a mafic volcanic field (100 km2) composed of andesite to basaltic andesite rocks that crop out 75 km to the east from the current arc, in the northern Puna of Argentina. The volcanic field comprises lavas and scoria cones resulting from three different eruptive phases developed without long interruptions between each other. Lavas and pyroclastic rocks are thought to be sourced from the same vents, located where orogen-parallel north-south faults crosscut transverse structures.The first eruptive phase involved the effusion of extensive andesitic flows, and minor Hawaiian-style fountaining which formed subordinate clastogenic lavas. The second phase represents the eruption of slightly less evolved andesite lavas and pyroclastic deposits, only distributed to the north and central sectors of the volcanic field. The third phase represents the discharge of basaltic andesite magmas which occurred as both pyroclastic eruptions and lava effusion from scattered vents distributed throughout the entire volcanic field. The interpreted facies model for scoria cones fits well with products of typical Strombolian-type activity, with minor fountaining episodes to the final stages of eruptions.Petrographic and chemical features suggest that the andesitic units (SiO2 > 57%) evolved by crystal fractionation. In contrast, characteristics of basaltic andesite rocks are inconsistent with residence in upper-crustal chambers, suggesting that batches of magmas with different origins or evolutive histories arrived at the surface and erupted coevally.Based on the eruptive styles and lack of volcanic quiescence gaps between eruptions, the Cerro Morado Andesites can be classified as a mafic volcanic field constructed from the concurrent activity of several small, probably short-lived, monogenetic centers.  相似文献   

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