Hydrothermal corundum, topaz, diaspore and alunite supergroup minerals in the advanced argillic alteration lithocap of the Kassiteres-Sapes porphyry-epithermal system, western Thrace, Greece

摘要

The Kassiteres-Sapes area is a telescoped porphyry Cu-Mo-Re ± Au and epithermal Au-Ag-Cu-Te system, hosted within Oligocene calc-alkaline to high-K calc-alkaline granodiorite-tonalite, microdiorite and quartz-feldspar porphyries and their volcanic equivalents. High- and intermediate sulfidation epithermal mineralization was deposited in advanced argillic lithocaps that overprint sodic/potassic and sericitic alteration of the intrusive rocks. Deep level, corundum-sericite-feldspar-, corundum-quartz-, and quartz-diaspore-topaz-APS minerals-pyrophyllite-sericite bearing assemblages occur in close proximity to porphyry-style mineralization at the Koryfes, Konos Hills and St Demetrios deposits. Rapid cooling of ascending magmatic-hydrothermal solutions at pressures below 0.6 kb and temperatures above 370℃ could dilute quartz resulting in silica undersaturation and the formation of corundum-sericite at Koryfes Hill. The corundum-quartz assemblage at St Demetrios was likely derived from previous diaspore-bearing advanced argillic alteration at moderate temperature (< 400℃ ) and low pressure, followed by rapid depressurization accompanying boiling of the hydrothermal fluid. The disequilibrium assemblage quartz+diaspore+topaz (± pyrophyllite + sericite) formed between 285° and 320℃ by vapor saturation pressure from acid solutions after condensation of vapors released from the porphyries. The presence of topaz reflects the presence of fluorine in the fluid. Alumino-phosphates-sulfates (woodhouseite, Sr-rich woodhouseite and Sr-Ce-rich woodhouseite) are present as pseudocubic crystals enclosed within diaspore and natroalunite. High level lithocaps include natroalunite/alunite-kaolinite/dickite-pyrite bearing assemblages of both replacement- and vein-type, which deposited at 300° to 200℃ by increasing acidity of the fluid at higher topographic levels and/or from direct input of magmatic vapors. Episodic magmatic-hydrothermal activity in the district is characterized by several stages of advanced argillic alteration and associated Au-Ag-Cu-Te high-sulfidation mineralization.