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New study proposes ‘Goldilocks’ temperature zones as factories for generation of mineral deposits

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Current understanding of the formation of porphyry deposits Porphyry deposits are a major source of copper (Cu), gold (Au), and molybdenum (Mo) (Fig. 1). These deposits contribute 75%, 50%, and 20% of the Cu-, Mo-, Au produced in the world, respectively 1 . Figure 1. Molybdenite (dark grey molybdenum-sulphide mineral) hosted in quartz (grey) veins in a porphyry deposit (Source 2 ) Typically, porphyry deposits form in magmatic arcs (oceanic and continental) above subduction zones in a series of processes beginning with (i) the expulsion of metal-enriched fluids from the subducting slab into the mantle wedge resulting in partial melting of the mantle wedge, followed by, (ii) the migration and stalling of such magmas in the magma chambers in the mid to lower crust, and subsequently, (iii) the migration of metal-rich fluids produced during the crystallisation of magmas in magma chambers into the upper crust where they form pipe- and dyke-like porphyry deposits  3 , 4 (Fig. 2). F...
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Controversies and knowledge gaps in Geology (Part 1): Formation of ‘Archean granitoids’ (TTGs)

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What are TTGs? A suite of granitoids comprising tonalites , trondhjemite and granodiorites collectively referred to as tonalite-trondhjemite-granodiorite (TTG) granitoids are the main constituents of Archean terranes 1 . TTGs were mostly emplaced in the Archean and are not generated in modern-day subduction zones 1 . Thus, these rocks have invariably undergone multiple episodes of metamorphism and usually occur as strongly deformed gneisses (Fig. 1). Figure 1. TTG specimen (Source: Wikipidea) Geochemically, TTGs are sodic (K 2 O/Na 2 O<0.5), contain high contents of light rare earth elements (LREE) and low contents of high rare earth elements (HREE) resulting in strongly fractionated REE patterns (average La/Yb N of 32.4), exhibit negative Nb-Ti anomalies, and lack pronounced Eu and Sr anomalies 2 . The fractionated REE patterns and the Nb-Ti anomalies are interpreted to be indicative of the presence of garnet, amphibole and/or rutile in the melt residue 3 , whereas the Sr and...
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Strange and fascinating rock formations on Earth

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Eye of Sahara Eye of Sahara (Source: NASA) The “Eye of Sahara”, formally known as the Richat Structure or the Guelb er Richât is an eroded circular structure located in the Sahara’s Adrar Plateau in Mauritania. The geological structure spans ~45 km (~28 mi) in diameter can be seen from space and has always captured the attention of astronauts. The “Eye of Sahara” is classified as a domed anticline (i.e., rocks at the “bullseye” or the centre of the structure are older than those on the flanks of the structure) made up of concentric ridges of Proterozoic to Paleozoic sedimentary rocks (sandstone and limestones) from the Taudeni Basin which have been intruded by igneous rocks (carbonatites, gabbros, rhyolite, and kimberlites) 1,2 . Though the genesis of the “Eye of Sahara” structure is largely accepted as a product of gradual erosion of a volcanic dome 2 , earlier models suggest that the structure is a product of a meteorite impact 3 . Giant Crystal Cave Giant Crystal Cave with giant gyp...
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