A new occurrence of garnetiferous skarn rocks in Saudi Arabia: a case study from Bahrah area, Jeddah–Makkah Al Mukaramah highway
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Keywords:
Titanite
Tremolite
Actinolite
Diopside
Hornblende
Andradite
Grossular
Titanite
Grossular
Cassiterite
Andradite
Diopside
Ilmenite
Supergene (geology)
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Grossular
Titanite
Almandine
Andradite
Pyrope
Cassiterite
Hornblende
Arsenopyrite
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Abstract The Enjerd skarn is located 28 km northwest of Ahar in East Azarbaijan Province, northwestern Iran. Based on mineral composition, paragenesis, and mineral relationships, two stages of hydrothermal alteration and related mineralization have been identified. Stage I was produced by early pulses of Fe-rich, magmatically derived fluids coincident with potassic alteration of the main stock. This event caused the formation of oscillatory zoned garnets, followed by pyroxene (diopside-heden-bergite) + wollastonite + magnetite and rare MoS at high temperatures (∼445°C). Molybdenite was the only sulfide mineral deposited at this stage. Stage II occurred at a lower temperature (∼360°C), and is characterized by tremolite-actinolite, epidote, quartz, calcite, apatite, titanite, and hematite crystallization and by copper deposition. Copper mineralization was due to decreasing fO2 and increasing pH as temperatures fell below 400°C. Fluctuations in the Al+3/Fe+3 ratio of hydrothermal fluids at the Enjerd skarn resulted in complex oscillatory zoning in the garnets from andradite100 to andradite76-grossular24. Individual zones are typically composed either of near-end-member andradite or andradite containing approximately 20 mole % grossular. Introduction of fresh batches of Fe-rich fluid caused crystallization of near-end-member andradite garnet because the system was buffered by the fluid. The Al in the system was derived from Cretaceous marls and limestone sequences, because the Al available from granitoid magmatic solutions was very low and almost constant.
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Paragenesis
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Molybdenite
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Tourmaline
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Garnet skarn mineralization was recently studied at the Trohanka locality near Prakovce (Gemeric Unit, Eastern Slovakia). Ca-skarn forms lenticular bodies in green schist environment. It mainly consists of zonal garnets, pyroxenes, amphiboles and magnetite accumulations. Studied garnets are rich in andradite component (up to 89.95 mol. %) with minor grossular component (6.83 - 39.67 mol. %). Strong oscillatory zoning in andradite is caused by substitution of Fe3+ and Al3+. Most pyroxenes are rich in the hedenbergite component. In some cases, euhedral diopside crystals with marginal transition zones (composed of diopside with lower content of Mg2+ and higher content of Fe2+) were found in hedenbergite matrix. Amphiboles are dominantly represented by ferro-actinolite and ferro-hornblende in association with isolated euhedral crystals of ferro-tschermakite and ferro-pargasite. Indistinct chemical zonality of amphibole euhedral crystals is caused by presence of ferro-pargasite in the central parts and ferro-tschermakite in the peripheral parts of crystals.
Andradite
Grossular
Amphibole
Diopside
Actinolite
Almandine
Hornblende
Formula unit
Pleochroism
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Andradite
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Pyroxene
Actinolite
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Titanite
Tremolite
Actinolite
Diopside
Hornblende
Andradite
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An extensive complex zoned skarn is developed at the contact of a leucoadamellite intrusive at Doradilla, NW New South Wales. The skarn is a disequilibrium assemblage resulting from a progressive sequence of replacement of a carbonate precursor. Early grossular‐clinopyroxene rocks are replaced by andradite with 0.5–3.5 wt.% SnO2 clinopyroxene and quartz. Later alteration along fractures and bedding planes of the garnet‐clinopyroxene quartz assemblage has produced calcite‐malayaite (CaSn0.95Ti0.05SiO5) veins. The final replacement stage was the overprinting of the silicate phases by assemblages containing sulphides, cassiterite, magnetite, titanite, fluorite, biotite and chlorite. The tin content of garent increases with increasing andradite component suggesting replacement of Fe3+ by Sn4+. Associated clinopyroxenes contain 0.1% SnO2. The coexistence of titanite and its tin isomorph malayaite with extremely limited solid solution indicates late stage skarn temperatures of less than 400°C.
Cassiterite
Titanite
Andradite
Grossular
Baddeleyite
Greisen
Arsenopyrite
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Lower Silesia hosts important European nephrite deposits of Jordanów and less-known of Złoty Stok. Nephrite artifacts were discovered in archaeological sites dated back to the Neolithic period, across Eurasia. Especially artifacts found in Poland, Italy and Bulgaria may originate from Polish nephrites. Nowadays, only one artifact is precisely linked to Jordanów. Petrographic study of nephrites and chemical analyses of constituents by means of EMPA allow accurate identification of the nephrites. The characteristic phases of Jordanów tremolite nephrite are rotated and cataclased diopside porphyroblasts with pressure shadows, chlorite layers and nests with interlocking non-pseudomorphic texture and prehnite veins. The presence of hydrogrossular, grossular, titanite, apatite with monazite inclusions, and zircon with pleochroic haloe is typical. Chlorites are usually represented by penninite, and minor clinochlore and diabantite. The characteristic features of Złoty Stok actinolite nephrite are löllingite and diopside crystals usually visible by the naked eye, with the presence of quartz and carbonates. Löllingite is chemically inhomogeneous and gold bearing. Most of the mineralogical-petrological features can be obtained using non-destructive methods
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Andradite
Actinolite
Grossular
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Greenschist
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