Emplacement Of Intrusions Research Articles

Volcanic record of the arc-to-rift transition onshore of the Guaymas basin in the Santa Rosalía area, Gulf of California, Baja California

AbstractThe Gulf of California is an archetype of continental rupture through transtensional rifting, and exploitation of a thermally weakened arc to produce a rift. Volcanic rocks of central Baja California record the transition from calcalkaline arc magmatism, due to subduction of the Farallon plate (ca. 24–12 Ma), to rift magmatism, related to the opening of the Gulf of California (<12 Ma). In addition, a suite of postsubduction rocks (<12 Ma), referred to as “bajaites,” are enriched in light rare-earth and other incompatible elements (e.g., Ba and Sr). These are further subdivided into high-magnesian andesite (with 50%–58% SiO2 and MgO >4%) and adakite (>56% SiO2 and MgO <3%). The bajaites correlate spatially with a fossil slab imaged under central Baja and are inferred to record postsubduction melting of the slab and subduction-modified mantle by asthenospheric upwelling associated with rifting or slab breakoff. We report on volcanic rocks of all three suites, which surround and underlie the Santa Rosalía sedimentary rift basin. This area represents the western margin of the Guaymas basin, the most magmatically robust segment of the Gulf of California rift, where seafloor spreading occurred in isolation for 3–4 m.y. (starting at 6 Ma) before transtensional pull-apart basins to the north and south ruptured the continental crust. Outcrops of the Santa Rosalía area thus offer the opportunity to understand the magmatic evolution of the Guaymas rift, which has been the focus of numerous oceanographic expeditions.We describe 21 distinct volcanic and hypabyssal map units in the Santa Rosalía area, using field characteristics, petrographic data, and major- and trace-element geochemical data, as well as zircon isotopic data and ten new 40Ar-39Ar ages. Lithofacies include lavas and lava domes, block-and-ash-flow tuffs, ignimbrites, and hypabyssal intrusions (plugs, dikes, and peperites). Calcalkaline volcanic rocks (13.81–10.11 Ma) pass conformably upsection, with no time gap, into volcanic rocks with rift transitional chemistry (9.69–8.84 Ma). The onset of rifting was marked by explosive eruption of silicic ignimbrite (tuff of El Morro), possibly from a caldera, similar to the onset of rifting or accelerated rifting in other parts of the Gulf of California. Epsilon Hf zircon data are consistent with a rift transitional setting for the tuff of El Morro. Arc and rift volcanic rocks were then juxtaposed by normal faults and tilted eastward toward a north-south fault that lay offshore, likely related to the north-south normal faults documented for the early history of the Guaymas basin, prior to the onset of northwest-southeast transtenional faulting. Magmatism in the Santa Rosalía area resumed with emplacement of high-magnesian andesite lavas and intrusions, at 6.06 Ma ± 0.27 Ma, coeval with the onset of seafloor spreading in the Guaymas basin at ca. 6 Ma.The 9.69–8.84 Ma rift transitional volcanic rocks underlying the Santa Rosalía sedimentary basin provide a maximum age on its basal fill. Evaporites in the Santa Rosalía sedimentary basin formed on the margin of the Guaymas basin, where thicker evaporites formed. Overlying coarse-grained clastic sedimentary fill of the Santa Rosalía basin and its stratiform Cu-Co-Zn-Mn sulfides may have accumulated rapidly, coeval with emplacement of 6.06 Ma high-magnesian andesite intrusions and the ca. 6 Ma onset of seafloor spreading in the Guaymas basin.

Coupled hydro-mechanical evolution of fracture permeability in sand injectite intrusions

Sandstone “injectite” intrusions are generally developed by the fluidization of weakly cemented sandstones and their subsequent injection into fractured reservoirs. In this work, a continuum coupled hydro-mechanical model TOUGH-FLAC3D is applied to simulate the discrete fracture network in large-scale sand injectite complexes. A sand production constitutive model is incorporated to consider the formation of sand through plastic deformation and its influence on evolution of fracture permeability. Overpressures in the fluidized sand slurry drives the injection with sand dikes intruded upwards, typically into previously low permeability “tight” mudstone formations. The contrast in poroelastic properties of the underlying weak sandstone and overlying injectite receptor directly affects the evolution of fracture aperture both during and after intrusion. Fluid drainage into the unconsolidated matrix may reduce the extent of fracture aperture growth, through the formation of shear bands. The results of this work have broad implications related to the emplacement of sandstone intrusions and subsequent hydrocarbon accumulation, maturation and then production.

In Situ Multiple Sulfur Isotope and S/Se Composition of Magmatic Sulfide Occurrences in the Labrador Trough, Northern Quebec

AbstractThe interaction between mafic-ultramafic magma and crustal sulfide is considered a key process in the formation of magmatic Ni-Cu-platinum group element (PGE) sulfide deposits. Integrated S/Se and multiple sulfur isotope studies are the most robust in constraining the role of crustal sulfur during ore genesis. In the present study, we report the first integrated S/Se and multiple sulfur isotope study of magmatic sulfide occurrences in the Labrador Trough, namely, on the recently discovered Idefix PGE-Cu and Huckleberry Cu-Ni-(PGE) prospects.Whole-rock and in situ S/Se values (~810–3115) of magmatic sulfides and their host rocks are consistent with S loss during postmagmatic hydrothermal alteration, negating their use in interpreting the origin of S. Values of ∆33S ~0 indicate no record of the assimilation of Archaean sulfur. Disseminated (–0.5 to +2.5‰) and globular (3.0–4.5‰) sulfides at Idefix as well as globular sulfides (2.1–9.6‰) at Huckleberry have δ34S values greater than the accepted mantle range, suggesting that crustal S played a role in the formation of these sulfides. In contrast, disseminated and net-textured sulfides at Huckleberry have variable δ34S values (–4.6 to +3.2‰) that are mostly within the accepted mantle range, excluding one anomalous sample that records relatively higher δ34S values (11.9–15.0‰). It is proposed that sulfide melt segregated in response to the addition of small proportions of crustal S prior to the final emplacement of the host intrusions, i.e., in a feeder conduit or staging chamber. Isotopic exchange between the sulfide melt and silicate magma has diluted and, in places, eradicated a crustal δ34S signature.

Crustal domains in the Western Barents Sea

SUMMARY The crustal architecture of the Barents Sea is still enigmatic due to complex evolution during the Timanian and Caledonian orogeny events, further complicated by several rifting episodes. In this study we present the new results on the crustal structure of the Caledonian–Timanian transition zone in the western Barents. We extend the work of Aarseth et al. (2017), by utilizing the seismic tomography approach to model Vp, Vs and Vp/Vs ratio, combined with the reprocessed seismic reflection line, and further complemented with gravity modelling. Based on our models we document in 3-D the position of the Caledonian nappes in the western Barents Sea. We find that the Caledonian domain is characterized by high crustal reflectivity, caused by strong deformation and/or emplacement of mafic intrusions within the crystalline crust. The Timanian domain shows semi-transparent crust with little internal reflectivity, suggesting less deformation. We find, that the eastern branch of the earlier proposed Caledonian suture, cannot be associated with the Caledonian event, but can rather be a relict from the Timanian terrane assemblance, marking one of the crustal microblocks. This crustal block may have an E–W striking southern boundary, along which the Caledonian nappes were offset. A high-velocity/density crustal body, adjacent to the Caledonian–Timanian contact zone, is interpreted as a zone of metamorphosed rocks based on the comparison with global compilations. The orientation of this body correlates with regional gravity maxima zone. Two scenarios for the origin of the body are proposed: mafic emplacement during the Timanian assembly, or massive mafic intrusions associated with the Devonian extension.

The Miocene Brahma porphyry Cu-Mo prospect in Central Chilean Andes (35°45′S): Geology, geochronology (U-Pb, Re-Os) and geochemistry

The Brahma prospect is a Neogene porphyry copper deposit located in the Chilean Andes at ~ 35°45′S, beyond the southerly extension of the previously defined Mio-Pliocene metallogenic belt of Central Chile. We present the results of a comprehensive study of this prospect based on drill hole logging, petrography, geochemistry, and geochronology, with the aim of constraining its paragenesis and formation within the metallogenic background of the region. The prospect is hosted by dioritic and granodioritic equigranular units mineralized by fluids from at least two hydrothermal pulses in turn related to the emplacement of porphyritic intrusives and hydrothermal breccias. Existing exploration drill holes only intercepted the phyllic and argillic zone of the system, consistent with the paucity of potassic alteration and the dominance of pyrite, sphalerite, chalcopyrite and molybdenite mineralization. Zircon U-Pb determinations of the main igneous units present yielded ages of ~ 17 Ma, 14 Ma, and 9 Ma. Mineralization ages of ~ 14 Ma, 12 Ma, and 10 Ma were obtained by Re-Os determinations on molybdenite from samples of different paragenetic stages. These data indicate that (i) the prospect is composed of at least three main magmatic events, (ii) early mineralization is associated with the ~ 14 Ma magmatic pulse, and (iii) economic mineralization occurred at ~ 10 Ma and lasted until at least 9 Ma, as indicated by the presence of a younger mineralized porphyritic unit of this age. The obtained ages are comparable to those reported for the Los Pelambres deposit, a Cu-Mo porphyry located at 31°43′S and belonging to the Mio-Pliocene Metallogenic Belt of Central Chile. For the main intrusive units recognized in the Brahma prospect, whole rock geochemical analyses reveal arc affinities, juvenile signatures, and the involvement of amphibole in the fractionating assemblages. In a whole, our results support the existence of a complex deep magmatic reservoir that fed the magmatic-hydrothermal system for a long period, of at least 4 to 5 m.y., a common characteristic in other Miocene-Pliocene deposits in Chile and also in porphyry Cu deposits worldwide. In addition, our results highlight an interesting exploratory potential for the Brahma prospect, a long-lived Cu-Mo porphyry deposit of early Miocene age which southerly expands until ~ 36°S the Mio-Pliocene Metallogenic Belt of Central Chile.

Weighted Photolineaments Factor (WPF): An Enhanced Method to Generate a Predictive Structural Evidential Map with Low Uncertainty, a Case Study in Chahargonbad Area, Iran

Decreasing the uncertainty and increasing the prediction rate of evidential maps are two important objectives in mineral prospectivity mapping (MPM). In this paper, we proposed weighted photolineaments factor (WPF) to generate a continuous weighted structural evidential map with high prediction rate and low uncertainty in Chahrgonbad area. The WPF which has four components was defined as weighted sum of the values related to four following characteristics of lineaments: (a) number, (b) length, (c) number of intersection and (d) number of directional sets. We used the extracted lineaments from geological map and processing of aeromagnetic data in our analysis to enhance the prediction rate of WPF map. The process was implemented using an effective multistage algorithm including: texture analysis, phase symmetry analysis, non-maximal suppression, thresholding, skeletonization and line fitting process. Afterward, values related to various characteristics of lineaments were extracted and transformed by a logistic function to improve their predictive ability. Then, using the concentration-area fractal model and prediction-area plot, the prediction rate of WPF components was derived and their importance weights were calculated using normalized density. Finally, the WPF map was generated and evaluated using location of 18 known mineral occurrences in the study area. The results demonstrated a good concordance with our previous geological knowledge and experience about Cu mineralization in Chahrgonbad area. The number of lineaments directional sets and the number of lineaments were recognized as structural criteria with the highest prediction rate for Cu prospectivity. The generated WPF map has a higher prediction rate (80%) in comparison with each individual component and delineated 20% of the study area as prospective target. Eventually, three zones with high structural permeability which are fertile environment for emplacement of intrusions, circulation of magmatic-hydrothermal fluids and Cu mineralization were delineated on the WPF map and proposed for further exploration.

The Seismic Factor in the Excitation of Endogenous and Exogenous Processes on Klyuchevskoi Volcano, Kamchatka

The Klyuchevskoi stratovolcano is one of the largest and most active volcanoes in the world; changes of varying sizes have affected the morphology of its surface during historical time. These rock failures mostly occur in long-lived geologic features, namely, volcano-tectonic chutes. Several researchers have made forecasts of failures based on the relief and morphology of the volcano, the system of tectonic discontinuities within its edifice, the localization of parasitic eruptions, the emplacement of intrusions and dikes that have dissected the edifice, glacier activity on the slopes, and other factors. The part played by seismic activity in the preparation and occurrence of major rockfalls and landslides on the volcano has not been previously studied in its own right. The present paper contains an analysis of Klyuchevskoi seismicity related to eruption periods as a key factor to induce deformation and major rockfalls and landslides due to various causes. According to the spatial distribution of earthquakes and the associated seismic energy, the northwestern and southeastern sectors of the volcano are prone to major collapses. This appears to be due to an extension of the edifice in relation to a north-northeast striking fault that traverses the edifice.

Geochemistry and Geochronology of Southern Norilsk Intrusions, SW Siberian Traps

The Norilsk ore region is characterized by the occurrence of numerous intrusions comprising the PGE–Cu–Ni deposits. The Turumakit area, within the Southern Norilsk Trough, also contains many mineralized mafic intrusions of probably similar economic potential to the known Norilsk deposits. We study igneous rocks from three boreholes within the Turumakit area, sampling gabbro-dolerites and trachydolerites related to the Norilsk and Ergalakh complexes, as well as an outcrop of the Daldykan gabbro-dolerite intrusion. Our petrographical, mineralogical and geochemical data, as well as the U–Pb dating of extracted baddeleyites and zircons, primarily discriminate between the sub-alkaline rocks of the main Turumakit area and the Ergalakh trachydolerites located in the Norilsk and Talnakh ore junctions. Coarser grained Turumakit trachydolerites (with pegmatite segregations) contrast finer grained Ergalakh trachydolerites by having: (1) higher TiO2 (up to 5.5 wt %) compared with 2.2 wt %–3.3 wt % in the typical Ergalakh rocks; (2) low U, lower La/Yb and La/Sm ratios (5–7), in contrast to 8–10 ppm, 2.5–2.6 and 3.0–3.3, respectively, for the Ergalakh trachydolerites; and (3) their age was determined by U–Pb methods on baddeleyite and zircon (244.8 ± 2.7 Ma), and it appears likely that the mafic rocks traditionally attributed to the Ergalakh complex within the Turumakit area are younger than the Norilsk intrusions (250 ± 1.4 Ma). These data strongly indicate an emplacement of Turumakit intrusions during the end of a ~5 Myr magmatic evolution of the Norilsk district. It is therefore proposed that the sub-alkaline rocks of the Turumakit area belong to a separate intrusive complex within the Norilsk district.

From 2D to 3D Modeling of Mineral Prospectivity Using Multi-source Geoscience Datasets, Wulong Gold District, China

The Wulong gold district (WGD) is located at the northeastern margin of the North China Carton (NCC) on the Liaodong Peninsula, China. The results of early mineral exploration, mining, and metallogenic modeling research show that gold deposition was associated with lithospheric thinning of the NCC and the emplacement of voluminous Early Cretaceous intrusions. In this study, multi-source information on the ore formation and geology (including geophysical, geochemical, and remote sensing data) was used to construct two-dimensional (2D) prospectivity maps at multiple levels (surface (0 m), − 500 m, − 1000 m, − 1500 m, − 2000 m, − 2500 m, and − 3000 m) for subsurface mineral exploration in the WGD. The methodology employed consists of four steps. Firstly, three-dimensional (3D) geological and geophysical models were constructed using UBC-GIF and SKUA-GOCAD software. 3D geophysical modeling included 3D gravity and magnetic inversions, and 3D resistivity models were based on 2D magnetotelluric (MT) inversion. Secondly, multi-source, ore-forming, and geoscientific data were integrated using radial basis function link net modeling technology within ArcGIS-SDM. Thirdly, multiple-level 2D prospectivity maps were integrated using the discrete smooth interpolation method within the SKUA-GOCAD software to produce 3D prospecting models. Finally, posterior probabilities were classified to delineate 3D exploration targets using the concentration–volume fractal method (GeoCube software, v. 2.0). The results suggest that: (1) the Early Cretaceous intrusions are key ore-forming and ore-controlling objects within the WGD, and a potential target is the western part of the Sanguliu intrusion at depths from 1000 to 3000 m, in the zone where the Wulong and Yangjia gold deposits cross; (2) there are potential targets within 2500 m of the Wulong gold deposit; however, (3) there are no potential targets from 1000 to 3000 m in the Sigdaogou gold deposit.

Multistage gold mineralization events in the Archean Tati greenstone Belt, northeast Botswana: Constraints from integrative white mica Ar/Ar, garnet U-Pb and sulfides Pb/Pb geochronology

The Tati Greenstone Belt (TGB) in northeastern Botswana hosts numerous Au deposits (Shashe, Mupane and Signal Hill) associated with sulfides, garnet and white mica. In this study, integrative white mica Ar/Ar, garnet U-Pb and sulfides Pb/Pb dating techniques were combined with whole rock and sulfide Pb isotope characteristics to track the source(s) of gold and constrain the timeframes of gold mineralization events. All sulfides and arsenopyrite samples from the TGB yielded overlapping Pb/Pb errorchron ages of 2227 ± 66 Ma and 2220 ± 73 Ma, respectively, which coincide with the Shashe sulfides Pb/Pb errorchron age of 2250 ± 110 Ma. These relatively imprecise Pb/Pb dates suggest heterogeneity in the initial Pb isotope ratios of sulfides. At Mupane, whereas Au mineralization-associated hydrothermal almandine garnet yielded overlapping Tera-Wasserburg lower intercept 206Pb/238U age and a concordia age of 2119 ± 18 Ma (MSWD = 1.03) and of 2105 ± 24 Ma (MSWD of concordance = 1.10), respectively, sulfides produced an errorchron Pb/Pb age of 2873 ± 140 Ma, which despite the large error remains geologically meaningful as it coincides within error with the first Neoarchean Limpopo-Liberian Orogeny (2.70–2.65 Ga), granitoids intrusion emplacement (2.65–2.73 Ga) and deposition of banded iron formation (2.73 ± 0.15 Ga). Ore-related white mica from Signal Hill yielded an overlapping Ar/Ar plateau age of 1987 ± 24 Ma and a weighted mean Ar/Ar age of 1987 ± 13 Ma (n = 15, MSWD = 4.3), which coincide within error with the 2.05–1.95 Ga second Limpopo-Liberian tectonic cycle, herein considered to have possibly triggered the Au mineralization in this area. The obtained radiometric dates point to at least three well constrained gold mineralization events, with two of them possibly triggered by two different regional tectonic events. Lead isotope compositions of most of the sulfides overlap with those of spatially associated schists and granitoids, thus suggesting these units possibly represent Pb and by inference Au rock sources. The genetic model of the TGB Au deposits is consistent with many greenstone-hosted gold deposits worldwide, suggesting our results are not solely of local/regional interest, but can be used to characterize greenstone-hosted gold deposits worldwide.

Paleoproterozoic Iron Oxide Apatite (IOA) and Iron Oxide-Copper-Gold (IOCG) mineralization in the East Arm Basin, Northwest Territories, Canada

The Paleoproterozoic East Arm Basin of Canada hosts polymetallic vein, iron oxide–apatite (IOA), and potential iron oxide–copper–gold (IOCG) mineral occurrences, mainly associated with a belt of ca. 1.87 Ga intermediate-composition sills termed the Compton intrusions. Advances in our knowledge of the East Arm Basin and of IOA and IOCG deposits within the broader context of iron oxide and alkali-calcic alteration systems enables a new regional analysis of this mineralization and facilitates comparison of these mineral occurrences and host rocks to the nearby Great Bear magmatic zone IOCG districts. The Compton intrusions and co-magmatic Pearson Formation volcanic rocks are comparable in age and composition to intrusive plus volcanic rocks of the Great Bear magmatic zone that host IOA–IOCG mineralization. Taking into account fault displacements, emplacement of Compton intrusions and Pearson Formation volcanic rocks are also consistent with the architecture of modern arcs, supporting a direct relationship with the Great Bear subduction zone. Trace element patterns of uraninite contained in IOA occurrences of the East Arm Basin are also similar to the patterns of uraninite from the Great Bear magmatic zone occurrences, consistent with both regions having experienced similar iron oxide and alkali-calcic alteration and mineralization. Our new results indicate that exploration for IOA, IOCG, and affiliated deposits in the East Arm Basin should focus on delineating increased potassic-iron alteration types and fault/breccia zones associated with these systems through field mapping and application of geochemical, radiometric, magnetic, and gravity surveys.

Transgressive Sills and Lateral Lava Flows: On the Visual Observation of Igneous Sheets in Rugged Mountainous Terrains and the Optical Illusion Factor

Transgressive sills are of common occurrences in rift-related offshore and onshore sedimentary settings worldwide and have been reported in onshore volcanic settings in e.g. flood basalt provinces as well. General geometries of individual sills and of entire sill complexes too are well documented from seismic images in many offshore sedimentary settings of exploration interest, but limitations in seismic resolution may inhibit correct interpretations when it comes to small-scale structures and evidences on sill intrusion modes. Hence, relevant and important details and clues on intrusion mechanisms during emplacement of sheet intrusions, which may be unclear or missing in seismic images from offshore sedimentary basins, can to some degree be clarified by means of thorough visual examination and measurements on exposed onshore sills and sill complexes. In the actual study, relevant visual observations and measurements of the Streymoy Sill and its feeders in particular and to a lesser degree the Eysturoy Sill of the Faroe Islands, as well as some local lava flows, are scrutinised in order to demonstrate the importance of correct and detailed mapping, associated measurements and interpretations at exposed sill margins. The actual study chiefly focuses on potential optical illusion factors, not uncommon for sheet intrusions in rugged mountainous terrain, and potential associated misinterpretations. It is shown that unless due care is taken in assuring that sub-horizontal visual observations from some distance in such terrains are duly accompanied by other observations in the sub-vertical and/or sub-lateral plane at other angles as well, i.e. proper 3-D considerations, noticeable errors could result, when it comes to interpretations on intrusion mechanisms and possible regional stresses that prevailed during emplacement of such sheet-like igneous bodies.

Paleomagnetism of 1.79 Ga Pará de Minas mafic dykes: Testing a São Francisco/Congo-North China-Rio de la Plata connection in Columbia

Paleogeographic connections between São Francisco/Congo, North China and Rio de la Plata Cratons through the Paleoproterozoic have been proposed on the basis of geological and paleomagnetic data. We conducted new paleomagnetic analyses for mafic dykes of the 1790 Ma Pará de Minas swarm, occurring in the southern São Francisco Craton. The data revealed south/southwestern, low inclination magnetic directions after alternating field (AF) and thermal demagnetization, providing a paleomagnetic pole at 39.8°S, 196.8°E (A95 = 17.0°). The characteristic remanent magnetization is interpreted as a thermo-remanent magnetization carried by stable ‘pseudo-single domain’ magnetite grains, being acquired during the cooling of the dykes at 1790 Ma, as attested by a positive baked contact test performed on one of the dykes. The new pole in conjunction with 1790–1750 Ma paleomagnetic poles available for other blocks allowed an improved reconstruction of Columbia/Nuna supercontinent at this time frame. Laurentia, Baltica, Siberia, proto-Amazonia, West Africa and proto-Australia were positioned as in previously proposed models. The main difference from our reconstruction concerns the link of the São Francisco/Congo, North China and Rio de la Plata cratons, implying that much of the Columbia supercontinent had already agglutinated around 1850–1800 Ma ago. This configuration is consistent with the onset of eventual taphrogenic episodes over most blocks, accompanied by the emplacement of mafic dyke swarms and granitic intrusions at 1790–1750 Ma.

Porphyry-related polymetallic Au-Ag vein deposit in the Central City district, Colorado: Mineral paragenesis and pyrite trace element chemistry

The Colorado Mineral Belt is a ~500 km long segment of calc-alkaline to alkaline Laramide-age intrusions and related porphyry and polymetallic vein deposits. The Central City historic Au-Ag mining district in the Front Range of Colorado, USA, has been one of the most productive Au deposits in the Colorado Mineral Belt, with a total of 119.1 t of Au produced between 1859 and 1959. Development of NE-trending Laramide strike-slip faults and re-activation of pre-existing Proterozoic brittle structures in Proterozoic gneiss provided the structural controls for the mineralization of Au and Ag in re-opened pyrite-quartz and base metal veins. Previous studies have related ore formation in the district to the emplacement of Late Cretaceous-Paleogene Laramide porphyritic alkaline intrusions. It remains, however, unclear how to reconcile the Au-Ag enrichment of this deposit with the historic vein descriptions and the evolution of a deep magmatic-hydrothermal porphyry system. Here, we document the crosscutting relationships and the mineral paragenesis of the different vein types and use the trace element chemistry in pyrite to fingerprint Au mineralization and the succession of different veining styles in the Central City district. Based on comparisons to porphyry and polymetallic-style veins worldwide, a revised vein classification is proposed, which distinguishes porphyry, transitional pyrite-quartz, and polymetallic veining styles. These veining styles led to the development of an almost concentric district-wide metal zoning, comprising a central and an intermediate zone, both enriched in Cu-Au-Ag, grading into a peripheral zone enriched in Zn-Pb-Ag. Rare porphyry-style veins include K-feldspar veins and quartz veins with molybdenite ribbons. The transitional-style veins include abundant pyrite-quartz veins that crosscut the earlier formed porphyry-style veins, and disseminated pyrite accompanied by phyllic alteration. Formation of the Au-Ag-bearing polymetallic-style veins was associated with the brecciation and re-opening of pyrite-quartz veins. Based on mineral paragenetic relationships, the polymetallic-style base metal veins can be subdivided into four subtypes, comprising sphalerite-chalcopyrite veins, tetrahedrite veins, enargite-pyrite-quartz veins, and galena-sphalerite-carbonate veins. The textures, mineral inclusions and trace element concentrations in pyrite are distinct for the different vein types and associated alteration halos. Pyrite in pyrite-quartz veins is enriched in Co and Ni, and contains quartz inclusions. In contrast, pyrite in polymetallic base metal veins is characterized by elevated Au (≤20 ppm), Cu, As, Pb, Ag and Sb concentrations, and contains inclusions of electrum and galena. The observed veining styles, paragenetic relationships, and associated pyrite chemistry in the Central City district confirm that ore formation is porphyry-related. The Central City district also shares several similarities with Cordilleran polymetallic-style Cu-Zn-Pb-Ag vein deposits associated with Cu-(Mo) porphyries. However, the high Au and low Cu-(Mo) grades make Central City an example of a newly recognized porphyry-related polymetallic Au-Ag vein deposit type.

Tracking the sources and the evolution of the late Neoproterozoic granitic intrusions in the Brusque Group, Dom Feliciano Belt, South Brazil: LA-ICP-MS and SHRIMP geochronology coupled to Hf isotopic analysis

The central Domain of the Neoproterozoic Dom Feliciano Belt in the Brazilian state of Santa Catarina is mainly composed of a metavolcano-sedimentary sequence, the Brusque Group, and its crystalline basement, such as the Camboriú Complex. This association was intruded in its late-deformational stage by voluminous granitic magmatism, related to the metamorphic climax of the area. The intrusions share a distinct crustal affinity and are divided into three suites, from oldest to youngest, as indicated by field criteria: São João Batista, Valsungana and Nova Trento. We present seven new LA-ICP-MS and two SHRIMP U-Pb zircon ages, coupled with in-situ analysis of Hf isotopes, and use them to discuss the magmatic sources and evolution of the intrusions, and propose an evolutionary model for this plutonic association. Intrusion mostly took place at 615–585 Ma, but the field stratigraphy is not necessarily reflected by U-Pb ages, which establish considerable overlaps between intrusions from different suites. Inherited zircon crystals are common, and reflect a long geological history that matches the geochronological record of the Brusque Group and the Camboriú Complex. Hf ratios are remarkably widespread, but strengthen the characteristic crustal signature of the magmatism, and partially overlap with whole-rock isotopic data for the same rocks. We propose that the intrusions were caused by a regional heating event within the central domain of the Dom Feliciano Belt in Santa Catarina, with limited direct mantellic contribution and more significant indirect thermal contribution through the voluminous magmatism of the recently juxtaposed Florianópolis Batholith. This triggered large-scale partial melting of the local continental crustal and formed the studied granites. The long-lasting magmatic event promoted the protracted generation and emplacement of numerous distinct intrusions synchronically, leading to contrasts between the field and U-Pb stratigraphy. The suites’ varied geochemical and isotopic characteristics probably reflect a combination of the heterogeneities of their source rocks with the impact of different magmatic processes during granitogenesis and emplacement. While the Valsungana and Nova Trento suites were probably generated within a deeper crustal level and underwent some degree of magmatic interaction, the peraluminous São João Batista Suite has an upper-crust signature that indicates the assimilation of metasedimentary rocks.

Metasomatic Vein Formation by Stationary Fluids in Carbonate Xenoliths at the Eastern Margin of the Bergell Intrusion, Val Sissone, Italy

AbstractFluid flow is an important mechanism associated with heat and mass transport within the Earth’s crust. The study of veins, which represent channelling of fluids, can thus be key in understanding these fluid movements, unravelling fluid composition and origin, paleo stress regimes, and the history of the host-rock. New stable isotope data on carbonates and silicates have been combined with phase petrology, mass balance, and field observations to evaluate the formation mechanism of metasomatic reaction veins in dolomitic xenoliths in the Bergell tonalites (Val Sissone, Italy). Multiple generations of extensional veins can be followed from the contact zone between the dolomites and the intrusion to a few meters within either the tonalites (with the epidote–quartz veins), or within the dolomites, where they terminate. Each type of vein contains a central zone, which is formed by open fracture crystallization. This central fracture is framed by relatively symmetric replacement zones, where the original dolomite reacted to form either forsterite, diopside, tremolite or talc, all accompanied by calcite in either a succession of reaction zones or in simpler bi-mineral (silicate + calcite) veins. The δ18O and δ13C values across the veins allow temperatures to be estimated from different mineral pairs (silicate + calcite), and which confirm vein formation along a retrograde cooling path of the intrusion. At least four different fluid infiltration events are required, the first one around 555 °C to form the forsterite–calcite veins, followed by the epidote–quartz veins at temperatures around 430 °C, then the tremolite–calcite veins at around 390 °C, and finally the talc–calcite veins at around 140 °C. The shape of the δ18O and δ13C profiles, which are flat across the central part and the replacement zones of the veins (buffered by the intrusion), change substantially over short distances. Both of these isotope profiles overlap with the equally sharp mineralogical front between the veins and the unreacted dolomites. These profiles are interpreted to be the result of an isotopic exchange mechanism driven by dissolution and re-precipitation reactions. All veins are oriented perpendicular to the contact with the intrusive body, except for the late talc veins. Elevated fluid pressures, above the confining pressures caused by the regional and intrusion emplacement stress field, are suggested to be responsible for the initial fracturing of the carbonates and intrusive rocks. The contact zones between the tonalites and carbonates likely served as fluid conduits, where fluids accumulated and the pressure built up until hydrofracturing occurred. We propose that the veins formed through episodic pulses of highly reactive fluids that remained stationary during reaction, rather than a system where fluids flushed through the veins. Based on the XCO2-constrained mass balance, the formation of the veins would only require a relatively small amount of fluid, which could potentially originate from the intrusive rocks in vicinity of the xenoliths. Veining is not ubiquitous around the Bergell intrusion, suggesting that it only may have been a localized event and thus there is no need to involve a larger convective hydrothermal system for their formation.

Comparative field study of shallow rhyolite intrusions in Iceland: Emplacement mechanisms and impact on country rocks

Shallow silicic intrusions are known to exist in many active volcanoes and can fuel both eruptions and hydrothermal fields. However, our knowledge of magma intrusions remains far from complete, and processes occurring at intrusion margins are poorly understood. In this field-based study, we characterise four shallows, dissected rhyolitic intrusions at three sites in Iceland (Njarðvík-Dyrfjöll, Krafla and Húsafell central volcanoes). We focus on the relationship between intrusion emplacement mechanisms and country rock response, employing scanline mapping of fractures and in-situ rock property measurements (hardness and permeability) along transects from the intrusion margins to damaged and undamaged country rocks.We identify various scenarios of shallow intrusion emplacement style, based upon their diverse geometry and lithofacies architecture. Additional information from rock properties and characteristics of fractures and vesicles, indicates that initial country rock properties strongly influence the emplacement style. We identify two discrete types of country rock response to magma injection. The matrix permeability of weak, porous and permeable lithologies (conglomerate and hyaloclastite) is reduced by >1 order of magnitude adjacent to intrusions due to pore occlusion. Stronger and denser, low-permeability lithologies (basalt and welded ignimbrite) undergo a decrease in hardness by a factor >2 related to an up to fivefold increase in fracture density, with no significant change in matrix permeability.Our observations highlight the importance of robust characterisation of the mechanical properties of caldera-filling or geothermal reservoir formations, for appropriate forecasting of magma mobility, geophysical data interpretation, and geothermal resources characterisation.

Nickel-Copper Sulfide Mineralization in the Ntaka Hill Ultramafic Complex, Nachingwea Region, Tanzania

Abstract The Ntaka Hill Ni-Cu deposit in Tanzania lies within the Mozambique belt, a complex, long-lived Neoproterozoic suture zone that formed during the amalgamation of Gondwana. The deposit is hosted within the moderately deformed Ntaka Ultramafic Complex, which was emplaced at ~660 Ma, around 20 m.y. before the East African orogeny. Due to strain partitioning into amphibolites and metapyroxenites at the margins of the sills, magmatic cumulate textures are very well preserved. The Ntaka Hill mineralization is predominantly disseminated sulfides within orthopyroxenites and harzburgites. Sulfides show typical magmatic mineralogy and occur as isolated blebs and interstitial networks as well as in soft-walled segregation veins that form localized shoot-like high-grade zones. Deformation had relatively little effect on the deposition of sulfides other than minor localization of strain within some preexisting sulfide-dominant veins. Grade shells define elongate tubular channels that coincide broadly with the more magnesian cumulate rocks. Base and precious metal tenors are widely variable, ranging from 1 to 17% Ni and 0.5 to 10% Cu with no particular correlation with host-rock type, although the tenor range in the more magnesian cumulates is more restricted, with the low-tenor component (<3% Ni, <1% Cu) missing. Olivine compositions vary widely, with forsterite contents ranging from Fo84 to Fo89 with up to 0.85% NiO. These unusually high Ni contents are attributed to equilibration with high-Ni-tenor sulfides percolating through the intercumulus pore space. Complex zoning patterns in the Cr content of orthopyroxenes record fluctuating crystallization conditions attributed to wall-rock assimilation. The intrusion emplacement setting is interpreted to be derived from a flow of xenolith-, crystal-, and sulfideladen magma through channelized sills where much of the sulfide was incompletely equilibrated with the host magma, resulting in highly variable R-factors. Despite the widespread presence of graphite in the marginal pyroxenites, olivine-sulfide Fe/Ni partitioning records typical redox conditions that are slightly more oxidized than quartz-fayalite-magnetite (QFM)—further evidence of a lack of extensive large-scale equilibrium within the intrusive complex. The Ntaka Ultramafic Complex preserves an arrested stage of the amalgamation of small sulfide-bearing sill-sediment complex intrusions into larger convecting magma chambers, forming one end of a process continuum including Kevitsa (Finland) as an intermediate stage and Mirabela (Brazil) as the layered intrusion end member.

Seismic reflection and petrographic interpretation of a buried monogenetic volcanic Field (part 1)

Buried volcanoes occur in great numbers within sedimentary basins globally. Knowledge of ancient buried volcanic systems has improved significantly over the past two decades. The in-depth understanding of these buried systems was mainly possible due to increasing availability of high-quality seismic reflection and subsurface borehole data. This paper examines a cluster of Miocene volcanoes now buried by ca. 1000 m of sedimentary strata in the Canterbury Basin, New Zealand. These volcanoes were imaged by 2D seismic lines and perforated by the Resolution-1 borehole. We refer to this group of volcanoes and related intrusive bodies as the Maahunui Volcanic Field (MVF). Here, we present detailed petrographic and seismic reflection interpretation of some representative volcanoes of the MVF, and of the strata that enclose them, to constrain the environments in which intrusions and eruptions occurred. Intrusive rocks penetrated by the Resolution-1 comprise a monzogabbro body with a saucer-shape geometry emplaced in organic-rich sedimentary layers. The monzogabbro contains miarolitic cavities and ophitic textures which, together with decompaction of its overburdened sedimentary strata, suggest an emplacement depth around 950 m below the paleo-seafloor. Seismic lines show an array of faults at the tips of the saucer-shaped monzogabbro. These faults are connected with the root of some volcanoes and may have formed feeder systems for eruptions and hydrothermal fluids onto the Miocene paleo-seafloor. Volcaniclastic rocks comprise abundant glassy shards, relics of bubble walls, spheroidal fragments enveloped in a palagonite film, broken phenocrysts, and lithics. These volcaniclastic rocks are interbedded with lower bathyal siltstones, indicating that eruptions near the location of the Resolution-1 occurred in a deep-submarine environment (1000–1500 m). Integration of petrographic, geochemical, and seismic reflection interpretation suggest that the volcaniclastic rocks have a genetic relationship with the saucer-shaped monzogabbro, which may have served as a shallow stationary magma chamber for some volcanoes in the MVF. The available data indicate processes of intense material fragmentation and particle dispersion, consistent with phreatomagmatic eruptions, although globally this eruptive style is rarely interpreted to occur at water depths > 1000 m. The emplacement of intrusions into organic-rich sedimentary rocks could incorporate thermogenic gases into the magmatic system, providing supplementary driving forces to form large deep-water pyroclastic eruptions.

Low δ18O rocks in the Belomorian belt, NW Russia, and Scourie dikes, NW Scotland: A record of ancient meteoric water captured by the early Paleoproterozoic global mafic magmatism

The 2.45–2.38 Ga intrusions and their host rocks in the Belomorian belt of NW Russia and the Scourie dikes of the Lewisian complex in NW Scotland are marked by extremely low δ18O values. Their isotope signatures reflect high-temperature exchange between rocks and low δ18O meteoric waters representing a record of active hydrologic cycle early in the Earth’s history. Here we explore triple oxygen and hydrogen isotope systematics (δD, δ18O and Δ17O) of these rocks to evaluate the isotope composition of meteoric water. The new occurrences from Belomorian belt range in δ18O from −14 to −2‰; combined with previously reported data, ranging between −27 and +3‰, the low δ18O rocks are traced across 500 km of the Belomorian belt. Based on spatial distribution of the values, hydrothermal alteration driven by the emplacement of the mafic intrusions is the most consistent explanation for the low δ18O values recorded in the Belomorian belt. The Δ17O systematics reveals contribution of distinct meteoric waters with δ18O of −38 and −9‰ recorded at different localities across the belt. We also present new data for the 2.41–2.38 Ga Scourie dikes, with the lowest δ18O value of −2.5‰ recorded in the amphiboles of Loch na h dike. Unlike the Belomorian belt, the low δ18O signature of Scourie dikes requires igneous incorporation of low δ18O hydrothermally altered rocks through contamination of mantle-derived melts. The Δ17O data analyzed at the Loch na h dike reveals that the incorporated material experienced water-rock interaction with meteoric water that had δ18O of −35 ± 10‰. The exact mechanism for incorporation of such low δ18O rocks in the mafic Scourie dike melts is equivocal and could involve subduction or assimilation of low δ18O hydrothermally altered mafic rocks. These reconstructed isotope compositions of meteoric waters are diverse and comparable to the modern-day precipitation of high-latitude regions (e.g. Greenland, δ18O = −35‰) and precipitation of mid-latitude regions (−9‰), possibly reflecting spatial δ18O gradients resulted from an active hydrologic cycle during the cold climate of the early Paleoproterozoic, as depicted by previous isotope-enabled global circulation models of snowball Earth state.