Marble-Hosted Submicroscopic Gold Mineralization at Asimotrypes Area, Mount Pangeon, Southern Rhodope Core Complex, Greece

Abstract

Gold mineralization at Asimotrypes, Mount Pangeon, Greece, occurs within amphibolite facies rocks of the Southern Rhodope Core Complex, one of the largest metamorphic core complexes in the world. Exhumation of the complex resulted from middle Eocene to middle Miocene northeast-southwest–oriented extension in the northern Aegean and was controlled by the Kerdylion detachment zone. Host rocks are mylonitic, impure dolomite marbles of marine provenance ( δ 13C = 1.9 – 2.9‰), which are intercalated with paramica schists, and amphibolites, and intruded by early Miocene syntectonic granitoids. In the Asimotrypes area, metamorphic rocks and granitoids exhibit flat mylonite-type ductile fabrics with consistent top-to-the-southwest sense of shear, as does the entire Complex. Two mineralogically similar and spatially coincident gold mineralization styles, with a supergene overprint, exist: (1) strata-bound replacement bodies that contain up to 17 ppm gold, concentrated at and controlled by the intersections of several hydrothermally altered top-to-the-southwest marble mylonites, with crosscutting northwest-southeast to east-west–trending high-angle brittle normal faults and fractures, and (2) structurally controlled quartz veins, pods, and lenses with 4 to 13 ppm gold, which occur along the northwest-oriented subvertical brittle structures; the latter crosscut the ductile shear foliation of the host rocks, together with the granitoids, and dip steeply to the southwest. Gold-bearing replacement bodies typically occur as discordant wide halo adjacent to the centrally located brittle structures. Ductile top-to-the-southwest shear zones and cross-cutting brittle structures are considered contemporaneous within the regional extensional deformation and exhumation history of the core complex host rocks. The distribution of gold mineralization is related to the geometry of the brittle structures, strongly suggesting that faults acted as major fluid feeder conduits during gold mineralization. Hydrothermal alteration associated with the auriferous bodies and veins, overprinting the metamorphic rocks at Asimotrypes, consists of quartz, muscovite (sericite), chlorite, calcite (dedolomite), and sulfide minerals (locally, as much as 35 vol %). Gangue quartz occurs as (1) peripheral banded quartz with mylonitic texture, (2) dominant gold-bearing jasperoidal quartz, and (3) late, fine drusy quartz. The gold assemblage consists of arsenopyrite (41.7–43 wt % As; electron microprobe analyses) and arsenian pyrite encompassed by jasperoidal quartz; chalcopyrite, galena, tetrahedrite-tennantite, and sphalerite are trace phases. Secondary ion mass spectrometry (SIMS) spot analyses revealed that gold in replacement ore is submicroscopic and occurs in arsenopyrite and arsenian pyrite (As = 0.01–3.8 wt %), ranging from 0.5 to 29 ppm (mean = 11.6 ppm), and 0.14 to 11 ppm (mean = 2.3 ppm), respectively. Supergene overprint includes colloidal goethite + hematite containing native gold grains (size range, 20–40 μ m) and lesser covellite, malachite, cerrussite, and chrysocolla. SIMS analysis also shows a primary, inhomogeneous distribution pattern for gold that appears to mimic finely banded growth twinning lamellae or growth zones. The form of the vein-related gold is unknown. Fluid inclusions trapped during gold mineralization are low to medium salinity (<10 wt % NaCl equiv), aqueous-carbonic H2O-CO2-NaCl inclusions with highly variable carbonic to aqueous contents and naturally decrepitated or reequilibrated inclusions, interpreted to represent decompression by concurrent exhumation of the Southern Rhodope Core Complex. Trail-bound aqueous H2O-NaCl inclusions are interpreted as a late extensional brittle deformation-related hydrothermal event unrelated to gold-bearing sulfide precipitation. Trapping temperatures of the H2O-CO2-NaCl inclusions and oxygen isotope equilibrium temperatures between quartz and muscovite (sericite) indicate formation of gold-bearing arsenopyrite, arsenian pyrite, and quartz bodies at ~270°C, with pressures of 1,800 to 2,000 bars corresponding to depths between 6 and 9 km under lithostatic load. Measured hydrogen and calculated oxygen isotope compositions of the aqueous-carbonic mineralizing fluids ( δ Dwater = −125 to −105‰, δ 18Owater = 11.9 to 13.7 ‰ at 250°C, and 13.9 and 15.7 ‰ at 300°C) indicate an evolved meteoric origin. Water/rock calculations indicate these values could derive from heated meteoric water that had undergone isotope exchange during circulation through marbles at low water/rock ratio. Gold-associated sulfide minerals have δ 34S values between 2.2 and 3.1 per mil, interpreted as leaching of the core complex Paleozoic marine metasedimentary (marble), and interlayered mafic metavolcanic (amphibolites) and Miocene granitic rocks through fluid/rock interaction. Gold was probably precipitated from an H2S-rich fluid when the host sulfide minerals were stabilized by pyritization of marble wall rock. Asimotrypes gold mineralization is interpreted as postmetamorphic and postmagmatic, and linked to late stages of exhumation of the Complex by middle Eocene to middle Miocene extension in the northern Aegean. The location and shape of gold orebodies are controlled by interactive ductile-then-brittle deformation, fluid flow and fluid-marble interaction processes. High paleogeothermal gradients associated with extension may have driven the dominantly meteoric fluid circulation downward within the exhuming Complex. Asimotrypes may represent a potential new gold mineralization style for the Rhodope region.