First zunyite-bearing lithocap in Greece: The case of Konos Hill Mo-Re-Cu-Au porphyry system.

Abstract

Zunyite is a rare F- and Cl- bearing mineral related to advanced argillic alteration zones of porphyry/epithermal style mineralization and is considered as a pathfinder mineral towards high-grade Au ores. We report here the first occurrence of zunyite along with alunite, quartz, APS minerals, diaspore, pyrophyllite and kaolinite in the metallogenic province of Western Thrace. The Konos Hill prospect in Western Thrace comprises a telescoped porphyry Mo-Re-Cu-Au system, overprinted by high-sulfidation mineralization. In low topographic levels, porphyry-style mineralization is exposed and comprises pyrite-chalcopyrite-bornite-molybdenite-rheniite-bearing quartz-stockwork. Host rocks are subvolcanic bodies of granodioritic composition that have suffered pervasive sericitic alteration. High-sulfidation epithermal-style alteration occupies the higher topographic levels and has caused significant overprinting of the porphyry-style mineralization and alteration. It consists of silicified zones related to N-S and E-W trending faults, which grade outwards to advanced argillic alteration assemblages. These assemblages are characterized by abundant alunite and quartz, with minor presence of diaspore, APS minerals, kaolinite, pyrophyllite and zunyite. Zunyite forms euhedral crystals that reach in size up to 300μm. They sometimes include minor quartz and are associated with alunite, APS minerals and pyrophyllite. EPMA data revealed variations in the F and Cl content of zunyite, that range between 3.62-6.54 wt.% and 2.65-3.15 wt.% respectively. Alunite supergroup minerals display a wide compositional range and are represented by members of the alunite, beudanite and plumbogummite subgroups. Alunite and natroalunite constitute the most common advanced-argillic alteration minerals and are found in both quartz+zunyite and quartz+diaspore+pyrophyllite assemblages. Available mineral-chemical data favor the existence of compositions that cover a complete solid-solution series between Na- and K-rich varieties. Common mode occurrences comprise euhedral, tabular-shaped and rarely pseudocubic crystals. APS minerals are usually found as pseudocubic crystals forming the cores of tabular alunites. Analyzed compositions comprise woodhouseite (Sr-, Ce- and Sr-Ce- rich members were found). Diaspore forms aggregates of euhedral, coarse-grain crystals scattered in strongly silicified rock. Finally, pyrophyllite when present, forms acicular aggregates in the matrix along with diaspore and quartz. Available data suggest that the formation of the studied advanced argillic alteration assemblages is hypogene and due to ascending magmatic fluids released by the subvolcanic bodies. Mineralogical variances in the different assemblages may reflect distinct degrees of hydrothermal alteration. Co-existence of zunyite, APS minerals and pyrophyllite could be used to set constraints on the physicochemical conditions of formation of the assemblage, as the volatile-rich nature of the minerals reflects a narrow range of pH and temperature in hydrothermal systems.