Search Results (8)

Epithermal deposits represent a significant category of gold occurrences, with their subsurface structure playing a key role in reserve assessments. Fujian Province, characterized by extensive Mesozoic volcanic activities, stands out as a noteworthy region for shallow hydrothermal mineralization in China. This paper focus on the Youxi area within Fujian Province, employing the dual-frequency induced polarization method (DFIP) and controlled-source audio-frequency magnetotelluric method (CSAMT) to investigate the target ore. The DFIP results revealed predominant northeast-oriented zones with high polarizability and notable apparent resistivity. The CSAMT data were inverted using the SCS2D software. Two-dimensional resistivity profiles reveal a three-layer electrical structure, comprising subsurface banded rhyolites influenced by fault zones, intermediate-low resistivity sandstone layers, and deep-seated high-resistivity conglomerates. The resistivity gradient zones and highly polarizable locations align closely with known local faults. We interpreted these resistivity gradient zones as prospective target areas for mineralization, a hypothesis subsequently validated by drilling results. Combining geochemical analyses of epithermal gold deposits with the electrical resistivity structure, we propose an explanatory model for the mechanism of the formation of epithermal gold–silver deposits in the Youxi area. The magmatic hydrothermal fluids ascended along the fault, underwent convection-driven interaction with meteoric waters, and subsequently metasomatized the host rocks. This integrated approach provides valuable insights into the geological processes governing epithermal gold–silver deposit formation in the Youxi region. Full article

Porphyry and epithermal deposits are important sources of base and precious metals. Most actively mined deposits have been exhumed such that ore bodies are relatively close to the surface and are therefore locatable and economic to extract. Identifying and characterizing concealed deposits, particularly more deeply buried porphyry deposits, represents a far greater challenge for mineral exploration, and will become progressively more important as near-surface resources are gradually exhausted over time. We report high-precision ⁴⁰Ar/³⁹Ar dates for coarsely crystalline alunite that precipitated from magmatic steam in open fractures in Oligocene dacitic volcanic rocks, and a SHRIMP ²⁰⁶Pb/²³⁸U zircon date for one of several rhyolite dikes present at Alunite Ridge and Deer Trail Mountain, Utah. Both the magmatic-steam alunite and rhyolite dikes are related to concealed intrusions. The rhyolite dike yielded an age of 30.72 ± 0.36 Ma, which is older than a commonly cited 27.1 Ma age estimate for the Three Creeks Tuff Member of the Bullion Canyon Volcanics that is cut by the dike. ⁴⁰Ar/³⁹Ar data for samples of magmatic-steam alunite and sericite from six mines and prospects provide evidence for at least two periods of episodic hydrothermal activity at ca. 15.7–15.1 Ma and ca. 14.7–13.8 Ma, with the older and younger pulses of activity recorded at the more eastern and western sites, respectively. These two periods of hydrothermal activity are consistent with previous interpretations that Alunite Ridge and Deer Trail Mountain are underlain by two concealed porphyry stocks. ⁴⁰Ar/³⁹Ar analyses of individual bands in a sample of massive, centimeter-scale banded vein alunite yield indistinguishable ages with a weighted mean of 13.98 ± 0.12 Ma, consistent with a short-lived (≲250 ka) magmatic event with episodic vapor discharge recurring on short timescales (≲36 ka). ⁴⁰Ar/³⁹Ar geochronology of magmatic-steam alunite is a valuable tool to constrain the timing and duration of magmatic hydrothermal activity associated with unexposed intrusions and potentially porphyry deposits, and therefore may be useful in exploration. Full article

This study evaluates the geological attributes of rocks within the Abu Ghalaga area using spatial, geochemical, and petrographic approaches. ASTER and Landsat imagery processed using band ratio and principal component analysis were used to map hydrothermal alterations, while a regional tectonic evaluation was based on automated extraction of lineaments from a digital elevation model. Geochemical and petrographic analyses were then employed for discrete scale evaluation of alteration patterns of rocks across the study location. Based on satellite image processing, alteration patterns across the study area are widespread, while evidence from lineament analysis suggests a dominant NW–SE tectonic trend accompanied by a less dominant ENE–WSW direction. The different rock units exposed in the studied district are arranged chronologically from oldest to youngest as arc metavolcanic group (basalt and rhyolite), arc metagabbro–diorite, gneissose granite (granodiorite and tonalite), and dykes (aplite and felsite). Various types of igneous and metamorphic rocks have propylitic, phyllic, and argillic zones. Geochemical data indicate that the studied rocks are classified into granite, granodiorite, gabbroic diorite, and gabbro. Geochemically, the rocks have a sub-alkaline magma type. The granodiorite–tonalite is derived from the calc–alkaline magma nature, while gabbro and diorite samples exhibit tholeiitic to calc–alkaline affinity. The tectonic setting of the studied rocks trends toward volcanic arc granite (VAG). Based on petrographic, geochemical, and remote analyses, sericitization, chloritization, epidotization, kaolinitization, carbonatization, and silicification are the main alteration types present in the study area. As a result of lineaments analysis, the existing fractures and structural planes form valid flow paths for mineral-bearing hydrothermal solutions. Full article

The North Makran domain (southeast Iran) is part of the Makran accretionary wedge and consists of an imbricate stack of continental and Neo-Tethyan oceanic tectonic units. Among these, the Band-e-Zeyarat ophiolite consists of (from bottom to top): ultramafic cumulates, layered gabbros, isotropic gabbros, a sheeted dyke complex, and a volcanic sequence. Sheeted dykes and volcanic rocks are mainly represented by basalts and minor andesites and rhyolites showing either normal-type (N) or enriched-type (E) mid-ocean ridge basalt affinities (MORB). These conclusions are also supported by mineral chemistry data. In addition, E-MORBs can be subdivided in distinct subtypes based on slightly different but significant light rare earth elements, Th, Nb, TiO₂, and Ta contents. These chemical differences point out for different partial melting conditions of their mantle sources, in terms of source composition, partial melting degrees, and melting depths. U-Pb geochronological data on zircons from intrusive rocks gave ages ranging from 122 to 129 Ma. We suggest that the Band-e-Zeyarat ophiolite represents an Early Cretaceous chemical composite oceanic crust formed in a mid-ocean ridge setting by partial melting of a depleted suboceanic mantle variably metasomatized by plume-type components. This ophiolite records, therefore, an Early Cretaceous plume–ridge interaction in the Makran Neo-Tethys. Full article

Agate—a spectacular form of SiO₂ and a famous gemstone—is commonly characterized as banded chalcedony. In detail, chalcedony layers in agates can be intergrown or intercalated with macrocrystalline quartz, quartzine, opal-A, opal-CT, cristobalite and/or moganite. In addition, agates often contain considerable amounts of mineral inclusions and water as both interstitial molecular H₂O and silanol groups. Most agate occurrences worldwide are related to SiO₂-rich (rhyolites, rhyodacites) and SiO₂-poor (andesites, basalts) volcanic rocks, but can also be formed as hydrothermal vein varieties or as silica accumulation during diagenesis in sedimentary rocks. It is assumed that the supply of silica for agate formation is often associated with late- or post-volcanic alteration of the volcanic host rocks. Evidence can be found in association with typical secondary minerals such as clay minerals, zeolites or iron oxides/hydroxides, frequent pseudomorphs (e.g., after carbonates or sulfates) as well as the chemical composition of the agates. For instance, elements of the volcanic rock matrix (Al, Ca, Fe, Na, K) are enriched, but extraordinary high contents of Ge (>90 ppm), B (>40 ppm) and U (>20 ppm) have also been detected. Calculations based on fluid inclusion and oxygen isotope studies point to a range between 20 and 230 °C for agate formation temperatures. The accumulation and condensation of silicic acid result in the formation of silica sols and proposed amorphous silica as precursors for the development of the typical agate micro-structure. The process of crystallisation often starts with spherulitic growth of chalcedony continuing into chalcedony fibers. High concentrations of lattice defects (oxygen and silicon vacancies, silanol groups) detected by cathodoluminescence (CL) and electron paramagnetic resonance (EPR) spectroscopy indicate a rapid crystallisation via an amorphous silica precursor under non-equilibrium conditions. It is assumed that the formation of the typical agate microstructure is governed by processes of self-organization. The resulting differences in crystallite size, porosity, kind of silica phase and incorporated color pigments finally cause the characteristic agate banding and colors. Full article

This work advances research on the role of hurricanes in degrading the rocky coastline within Mexico’s Gulf of California, most commonly formed by widespread igneous rocks. Under evaluation is a distinct coastal boulder bed (CBB) derived from banded rhyolite with boulders arrayed in a partial-ring configuration against one side of the headland on Ensenada Almeja (Clam Bay) north of Loreto. Preconditions related to the thickness of rhyolite flows and vertical fissures that intersect the flows at right angles along with the specific gravity of banded rhyolite delimit the size, shape and weight of boulders in the Almeja CBB. Mathematical formulae are applied to calculate the wave height generated by storm surge impacting the headland. The average weight of the 25 largest boulders from a transect nearest the bedrock source amounts to 1200 kg but only 30% of the sample is estimated to exceed a full metric ton in weight. The wave height calculated to move those boulders is close to 8 m. Additional localities with CBBs composed of layered rock types such as basalt and andesite are proposed for future studies within the Gulf of California. Comparisons with selected CBBs in other parts of the world are made. Full article

San Basilio basin in Baja California Sur (Mexico) exhibits distinct styles of volcanism that interrupted phases of normal sedimentation correlated with the Zanclean Stage (Lower Pliocene). Sea cliffs around a 4-km² bay opening onto the Gulf of California are dominated by rhyolite, mudstone, sandstone, and limestone. Volcanism associated with re-sedimented hyaloclastite is regionally uncommon and the goal was to investigate interactions between volcanic events and intervals of stability represented by fossil-rich strata. Methods of study involved a combination of microfossil and macrofossil analyses. Relating the basin’s faults to Pliocene development in the greater Gulf of California was a secondary goal. Microfossils Bolivina bicostata and B. interjuncta recovered from mudstone indicate an initial water column of 150 m. An abrupt hydromagmatic explosion ruptured the mudstone cover, followed by banded rhyolite flows inter-bedded with sandstone. Outlying limestone beds with the index fossil Clypeaster bowersi are separated from rhyolite by conglomerate eroded under intertidal conditions. A renewed phase of activity saw eruption of smaller volcanoes in the basin center semi-contemporaneous with pecten limestone deposited on unstable slopes. Normal faults conform to a pattern of extensional rifting in the proto-gulf, followed by cross-cutting faults indicating the onset of transtensional tectonics beginning about 3.5 Ma. Full article

The western part of the island of Milos, Greece has undergone widespread, intense alteration associated with a range of mineralization, including seafloor Mn-Fe-Ba, sub seafloor Pb-Zn-Ag, and epithermal Au-Ag. The surrounding country rocks are a mixture of submarine and subaerial calc-alkaline volcanic rocks ranging from basaltic andesite to rhyolite in composition, but are predominantly andesites and dacites. The current surface spatial distribution of the alteration mineralogy is a function not only of the original hydrothermal, but also subsequent tectonic and erosional processes. The high relief and the excellent rock exposure provide ideal conditions to evaluate the potential of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite remote sensing data to identify and differentiate the different styles of alteration mineralisation. Laboratory spectral reflectance and calculated emittance measurements of field samples, supported by XRD analysis and field mapping, were used to support the analysis. Band ratio and spectral matching techniques were applied to the shortwave-infrared (SWIR) reflectance and thermal-infrared (TIR) emissivity imagery separately and were then integrated with topographic data. The band ratio and spectral matching approaches produced similar results in both the SWIR and TIR imagery. In the SWIR imagery, the advanced argillic, argillic and hydrous silica alteration zones were clearly identifiable, while in the TIR imagery, the silicic and advanced argillic alteration zones, along with the country rock, were differentiable. The integrated mineralogical–topographic datasets provided an enhanced understanding of the spatial and altitude distribution of the alteration zones when combined with conceptual models of their genesis, which provides a methodology for the differentiation of the multiple styles of alteration. Full article