18-Geol-A1 Mineralogy and Petrology · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A1 Mineralogy and Petrology, 2019-Dec. Closed book; no calculator permitted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate structural classification, mineral chemistry and substitution, crystal systems, sulfide/carbonate ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, tectonic settings of magmatism, metamorphic/metasomatic processes, volcanic and pyroclastic processes, plate-tectonic cycle).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
The distinction is fundamentally about mass balance across the rock's boundary during recrystallization.
Ordinary metamorphism is essentially a closed-system process: mineralogy and texture re-equilibrate in the solid state in response to changed pressure, temperature and/or differential stress, but the rock's bulk chemical composition is conserved (apart from minor loss of volatiles such as $\text{H}_2\text{O}$ and $\text{CO}_2$ during dehydration/decarbonation reactions). A shale metamorphosed to a slate/phyllite/schist carries essentially the same major-element chemistry throughout, just reorganized into new, higher-grade minerals.
Metasomatism is open-system: a chemically active fluid phase actively adds and/or removes components across the rock's boundary, so the rock's bulk composition genuinely changes, not just its mineralogy/texture. This requires sustained fluid flow along a permeable pathway (a fracture network, or a reactive lithologic contact) to carry material in and out.
The clearest field expression is a skarn: at an igneous–carbonate contact, a fluid derived largely from the cooling intrusion carries Si, Fe, Al and Mg outward into the limestone while Ca and $\text{CO}_2$ diffuse back inward, producing a Ca–Fe–Mg–Mn calc-silicate rock (garnet–pyroxene–wollastonite–epidote) that could never form by isochemical recrystallization of either protolith alone. Without that cross-boundary mass transfer, the identical intrusion-into-limestone contact instead produces only a thermally-recrystallized but compositionally unchanged marble or hornfels — the isochemical, metamorphism-only outcome.
The two processes are not mutually exclusive events but end-members of a spectrum that a single contact aureole can pass through in sequence: early, dominantly conductive heating produces isochemical hornfels/marble across most of the aureole, while later, focused fluid flow along the most permeable pathways — typically the intrusion-carbonate contact itself, or a fracture network within it — superimposes localized metasomatic skarn on top of that earlier isochemical baseline. Recognizing this sequence in the field (unaltered marble grading inward into a sharply-bounded skarn zone at the contact) is itself diagnostic evidence that fluid flow, not just heat, was concentrated along that specific pathway.
