Adjacent Wall Rocks Research Articles

Isotopic investigations of the Nova-Bollinger Ni–Cu–Co deposit in the Fraser Zone, Albany-Fraser Orogen, Western Australia

The Fraser Zone of the Albany-Fraser Orogen of Western Australia contains an extensive suite of mafic and mafic–ultramafic intrusions. This includes the Nova-Bollinger intrusions (ca 1300 Ma), which host economic Ni-sulfide mineralisation, a prime example of magmatic sulfide liquids emplaced into the deep crust under ambient granulite facies conditions. Sulfur, Sr and Sm–Nd isotopes, and trace elements have been used to evaluate the role of crustal contamination of the parental magmas to Nova-Bollinger and other sulfide-bearing mafic–ultramafic bodies within the Fraser gabbro suite. All ore types have δ34S values between +1.6 and +8.0‰ VCDT with a positively skewed distribution and a median value of +2.3‰ VCDT, anomalous relative to uncontaminated mantle values near 0‰ VCDT. No significant Δ33S anomalism was observed. The initial 87Sr/86Sr and 147Sm/143Nd ratios of most of the Nova-Bollinger igneous rocks have an age of formation of 1300 Ma, which is similar to those of CHUR1300Ma and record limited contamination by lower continental crustal material. The country rocks of the Snowys Dam Formation, locally enriched in iron and sulfur, are likely candidates as crustal contaminants. These metasedimentary sulfides show similar ranges in values for both δ34S and Δ33S to the ores, but their Sr isotopic range is greater than observed in the host intrusions at Nova-Bollinger and in most of the other sampled intrusions. Bollinger globular sulfides with ɛSr values of 63.1 and 144.5 and ɛNd values of −5.7 and −1.8 are suggestive of localised crustal contamination from adjacent wall rock marbles, but otherwise high ɛSr country rocks do not appear to have been dominant contaminants. Bulk assimilation of the country rocks, as modelled in both silicate and sulfide portions of the system, of up to 30% contamination of a primitive mantle melt by siliceous country gneisses, would have been sufficient to produce the degree of crustal contamination recorded in the Nova-Bollinger ores, and more regionally within the Fraser gabbros. Beyond indications that S was crustally derived, it is not possible to identify a specific source, but Snowys Dam sediments are the most likely. There is no compelling evidence for Archean S in the ores, unsurprisingly given the mid-Proterozoic age. It is likely that much of the contamination of the magma and generation of sulfide liquid took place during lateral flow through linked sill complexes within the deeply buried Snowys Dam metasediments, prior to the emplacement of the magmas to form the orebodies. KEY POINTS Nova-Bollinger Ni–Cu–Co sulfide ores have δ34S values between +1.6 and +8.0‰ VCDT, with a median value of +2.3‰ VCDT, anomalous relative to uncontaminated mantle values but similar to the range in the country rock metasediments. Values are consistent with equilibration of sulfide xenomelts at R factors of 200–700. There is no detectable mass-independent fractionation of δ33S in the Nova-Bollinger ores. The initial 87Sr/86Sr and 147Sm/143Nd ratios of most of the Nova-Bollinger igneous rocks at the age of formation of 1300 Ma are similar to those of CHUR1300 Ma and record contamination of a primitive mantle melt by up to 30% lower continental crustal material. Unusual globular ores at Bollinger have anomalous ɛSr indicating localised contamination from adjacent wall rock marble; however, this contamination is not pervasive through the orebody. It is likely that much of the contamination of the magma and generation of sulfide liquid took place during lateral flow through linked sill complexes within the deeply buried Snowys Dam metasediments, prior to the emplacement of the magmas to form the orebodies.

Outgassing through magmatic fractures enables effusive eruption of silicic magma

Several mechanisms have been proposed to allow highly viscous silicic magma to outgas efficiently enough to erupt effusively. There is increasing evidence that challenges the classic foam-collapse model in which gas escapes through permeable bubble networks, and instead suggests that magmatic fracturing and/or accompanying localized fragmentation and welding within the conduit play an important role in outgassing. The 2011–2012 eruption at Cordón Caulle volcano, Chile, provides direct observations of the role of magmatic fractures. This eruption exhibited a months-long hybrid phase, in which rhyolitic lava extrusion was accompanied by vigorous gas-and-tephra venting through fractures in the lava dome surface. Some of these fractures were preserved as tuffisites (tephra-filled veins) in erupted lava and bombs. We integrate constraints from petrologic analyses of erupted products and video analyses of gas-and-tephra venting to construct a model for magma ascent in a conduit. The one-dimensional, two-phase, steady-state model considers outgassing through deforming permeable bubble networks, magmatic fractures, and adjacent wall rock. Simulations for a range of plausible magma ascent conditions indicate that the eruption of low-porosity lava observed at Cordón Caulle volcano occurs because of significant gas flux through fracture networks in the upper conduit. This modeling emphasizes the important role that outgassing through magmatic fractures plays in sustaining effusive or hybrid eruptions of silicic magma and in facilitating explosive-effusive transitions.

Mineralogy and petrogenesis of fracture coatings in Athabasca Group sandstones from the McArthur River uranium deposit

ABSTRACT The McArthur River unconformity-related uranium deposit, located in the Athabasca Basin of Saskatchewan, Canada, is structurally hosted near the unconformity between Archean to Paleoproterozoic metasedimentary basement and the Proterozoic Athabasca Group sandstones. In this study, the mineralogy and geochemistry of fracture materials within the entire ca. 550 m thickness of the Athabasca Group sandstones and the metasedimentary (host) rocks from the McArthur River area were used to determine the paragenetic sequence and origin of minerals in and near the fractures. Our work sought to determine if the host minerals record elements associated with the uranium deposit at depth and if they could be used to guide exploration (vectoring). Fracture orientations indicate that most are moderately dipping (<50°) and provided permeable pathways for fluid movement within the basin, from below, and through the overlying sedimentary rocks. Many of the fractures and adjacent wall rocks record evidence of multiple distinct fluid events. Seven types of fracture fillings were identified from drill core intersecting the Athabasca Basin and present distinct colors, mineralogy, and chemical features. Brown (Type 1) and pink (Type 7) fractures host paragenetically late botryoidal goethite, Mn oxide minerals, and poorly crystallized kaolinite that formed from relatively recent low-temperature meteoric fluids, as indicated by poor crystallinity and low δ2H values of –198 to –115‰. These minerals variably replaced higher temperature minerals that are rarely preserved on the fractures or in wall rock near the fractures. Hydrothermal alteration associated with the mineralizing system at ca. 200 °C is recorded in assemblages of dickite, well-crystallized kaolinite, and spherulitic dravite in some white and yellow (Type 2) and white (Type 3) fractures, as reflected by the crystal habits and variable δ2H values of –85 to –44‰. Fibrous goethite in white and yellow (Type 2) and black and orange (Type 5) fractures and microfibrous Mn oxy-hydroxide minerals in black (Type 4) fractures also crystallized from hydrothermal fluids, but at temperatures less than 200 °C. White and yellow fractures (Type 2) containing fibrous goethite reflect fracture networks indicative of hydrothermal fluids associated with the mineralizing system during primary dispersion of pathfinder elements and therefore extend the deposit footprint. Brown (Type 1) and pink (Type 7) fractures have low δ2H values in botryoidal goethite and poorly crystallized kaolinite and are indicative of the movement of meteoric waters. Secondary dispersion of elements from the deposit to the surface on some fractures is evidence that fractures are pathways for element migration from the deposit to the surface, over distances exceeding ∼500 m.

Nature of the mineralizing fluids in the Balda and Motiya W-prospects, western India: Constraints from chemical and B-isotope composition of tourmaline

In the Proterozoic tungsten belts of Balda and Motiya in western India, tungsten mineralization is hosted in tourmaline-bearing quartz veins intrusive into pelitic schists and granites. Tourmaline is a ubiquitous phase in all rock types, and in this study, we use its major, trace element and B-isotope composition to constrain the nature of the tungsten (W)-bearing hydrothermal fluid and the processes involved in the precipitation of wolframite. We have also reconstructed the compositions of the W-precipitating fluids from mineral-fluid trace element partition coefficients that were extrapolated using the Lattice Strain Model. The tourmalines from both belts are of schorl composition and have high alkali, low Ca content, and moderate X-site vacancies. High Li, Mn, Zn, and Sn in the fluid in Motiya is suggestive of relatively saline fluid possibly derived from a granitic source. The high V/Sc ratios of tourmalines in the mineralized veins and wall-rock tourmalinites indicate an important role of biotite dissolution and fluid-rock interaction that contributed Fe–Mn for the precipitation of tourmaline and wolframite. The tourmalines in the mineralized veins at Balda (δ11Btur = −10.9 ± 0.7‰, 2σ; n = 10) and those in the associated granites (δ11Btur = −11.5 ± 0.7‰, 2σ; n = 6) have similar B-isotope composition, while those of the associated topaz granites and pegmatites (−13.9 ± 0.7‰, 2σ; n = 19) are isotopically lighter than the granites. The B-isotopic variation in the granite-pegmatite-vein system can be explained by fluid exsolution with the mineralized vein forming from exsolved fluid and the topaz-bearing granites and the pegmatites crystallizing from the residual melts. The δ11B of the mineralizing fluid is estimated to be ca. −7.4‰ at Balda and ca. −7.6‰ at Motiya, and were possibly derived from granites, consistent with extensive tourmalinization and muscovitization of the adjacent wall rocks, and high concentration of elements such as F, Li, B, Sn, Mn in the tourmalines and the reconstructed fluid. The chemistry and B-isotope composition of tourmalines in both the belts support the hypothesis that W-bearing hydrothermal fluid was primarily derived from a fractionated granitic source, and that precipitation of wolframite and tourmaline involved interaction of the granitic fluid with surrounding pelitic rocks.

The Alteration and Mineralization Characteristics of Miocene Porphyry Cu-Au Deposits of Chagai Magmatic Belt, District Chagai, Balochistan, Pakistan

Subduction related Miocene porphyry type deposits are found in the east-west trending Chagai magmatic belt (CMB) in Pakistan's western margin, Balochistan. This arc exists on the west segment of the Tethyan metallogenic belt in the south-west of Pakistan. Tethyan metallogenic belt is widely spread over 12,000 km from east to west direction from Indochina, Tibet, Pakistan, Iran, Turkey and Alpine mountain range in Europe. During the last thirty to forty years several porphyry deposits have been reported in the Chagai magmatic arc, including the very large Reko Diq H14-H15, large Saindak, Tanjeel, H35, H8 and medium Dasht-e-Kain porphyry deposits and many small porphyry copper deposits. These porphyry deposits were developed within the phase of calc-alkaline type magmatism in the Chagai arc. Tonalite, quartz diorite, and monzonite host the porphyry deposits within the adjacent sedimentary wall rock units of Sinjrani Volcanic Group, Juzzak, Saindak, and Amalaf Formations. The concentric zonal pattern of hydrothermal alteration in these porphyry deposits of the Chagai magmatic arc follows the world's major porphyry deposits' alteration pattern. Zones of hydrothermal alteration from distal to proximal part includesa potassic alteration, sericitic-clay-chlorite alteration, sericitic alteration, argillic alteration and propylitic alteration. Major ore mineralization in these deposits is of copper, gold, silver, molybdenum, and minor constituents of other base metals that have been reported to occur within hydrothermal alteration zones in the Miocene porphyry Chagai magmatic arc

Element enrichment/depletion during faulting in shale-rich surface exposures of the 2008 Wenchuan earthquake (Mw 7.9) and implications for coseismic temperature

The coseismic frictional temperature is increasingly recognised as an important factor for the transformation mechanism in shallow crustal faulting. We report changes in the mineralogical and geochemical compositions across the fault zone in the Shaba (SB) exposure of the 2008 Wenchuan earthquake (Mw 7.9), which contained the maximum value of vertical displacement and cut through shale-rich strata by the principal slip surface. Assuming Yb to be an immobile trace element, with respect to those in the adjacent damaged zone and wall rocks, Li, Rb, Cs and most light rare earth elements (LREE) exhibited marked enrichments of 143%, 247%, 475% and >60%, respectively; and a 40% depletion of Sr and <5% depletion of most heavy rare earth elements (HREE) was found, responding to mineralogical transformation such as alteration and dissolution of feldspar and formation of abundant clay minerals. In addition, heterogeneous adsorption of clay minerals could result in fractionation between and within LREE and HREE as well as negative Eu anomalies and positive Ce anomalies. The element enrichment/depletion model of the northern segment of the Yingxiu–Beichuan (YX–BC) fault zone was quite different from the middle/southern segments characterising faults in carbonate and granitic rocks. Through model calculation, the calculated trace element compositions (Li, Rb, Sr, Cs, Ba, La, Sm and Pb) of the fault gouge were obtained at 250 °C, 300 °C and 350 °C, and were produced by frictional heating. By comparing with the observed values, we suggest that high temperatures derived from frictional heating or due to buffering of the slip zone temperature by evaporation of pore water during the earthquake within the principal slip surface in the northern segment of the YX–BC fault zone may not have existed. Mechanical crushing and chemical alteration (not under high-temperature conditions) within the fault zone during the coseismic stage may result in mass removal and element enrichment/depletion. KEY POINTS Mineralogical and geochemical compositions were reported in the Shaba exposure of the 2008 Wenchuan earthquake. Li, Rb, Cs and most LREE exhibited marked enrichments in the fault gouge. Fractionation between and within LREE and HREE, negative Eu anomalies and positive Ce anomalies occur in the fault gouge. High temperatures may not have existed within the principal slip surface of the northern segment of the YX–BC fault zone.

High‐Resolution Numerical Modeling of Heat and Volatile Transfer from Basalt to Wall Rock: Application to the Crustal Column beneath Long Valley Caldera, CA

AbstractWe present a high‐resolution numerical model of the thermal evolution of the crustal column beneath Long Valley caldera, California, from which &gt;800 km3 rhyolite erupted over the last 2.2 Myr. We examine how randomly emplaced basaltic sills of variable thickness (10, 50, or 100 m) at various depth intervals (10–25 km) and at variable emplacement rates (5–50 m/kyr) gradually heat the crust and lead to a variably mixed crustal lithology (solidified mafic sills and preexisting granitoid). We additionally explore the time scales over which dissolved water (~3 wt%) in a newly emplaced basaltic sill exsolves during crystallization and is transferred to adjacent wall rock that is undergoing partial melting. We employ a finite‐difference‐based technique, with variable spatial (≥1 m to ≥10 km) and temporal (&lt;100 and &gt; 106 years) resolution, that enables dense analysis within and directly adjacent to a newly emplaced sill. Our results show that once ambient crustal temperatures reach ~500–600 °C, subsequent injections of basaltic sills lead to significant partial melting of adjacent wall rock (granitoid and solidified mafic sills) on time scales (101–102 years) that match those of exsolution of H2O‐rich fluid from basaltic sills. Large volumes of fusion (&gt;10%) during fluid‐undersaturated partial melting, combined with the preexisting occurrence of aplite dikes, facilitates the development of melt‐filled fractures that exceed the critical length for self‐propagation. The advection of wall‐rock partial melts (with a combined mantle‐derived and crustal geochemical signature) to shallower depths will alter both the thermal and compositional profile of the middle‐upper crust.

Direct evidence for fluid overpressure during hydrocarbon generation and expulsion from organic-rich shales

Abstract Fluid overpressures are widely expected during hydrocarbon generation and expulsion from source rocks, yet direct evidence for this phenomenon is lacking in the case of organic-rich shales. Here we show that formation of bed-parallel fibrous calcite veins in mature laminated organic-rich shales in the Eocene Dongying depression, Bohai Bay Basin, east China, occurred in direct response to fluid overpressure due to hydrocarbon generation. The evidence for overpressure is recorded by coexisting primary aqueous and petroleum inclusions in the calcite fibers. Our results show that all analyzed fluid-inclusion assemblages record variable degrees of overpressure during vein dilation, ranging from only modestly in excess of hydrostatic, to approaching and perhaps exceeding lithostatic. Thus, our results indicate that fluid pressures during dilation of horizontal veins are not necessarily equal to the opposing force of overburden throughout the history of opening. This suggests that at least some of the vein dilation is accommodated by concomitant narrowing of the adjacent wall-rock laminae, likely by scavenging (dissolution and reprecipitation) of CaCO3 from the adjacent wall rock.

Structures and Implications for Fluid Migration in the Jiadi Carlin‐Type Gold Deposit, Guizhou Province, Southwest China

AbstractThe Jiadi gold deposit is a newly discovered Carlin‐type gold deposit in the Guizhou Province, Southwest China. This deposit is structurally controlled by a shallow fold–fault superimposed system along the Lianhuashan trend. Field geological investigations, structural analysis, and mathematical research are conducted to study its structures and hydrothermal fluid flow process. Geological investigations (i.e. sections, stope, and drill hole) indicate that the structures are dominated by NE‐trending folds, NWW‐trending folds, and faults. A trend‐surface analysis of the low interformational fracture zone suggests that the overall distribution of the Lianhuashan trend is controlled by the NE‐trending Lianhuashan anticline. Nearly all primary Carlin‐type gold deposits are distributed along the southeastern flank where the fold axis changes from NE to EW. Gold orebodies are hosted by the interformational fracture zones and primarily situated at the transitions from the high‐value areas to the low‐value areas of the interformational fracture zones. A stress analysis on the hydrothermal veins indicates that they are of tenso‐shear properties. The ore‐forming elements (Au, As, Sb, Hg, and Tl) of the hydrothermal veins from the interformational fracture zones and intrusive breccia body present strong positive anomalies compared with those from the adjacent wall rocks. According to the development patterns of the ore‐forming elements and hydrothermal veins, a migration process of the ore‐forming fluids is proposed: the deeply sourced ore‐forming fluids migrated vertically to shallow crust along an intrusive breccia body, subsequently flowed horizontally along the interformational fracture zones accompanied by gold precipitation in the early stage, and finally migrated outward along steep micro‐fractures during following stages.

In Situ Stress and Pore Pressure in the Deep Interior of the Nankai Accretionary Prism, Integrated Ocean Drilling Program Site C0002

AbstractStrength and slip behaviors of subduction megathrusts are controlled by the physical properties and stress state not only of the fault zones themselves but also the adjacent wall rocks in overriding and subducting plates. In the Nankai Trough, a 3 km deep riser borehole at Integrated Ocean Drilling Program Site C0002 was drilled into the Kumano fore‐arc basin and the underlying accretionary prism. We quantify the full in situ stress tensor and pore pressure at Site C0002 by integrating drilling data, geophysical logging, and downhole measurements using empirical relations that describe sediment compaction behavior as a function of loading path. The Kumano Basin is loaded in uniaxial vertical strain conditions, whereas the prism below is loaded with greater differential stress, has a modestly elevated pore pressure, and lies in a strike‐slip faulting regime. This can be reconciled with slip on the megathrust fault if the horizontal stresses vary with time or depth and/or if shear stress along the megathrust is low.

The mechanisms and time scale of alteration halos in vein-type tungsten deposits in southern China

Alteration halos are the fingerprints left by hydrothermal flow in rocks and indicative of ore forming processes. The Nanling Range in southern China is a world-class tungsten province. However, it is poorly understood why alteration halos normal to veins in many tungsten deposits in the area decrease with increasing depth, and the time it takes to form the halos. Finite element based numerical experiments were conducted to investigate hydrothermal flow and species diffusion processes from fractures to adjacent wallrock in the tungsten deposits. The vertical distribution of alteration halos is influenced by wallrock porosity, fracture permeability, and the duration of hydrothermal flow in fractures. When hydrothermal fluids flow upwards, wider alteration halos are formed at deeper levels, when the wallrock has a constant porosity for a depth of around one kilometre. This is inconsistent with the characteristics of alteration halos in the tungsten deposits in southern China. Alteration halos in the tungsten deposits could be formed when the wallrock porosity decreases with increasing depth. The silicification and greisenization of the tungsten deposits could form over a period from tens of years to a few thousand years. Such a period is much shorter than the uncertainties of existing isochronal ages and may represent the duration of mineralizing pulses rather than all the geological events associated with tungsten mineralization. Tungsten diffusion from fractures to adjacent wallrock cannot produce large-scale disseminated tungsten mineralization during such a short period of time.

Influences of fluid properties on the hydrothermal fluid flow and alteration halos at the Dajishan tungsten deposit, China

The Nanling Range located in South China is a world-class tungsten province. Those tungsten deposits are spatially and temporally associated with the granitic magmatism during the Late Mesozoic (J2-K2). It is common that alteration halos decrease with increasing depth at the vein-type tungsten deposits. However, the mechanisms forming the alteration characteristics are still enigmatic. In this contribution, we investigate the influences of variable fluid density and viscosity on the fluid flow and species diffusion from fractures to adjacent wallrock at the Dajishan tungsten deposit using finite element based numerical experiments. Fluid density has a negative influence on the transient pressure-driven fluid flow at Dajishan. Large-density fluids slow fluid flow and heat transfer. High-viscosity fluids flow slowly. Variations of fluid viscosity at Dajishan are large and influence hydrothermal fluid flow and species diffusion from fractures to surrounding wallrock. Depth-dependent porosity and permeability of wallrock are effective mechanisms forming the alteration halos at Dajishan. Hydraulic and geometric characteristics of joint zones influence diffusion of chemical species at the initial stages of hydrothermal fluids injected into fractures. High-permeability narrow joint zones favor formation of thick alteration halos at shallower levels. Depth-dependent wallrock porosity also impedes diffusion of metals from fractures to adjacent wallrock increasing ore formation efficiency at Dajishan.

Archean turbidite hosted orogenic gold mineralization in the Gadag greenstone belt, Western Dharwar Craton, Peninsular India

Turbidite hosted orogenic gold mineralization in the Archean Gadag greenstone belt of the Western Dharwar Craton, forms a major auriferous zone (Central Auriferous Zone) extending over a strike length of about 12km in the Gadag duplex. The turbidite sequence comprises thick inter-bedded, medium to coarse grained lithic graywacke and thin laminated layers of fine grained carbonaceous phyllite. Gold bearing quartz veins impregnate preferentially along the en-echelon shear planes, fractures and schistosity planes. Auriferous quartz veins are enveloped by the altered wall rocks.Mineralogy of the auriferous zone is dominated by gangue minerals like quartz, ankerite, chlorite, sericite and carbonaceous matter, with subordinate plagioclase. Monazite and xenotime are the important accessory minerals. Arsenopyrite and pyrite are the major sulfide minerals, but pyrrhotite, chalcopyrite, sphalerite, galena and scheelite are also present. Gold in native state occurs within quartz, silicates and arsenopyrite.Notable distinctions in mineral assemblage, texture and in chemical compositions of altered wall rocks compared to the precursor host rock in the study area implies that the metasomatism and wall rock alterations are the results of pervasive infiltration and intense interaction between hydrothermal fluids and the surrounding host rocks over a prolonged period.Sulfides, carbonates, carbonaceous matter, K2O, MgO, CaO, Cr, Ni, Cu, Pb, Zn, As and higher values of gold (0.98–4.72ppm) are added into the altered wall rocks, immediately enveloping the auriferous quartz vein bodies. The chondrite normalized REE pattern of altered wall rocks exhibits enriched LREE (LaN/YbN=av. 9.54), with prominent negative Eu anomaly. The observed variation in geochemical characteristics and mineral assemblages in the alteration zones indicates differential response of the host rock and intensity of alteration depending on the composition of host rocks and hydrothermal fluids.The auriferous hydrothermal fluids were of low salinity (2.0 to 6.6wt.% NaCl), dominated by CO2–H2O (about 30mol% CO2) with moderate densities (0.7 to 1.04g/cm3), and gold deposition occurred over a wide temperature range between 175°C and 325°C. Gold deposition was influenced by fluid mixing, phase separation and redox reactions. Mixing between CO2–H2O fluids and more reduced fluids, which evolved during fluid reaction with adjacent carbonaceous wall rocks, was the key factor causing gold deposition.The formation of the Gadag duplex, deformation, folds and reverse strike slip faults (discontinuities) was caused by the compression associated with subduction related tectonic processes. During the initial period of intrusive magmatism (2,555±6Ma), regional metamorphism occurred in the entire greenstone belt, while during later period, hydrothermal fluids responsible for gold mineralization probably were derived from metamorphic processes as well as from intrusive granites. Such fluids channeled through the thrust in host turbidite sequence carrying dissolved gold, associated metals and sulfur, ultimately were precipitated in a reducing environment in the splays to the thrust in the Gadag duplex at about 2,522±6Ma, resulting in retrograde alteration assemblages.

Identification of hydrothermal paleofluid pathways, the pathfinders in the exploration of mineral deposits: A case study from the Sukumaland Greenstone Belt, Lake Victoria Gold Field, Tanzania

Hydrothermal fluids play a key role in the process of metalliferous mineralization such as gold deposits. The modern exploration indicators for such deposits are tectonic structures and characteristic alteration minerals observed as detectable halos adjacent to mineral deposits. Tectonic fractures are the conduits to these hydrothermal fluids and thus control the spatial locations for the formation of mineral deposits. Along crustal structures, hydrothermal fluids commonly induce mineral alteration in the adjacent wall rocks depending on the physical–chemical conditions. These alteration patterns, which are the pathfinders for the proxies in the modern mineral exploration, can be detected by innovative application of combined remote sensing techniques. The study area has experienced intense tectonic deformations, which resulted to two major sets of structures, the NW–SE and NE–SW-trending structures. The knowledge-based analysis applied to SRTM data was useful in identifying crustal lineaments, which the above two set of structures, truncating lithological units of the Sukumaland Greenstone Belt were identified. The Feature Oriented Principal Component Selection (FPCS) together with the GIS functions applied to Landsat 7ETM+ data, were useful to enhance signals from hydrothermal alteration minerals. Results have revealed that the Sukumaland Greenstone Belt is intensively fractured, in a systematic pattern, and has apparently been “injected” with large volumes of hydrothermal fluids. Both processes together have resulted in the systematic and structurally controlled hydrothermal alteration patterns. In this study linear alteration patches are interpreted to represent the hydrothermal paleofluid pathways. Alteration patches coincide spatially with regional and local tectonic structures and are consistent with major gold occurrences and gold mines. This study indicate that careful analysis of SRTM and Landsat ETM+ data can identify crustal lineaments, the likely hydrothermal paleofluid conduits, which may lead to the discovery of potential ore deposits.

Metal and fluid sources in a potential world-class gold deposit: El-Sid mine, Egypt

Lode gold mineralization at the El-Sid mine area is associated with the ca. 600 Ma Fawakhir granite intrusion, which cuts the ~737 Ma ophiolite nappes in the Central Eastern Desert of Egypt. The mineralized quartz veins are hosted by ~E- and NE-trending fault/fracture sets cutting the western boundary of the intrusion and sheared ophiolites. The results of electron microprobe analyses of gold-associated hydrothermal sulfide and silicate minerals suggest that Au was mobilized alongside Ni, Co, Cr and As from the adjacent ophiolitic serpentinite. After granite emplacement, hydrothermal fluids interacted with the sheared serpentinite, leaching metals and re-depositing them in the faults/fractures and adjacent wall rock in a cyclic process. Low-salinity aqueous-carbonic fluids with significant quantities of volatile species (CO2, CH4, and N2 ± H2S) leached and transported Au from deep to shallow crustal levels. Carbon dioxide had a buffering effect on the Au-bearing hydrothermal solution, maintaining its pH within a narrow near-neutral range, where elevated gold concentration was transported by complexation with reduced magmatic sulfur in a reducing environment. Gold deposition along fault/fracture conduits in the Fawakhir granite and adjacent serpentinite resulted from interplay of pressure drop, fluctuations in oxygen and sulfur fugacities, and exsolution of the volatile phases. Infiltration of meteoric water may have contributed to the formation of the late stage gold-sulfide mineralization that formed at shallower levels during terrane uplift. Sulfidation of the Fe-rich magmatic minerals was, on the other hand, the overriding process in the wall rock as evidenced by abundant disseminated sulfides with gold inclusions. Considering the structural control by regional shear zones (fluid conduits) and the voluminous granitic and ophiolitic rocks (metal sources), a high tonnage gold deposit amenable to open pit mining at the El-Sid mine area is very likely.

Comparison of evolutionary and static modeling of stresses around a salt diapir

We compare an evolutionary with a static approach for modeling stress and deformation around a salt diapir; we show that the two approaches predict different stress histories and very different strains within adjacent wall rocks. Near the base of a rising salt diapir, significantly higher shear stresses develop when the evolutionary analysis is used. In addition, the static approach is not able to capture the decrease in the hoop stress caused by the circumferential diapir expansion, nor the increase in the horizontal stress caused by the rise of the diapir. Hence, only the evolutionary approach is able to predict a sudden decrease in the fracture gradient and identify areas of borehole instability near salt. Furthermore, the evolutionary model predicts strains an order of magnitude higher than the strains within the static model. More importantly, the evolutionary model shows significant shearing in the horizontal plane as a result of radial shortening accompanied by an almost-equivalent hoop extension. The evolutionary analysis is performed with ELFEN, and the static analysis with ABAQUS. We model the sediments using a poro-elastoplastic model. Overall, our results highlight the ability of forward evolutionary modeling to capture the stress history of mudrocks close to salt diapirs, which is essential for estimating the present strength and anisotropic characteristics of these sediments.

AGE AND ORIGIN OF QUARTZ-CARBONATE VEINS ASSOCIATED WITH THE COEUR D'ALENE MINING DISTRICT, IDAHO AND WESTERN MONTANA: INSIGHTS FROM ISOTOPES AND RARE-EARTH ELEMENTS

Sm-Nd, Pb, and Sr isotope and rare-earth element (REE) analyses of quartz-carbonate veins associated with the Coeur d’Alene mining district, Idaho and western Montana, constrain the age and origin of Coeur d’Alene veining. An Sm-Nd isochron age of siderites from ore-bearing veins (1511 ± 45 Ma) and a Pb-Pb isochron age of siderites, ankerites, and calcites from ore-barren veins (1523 ± 41 Ma) are similar to the model age of Coeur d’Alene-type Pb (~1450 Ma) and constrain Mesoproterozoic events in the earliest history of the Belt-Purcell Basin. Xenotime, an accessory mineral found in all veins, is zoned with a core laser ablation age of 1420 ± 90 Ma, confirming vein emplacement early in the diagenetic and/or metamorphic history of the Belt-Purcell Basin, and an overgrowth age of 990 ± 130 Ma results from later, Grenville-age metamorphism. Pb isotope ratios of carbonates from both ore-bearing and ore-barren veins define a single linear array resulting from Mesoproterozoic vein emplacement with an initial Pb isotope ratio similar to that of Coeur d’Alene-type Pb observed in galena, while moderately radiogenic Pb isotope ratios originate from variable amounts of accompanying xenotime. This array is similar to the previously determined “more radiogenic” array of Leach et al. (1998a). Ore-barren veins are characterized by initial 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios of 0.5108 and 0.769, respectively, which may be partially inherited from Belt-Purcell Supergroup and/or Archean rocks. In contrast, initial 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios of 0.5083 and 1.146 of ore-bearing veins require a source with highly unradiogenic Nd and highly radiogenic Sr, most likely Archean crust. Ore minerals may result from interaction of hydrothermal fluids from Archean sources with potential Belt-Purcell Basin sedimentary exhalative (sedex) deposits. REEs are concentrated in carbonate fractions of veins. Siderite and ankerite are light REE depleted and heavy REE enriched, resulting in significant Sm/Nd fractionation from the dual effects of REE complexation in carbonate fluids and mineralogical control, which in the case of siderite and ankerite favors the heavy REEs. Positive Eu anomalies characterize ore-bearing veins, whereas ore-barren veins retain negative Eu anomalies. Normalization of REE patterns of ore-barren veins to those of adjacent Belt-Purcell wall rocks results in a near-zero anomaly, implying that REEs from Belt-Purcell metasediments were locally scavenged by carbonate-rich fluids responsible for ore-barren veins. In contrast, leaching of Archean gneisses followed by probable interaction with sedex deposits produced fluids that formed ore-bearing deposits. Further faulting and deposition from successive fluxes of carbonate-rich solutions derived in part from leaching Belt-Purcell metasediments generated ore-barren siderite-, ankerite-, and, finally, calcite-dominant veins.

Frictional properties and sliding stability of the San Andreas fault from deep drill core

Research Article| August 01, 2012 Frictional properties and sliding stability of the San Andreas fault from deep drill core B.M. Carpenter; B.M. Carpenter Department of Geosciences and Energy Institute Center for Geomechanics, Geofluids, and Geohazards, The Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar D.M. Saffer; D.M. Saffer Department of Geosciences and Energy Institute Center for Geomechanics, Geofluids, and Geohazards, The Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar C. Marone C. Marone Department of Geosciences and Energy Institute Center for Geomechanics, Geofluids, and Geohazards, The Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (8): 759–762. https://doi.org/10.1130/G33007.1 Article history received: 08 Nov 2011 rev-recd: 09 Mar 2012 accepted: 16 Mar 2012 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation B.M. Carpenter, D.M. Saffer, C. Marone; Frictional properties and sliding stability of the San Andreas fault from deep drill core. Geology 2012;; 40 (8): 759–762. doi: https://doi.org/10.1130/G33007.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The strength of tectonic faults and the processes that control earthquake rupture remain central questions in fault mechanics and earthquake science. We report on the frictional strength and constitutive properties of intact samples across the main creeping strand of the San Andreas fault (SAF; California, United States) recovered by deep drilling. We find that the fault is extremely weak (friction coefficient, μ = ∼ 0.10), and exhibits both velocity strengthening frictional behavior and anomalously low rates of frictional healing, consistent with aseismic creep. In contrast, wall rock to the northeast shows velocity weakening frictional behavior and positive healing rates, consistent with observed repeating earthquakes on nearby fault strands. We also document a sharp increase in strength to values of μ > ∼0.40 over <1 m distance at the boundary between the fault and adjacent wall rock. The friction values for the SAF are sufficiently low to explain its apparent weakness as inferred from heat flow and stress orientation data. Our results may also indicate that the shear strength of the SAF should remain approximately constant at ∼10 MPa in the upper 5–8 km, rather than increasing linearly with depth, as is commonly assumed. Taken together, our data explain why the main strand of the SAF in central California is weak, extremely localized, and exhibits aseismic creep, while nearby fault strands host repeating earthquakes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dehydration of subducting serpentinite: Implications for halogen mobility in subduction zones and the deep halogen cycle

We investigated the halogen (Cl, F, Br, and I) chemistry of serpentinites that record progressive dehydration during subduction from shallow oceanic environments via increased pressure and temperature conditions to complete breakdown of antigorite. The aim is to evaluate the relevance of serpentinites for halogen recycling in subduction zones and for deep mantle recharge of these elements. The halogen compositions of the analyzed samples indicate input from seawater and sedimentary sources during initial serpentinization of either subducting lithospheric mantle during slab bending or forearc mantle by uprising slab fluids. During the first dehydration stage (antigorite + brucite → olivine + H 2O), fluids with high Br/Cl and I/Cl ratios are released resulting in residual serpentinites with lower Br/Cl and I/Cl ratios. Veins associated with this event and with the final antigorite breakdown (antigorite → olivine + orthopyroxene + H 2O) show higher halogen ratios compared to their adjacent wall rocks, and they are similar to those found in arc volcanoes (F/Cl and I/Cl between ca. 0.083–1.5, and ca. 0.00038–0.0013, respectively). All measured deserpentinization samples show a narrow range in δ 37Cl values (between − 0.42‰ and + 0.92‰) overlapping the δ 37Cl values of seafloor serpentinites and confirming that no significant Cl isotope fractionation occurs during subduction dehydration of serpentinites. Our findings document the conservative behavior of halogens during subduction. Mass balance constraints reveal that serpentinites strongly control the halogen chemistry of deep subduction zone fluids and that descent of rock residues after deserpentinization strongly affects the halogen budget of the mantle.

Thermochronology of the Salt Spring fault: Constraints on the evolution of the South Virgin–White Hills detachment system, Nevada and Arizona, USA

We present new clay mineralogy and muscovite and illite 40 Ar/ 39 Ar data from fault gouge and immediately adjacent wall rocks from the Salt Spring fault, the central portion of the Miocene South Virgin–White Hills detachment system in southern Nevada and northern Arizona. In combination with a wealth of published regional thermochronology data, we find that useful age information can be obtained from 40 Ar/ 39 Ar step-heating spectra of fine-grained clays in both sedimentary rocks and fault gouge derived from them. This information can be used to investigate the provenance of detrital clays in lowgrade sediments, the source of clays in fault gouge, and potentially to constrain thermal histories. A new muscovite 40 Ar/ 39 Ar age from the footwall near the Salt Spring fault is ca. 900 Ma, in contrast with previously reported ages of ca. 90 Ma from near the South Virgin–White Hills detachment system at Gold Butte, Nevada, located 20 km along strike to the north. This discrepancy in muscovite cooling ages supports prior interpretations of significant along-strike variations in the magnitude of footwall exhumation. 40 Ar/ 39 Ar data from clay-size sediment in a supradetachment basin at the Salt Spring Wash area show no influence from detrital ca. 900 Ma or older muscovite, but do show an influence from detrital muscovite with Cretaceous apparent ages, suggesting that these sediments were derived from Gold Butte to the north, and not from directly updip areas to the east. These observations suggest a Miocene paleotopographic configuration wherein Gold Butte formed an upland area that shed detritus southward into the supradetachment basin during exhumation of the Salt Spring footwall. Fault gouge from the Salt Spring detachment is similar to fine-grained sediment from the adjacent supradetachment basin both in terms of clay mineralogy and 40 Ar/ 39 Ar results. The gouge is rich in illite and smectite, clays that have low frictional coefficients, are relatively impermeable, and would have reduced the shear strength of the detachment at shallow depths. Apatite grains entrained in the gouge have a mean fission-track age of 15 Ma and mean track lengths that are statistically indistinguishable from previously published data from footwall samples. Collectively, these observations suggest that clay-rich gouge along this segment of the fault formed principally by scraping and incorporating clays and other minerals from the base of the supradetachment basin and that the fault zone did not experience temperatures measurably greater than the footwall.