Geochronology of Mexican mineral deposits. II: Veta Madre and Sierra epithermal vein systems, Guanajuato district

This paper presents two new high-resolution geochronological determinations for the epithermal deposits in the World-class Guanajuato mining district, in central Mexico. These are a Rb-Sr isochron age in illite at 28.47 ± 0.55 Ma for the Villalpando and San Juan de Dios low sulfidation veins of the Sierra group of veins, and a 40Ar/39Ar plateau age in adularia ("valencianite") at 30.20 ± 0.17 Ma for the La Valenciana ore shoot of the famous Veta Madre intermediate sulfidation vein. These determinations have greater accuracy, precision and trueness than the preexisting K-Ar determinations for similar adularia samples. The accuracy of such determinations supports the idea of a diachronic emplacement of intermediate and low sulfidation deposits in this district, the former being older than the latter, similar to other epithermal deposits in Mexico. Also, the ~2 m.yr. span between the Veta Madre and Sierra groups of epithermal veins is in agreement with other case studies, regardless of the size of the deposit

Diagenesis- and thermal maturity-evolution of the Silurian unconventional hydrocarbon deposits (Tassili n’Ajjer plateau, Algeria): Clay mineralogy, graptolite reflectance, and K–Ar dating

peer reviewedThe need for determining the thermal maturity of Lower Paleozoic rocks has increased with the intensified unconventional shale oil/gas resources exploration within North Africa, Arabian Peninsula, and the adjoining regions. Indeed, Lower Paleozoic strata represent an increasingly important resource that is related to the globally widespread occurrence of graptolite-bearing marine sediments in an expansive shelf area of the Gondwana. However, some crucial information for basin analysis remains poorly understood even across the largest hydrocarbon-producing province in Algeria. The present study provides a solid understanding of the diagenesis- and thermal maturity-evolution from the Tassili n’Ajjer plateau while involving multiple and independent approaches, such as clay mineralogy, petrography, graptolite reflectance, source rock maturity, as well as illite K−Ar geochronology. The combined use of X-ray analysis and field emission scanning electron microscope evidenced kaolinite, illite, and iron-rich chlorite as the main authigenic mineral phases. K−Ar data indicate that episodic in situ illite crystallization occurred at different times, the oldest illite at about 335 Ma and the youngest illite between 238 to 179 Ma, under diagenetic-to-hydrothermal conditions. Paleotemperature estimates (~165–232°C) derived from illite crystallinity (0.37–1.58 Δ°2θ), and graptolite reflectance (VReqv, 1.09–1.84%) values indicate deep diagenetic-low anchizone boundary conditions. This is broadly suggestive of late oil-to-dry gas zones of hydrocarbon generation and destruction, notably in the western sector of the study area. At least two heating events and diagenetic fluid flow processes are identified mainly in response to various tectonic events. They are largely due to fault reactivations, and migration of hot, potassium-rich, fluids throughout the Phanerozoic. Additionally, these events and processes had a later influence on the hydrocarbon maturation, migration, and/or entrapment, especially along the inherited N-S lineaments and Hoggar Massif mega-shear zones in the westernmost part of the Tassili n’Ajjer plateau. Lastly, it is important to highlight that the promising areas offering the highest potential for future unconventional hydrocarbon resource exploration could be most likely those bordering major lineaments, coupled with subsequent igneous activity, where unrestricted hydrothermal fluids are frequently reported, as well as the maturation, being much more advanced.Silurian succession from North Africa - Sedimentology, ichnology and thermal history for a new era of hydrocarbon exploratio

Geochemistry of fluid inclusions in travertines from Western and Northern Turkey: inferences on the role of active faults in fluids circulation

The understanding of the relationship between the geochemistry of fluids circulating during travertine deposition and the presence of active faults is crucial for evaluating the seismogenetic potential of an area. Here we investigate travertines from Pamukkale and Reşadiye (Turkey), sited in seismic regions and next to thermal springs. These travertines formed ~24,500–50,000 (Pamukkale) and ~240–14,600 years (Reşadiye) BP. We characterize fluid inclusions (FIs) and studied concentration of H2O, CO2, O2 + N2, and 3He, 4He, 20Ne, and 40Ar, and bulk composition (trace elements and δ13C‐δ18O). FIs from both localities are mainly primary with low salinity and homogenization temperature around 136–140 °C. H2O is the major component followed by CO2, with the highest gas content measured in Pamukkale travertines. Concentrations of Ne‐Ar together with O2 + N2 indicate that travertines from both areas precipitated from atmosphere‐derived fluids. The 3He/4He is 0.5–1.3 Ra in Pamukkale and 0.9–4.4 Ra in Reşadiye. Samples with R/Ra > 1 are modified by cosmogenic 3He addition during exposure to cosmic rays. Excluding these data, FIs of Reşadiye are mostly atmosphere‐derived. This implies a shallow formation where the circulation was dominated by meteoric waters, which is consistent with their young age. Instead, FIs of Pamukkale show mixing of mantle‐, crustal‐, and atmosphere‐derived He, indicating that these travertines formed in lithospheric fractures. Based on the δ13CCO2 and δ18O of bulk rocks, we infer that travertines formed involving crustal‐ (mechanochemical rather than organic) and mantle‐derived CO2. Trace elements of Pamukkale and Reşadiye show comparable rare earth element patterns. We conclude that travertines formed in response of seismogenetic activity.Published5473-54982T. Deformazione crostale attiva7T. Variazioni delle caratteristiche crostali e precursori sismici6A. Geochimica per l'ambiente e geologia medicaJCR Journa

Illite occurrences related to volcanic-hosted hydrothermal mineralization in the biga peninsula, NW Turkey: Implications for the age and origin of fluids

A different approach to investigate the origin of fluids, temperature conditions, age of hydrothermal activity of mineralization in the Biga Peninsula, (Koru, Tesbihdere and Kumarlar) employed mineralogical (illite Kübler index, b cell dimension, polytype) and geochemical (major, trace/REE, O-H stable isotope and Rb/Sr dating) methods. The Kübler Index (KI) values of illites indicate different temperature conditions, such as low temperature (high-grade diagenesis) for Koru deposit, and high temperature (anchizone) for the Tesbihdere and Kumarlar deposits. The textural, mineralogical and geochemical data from illites show that these have potential for estimating the age of hydrothermal activity and fluid characteristics. Both mineralogical (high grade diagenetic to anchizonal KI, 1 M polytype, low d060 values) and geochemical (similar major and trace element composition to host-rocks, low octahedral Mg + Fe contents, oxygen and hydrogen isotope composition) data are compatible with commonly known hydrothermal illites. Stable isotope data of illites are well matched to similar data from fluid inclusions, which indicate mainly magmatic fluids. The Rb/Sr age (22.4 ± 2.3 Ma: latest Oligocene and lowest Miocene) of the illites coincides with plutonic intrusions that are the main instigators of hydrothermal activities related to the extensional tectonic regime in the Biga Peninsula. The mineralogical and geochemical data of illites have some important advantages with respect to the use of fluid inclusions in determining δD of hydrothermal fluids thereby leading to better understanding ore-forming hydrothermal condition

Hydrogen and 40Ar/39Ar isotope evidence for multiple and protracted paleofluid flow events within the long‐lived North Anatolian Keirogen (Turkey)

We present a new approach to identifying the source and age of paleofluids associated with low‐temperature deformation in the brittle crust, using hydrogen isotopic compositions (δD) and 40Ar/39Ar geochronology of authigenic illite in clay gouge‐bearing fault zones. The procedure involves grain‐size separation, polytype modeling, and isotopic analysis, creating a mixing line that is used to extrapolate to δD and age of pure authigenic and detrital material. We use this method on samples collected along the surface trace of today's North Anatolian Fault (NAF). δD values of the authigenic illite population, obtained by extrapolation, are −89 ± 3‰, −90 ± 2‰, and −97 ± 2‰ (VSMOW) for samples KSL, RES4‐1, and G1G2, respectively. These correspond to δD fluid values of −62‰ to −85‰ for the temperature range of 125°C ± 25°, indistinguishable from present‐day precipitation values. δD values of the detrital illite population are −45 ± 13‰, −60 ± 6‰, and −64 ± 6‰ for samples KSL, G1G2, and RES4‐1, respectively. Corresponding δD fluid values at 300°C are −26‰ to −45‰ and match values from adjacent metamorphic terranes. Corresponding clay gouge ages are 41.4 ± 3.4 Ma (authigenic) and 95.8 ± 7.7 Ma (detrital) for sample G2 and 24.6 ± 1.6 Ma (authigenic) and 96.5 ± 3.8 Ma (detrital) for sample RES4‐1, demonstrating a long history of meteoric fluid infiltration in the area. We conclude that today's NAF incorporated preexisting, weak clay‐rich rocks that represent earlier mineralizing fluid events. The samples preserve at least three fluid flow pulses since the Eocene and indicate that meteoric fluid has been circulating in the upper crust in the North Anatolian Keirogen since that time.Key Points:Illite preserves the hydrogen isotopic signature and age of paleofluids in the earth's upper crustThree fluid events are pinpointed in the NAKThe NAF exploited zones of preexisting weak clay material during its formationPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/112210/1/ggge20754.pd

Geophysical and structural criteria for the identification of buried impact structures, with reference to Australia

The discovery of large asteroid impact structures, likely and possible impact structures, onshore and offshore the Australian continent (Woodleigh [120. km; ~. 360. Ma], Gnargoo [75. km; Lower Permian - upper Cretaceous], Tookoonooka [55-65. km; ~. 125. Ma], Talundilly [~. 84. km; ~. 125. Ma], Mount Ashmore [>. 100. km; end-Eocene] and Warburton twin structures [>. 400. km; pre-end Carboniferous]) requires re-examination of the diagnostic criteria used for their identification. Bouguer anomalies of established impact structures (Chicxulub [170. km; 64.98. ±. 0.05. Ma], Woodleigh impact structure and Gnargoo probable impact structure display a unique structural architecture where pre-impact structural ridges are intersected and truncated by the outer ring of the circular structure. Seismic reflection data outline circular central uplift domes, basement plugs and rim synclines. Sharp circular seismic tomography anomalies indicate low velocity columns under both the Woodleigh impact structure and Warburton probable impact, hinting at deep crustal fracturing. Deformed, curved and clouded intra-crystalline planar deformation features in quartz (Qz/PDFs), displaying Miller indices ({10-11}, {10-12}, {10-13}) diagnostic of shock metamorphism, abound around exposed established impact structures (Vredefort [298. km; 2023. ±. 4. Ma], Sudbury [~. 250. km; 1850. ±. 3. Ma], Charlevoix [54. km; 342. ±. 15. Ma], Manicouagan [100. km; 214. ±. 1. Ma]), Tookoonooka and Talundilly). Deformed Qz/PDFs allow recognition of shock metamorphism in buried impact structures, where original Qz/PDFs were bent, recrystallized and/or clouded during formation of the central uplift and hydrothermal activity triggered by the impact. Planar deformation in quartz can also occur in explosive pyroclastic units but are limited to Boehm lamella (Brazil twins) with single lamella sets {0001}. It has been suggested that a class of microstructures in quartz, referred to as metamorphic deformation lamella (Qz/MDL), occur in endogenic tectonic-metamorphic terrains. However, no type locality has been established for Qz/MDL of non-impact origin

Late Jurassic intraplate faulting in eastern Australia: a link to subduction in eastern Gondwana and plate tectonic reorganisation

Eastern Gondwana was subjected to subduction processes during the Middle-Late Jurassic, but how these processes affected intraplate deformation in eastern Australia is poorly understood. Here we present Ar/Ar, K-Ar, and Rb-Sr geochronological data from illitic clay-bearing fault gouges associated with the northern part of the 200 km long, N-striking, dextral strike-slip, Demon Fault in eastern Australia. We show a major range of geochronological ages at 162.99 ± 0.74–152.1 ± 4.8 Ma, indicating that the Demon Fault was active during the Late Jurassic. This period partially coincides with the Middle-Late Jurassic deposition of widespread ash-fall tuffs in the Clarence-Moreton, Surat, and Eromanga basins. We propose that Middle-Late Jurassic intraplate tectonism in eastern Australia was influenced by subduction processes farther east, which produced extensive calc-alkaline magmatism in New Zealand from ~170 Ma. A global plate reorganisation event, related to the development of Early-Middle Jurassic sea-floor spreading of the Pacific Plate, possibly acted as the driving mechanism responsible for the intensification of magmatism and intraplate faulting in eastern Gondwana

Elemental and K-Ar Isotopic Signatures of Glauconite/Celadonite Pellets from a Metallic Deposit of Missouri: Genetic Implications for the Local Deposits

In the course of attempting to date the host rocks of Viburnum metal deposits from the US state of Missouri, the purpose was here a detailed examination and contribution of the constitutive minerals of glauconite-rich pellets to the isotopic dating of these deposits. The glauconite pellets of Cambrian sediments hosting metal concentrates were dated here by the K-Ar method to complement earlier published Rb-Sr data. The study confirmed that the preparation and purification step of such glauconite pellets is especially critical with the need for a specific cleaning step to not only remove the detrital counterparts but also all Sr-rich components occurring as accessory minerals such as the carbonates, sulfates and oxides that apparently “contaminated” the Rb-Sr results. The K-Ar data and the previously released Rb-Sr results obtained on strictly the same glauconite-rich separates outline clear age discrepancies that can be summarized by higher, “older” K-Ar age data at about 440, 415 and 390 Ma, and lower, “younger” Rb-Sr data at about 400 and 370 Ma. The glauconite separates of most samples being apparently not contaminated by various detrital K-rich crystals, the two dating methods should have been affected similarly. The analytical dispersion seems, then, to result from a diagenetic event that affected the Rb-Sr system more than the K-Ar system by a plausible addition/subtraction of one or several Sr-rich and Rb-poor and, therefore, K-poor minerals. In turn, the studied pellets were apparently impregnated after deposition by flowing metal-rich fluids in a low-temperature environment not affected by a significant thermal impact. The Bonneterre Formation acted apparently as a regional drain for metal-rich fluids that percolated throughout the region at a probable burial depth of less than 2000 m

The evolution of the Cappadocia Geothermal Province, Anatolia (Turkey): geochemical and geochronological evidence (土耳其)卡帕多西亚地热区的演化:地球化学和地质年代学证据 Evolution de la province géothermale de Cappadoce, Anatolie (Turquie): Evidences géochimiques et géochronologiques Evolução da Província Geotérmica de Capadócia, Anatólia (Turquia): evidências geoquímicas e geocronológicas La evolución de la Cappadocia Geothermal Province, Anatolia (Turquía): pruebas geoquímicas y geocronológicas

Cappadocia Geothermal Province (CGP), central Turkey, consists of nine individual geothermal regions controlled by active regional fault systems. This paper examines the age dating of alteration minerals and the geochemistry (trace elements and isotopes) of the alteration minerals and geothermal waters, to assess the evolution of CGP in relation to regional tectonics. Ar–Ar age data of jarosite and alunite show that the host rocks were exposed to oxidizing conditions near the Earth’s surface at about 5.30 Ma. Based on the δO–δD relationhip, water samples had a high altitude meteoric origin. The δS values of jarosite and alunite indicate that water samples from the southern part of the study area reached the surface after circulation through volcanic rocks, while northern samples had traveled to the surface after interacting with evaporates at greater depths. REY (rare earth elements and yttrium) diagrams of alteration minerals (especially illite, jarosite and alunite) from rock samples, taken from the same locations as the water samples, display a similar REY pattern to water samples. This suggests that thermal fluids, which reached the surface along a fault zone and caused the mineral alteration in the past, had similar chemical composition to the current geothermal water. The geothermal conceptual model, which defines a volcanically heated reservoir and cap rocks, suggests there are no structural drawbacks to the use of the CGP geothermal system as a resource. However, fluid is insufficient to drive the geothermal system as a result of scanty supply of meteoric water due to evaporation significantly exceeding rainfall

Ultrasonic Shaking of Glauconite Pellets with Diverse Reagents for a Comparison of Their K–Ar with Already Published Rb–Sr Results

International audienceA combined ultrasonic treatment, with de-ionized H2O, dilute HAc or dilute HCl, of three Mid-Miocene glauconite samples was applied to K–Ar date the different separates in order to compare the results with those obtained by the Rb–Sr method using the same three samples and that were analyzed strictly in the same way. Two aliquots yield opposite elemental and K–Ar trends, which suggests different initial mineral compositions for the various pellets. The K–Ar data of two untreated and leached L7 and L8 aliquots are almost within analytical uncertainty from 17.3 ± 0.6 Ma to 19.6 ± 0.7 Ma (2σ), while those of the third L10 sample are slightly higher at 22.1 ± 1.2 Ma (2σ). Comparatively, the earlier published Rb–Sr ages of the three untreated samples and of the leached aliquots gave similar data for the L7 aliquots by an isochron at 18.1 ± 3.1 (2σ) Ma and for the sample L8 by an isochron with an age of 19.6 ± 1.8 (2σ) Ma, while the untreated L10 aliquot yields a very high Rb–Sr date of 42.1 ± 1.6 (2σ) Ma. This untreated L10 glauconite fraction contains blödite, a Sr-rich carbonate that impacted the two isotopic systems differently. Generally, dilute HCl or HAc acids dissolve carbonates, sulfates, sulfites and oxides, while they do not affect the clay-type crystals such as glauconites. These soluble minerals can be identified indirectly, as here, by X-ray diffraction and the amounts of leached Na2O, CaO and Fe2O3 contents. Together with the leaching of some metallic trace elements, those of NaO confirm the leaching of metals and of blödite that are both hosted by the glauconite pellets. The occurrence of this Sr-enriched mineral explains the age differences of the non-treated aliquots and suggests a systematic leaching of any glauconite separate before isotope determination and, possibly, a comparison of the Rb–Sr and K–Ar results. Ultrasonic shaking appears appropriate for physical disaggregation of any contaminating grains that may remain hosted within the pellets, even after a preliminary H2O wash, which may dissolve and remove the soluble minerals but not the H2O-insoluble silicates. The K–Ar study completed here as a complement to a previous Rb–Sr study highlights, again, the importance of the preparation step in isotopic studies of glauconite-type and, by extension, of any clay material, as all occurring minerals can interfere in the final age determinations and, therefore, differently in the mineral assemblages. All those not in isotopic equilibrium need to be removed before analysis, including the soluble Sr or alkali-enriched ones