ALBERT

Description: Realistic simulation of structurally complex reservoirs (SCR) is challenging in at least three ways: (1) geological structures must be represented and discretized accurately on vastly different length scales; (2) extreme ranges and discontinuous variations of material properties have to be associated with the discretized structures and accounted for in the computations; and (3) episodic, highly transient and often localized events such as well shut-in have to be resolved adequately within the overall production history, necessitating a highly adaptive resolution of time. To facilitate numerical experiments that elucidate the emergent properties, typical states and state transitions of SCRs, an application programmer interface (API) called complex systems modelling platform (CSMP++) has been engineered in ANSI/ISO C++. It implements a geometry and process-based SCR decomposition in space and time, and uses an algebraic multigrid solver (SAMG) for the spatio-temporal integration of the governing partial differential equations. This paper describes a new SCR simulation workflow including a two-phase fluid flow model that is compared with ECLIPSE in a single-fracture flow simulation. Geologically realistic application examples are presented for incompressible 2-phase flow, compressible 3-phase flow, and pressure-diffusion in a sector-scale model of a structurally complex reservoir.

Description: [1]  The origin of crustal-scale silicic magmatism remains a matter of debate, and notable uncertainty exists concerning the physical mechanisms that drive ascent and emplacement of felsic magmas in upper crustal regions. A 2D numerical model demonstrates that injection of mantle-derived mafic magma into a partially molten hot zone in the lower crust can drive felsic magma ascent and intrusion into upper crustal levels. The injection of mafic magma induces over pressure in the reservoir, which increases crustal stresses and triggers development of brittle/plastic shear zones, and can drive significant surface uplift. The emerging topography causes a non-uniform over pressure distribution in the reservoir and can trigger felsic magma ascent along crustal shear zones. Based on systematic numerical experiments we investigate the influence of crustal strength and injection rate. The initial upper crustal strength controls the degree of crustal faulting and surface uplift and, therefore, whether felsic magma ascent can be initiated or not. The final upper crustal strength influences the depth and final style of felsic intrusion. The injection rate of mafic magma determines the time scale of overpressure growth and surface uplift stage. In contrast, the duration of the subsequent felsic ascent and intrusion emplacement stages remains nearly constant. Our results imply that mafic underplating and intrusion into the lower crust may not only be a prime control for the generation of felsic magmas in the lower crust, but may also be an important physical driving mechanism for felsic magma ascent and intrusion into upper crustal levels.

Description: The formation of porphyry copper deposits requires a focused flux of magmatic fluid, expelled from a large reservoir of water-, metal-, and sulfur-rich magma. The dimensions of this usually hidden magma reservoir are difficult to determine but can be constrained by combining geophysical observations with thermal constraints and the mass balance imposed by the chemical enrichment of elements in the deposit. Here we show that an internally consistent scenario can be derived for the world-class Cu-Mo-Au deposit at Bingham Canyon (Utah, United States), which quantifies the essential characteristics, approximate dimension, and temporal evolution of a large pluton that generated the deposit. The mineralized district shows a distinct WSW-ENE–striking magnetic anomaly indicating a large intrusive body underlying the sedimentary host rocks of the Oquirrh Mountains. Modeling the deep body by geomagnetic methods is possible because of the high contrast in magnetic susceptibility between sedimentary host rocks and intrusive rocks and because a former volcanic edifice is largely eroded. Additional constraints from drilled geology and district-wide outcropping rocks, including partial demagnetization by hydrothermal alteration on the mine scale, restrict the range of possible solutions to a broadly laccolith-shaped intrusion with a volume of approximately 1,400 to 3,000 km 3 . From the roof of the laccolith, several smaller subvolcanic stocks and dikes protrude to the present surface, of which a major one is hosting the Bingham Canyon deposit. The roof of the laccolith probably lies between 2 and 3.5 km below the bottom of the present open-pit mine, and the average thickness of the laccolith is constrained between 2 and 3.5 km. Thermal modeling, using pluton dimensions derived from the geologic and geomagnetic modeling, indicates that a single laccolith with a magma volume of ~2,000 km 3 beneath Bingham would have solidified within about 230,000 years or less. Comparison of the thermal models with published high-precision geochronologic data and petrologic constraints suggests a scenario in which about 1,000 km 3 of magma was encapsulated by inward crystallization of the pluton after the preore equigranular monzonite stocks solidified and extrusive volcanism was probably terminated. This encapsulated reservoir was close to water saturation and contained approximately 150 billion metric tons (Gt) of magmatic water for subsequent closed-system fractionation and eventual fluid expulsion driving porphyry copper mineralization. Chemical mass balance shows that the known metal endowment and mapped mass of vein quartz within the deposit can be advected and precipitated by a fluid mass that is slightly smaller than the available 150 Gt of water. A conservative estimate indicates that 115 Gt of water is sufficient to precipitate all the quartz associated with successive Cu-Au-and Mo-stage veins as well as their barren precursors. According to our thermal model, approximately 250 km 3 of quartz monzonite magma with a temperature of about 690°C remained partially liquid some 215,000 years after initial intrusion of the laccolith. At that point, it expelled almost simultaneously the quartz monzonite porphyry and the main mass of accumulated fluid, generating most of the vein quartz in the quenched porphyry and the adjacent older rocks. Petrographic evidence indicates that the ore metals precipitated near the end of individual pulses of quartz veining that followed recurrent but waning pulses of porphyry intrusion. Considering published experimental solubility data as well as ore metal contents in fluid inclusions, a small fraction of the available fluid mass is sufficient to transport and precipitate all the ore metals after an initial fluid pulse precipitated most of the quartz. However, the total amount of sulfur present in the deposit, which includes Cu and Mo sulfides as well as a major addition of pyrite, would be facilitated by addition of a mafic magma input into the residual magma chamber that contained the evolved felsic magma. This magmatic injection probably triggered the emplacement of the mineralized porphyries, consistent with the more mafic composition of some of the latest porphyry dikes and the CO 2 -rich nature of ore-related fluid inclusions.

Description: The late Alpine evolution of the Rhodope Massif in southern Bulgaria and northern Greece involved postcollisional extension, which generated detachment faults, syndeformational sedimentary basins, and exhumation of a large metamorphic core complex composed of gneisses and marbles: the Central Rhodopian dome. Closely associated with this complex, subvolcanic rhyolite dikes and extrusive rocks were emplaced, shortly followed by major swarms of epithermal to mesothermal Pb-Zn veins and carbonate replacement orebodies. High-precision geochronology using complementary Ar-Ar, Rb-Sr, and U-Pb dating methods resolves how this process of tectonic denudation from deep crustal metamorphism to near-surface epithermal ore formation occurred within a period of about 12 m.y. After an early Alpine phase of accretion, eclogite-facies metamorphism, and orogenic nappe stacking, the late Alpine postcollisional evolution of the Central Rhodopian dome started with the intrusion of granitic bodies at about 42 to 41 Ma, probably marking the beginning of extension and core complex formation. The early stages of extension were characterized by normal faulting, rotation of fault blocks, and thinning that caused cooling of the hanging wall through ~300°C at about 40 to 38 Ma, as dated by Rb-Sr and Ar-Ar geochronology of metamorphic biotite. The main extensional phase occurred between 38 and 36 Ma and led to horizontal displacements of tens of kilometers in the hanging wall. In the footwall, high metamorphic temperatures and decompression persisted and resulted in partial melting and the formation of migmatites at 37 Ma and vuggy pegmatites at about 36 Ma. Cooling of the footwall below ~300°C occurred between 36 and 34 Ma, followed by emplacement of undeformed rhyolite porphyry dikes and the extrusion of volcanic products deposited onto the surface-exposed center of the dome at about 33 to 30 Ma. The hydrothermal ores were formed ca. 30.5 Ma in the south and ca. 29.3 Ma in the northern part of the dome during the last major event of focused heating to 270° to 330°C of near-surface rocks by hydrothermal fluid advection. Ore formation and localized, later fluid processes caused disturbance and younging of some Rb-Sr ages in the footwall of the dome. Field and geochronologic constraints indicate that the formation of the Pb-Zn deposits (~31–29 Ma) is up to 2 m.y. younger than the local rhyolitic magmatism, which is volumetrically minor in the mineralized core complex. This contrasts with ore formation related to calc-alkaline magmatism in the Eastern Rhodopes, where polymetallic Cu-Au-Ag-Pb-Zn mineralization was found to be coeval with the latest phases of igneous activity (~32 Ma). The chemically simpler but considerably larger metamorphic-hosted Pb-Zn deposits of the Central Rhodopian dome were generated by large-scale hydrothermal fluid circulation, driven by the high heat flow attending core complex formation, exhumation, and final fracturing of a rapidly thinned crust.

Description: The magmatic to hydrothermal transition in the Late Cretaceous Elatsite porphyry Cu-Au-(Mo-platinum group element) deposit has been studied in a suite of samples with clear timing relations between porphyry dikes, magmatic-hydrothermal veins, silicate melt inclusions in quartz veins, fluid inclusion generations, and ore minerals. Ore mineralization occurs late, at temperatures ~200° to 300°C below those of the early, multistage interplay between magmatic and hydrothermal processes at near-magmatic temperature-pressure conditions. Crosscutting relations and petrography indicate that shortly after the intrusion of the earliest, monzodioritic dikes, fluids precipitated a first generation of granular quartz veins accompanied by potassic alteration. The second vein generation with crystalline quartz textures and K-feldspar alteration halos formed at the same time as the second, granodioritic pulse of porphyry intrusions. Cathodoluminescence imaging of quartz growth textures reveals that the earliest fluid inclusions in the crystalline quartz veins are of intermediate density and ~8 wt % NaCl equiv salinity, probably trapped at near-lithostatic pressures of ~1,200 to 1,300 bars at near-magmatic temperatures (≤730°C), and implies a depth of ~4 to 5 km. Depressurization led to phase separation, indicated by a first generation of coexisting brine and vapor inclusions trapped at temperatures of ≥640°C and suprahydrostatic pressures of ≥920 bars. A second quartz generation in crystalline quartz veins precipitated during progressive depressurization and hosts assemblages of coexisting brine, vapor, and silicate melt inclusions, trapped at temperatures in excess of 600°C and suprahydrostatic pressures of 630 to 880 bars. Some open-spaced quartz veins were filled with aplite during this stage. Field relations and geochemical evidence suggest that the aplites as well as the silicate melt inclusions in hydrothermal quartz veins represent volumetrically minor residual melts that evolved directly from granodiorite porphyries at the level of deposit formation, and do not represent aliquots of metal-supplying magma at depth. Fluid inclusions coexisting with the silicate melt inclusions are metal rich, but these fluids predate sulfide precipitation and are, therefore, not the dominant fluids responsible for the Cu-Au mineralization at Elatsite. Melt-fluid-metal separation processes recorded in these co-trapped silicate melt and fluid inclusions in vein quartz are small-scale, local phenomena and do not appear to be suited for understanding and quantifying metal segregation in the deeper source. Microthermometry data of fluid inclusions, trapped during the later stages of the formation of the second quartz generation, show further depressurization to ~260 to 325 bars and cooling to ~460°C, representing a hot hydrostatic regime. Bornite, chalcopyrite, and magnetite seem to be slightly later precipitated at temperatures ≤460°C in separate veins or opened spaces of preexisting veins. Dissolution textures of the second quartz generation indicate a subsequent local redissolution of vein quartz, most likely as a result of passing through a window of retrograde quartz solubility upon further cooling below 460°C. The next, economically most important chalcopyrite-pyrite stage is largely devoid of quartz precipitation and is mostly expressed as "paint veins." Due to the lack of fluid inclusions, the temperature-pressure conditions of formation of these veins could not be constrained. The waning phase of hydrothermal activity is represented by a quartz-carbonate-zeolite stage, formed at low temperature (~145°C), as indicated by microthermometry of fluid inclusions trapped in quartz from this stage. Laser ablation-inductively coupled plasma-mass spectrometry analyses of fluid inclusions show very high Cu contents in early intermediate-density fluid inclusions and in the first vapor inclusion generation, followed by a drastic decrease in the second generation of vapor inclusions. The decrease correlates with the appearance of anhydrite inclusions in the second quartz generation and may indicate that a lack of sulfur in these later vapor inclusions led to Cu partitioning into the brine. Alternatively, it is also possible that, unlike early vapor inclusions, the later vapor inclusions were not susceptible to postentrapment copper enrichment, in accordance with recent experiments. A progressive Cu enrichment in the brine phase correlates well with depressurization prior to mineralization. Mass balance considerations based on analyzed fluid components and fluid phase relations indicate that brine was volumetrically minor and therefore likely stagnant, but this may have prepared ore precipitation by accumulating Cu stripped from ascending vapor.

Description: Numerical, multiphase pure-water simulations were performed to study the first-order geologic and physical parameters controlling the style and distribution of hydrothermal venting at Brothers volcano, southern Kermadec arc, New Zealand. By comparing the results for different permeability scenarios, we can show that the location of venting on the inner slopes of the caldera (e.g., at the NW caldera site) requires the presence of higher permeability faults. Venting at the top of the Upper cone develops naturally by hydrothermal flow in porous rocks above an underlying magma body. However, this magmatic reservoir cannot alone account for present-day hydrothermal venting at the NW caldera site, which implies that a larger magma chamber, which was responsible for caldera collapse, is still active. Modeled venting temperatures for scenarios with homogeneous host-rock permeability correspond well with formation temperatures determined by sulfate-sulfide mineral pairs from different vent sites at Brothers volcano. Direct measurements of vent fluids at the NW caldera site today, however, show higher temperatures than modeled. This may be due to rapid ascent of hot fluids in individual fractures that are not resolved in the simulations. At the cone sites, measured temperatures are lower than modeled, likely the result of mixing with ambient seawater in near-surface permeable rocks. The inferred presence of a constant magmatic fluid source underneath the volcanic edifice leads to a more rapid development of the hydrothermal circulation and stabilizes the system at higher temperatures. We suggest that the hydrothermal evolution and fluid-flow patterns at Brothers volcano are controlled by episodes of varying magmatic fluid input into a seawater-dominated convection system.

Description: Maureen is the largest among several U (-Mo-F) prospects occurring along a Late Devonian unconformity in the Georgetown area, northern Queensland, Australia. Mineralization is structurally controlled by the intersection of steep east-west fractures with an unconformity between a Proterozoic basement and a Paleozoic cover sequence of continental sedimentary rocks and abundant rhyolitic volcanic rocks. The mineralogical composition of high-grade ore (pitchblende + Fe-rich molybdenite + arsenopyrite + arsenian pyrite + fluorite + dickite + chlorite + goyazite ± graphite or hematite), postlithification brecciation, and quartz dissolution indicate a strong chemical gradient and disequilibrium during mineralization between the reduced basement and the largely oxidized cover sequence. Elemental and mineral zonation centered to faults, and fractures cutting the unconformity, indicates locally reducing and quartz-dissolving conditions during the formation of tabular U-Mo orebodies, which are surrounded by a halo rich in fluorite. A detailed petrographic study, microthermometry, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of quartz- and fluorite-hosted fluid inclusions provide concentrations of ore-forming components in fluids related to hydrothermal uranium mineralization. Three fluids were involved in the mineralizing process at Maureen: a likely oxidized saline (L3) fluid, an aqueous (L1) fluid, and a CH 4 -bearing carbonic vapor (cV). The oxidized saline (L3) fluids trapped in fluorite and dickite-bearing hydrothermal quartz veinlets crosscutting detrital quartz show the highest average concentrations of U (10–47 ppm), Mo (489–888 ppm), and As (318–777 ppm), with element ratios close to those of high-grade mineralized rocks. We suggest that aqueous (L1) fluids mixed in the cover sequence with oxidized saline fluids (L3) to precipitate fluorite as a distal halo surrounding the east-west fractures. Their mixture, an oxidized moderately saline fluid (L2), reacted with methane-bearing carbonic vapor (cV) to precipitate U and Mo by reduction, close to the intersection of the fractures with the unconformity. Geochemical analysis of coexisting vapor and liquid assemblages indicate mixing of oxidized saline fluids with reduced carbonic vapors to be the driving force for uranium precipitation. The Maureen deposit shares essential geologic characteristics and hydrothermal processes with Proterozoic unconformity-related U deposits, but probably owes its unusual element association of U with (equally redox sensitive) Mo and abundant F to a source region in the volcano-sedimentary cover sequence that was enriched in these elements, due to the presence of highly fractionated, easily leachable, and oxidizable felsic volcanics.

Description: Magmatic-hydrothermal systems associated with upper crustal plutons strongly influence volcanic and geothermal processes and form important mineral deposits. Fluids released from plutons are commonly saline and undergo phase separation into high-salinity brines and low-salinity vapors upon ascent. While brine-vapor immiscibility has been extensively studied, precipitation of solid salt during phase separation in magmatic-hydrothermal systems has generally been considered a rare phenomenon. Here we show that most porphyry deposits exhibit fluid inclusion evidence best interpreted by solid salt precipitation from ore-forming solutions. This interpretation naturally links thermodynamics, numerical simulations, and independent estimates of porphyry ore formation depths. Salt precipitation imposes major changes on the permeability of the system. Moreover, salt precipitation has implications for ore formation along the liquid-vapor-halite curve. The recognition of salt-saturated systems is challenging, but very relevant for understanding the evolution of magmatic-hydrothermal systems.