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Metamorphic Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Metamorphism often involves fluids, most commonly water-rich but sometimes dominated by CO2, sulfur, or other components. The fluids may be magmatic (expelled by magmas as they crystallize), meteoric (derived from precipitation that infiltrates the ground), released during subduction of wet lithosphere, or products of reactions that release H2O or CO2 from minerals. Hydrothermal metamorphism occurs when fluids significantly alter protolith rocks. This kind of metamorphism can affect large areas and be part of regional metamorphism, or it can be localized and part of contact metamorphism. In either case, the metamorphism involves hot, generally water-rich fluids that flow through cracks and along grain boundaries. The fluids act as catalysts and fluxes that promote reactions and large crystal growth. More important, the fluids may change the composition of the protolith by adding or removing specific elements, and the resulting rock may be of a much different composition than its parent. This composition-changing process, called metasomatism, creates many kinds of products. Metasomatism can, for example, create ore deposits by concentrating minerals (most commonly copper, iron, or lead sulfides) in host rocks where they did not exist previously. And metasomatism was responsible for the wollastonite ores in the Adirondacks.
Metamorphism
Published in Aurèle Parriaux, Geology, 2018
High-temperature magmatic fluids cause metasomatism as they infiltrate into the surrounding rocks. A typical case is a granitic intrusion into a Mg-bearing limestone rock. This creates a skarn, a marble that contains silicate minerals. Fluids from the magma enrich the surrounding rock in major elements (Si, Al and Fe) and minor elements (Cl, F, and B). This process may form rare minerals (garnet, pyroxene, and calcic amphibole), and chemical reactions may produce exploitable polymetallic deposits.
Metamorphism
Published in Aurèle Parriaux, Geology, 2018
High-temperature magmatic fluids cause metasomatism as they infiltrate into the surrounding rocks. A typical case is a granitic intrusion into a Mg-bearing limestone rock. This creates a skarn, a marble that contains silicate minerals. Fluids from the magma enrich the surrounding rock in major elements (Si, Al and Fe) and minor elements (Cl, F, and B). This process may form rare minerals (garnet, pyroxene, and calcic amphibole), and chemical reactions may produce exploitable polymetallic deposits.
Improving geological logging of drill holes using geochemical data and data analytics for mineral exploration in the Gawler Ranges, South Australia
Published in Australian Journal of Earth Sciences, 2021
E. J. Hill, A. Fabris, Y. Uvarova, C. Tiddy
Metamorphic and metasomatic rocks present several challenges for geochemical discrimination. Metamorphism occurs in a closed, or near-closed chemical system, and therefore metamorphic rocks have similar chemistry to their protolith (Putnis & Austrheim, 2010). For instance, an amphibolite facies pelite may have the same chemical composition as a mudstone. In contrast, metasomatism results in a change in rock composition and texture owing to interaction with hydrothermal or other fluids. The resultant metasomatic effect on the rock is therefore influenced by the chemistry of the altering fluids and can result in dramatic changes to its chemical composition (Putnis & Austrheim, 2010). Drill hole MSDP11 intersected several metasomatised intervals, logged as skarn, between 320 and 450 m. The effect on the chosen compositional variables ranges from dramatic to more subtle; however, all intervals are delineated in the multivariate mosaic plots (Figure 17). The main skarn zone between ∼320 and 390 m is identified in the multivariate mosaic plot at a scale of <90, whereas narrow intervals logged as skarn below 408 m show a weaker chemical contrast to surrounding lithologies and are identified at a scale of <10. The variation in chemical composition of the skarn highlighted in the multivariate plots demonstrates an advantage of integrating geochemistry with logging. Broad geological classifications such as the use of the term skarn can more easily be refined to provide additional sub-classification. For example, intervals of skarn in drill hole MSDP11 could clearly be sub-divided using tessellation of geochemical data to provide improved lithological classification (Figure 17).