Quartz dissolution associated with magnesium silicate hydrate cement precipitation

Quartz has been replaced by magnesium silicate hydrate cement at the Feragen ultramafic body in south-east Norway. This occurs in deformed and recrystallized quartz grains deposited as glacial till covering part of the ultramafic body. Where the ultramafic body is exposed, weathering leads to high-pH (∼ 10), Mg-rich fluids. The dissolution rate of the quartz is about 3 orders of magnitude higher than experimentally derived rate equations suggest under the prevailing conditions. Quartz dissolution and cement precipitation start at intergranular grain boundaries that act as fluid pathways through the recrystallized quartz. Etch pits are also extensively present at the quartz surfaces as a result of preferential dissolution at dislocation sites. Transmission electron microscopy revealed an amorphous silica layer with a thickness of 100–200 nm around weathered quartz grains. We suggest that the amorphous silica is a product of interface-coupled dissolution–precipitation and that the amorphous silica subsequently reacts with the Mg-rich, high-pH bulk fluid to precipitate magnesium silicate hydrate cement, allowing for further quartz dissolution and locally a complete replacement of quartz by cement. The cement is the natural equivalent of magnesium silicate hydrate cement (M-S-H), which is currently of interest for nuclear waste encapsulation and for environmentally friendly building cement, but it has not yet been developed for commercial use. This study provides new insights that could potentially contribute to the further development of M-S-H cement.

Auriferous Quartz Veining Due to CO2 Content Variations and Decompressional Cooling, Revealed by Quartz Solubility, SEM-CL and Fluid Inclusion Analyses (The Linglong Goldfield, Jiaodong)

Quartz is the most common gangue mineral in hydrothermal veins. Coupled with capacities of hosting fluid inclusions and recording varieties of microtextures, its solubility behavior may provide unparalleled insights into hydrothermal processes. In this study, the Linglong goldfield in Jiaodong is targeted to investigate gold-producing quartz veining process. Scanning electron microscope (SEM)-cathodoluminescence (CL) imaging uncovered three episodes of quartz deposition, intervened by an episode of quartz dissolution. Based on newly-developed quartz solubility diagrams and CL-aided fluid inclusion microthermometry, it is proposed that precipitation of the earliest quartz (Qz1) was controlled by CO2 content increase and subordinately affected by decompressional cooling, leading to the formation of the early thick gold-barren veins (V1); the second generation of quartz (Qz2a) was formed by the same fluids that may have been diluted and cooled by meteoric water, leading to a greatly reduced quantity of quartz and the deposition of pyrite and gold; and the third generation of quartz (Qz2b) was deposited along with polymetallic sulfides, due to fluid cooling following a quartz dissolution event likely induced by cooling in retrograde solubility region and/or CO2 content decrease. This research may elucidate gold formation processes in orogenic intrusion—related deposits, and points to imperative CL-based in situ analyses for future studies.

Quartz dissolution associated with magnesium silicate hydrate cement precipitation

Quartz has been replaced by magnesium silicate hydrate cement at the Feragen ultramafic body in south-east Norway. This occurs in deformed and recrystallized quartz grains deposited as glacial till covering part of the ultramafic body. Where the ultramafic body is exposed, weathering leads to high pH (~10), Mg-rich fluids. The dissolution rate of the quartz is about 3 orders of magnitude higher than experimentally derived rate equations suggest under the prevailing conditions. Quartz dissolution and cement precipitation starts at intergranular grain boundaries that act as fluid pathways through the recrystallized quartz. Etch pits are also extensively present at the quartz surfaces as result of preferential dissolution at dislocation sites. Transmission electron microscopy revealed an amorphous silica layer with a thickness of 100–200 nm around weathered quartz grains. We suggest that the amorphous silica is a product of interface-coupled dissolution-precipitation and that the amorphous silica subsequently reacts with the Mg-rich, high pH bulk fluid to precipitate magnesium silicate hydrate cement, allowing for further quartz dissolution and locally a complete replacement of quartz by cement. The cement is the natural equivalent of magnesium silicate hydrate cement (M-S-H), which is currently of interest for nuclear waste encapsulation or for environmentally friendly building cement, but not yet developed for commercial use. This study provides new insights that could potentially contribute in the further development of M-S-H cement.

Reactive Flow Model for Porosity Reduction by Quartz Dissolution/Precipitation

<p>Quartz dissolution and precipitation is an important pore reducing process in geothermal reservoirs. We present a single-phase reactive flow model coupled with hydrodynamic flow and heat transfer components and implement it into COMSOL Multiphysics. The model includes diffusion and advection, and analytical equations are used to describe quartz kinetics and equilibrium concentrations with respect to the silicate phases. The numerical model can <em>a priori</em> be used to analyze the evolution of the porosity/permeability, and hence the productivity of the reservoir induced by heat extraction in geothermal reservoirs. A geothermal reservoir is modeled with realistic time steps, where its geometry is represented as a porous medium block in which chemical reactions occur between the pore fluid and the rock matrix. Future developments include adding a fracture and fracture networks to the system and analyzing the changes in effective stresses in the presence of reactive flow. Economic reservoir development requires a combined analysis of the thermo-hydro-mechanical and chemical processes, and precipitation processes may be important in post-seismic fluid flow processes.</p>

Effect of alkaline diagenesis on sandstone reservoir quality: Insights from the Lower Cretaceous Erlian Basin, China

The Tengger Formation in the Baiyinchagan sag of the Erlian Basin has behaved as a low-permeability petroleum system during its diagenetic history. Through the observation and examination of thin section, scanning electron microscopy, and X-ray diffraction data, this study found the existence of alkaline diagenesis, as indicated by the dissolution of quartz, the precipitation of authigenic illite and chlorite, and the formation of carbonate and authigenic albite cements. There were two types of alkaline diagenetic conditions: the early alkaline diagenetic conditions were controlled by the syndepositional environment, and the multiphase burial alkaline diagenetic conditions were controlled by the evolution of organic acids and thermal fluids. The quantitative model of the evolution of porosity over geologic time developed in this study indicates that quartz dissolution increased the porosity by a total of 0.85%. This work may provide significant advances in the understanding of the low-permeability reservoirs in the Erlian Basin and lays the scientific foundation for oil and gas exploitation.