M.Sc. Thesis Late-stage B-bearing fluid circulation in the Kavala pluton: Evidence from mineralogical, spectroscopic and geochemical data in tourmaline-rich fault-related rocks

Τα τουρμαλινικά πετρώμτα της Καβάλας είναι προϊόντα κρυστάλλωσης υδροθερμικών ρευστών, εμπλουτισμένων σε Βόριο, που διαχωρίστηκαν από γρανιτικό μάγμα.Το μάγμα που δημιουργήθηκε με διαδικασίες τήξης του φλοιού, κατά τη διάρκεια της εκταφής του συμπλέγματος μεταμορφικού πυρήνα, διαφοροποιήθηκε και απελευθέρωσε μέσω βρασμού μια ρευστή φάση πλούσια σε B. Κατά το βρασμό η υδροστατική πίεση,ξεπέρασε την λιθο-στατική σε περιβάλλον ανώτερου τεκτονικού ορόφου, όπου επικρατεί η θραυσιγενής παραμόρφωση.Πολλαπλά στάδια υδραυλικής ρωγμάτωσης έλαβαν χώρα κατά μήκος των ΒΑ-ΝΔ αξονικών επιπέδων της φύλλωσης και πτύχωσης του γρανοδιορίτη, σε μεγάλες γωνίες κλίσης. Η διαδικασία αυτή πιθανόν να επαναλήφθηκε τουλάχιστον δύο φορές, ύστερα από επιπλέον διαφοροποίηση και απόμειξη. Η κυκλοφορία των υδροθερμικών ρευστών, είχε ως αποτέλεσμα την μετασωματική εξαλλοίωση του γρανοδιορίτη και τη δημιουργία τουρμαλινικών πετρωμάτων. Οι τουρμαλινίτες, στους οποίους το ποσοστό συμμετοχής του τουρμαλίνη ξεπερνά το 50% κ.ο., δημιουργήθηκαν σε υψηλούς λόγους αντίδρασης ρευστού/πετρώματος, ενώ τα τουρμαλινικά λατυποπαγή σε μικρότερους. Τα τουρμαλινικά λατυποπαγή, αποτελούμενα από γωνιώδεις κλάστες του περιβάλλοντος πετρώματος και κύρια μάζα από κρυπτοκρυσταλλικό τουρμαλίνη,υπερέχουν, και δείχνουν πως η διαδικασία ήταν υψηλής θερμοκρασίας και αλατότητας. Σταδιακός εμπλουτισμός σε βόριο και άλλα οξείδια συμβατά με τον τουρμαλίνη, σε συνδυασμό με την απόπλυση σε REE, παράγουν το γεωχημικό αποτύπωμα της τουρμαλινώσης, υποδεικνύοντας πολύ χαμηλό pH, κατά τη συνολική διαδικασία, το οποίο ίσως αποτελεί κύριο αποτρπετικό παράγοντα για την παρουσία μεταλλοφορίας θειούχων μέσα στα λατυποπαγή.The tourmaline-rich rocks in Kavala are the products of juvenile late B-rich hydrothermal fluids exsolved from a granitic magma. In other words, this granitic magma that generated after anatectic processes in a thickened crust, during its emplacement in an ascending core-complex margin was fractionated leading to the exsolution through boiling of an immiscible volatile B-rich phase. The boiling pressure of the fluid exceeded the relatively low lithostatic pressure as it happened in a brittle setting of upper tectonic floors. Hydraulic, almost vertical, fracturing of various degrees and in dispersed places occurred, which mostly located along the NE-SW axial planes of the foliated and folded granitic pluton. This process may have occurred at least two times after further magmatic fractionation and exsolution. Hydrothermal fluid infiltration, triggered metasomatic alteration of the host granodiotite, resulting in the formation of tourmaline-rich rocks. Tourmalinites, in which tourmaline exceeded 50% vol, were formed at high fluid/rock ratios, while tourmaline breccias formed at lower ones. The latter, which comprise abundant angular to sub-rounded clasts of country rock infilled mostly by cryptocrystalline tourmaline, prevailed, indicating that the process was of high temperature and salinity.Progressive enrichment in Boron and other oxides compatible in tourmaline, coupled with depletion in REE’s produced a geochemical signature of typical toyrmalinization, suggesting an acidic pH, for the overall procces, which is maybe a dominat factor for the absence of sulfide mineralization within the breccias.

Temporal evolution of the Majuba Hill Cu-(Mo)-(Sn) deposit, Pershing County, Nevada

The Majuba Hill intrusive complex of northwestern Nevada exhibits two superimposed hydrothermal mineral deposits. Quartz vein-hosted Cu-(Mo) mineralization occurring in Jurassic (~160 Ma) granodiorite was later overprinted by Oligocene (~25–28 Ma) subvolcanic rhyolite magmatism and related Cu-(Mo)- (Sn) mineralization. The Jurassic hydrothermal system is characterized by early barren to molybdenite-bearing granular quartz veins and barren comb-textured quartz veins reopened by later tourmaline, calcite, and chlorite. Fractures in early veins provided pathways for tourmaline, calcite, chlorite, and later sulfide deposition. Hydrothermal Cu occurs as anhedral chalcopyrite overprinting earlier veins and gangue minerals. Oligocene hydrothermal deposition commenced with granular quartz + (molybdenite) veins and barren comb-textured quartz veins crosscutting subvolcanic rhyolites. Euhedral cassiterite + tourmaline + quartz mineralization (~25 Ma) followed and accompanied significant tourmaline + quartz + sericite + fluorite deposition and alteration of porphyritic rhyolites. Anhedral chalcopyrite deposited as disseminated crystals overprinting altered K-feldspar phenocrysts and by infilling portions of open space-filling veins. Chalcopyrite also crosscuts euhedral quartz-arsenopyrite veins. Subequal amounts of pyrrhotite and minor sphalerite are spatially associated with chalcopyrite but occur earlier in the paragenetic sequence. Shallow portions of the Majuba Hill deposit were affected by a supergene fluid that deposited digenite + (covellite) along the margins of hypogene chalcopyrite crystals. Minor cuprite and native Cu were also observed in drill hole MHB9 but appear less common than digenite and covellite. Paragenetic sequences from both the Jurassic and Oligocene hydrothermal systems suggest the sulfidation state of parent hydrothermal fluid(s) increased over the duration of each system. The genetic association between cassiterite, hydrothermal tourmaline + quartz, and emplacement of subvolcanic rhyolites suggests Sn endowment may be derived from a peraluminous and fractionated parent intrusion. These genetic associations are similar to those observed in Bolivian rhyolite-hosted Sn deposits. Significant hypogene and supergene hydrothermal Cu systems like those at Majuba Hill, however, are absent in the Bolivian deposits

Using Tourmaline As An Indicator Of Provenance: Development And Application Of A Statistical Approach Using Random Forests

Tourmaline is a petrologic indicator mineral that is the major repository of boron in the earth’s crust. It forms readily when boron is present, accommodating multiple cations and anions with multiple possible substitutions for each site in the crystal structure. It is stable over a wide variety of pressures and temperatures, from near-surface P/T conditions to greater than 950 C and 7 GPa. It records information about conditions of formation, as well as pressure and temperature. Due to its resistance to chemical or physical weathering, and the negligible diffusion of elements in the crystal lattice, information about provenance is preserved. In Henry and Guidotti (1985), major elements of tourmaline were used to construct ternary diagrams that classify tourmalines according to provenance. However, this technique does not make use of the entirety of available chemical data. New statistical techniques can make use of all available chemical information and provide information about element importance. Using a novel application of an existing statistical method, random forests, to high-dimensional tourmaline data, provenance information is obtained. Existing chemical analyses are assembled into a database and labeled with their provenance. A random forest is ‘grown’ using a full database of tourmaline data, producing a set of rules for classifying tourmalines according to provenance. The random forest method has internal controls on accuracy and fitting of the data, and is capable of classifying tourmalines at a level of between 90 and 95% accuracy. As an independent test, a random forest built from this database is used to successfully classify tourmalines according to provenance

Textures and composition of hydrothermal tourmaline in the Chacaltaya-Kellhuani-Milluni district, La Paz, Bolivia

Los minerales pertenecientes al supergrupo de la turmalina son muy útiles para registrar las condiciones fisicoquímicas de su cristalización y rastrear los procesos geológicos asociados a la formación de depósitos minerales. Este estudio presenta nuevos resultados sobre la petrografía y geoquímica de turmalina en el distrito Triásico de Chacaltaya-Kellhuani-Milluni (oeste de Bolivia), ubicado en el segmento norte de la Cordillera Real dentro del Cinturón Estannífero de los Andes Centrales. El área de estudio se encuentra alrededor del stock granítico de Chacaltaya, aproximadamente 5 km al sur del granito de Huayna-Potosí. En este distrito, la mineralización hidrotermal está asociada a greisen, brechas y vetas. Se distinguen tres tipos principales de turmalina hidrotermal, de acuerdo a la petrografía: i) Tur-1, que presenta un color naranja y se manifiesta como cristales esqueléticos dispersos o como reemplazamiento pseudomórfico de cristales primarios de feldespato potásico en el granito greisenizado de Chacaltaya, como cristales euhedrales a subhedrales diseminados que a menudo forman agregados radiales en el greisen, y como fragmentos de cristales en brechas cementadas por turmalina; ii) Tur-2, que se presenta como cristales aciculares muy finos de color verde oscuro que componen el cemento de las brechas; y iii) Tur-3, que forma cristales elongados subhedrales de color verde con tonalidades marrones con zonación oscilatoria dentro de las vetas y cristales anhedrales muy finos en el halo de las vetas. Tanto Tur-2 como Tur-3 muestran evidencia textural de co-cristalización con casiterita tanto en brechas hidrotermales como en vetas. Los tres tipos petrográficos pertenecen al grupo de las turmalinas alcalinas y se caracterizan por ser ricas en Fe, en su mayoría cercanas a la composición del chorlo, extendiéndose en parte a los campos composicionales de la foitita y dravita. La superposición composicional de los tres tipos petrográficos de turmalina sugiere un continuo en la evolución del fluido hidrotermal que reflejaría un enfriamiento progresivo del sistema desde Tur-1 a Tur3, lo que finalmente habría conducido a la cristalización de casiterita coetáneamente a la cristalización de Tur-2 y Tur-3.Minerals of the tourmaline supergroup are useful to decipher the physicochemical conditions of its crystallization and to trace geologic processes associated with the formation of ore deposits. This study presents new results on the petrography and geochemistry of tourmaline from the Triassic Chacaltaya-Kellhuani-Milluni district (western Bolivia), located in the northern segment of the Cordillera Real within the Central Andean tin belt. The study area lies around the Chacaltaya granitic stock about 5 km south of the Huayna-Potosí granite. Here, hydrothermal mineralization is associated with greisen, breccia, and veins. Three main petrographic types of hydrothermal tourmaline are distinguished: i) Tur-1, which is orange in color and occurs as scattered skeletal crystals or pseudomorphic replacement of primary Kfeldspar in the greisenized Chacaltaya granite, as disseminated euhedral to subhedral crystals mostly forming radial aggregates in greisen, and as crystal fragments in tourmaline-cemented breccias; ii) Tur-2, which appears as very fine acicular grains with a dark-green color composing the cement of breccias; and iii) Tur-3, which forms elongated green-brownish subhedral, oscillatory-zoned crystals within veins, and anhedral, very fine-grained crystals in the halos of veins. Both Tur-2 and Tur-3 show textural evidence of co-crystallization with cassiterite in both hydrothermal breccias and veins. The three petrographic types of tourmaline belong to the alkali group and are characterized by Fe-rich compositions, in majority close to the schorl endmember, and partly extending into the compositional fields of foitite and dravite. Overlapping tourmaline compositions suggest a progressive cooling of the ore-forming system from Tur-1 to Tur-3, eventually leading to cassiterite deposition during the crystallization of Tur-2 and Tur-3