Martin S. Appold

Anomalously Metal-Rich Fluids Form Hydrothermal Ore Deposits

Hydrothermal ore deposits form when metals, often as sulfides, precipitate in abundance from aqueous solutions in Earths crust. Much of our knowledge of the fluids involved comes from studies of fluid inclusions trapped in silicates or carbonates that are believed to represent aliquots of the same solutions that precipitated the ores. We used laser ablation inductively coupled plasma mass spectrometry to test this paradigm by analysis of fluid inclusions in sphalerite from two contrasting zinc-lead ore systems. Metal contents in these inclusions are up to two orders of magnitude greater than those in quartz-hosted inclusions and are much higher than previously thought, suggesting that ore formation is linked to influx of anomalously metal-rich fluids into systems dominated by barren fluids for much of their life.

Geochemistry of Mississippi Valley–type mineralizing fluids of the Ozark Plateau: A regional synthesis

The compositions of fluid inclusions hosted in ore and gangue minerals from Mississippi Valley–type (MVT) Pb-Zn-Ba deposits of the Ozark Plateau region were measured to develop a regional hydro-geochemical conceptual model for ore emplacement. This model may explain the diverse compositions of fluids involved in mineral precipitation, the ore precipitation mechanism, and the temporal change in composition of fluids invading the ore districts. The conceptual model additionally provides evidence for what factors may have controlled deposit size, stratigraphic location, and Zn/Pb ratio. High Pb concentrations up to 1,000s of ppm were identified in sphalerite-hosted fluid inclusions from all of the regions districts. If these high Pb concentrations were transported in the same fluid with sulfide, then total sulfur concentration in the fluid must have been low. Mass balance calculations demonstrate that the Arkoma Basin, a presumed source basin for the mineralizing fluids, is too small to have contained enough fluid to precipitate the observed masses of sulfur in the larger MVT districts, given the low sulfide concentrations that could coexist in the fluid with such high concentrations of Pb. High methane concentrations in sphalerite-hosted fluid inclusions from all of the regions districts, in dolomite-hosted fluid inclusions from the Tri-State and Northern Arkansas districts, and quartz-hosted fluid inclusions from the Northern Arkansas district suggest that the prevailing redox conditions during MVT mineralization were reducing, making it unlikely that sulfate was transported in large concentrations in the fluids. During sphalerite precipitation, assuming saturation with respect to carbon dioxide, measured methane concentrations in sphalerite-hosted fluid inclusions would have required oxygen fugacity to have been at least two log units below the sulfate predominance field boundary. Fluid inclusion methane concentrations can also be used to estimate minimum burial depths of mineralization of about 0.08 to 1.2 km. Available evidence indicates that sulfide mineral precipitation in the Ozark Plateau MVT districts most likely occurred primarily as a result of the introduction of sulfide into a Pb- and Zn-rich ore fluid. In the two larger MVT districts, sulfide may have been supplied by local organic- and sulfur-rich carbonate facies. An apparent mixing line between high Ca/Na ratio and low Ca/Na ratio fluids hosted predominantly in main stage sulfide minerals and paragenetically late minerals, respectively, indicates the ore fluid was relatively Ca enriched. The lack of continuity in high Pb concentrations in fluid inclusions in sulfide and nonsulfide minerals from across the mineral parageneses suggests that the ore fluids entered the districts intermittently and/or had variable metal contents over time.

Numerical modeling of porosity waves in the Nankai accretionary wedge décollement, Japan: implications for aseismic slip

Seismic and hydrologic observations of the Nankai accretionary wedge décollement, Japan, show that overpressures at depths greater than ∼2 km beneath the seafloor could have increased to near lithostatic values due to sediment compaction and diagenesis, clay dehydration, and shearing. The resultant high overpressures are hypothesized then to have migrated in rapid surges or pulses called ‘porosity waves’ up the dip of the décollement. Such high velocities—much higher than expected Darcy fluxes—are possible for porosity waves if the porous media through which the waves travel are deformable enough for porosity and permeability to increase strongly with increasing fluid pressure. The present study aimed to test the hypothesis that porosity waves can travel at rates (kilometers per day) fast enough to cause aseismic slip in the Nankai décollement. The hypothesis was tested using a one-dimensional numerical solution to the fluid mass conservation equation for elastic porous media. Results show that porosity waves generated at depths of ∼2 km from overpressures in excess of lithostatic pressure can propagate at rates sufficient to account for aseismic slip along the décollement over a wide range of hydrogeological conditions. Sensitivity analysis showed porosity wave velocity to be strongly dependent on specific storage, fluid viscosity, and the permeability–depth gradient. Overpressure slightly less than lithostatic pressure could also produce porosity waves capable of traveling at velocities sufficient to cause aseismic slip, provided that hydrogeologic properties of the décollement are near the limits of their geologically reasonable ranges.RésuméLes observations sismiques et hydrologiques sur le décollement du prisme d’accrétion de Nankaï, Japon, montrent que les surpressions à des profondeurs de plus de 2 km en-dessous du plancher océanique pourraient avoir augmenté presque jusqu’aux valeurs lithostatiques dues à la compaction du sédiment et à la diagenèse, à la déshydratation des argiles et au cisaillement. Les fortes surpressions qui en résultent sont alors supposées avoir migré au-dessus du plan du décollement par remontées ou impulsions rapides appelées “ondes de porosité”. Des vitesses aussi élevées—plus fortes que les flux de Darcy attendus—sont possibles pour des ondes de porosité si les milieux poreux à travers lesquels les ondes se propagent sont suffisamment déformables pour que la porosité et la perméabilité augmentent fortement avec l’accroissement de la pression du fluide. La présente étude avait pour but de tester l’hypothèse selon laquelle des ondes de porosité peuvent se propager à des vitesses (en kilomètres par jour) assez élevées pour provoquer un glissement asismique au droit du décollement de Nankaï. Cette hypothèse a été testée en utilisa nt une solution numérique à une dimension de l’équation de conservation de la masse du fluide dans un milieu poreux élastique. Les résultats montrent que les ondes de porosité générées à des profondeurs de ∼2 km par des surpressions dépassant la pression lithostatique peuvent se propager à des vitesses suffisantes pour expliquer un glissement asismique le long du décollement pour une large gamme de conditions hydrogéologiques. Une analyse de sensibilité a montré que la vitesse de l’onde de porosité était fortement dépendante de l’emmagasinement spécifique, de la viscosité du fluide et du gradient de perméabilité-profondeur. Une surpression légèrement inférieure à la pression lithostatique pourrait elle aussi produire des ondes de porosité capables de se propager à des vitesses suffisantes pour causer un glissement asismique, à condition que les propriétés hydrogéologiques du décollement soient proches des bornes d’une gamme de valeurs géologiquement vraisemblables.ResumenLas observaciones sísmicas e hidrológicas en el desprendimiento de la cuña de de acreción de Nankai, Japón, muestran que las sobrepresiones a profundidades mayores de ∼2 km por debajo del nivel del mar podrían incrementarse hasta cerca de los valores de la litostática debido a la compactación y diagénesis de sedimentos, a la deshidratación de las arcillas y al cizallamiento. Se plantea la hipótesis de que las elevadas sobrepresiones resultantes hayan migrado en rápidas ondas o pulsos llamadas “ondas de porosidad” hasta arriba de le pendiente del desprendimiento. Tales velocidades tan altas, muchos mayores que los esperados flujos de Darcy, son posibles para las ondas de porosidad si los medios porosos a través de los cuales viajan las ondas son lo suficientemente deformables para que la porosidad y la permeabilidad se incrementen fuertemente con el aumento de la presión del fluido. El presente estudio tuvo como objetivo ensayas la hipótesis de que las ondas de porosidad pueden viajar a velocidades (kilómetros por día) lo suficientemente rápido como para causar deslizamientos asísmicos en el desprendimiento de Nankai. La hipótesis se ensayó usando una solución numérica unidimensional de la ecuación de conservación de la masa de fluido para medios porosos elásticos. Los resultados muestran que las ondas de porosidad generadas a profundidades de ∼2 km a partir de sobrepresiones por encima de la presión litostática se pueden propagar a velocidades suficientes para dar cuenta del deslizamiento asísmico a lo largo del desprendimiento en un amplio intervalo de condiciones hidrogeológicas. Los análisis de sensibilidad demostraron que las velocidades de las ondas de porosidad son fuertemente dependientes del almacenamiento específico, de la viscosidad del fluido, y del gradiente de la permeabilidad en profundidad. La sobrepresión ligeramente menor a la presión litostática también podría producir ondas de porosidad capaz de circular a velocidades suficientes para causar deslizamientos asísmicos, siempre y cuando que las propiedades hidrogeológicas del desprendimiento estén cerca de los límites de rangos geológicamente razonables.摘要对日本Nankai增积岩体滑脱构造的地震和水文观测结果显示,由于沉积物的压实和成岩作用、粘土脱水及剪切作用,海床之下超过2公里深度的超压可能会增加到接近地压值。然后假定所造成的强超压在快速波涌中或滑脱构造倾斜之上被称为 “孔隙波”的脉冲中运移。如果波通过的孔隙介质能够产生足够的变形,使孔隙度和渗透性随着流体压力的增加而大大增强,如此高的速度---比预料的达西通量高很多的速度---对于孔隙波来说是可能的。目前的研究旨在检验孔隙波可以在足够快的速度(每天若干公里)行进而在Nankai滑脱构造中引起抗震滑动这一假设。对弹性孔隙介质流体质量守恒方程采用一维数值解检验了这一假设。结果显示,由于超过岩石静压力的超压,在2公里深度处产生的孔隙波会传导,其速度足以解释很宽的水文地质条件范围内沿滑脱构造的抗震滑动。灵敏度分析显示,孔隙波速度主要取决于单位储水量、流体粘度及渗透性—深度梯度。假如滑脱构造的水文地质特性接近其地质学上的合理范围,稍微少于岩石静压力的超压也会产生孔隙波,并且孔隙波能够以足够引起抗震滑动的速度行进。ResumoObservações sísmicas e hidrológicas no décollement de cunha acrescionária de Nankai, Japão, mostram que sobrepressões em profundidades acima de ∼2 km sob o assoalho oceânico podem ter aumentado até aproximadamente os valores litostáticos devido a compactação de sedimentos e diagênese, desidratação de argilas e cisalhamento. A hipótese é de que as altas sobrepressões resultantes teriam migrado rapidamente em pulsos chamados “ondas de porosidade” ao longo do plano de mergulho do décollement. Tais altas velocidades—muito maiores do que os fluxos Darcianos esperados—são possíveis para ondas de porosidade, contanto que o meio poroso no qual as ondas se propagam seja suficientemente deformável para a porosidade e a permeabilidade aumentarem com o aumento da pressão de fluído. O presente estudo objetivou testar a hipótese de que as ondas de porosidade são capazes de se propagar em velocidades (quilômetros por dia) suficientemente rápidas para causar deslizamentos asísmicos no décollement de Nankai. A hipótese foi testada utilizando uma solução numérica unidimensional para a equação de conservação de massa de fluído em meio poroso elástico. Os resultados mostram que as ondas de porosidade geradas a profundidades de ∼2 km a partir de sobrepressões em excesso de pressão litostática podem propagar-se em velocidades suficientes para causar deslizamento asísmico pelo décollement em uma variedade de condições hidrogeológicas. A análise de sensibilidade sugeriu que velocidade da onda de porosidade é fortemente dependente do armazenamento específico, viscosidade de fluído e da permeabilidade-gradiente de profundidade. Sobrepressões ligeiramente inferiores do que a pressão litostática também poderiam produzir ondas de porosidade capazes de viajar a velocidades suficientes para causar deslizamentos asísmicos, no caso das propriedades hidrogeológicas do décollement se aproximarem dos limites dos cenários geologicamente razoáveis.

Book review: Frontiers in Geofluids, edited by Bruce Yardley, Craig Manning, and Grant Garven (Wiley-Blackwell, 2011)

Frontiers in Geofluids (Yardley et al. 2011) is a compilation of 19 papers originally published in the journal Geofluids in 2010 (volume 10, numbers 1–2) to commemorate the tenth anniversary of the journal’s founding. The papers highlight several areas of current research in hydrology and geochemistry pertinent to a wide range of depths in the Earth’s crust. The book is valuable as a reference for researchers and as a teaching resource for upper level graduate courses. The first four papers concern theoretical aqueous geochemistry. They include: a review of the properties of water as a function of pressure, temperature, and concentration of selected silicates; two studies that challenge Born theory in aqueous solution models, proposing alternative models based in one case on free energy change associated with mineral lattice breakdown, formation of the solute species hydration shell, and the volume reduction of the hydration shell, and in the other case based on a quantum mechanical description of the interactions between cations, ligands, and the solvent; and a study of the effects of salinity on high grade metamorphic mineral solubility. Collectively these papers highlight some of the challenges in quantifying chemical reaction and mass transport at mid-to-lower crustal pressures and temperatures and identify areas where more research is needed. The limitations of Born theory near the critical point of water and at higher temperatures are substantial, but the alternative models proposed here will need to be developedmuch further before they will have the practical applicability that Born theory-based models like the Helgeson-Kirkham-Flowers model (Helgeson et al. 1981; Tanger and Helgeson 1988) have had for shallow crustal conditions. The next three papers examine fluid flow in hydrocarbon-rich sedimentary basins. They include: a fluid inclusion and Ar–Ar dating study that documents episodic flow of hot hydrocarbon fluids through fractured rocks, offshore Scotland, UK; a study of the origin and distribution of pore fluid salinity in deep water sediments of the northern Gulf of Mexico basin, offshore Louisiana and Texas; and a critique of common approaches in petroleum industry reservoir models for treating fluid flow through faults. The first two papers affirm the ongoing importance of long-standing geochemical techniques in unraveling complex geologic histories and constraining numerical models. The third paper’s analysis of fault permeability as a function of shale gouge ratio, reservoir pressure depletion during hydrocarbon production, and the presence of anisotropic relay zones (essentially high permeability conduits within faults) shows the value of numerical modeling in hypothesis testing. Four papers are concerned with hydrothermal fluids in oceanic crust. The first presents a numerical modeling study of fluid flow in the Nankai accretionary wedge, offshore Japan. The paper shows that portions of the wedge that contain permeable basal turbidites support lower fluid pressures and higher shear strengths that allowed larger wedge taper angles compared to portions of the wedge that contain predominantly low permeability basal hemipelagic muds. The second paper presents a numerical modeling study of hydrothermal plume behavior at mid ocean ridges using recently developed nonlinear correlations of physical and thermodynamic properties of water to pressure, temperature, and NaCl content (Driesner 2007; Driesner and Heinrich 2007). Application of these new correlations has led to numerous insights into mid-ocean ridge hydrothermal plume behavior (e.g. Coumou et al. 2008), and in the present study identified a temperature maximum of about 400 °C achieved in these systems at a permeability of about 10m. The third paper reviews experimental methods for determining heat flow on the seafloor and uses heat flow data from the northern Juan de Fuca plate and eastern Cocos plate to map fluid circulation patterns in the shallow oceanic crust. The results of each of these studies are strongly dependent on permeability—in governing the wedge taper angle, the temperature of the discharging hydrothermal plume, or the degree to which seafloor heat flow represents thermal conditions in the underlying crust and has resisted dilution by infiltration of cool seawater—and illustrate the ever present need in hydrologic studies to determine permeability accurately. The fourth paper in this group presents a suite of geochemical models calculating the affinities of abiotic formation reactions for various organic compounds Received: 31 August 2012 /Accepted: 30 October 2012 Published online: 22 November 2012