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International audienceSets of fractures and breccia sealed by Ni-rich silicates and quartz occur within saprock of the New Caledonian regolith developed over ultramafic rocks. The crystallization sequence in fractures is as follows: (1) serpentine stage: lizardite > polygonal serpentine > white lizardite; (2) Ni stage: Ni-Mg kerolite followed by red-brown microcrystalline quartz; and (3) supergene stages. The red-brown microcrystalline quartz corresponds to the very last stage of the Ni sequence and is inferred to have precipitated within the 50–95 °C temperature range. It constitutes also the main cement of breccia that has all the typical features of hydraulic fracturing. The whole sequence is therefore interpreted as the result of hydrothermal fluid circulation under medium to low temperature and fluctuating fluid pressure. Although frequently described as the result of a single downward redistribution of Ni and Mg leached in the upper part of the regolith under ambient temperature, the Ni silicate veins thus appear as the result of recurrent crack and seal process, corresponding to upward medium temperature fluid convection, hydraulic fracturing and subsequent fluid mixing, and mineral deposition

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Natural H2 emissions from the ground have now been measured in many places worldwide. These emissions can be localized on faults or be more diffuse in some sedimentary basins, usually of Proterozoic age. In such a case, emanation zones are often visible from aerial images or on high-resolution topographic maps since they correspond to slight depressions of circular to elliptic shape. Furthermore, the rounded depressions are covered with a scrubby vegetation which often contrasts with the surrounding vegetation. Although the emission structure displays a very regular shape, the distribution of H2 concentration in the first meter of soil in such a structure does show a clear pattern. For example, the maximum concentration is almost never measured in the center of the structure and the few time-resolved data show that the soil H2 concentration is variable with time. Here, the time and space evolution of H2 concentration is simulated using a 2-D advective-diffusive model of H2 transport in porous media. Several parameters have been tested as the depth and periodicity of the H2 point source (pulsed), bacterial H2 consumption and permeability heterogeneities of the soil. The radius of the structure is linked to the time spent by the H2 in the soil that depends on the soil permeability, the depth of the gas leakage point and the pressure of the bubble. To account for field observations, the case of a shaly, less permeable, heterogeneity in the center of the structures has been modeled. It resulted in an increase of the concentration toward the rim of the structure and a close to zero signal in its center. If the deep signal is periodic with a frequency smaller than a few hours, H2 concentration within the soil is almost constant; in other cases, the near surface concentration wave reflects the concentration periodicity of the source with a delay (in the range of 12 h for 30 m of soil) and so the near surface H2 concentration values will be highly dependent on the time at which the measurement is performed. H2 monitoring through a sensor network is thus mandatory to characterize the H2 dynamics in the soil of fairy circles.

This study is devoted to understanding the impact of topography and hydrodynamics on the formation of high‐grade supergene nickel deposits. The investigation proposes a new conceptual mineralization model that describes the formation of various exceptionally Ni‐enriched hot spots observed in lateritic profiles. Numerical analysis of the effects of local hydrodynamics on deposits formation is performed by means of PHREEQC geochemical simulator and COMSOL Multiphysics software. These are coupled through an iCP Java interface that allows to code their level of interaction and facilitates the exchange of parameters. The model developed extends the currently existing geochemical formulation of nickeliferous laterite formation from peridotite carried out in 1‐D and is additionally capable of simulating mass solute transport and geochemical processes within complex fractured‐porous systems. The simulations improve our understanding of metal enrichment in saprolite and bedrock zones. It was shown that, although the initial development of nickel lateritic ores takes a few million years, they are prone to relatively quick leaching and subsequent redistribution of Ni when the topography changes in response to tectonic processes. The latter leads to the formation of rich nickel deposits at the bottom of the slope, mostly due to leaching of the saprolite material. In addition to the role of changes in topography, the critical impact of fractures and fracture networks on metal mobility and distribution was identified. The model developed provides significant insight into the distribution of mineral resources, in particular Ni deposits, and can be of great help for future mineral prospecting in industry.

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This paper examines numerical approaches to model operation of gas storage in salt caverns. The emphasis is on taking into account the thermal exchanges between the well, the cavern, and the rock mass, as well as the modelling of the volume losses of the cavern, while keeping the model fast, simple, and operationally usable. The use of this model in the context of monitoring storage operations is illustrated.