Fluid evolution and Re enrichment in the Maronia porphyry system, NE Greece: Insights from pyrite, molybdenite and quartz micro-analysis

The monzonitic-granitic intrusion of Maronia in NE Greece hosts an anomalously Re-rich Cu-Mo ± Au porphyry system. The mineralization is dominated by pyrite, molybdenite, chalcopyrite, native Au, and rheniite (ReS2) in quartz-bearing hydrothermal veins associated with potassic, sericitic, and advanced argillic alteration. To decipher the hydrothermal processes leading to the enrichment of Re, Mo, and Au, we used detailed imaging techniques such as BSE-SEM and CL-SEM combined with in-situ trace element analysis of pyrite, molybdenite, and quartz by EPMA and LA-ICP-MS, in-situ δ34S analysis of pyrite by LA-ICP-MS and fluid inclusion microthermometry in quartz from different veins and miarolitic cavities.

Hydrothermal quartz CL intensity, textures and Al/Ti and Ge/Ti ratios are characteristic for each vein type and Ti-in-quartz thermometry indicates a temperature decrease from 600°C to 300°C during the porphyry-epithermal transition. Arsenic, Au, Ag, Te, Pb, Cd, and Se contents and ratios of As/Sb, Co/As, Se/Tl, Se/Te and Se/Ge in pyrite record changes in the physicochemical fluid conditions. Extreme Re enrichment in molybdenite at Maronia (⌀=3922 ppm) compared to the global average from porphyry deposits (⌀=779 ppm) is likely controlled by a district-wide Re-rich magmatic source, whereas the enrichment of Re in B-type (⌀=5532 ppm) relative to D-type (⌀=1954 ppm) veins seems to be related to changes in fluid temperature, salinity and fO2. Our multi-mineral trace element approach allows us to constrain the framework of the physicochemical fluid parameters during the porphyry-epithermal transition favorable for the enrichment of Re and related precious elements like Au, Ag and Te at Maronia.