Contrasting constraints on the temporal and spatial extents of normal faults from the Hilltop and Lewis mining districts, northern Shoshone Range, Nevada, USA

Vertical mapping of hydrothermal fluids and alteration from bulk conductivity: Simple interpretation on the USDP-1 site, Unzen Volcano, SW Japan

Exploring for structurally concealed Carlin-type mineralization: A case study from the northern Shoshone Range, Nevada, USA

Because major mineral deposits in north-central Nevada predate significant Basin and Range extension, a detailed understanding of the timing and kinematics of extensional faulting is necessary to place these deposits in their original structural context. The complexity of pre-Cenozoic deformation in northern Nevada makes restoring Basin and Range faulting difficult without locating well-dated, regionally extensive Cenozoic units that can be used to restore slip along normal faults. The goal of this study is to reconstruct extensional faulting in the Shoshone and northern Toiyabe Ranges by using Cenozoic rocks in and around the Caetano caldera, which formed ca. 33.8 Ma during eruption of the Caetano Tuff. The caldera filled with more than 4 km of intracaldera tuff during initial caldera-forming eruptions, and additional sedimentary and volcanic rocks subsequently filled the topographic depression left by the caldera collapse. These rocks are conformable over the interval 34–25 Ma, consistent with little, if any, extension during that time. The 34–25 Ma rocks were later cut by a set of closely spaced (1–3 km) normal faults that accommodated significant extension and foot-wall rotations of 40°–50°. Restored structural cross sections indicate that the present ∼42 km (east-west) width of the Caetano caldera has been extended 110%, resulting in 22 ± 3 km westward translation of the Fish Creek Mountains relative to the southern Cortez Range. Major normal faults mapped within the caldera continue south and north along strike into the surrounding Paleozoic basement rocks; therefore it is likely that parts of surrounding areas are also significantly extended. Miocene (16–12 Ma) sedimentary rocks in the hanging walls of major normal faults include both fluvial/lacustrine facies and coarser alluvial fan deposits. Where exposed, the bases of the Miocene sedimentary sections are in angular conformity with underlying ∼40°E tilted 34–25 Ma volcanic and sedimentary rocks. The distribution, composition, and geometry of these deposits are best explained by accumulation in a set of half-graben basins that formed in response to slip on basin-bounding faults. Extension thus appears to have taken place in the middle Miocene, beginning at or shortly after 16 Ma, and was mostly completed by 10–12 Ma. Fault blocks and basins formed during middle Miocene extension are cut by younger, more widely spaced, high-angle normal faults that began forming more recently than 10–12 Ma. These faults outline the modern basins and ranges in the study area and some have remained active into the Holocene.

Geochemical exploration models for sedimentary uranium deposits

The weathered layer has been shown to contain some of the greatest values of shear wave velocity anisotropy in the earth (Lynn, 1991, Crampin, 1990). The cause of the shear wave velocity anisotropy is most often either due to particle layering in unconsolidated sediments and sedimentary rocks or due to aligned weaknesses commonly manifest as aligned open fractures. The aligned open fractures can be vertical and cross cut bedding in sedimentary rocks or can be horizontal to sub-horizontal and are aligned with bedding. The purpose of this study was to measure the variation in properties associated with the vertical or sub-vertical fractures as these fractures often provide preferential pathways to fluid migration and can also influence recording of deeper seismic shear wave reflection data for oil and gas exploration.

Silver veins of Proterozoic age in the Cobalt area, Ontario, are associated with the thick Nipissing diabase sheet, and those at Great Bear Lake, N.W.T., are also, in part, associated with a diabase sheet. Host rocks in the Cobalt area are flat-lying Aphebian sedimentary rocks, steeply dipping Archean volcanic and associated sedimentary rocks, and diabase, while host rocks in the Great Bear Lake area are intermediate and felsic volcanic rocks and tuffs of the Echo Bay group.Model-lead ages for ordinary galenas are about 1,630 and 2,280 m.y. for the Great Bear Lake and Cobalt veins, respectively, whereas the pitchblende U-Pb age for the Great Bear Lake veins is about 1,445 m.y., and the apparent Rb-Sr age for the Cobalt veins is about 2,160 m.y. An Rb-Sr isochron age of 1,425 + or - 48 m.y. has been obtained for the diabase sill in the Port Radium area, Great Bear Lake.In the Cobalt area essentially identical lead isotope compositions were obtained for interflow base metal mineralization in Archean volcanic rocks, galena along bedding in sedimentary rocks of the Cobalt group, and some late-stage sulfide veins. One interflow sample with a more primitive composition has a model age of at least 2,805 m.y. Mineralization, including deposition of most of the lead in Archean interflow beds, is interpreted to have taken place at about 2,160 m.y. and to be genetically related to the Nipissing diabase. In addition, two anomalous lead lines are defined, one of which suggests an age of 3,260 + or - 100 m.y. for Archean volcanic rocks. A shallow line, represented in only late-stage sulfide veins, is difficult to interpret but could be due to the late addition of a radiogenic component during Paleozoic or younger events not normally considered as causing mineralization, or even from ground water.Leads from the Great Bear Lake area could represent a single anomalous lead line with a slope of 0.1085 + or - 0.0070, and thus have a maximum possible age of 1,130 m.y. Geological events younger than this are known in the Bear Province, but not in the vicinity of the veins, and the same problems of interpretation exist as for the Cobalt area.The source of lead is not defined in either area. However, a source with homogeneous lead is required and the lack of extensive wall-rock alteration suggests that the lead was not leached from the country rocks. The diabase is tentatively favored as the source, but a connate brine or other source is possible.

We describe and interpret a system of well-preserved normal and reverse faults in the Kayo Formation of the Miocene Shimanto belt, an exhumed accretionary complex exposed on Okinawa Island. The normal and reverse fault systems both strike NE-SW, suggesting systematic horizontal stress variations between compression and extension. Temperature and pressure conditions for the normal and reverse fault systems were estimated from the densities of water in fluid inclusions in the veins along the faults, and previously reported maximum paleotemperature based on values of vitrinite reflectance and illite crystallinity. The fluid inclusion analyses yielded similar estimates for water density in both normal and reverse fault systems. The minimum geothermal gradient was constrained to a narrow range of 40–50 °C/km. These results suggest that the normal and reverse fault systems developed at a similar depth within the seismogenic zone. This can be interpreted as a change between horizontal compression and horizontal extension occurring at a maximum depth of 3.8–7.5 km below the seafloor, assuming lithostatic fluid pressure. This 90° rotation of the principal stress could be controlled by the seismic cycle, as exemplified by the rotation of stresses that occurred after the Tohoku-Oki earthquake.

Remote sensing investigations combined with Geographical investigation systems (GIS) have become more important for the study of geological mineralization and structural geology like lineaments. Automatic extraction of lineaments from satellite imagery help in giving an overview of the tectonic events in the study area. The main objective of the study was to map hydrothermally altered rocks and geological structures that may be associated with mineral deposits in Mwitika-Makongo area. The study involves the use of Landsat- 8/OLI image. The satellite images were classified using ENVI 5.3 and ArcGIS 10.5. PCI geomatics 2016 was used for extraction of lineaments while Rose diagram was generated using Rockworks 16. Different remote sensing techniques, such as colour composite, band ratio and principal component analysis were applied to identify geological units and features related to economic mineralization. Colour composite band combination (5, 6, 7) showed hydrothermally altered geological units as blue. Band ratio combination (4/2, 6/7, 6/5) showed the areas that were hydrothermally altered as blue. Lineament mapping was done using PCA, with the first three PCs having the highest percentage of Eigenvalues. A lineament density map showed higher values in Makongo hill followed by Kalima Kathei hill. The structural trend from the rose diagram is in the NE direction. The results of image analysis and lineament extraction show that the economic minerals like iron ore are located in areas of hydrothermal alterations where intrusive rocks like Pyroxenite were found. Keywords: Colour composite, Geographical investigation systems, Landsat- 8/OLI, Lineaments, Principal component analysis, hydrothermally altered. DOI : 10.7176/JEES/9-11-03 Publication date: November 30 th 2019

Hummocky cross-stratification is a type of bedding in sedimentary rocks caused by the draping of sand around low-steepness three-dimensional bedforms. This distinctive form of stratification has been thought to be diagnostic of the former action of storm waves and hence its occurrence has been used to reconstruct ancient palaeogeographies and marine processes. Here I examine theoretical and empirical aspects of hummocky cross-stratification pertaining to its origin and argue that the notion that it was produced purely under progressive storm waves is incompatible with observed hummock spacings and possible values of the ratio a/λ under waves (a is the amplitude of oscillation near the bed and λ is the bedform spacing). My view is supported by recent observations on naturally-occurring hummocky megaripples from the sea floor of the inner Atlantic Shelf of North America.

Layering in rapakivi granite occurs locally in the islet Soderoren, Aland, SW Finland. The layering consists of 3—5 cm thick, rhythmically repeated layers with high concentrations of biotite, hornblende, allanite and olivine. Almost all layers are asymmetric with sharp lower contacts but gradually pass-ng into the overlying light layers. Many layers show cross-cutting structures resembling current bedding in sedimentary rocks. It is concluded that the layering is a gravity stratification probably due to convective magmatic currents in the roof zone of a granitic magma.

Colloidal clay of the montmorillonite-beidellite group extracted from the Putnam soil of Missouri was studied under the petrographic microscope while undergoing flocculation. The floccules were relatively coarse aggregates so well oriented as to resemble crystals. They were platy in shape, resembling in that respect the mineral in a bedded clay shale. It is suggested that bedding in shale may originate in part in the colloids contributed to it, that clay may be carried far seaward as a colloid and be flocculated and deposited in coarser floccules, and that clay partings and films in limestone may originate from flocculated clay colloids. The texture of fire clays is contrasted with that of shale.

The Main Ethiopian Rift (MER) is characterized by extensional tectonics and volcanism, associated with active hydrothermal systems, hydrothermal alteration and fumarolic deposits. The spatial distribution of these hydrothermal products and their link with faults and rock types provides important clues to what controls fluid flow in the subsurface. However, little is known about this in the East African Rift. We address this issue with a multidisciplinary approach in the Fentale-Dofan magmatic segment of the MER, an area characterized by intense volcanic and tectonic activity and a geothermal prospect. Primarily we conduct mapping of hydrothermal alteration and fumarolic deposits, and rock lithologies using a surface feature classification technique of multispectral satellite images. Then we interpret the map using a new database of faults and active hydrothermal manifestations such as hot-springs and fumaroles. We find that the surface hydrothermal alteration and deposits are mainly focused near Fentale and the Dofan Volcanic Complex (DVC). At DVC the hydrothermal products are focused on rhyolites on the western side of the volcano, in an area of intense NNE striking, rift parallel faults. At Fentale volcano the hydrothermal products are mainly associated with ignimbrite and show a circular pattern around the volcanic edifice, but also in places follow the NNE striking faults. At Fentale, the more complex association of hydrothermal products and active manifestations around the edge of the ignimbrite suggests formation contacts may also localize fluid flow in places. At both volcanoes the association between hydrothermal products with either the rhyolites and ignimbrites is likely due to them being relatively easily altered (in comparison to basalt), and also their brittle nature allows for fracturing through which localized fluid flow can occur (as opposed to the sediments). The general pattern of hydrothermal products suggests a stronger structural influence at the DVC with respect to Fentale. The presence of hydrothermal products and active hydrothermal manifestations, along with other lines of evidence such as locus of subsurface dike intrusion at the volcanic centres, suggest that discrete and localized magma reservoirs beneath Fentale and the DVC are the heat source for hydrothermal circulation. Our study also demonstrates that geology, including hydrothermal deposits, can be successfully mapped using automated remote sensing based classification.

New geologic mapping, structural studies, and geochronology of Miocene volcanic and sedimentary rocks in the southern Sierra Juarez, Baja California, shed light on the extensional history of the Gulf Extensional Province prior to sea-floor spreading in the Gulf of California. The southern Sierra Juarez is underlain by lower–middle Miocene rocks including fluvial strata, intermediate composition volcanic deposits, basalt lava flows and cinder cones, and dacite pyroclastic deposits and lavas that nonconformably overlie the Cretaceous Peninsular Ranges batholith. The 40 Ar/ 39 Ar geochronology indicates that basaltic rocks are 16.90 ± 0.05 Ma and dacite pyroclastic deposits are between 16.69 ± 0.11 Ma and 15.98 ± 0.13 Ma. These strata were subsequently cut by two generations of faults. First generation faults comprise a dominant set of north-south–striking, west-dipping normal faults, a secondary set of north-south–striking, east-dipping normal faults, and a lesser set of variably oriented strike-slip faults. All three fault sets are temporally and spatially related and were produced by east-west extension. The dominant west-dipping faults, which are antithetic to and oblique to the east-dipping Main Gulf Escarpment, may have been a precursor or an early phase accommodation zone along the escarpment. West-dipping normal faults are cut by a 10.96 ± 0.05 Ma dacite hypabyssal intrusion, thus bracketing the age of east-west extension between 15.98 ± 0.13 Ma and 10.96 ± 0.05 Ma. Hence, this faulting event clearly indicates a period of extension that predates the onset of oceanic rifting and even predates other dated Miocene extension within Baja California. Second generation faults, which are comprised of east-west–striking strike-slip faults that cut first generation faults and associated northwest-striking, northeast-dipping normal faults, may be related to early development of the Transpeninsular Strike-slip Province. Global plate reconstructions suggest that transtensional motion between the North American and Pacific plates along the western margin of Baja California began during middle Miocene time, coeval with east-west extension in the southern Sierra Juarez. This observation supports a hypothesis that middle Miocene transtensional plate motion was partitioned into two components: a strike-slip component parallel to active faults along the western offshore margin of Baja California, and an extensional component normal to the margin, but located in what is now the Gulf Extensional Province. Hence, the onset of extension within the circum-gulf region was in response to plate boundary processes.

Remote sensing (RS) technologies have emerged as efficient and cost-effective tools for the rapid identification of hydrothermal alteration zones, thereby supporting mineral exploration. The Zafarghand area in northeastern Isfahan Province, situated within the Central Iran structural zone of the Urmia–Dokhtar magmatic arc, hosts a porphyry system with multiple alteration types, including phyllic, propylitic, argillic, potassic, and silicic zones. This study aimed to develop an unsupervised and data-driven framework for mapping hydrothermal alteration using high-resolution satellite imagery. To achieve this, Sentinel-2 data were processed through a β-Variational Autoencoder (β-VAE) Deep Learning (DL) model, enriched by Discrete Wavelet Transform (DWT), and subsequently clustered with a Gaussian Mixture Model (GMM). The proposed approach successfully identified alteration zones and iron-bearing minerals (Fe²⁺, Fe³⁺, and iron oxides) at a 20-m spatial resolution. Validation using geochemical data showed classification accuracies of 94.74% and 95.74% for the phyllic and propylitic zones, respectively. In addition, buffered pixel-based validation using confusion matrices yielded overall accuracies of 94.5% and 86.9% for the phyllic and propylitic maps, confirming the statistical robustness of the proposed framework. Due to the limited distribution of argillic and silicic alteration, the depth of the potassic zone, and the lack of sufficient geochemical data for their validation, this study mainly focused on identifying phyllic and propylitic alteration along with iron oxides.These findings demonstrate that integrating DL, wavelet analysis, and probabilistic clustering with Sentinel-2 imagery provides a robust framework for detecting hydrothermal alteration zones. While the current study primarily focused on phyllic, propylitic, and iron-oxide zones due to limitations in data availability, the framework is transferable to other porphyry systems, provided that suitable reference data exist for validation.

منطقه مورد بررسی، بخشی از نوار ماگمایی ارومیه- دختر در زون ساختاری ایران مرکزی است که در جنوب غرب شهرستان بوئین‌زهرا و جنوب بخش دانسفهان قرار دارد و فاصله آن تا مرکز استان حدود 60 کیلومتر است. منطقه رامند، عمدتا شامل سنگهای آذرین ریوداسیتی و ریولیتی، توف ریوداسیتی، توف بلورین و گدازه‌های جریانی ریوداسیتی است. در این منطقه دگرسانی وسیعی به روشنی در پردازش تصاویر ماهواره‌ای قابل ردیابی و پی‌گیری است. حضورکانیهای رسی، بر وجود دگرسانی آرژیلیک در منطقه دلالت دارد؛ که می‌تواند نشانه خوبی به لحاظ کانی‌سازی باشد. وجود این دگرسانی در بازدیدهای صحرایی، مطالعه مقاطع نازک و آنالیز XRD نیز به‌خوبی مشخص است .واحدهای آذرآواری و آتشفشانی توسط گسلهای اصلی این منطقه شامل گسل کورچشمه، گسل حسن‌آباد و گسلهای فرعی، قطع شده‌اند. این گسلها در امتداد شمال‌غرب ـ جنوب‌شرق در منطقه وجود دارند و دگرسانیهای گسترده‌ای در طول گسلها و در سنگهای دیواره رخ‌داده است. دگرسانیها شامل دگرسانی آرژیلیک، سریسیتیک، پروپیلیتیک و سیلیسی هستند؛ که به‌صورت قائم و جانبی منطقه‌بندی نشان می‌دهند. بارزسازی زون‌های احتمالی دگرسانی در محدوده مورد نظر در مراحل اولیه، به‌کمک پردازش داده‌های سنجنده ETM به روش کروستا صورت گرفت. همچنین برای تفکیک زون دگرسانی آرژیلیک منطقه، از بین ره‌یافتهای مختلف، روش مؤلفه اصلی انتخاب شده است. علاوه‌بر هاله دگرسانی آرژیلیک، پدیده سیلیسی‌شدن سنگ میزبان دارای اهمیت بالایی بوده است و شرایط میزبانی فلزات پایه و گرانبها در درون این رخساره تأمین می‌شود. برای شناسایی دگرسانی سیلیسی، از داده‌های سنجنده ASTER استفاده شده که با اعمال روش نسبت باندی، نواحی با احتمال کانی‌سازی سیلیسی مشخص شده‌اند. نتایج مبتنی بر آنالیز دستگاهی (XRD)، تأییدکننده بررسیهای سنجش از دور است. با توجه به بررسیهای انجام شده در این منطقه، دگرسانیهای توأم رسی، اکسیدهای آهن و سیلیسی‌شدن سنگ دیواره برای پی‌جویی ذخایر فلزات پایه و گرانبها حایز اهمیت است.