Physicochemical constraints on indium-, tin-, germanium-, gallium-, gold-, and tellurium-bearing mineralizations in the Pefka and St Philippos polymetallic vein- and breccia-type deposits, Greece

Abstract

The Pefka Cu-Au-Te-In-Se and nearby St Philippos Pb-Zn-Bi-Sn-Ge-Ga-In vein- and breccia-type deposits in western Thrace, Greece, display strong similarities, but also differences in terms of mineralization style, ore mineralogy, and chemistry, and host rock compositions. The Pefka mineralization consists of two crosscutting vein systems with high sulfidation (HS)- and intermediate-sulfidation (IS) assemblages hosted by andesitic lavas and is unusually enriched in In (up to 700 ppm), Te (>1000 ppm), Se (>100 ppm), and Cu (>1 wt%). The main In-carriers are roquesite (CuInS2) and In-bearing “tennantite-(Cu)” and Cu-rich “tennantite-(In)” which contains up to 6.5 wt% In, substituting into the C site. Roquesite is associated with enargite and arsenosulvanite/colusite, as part of the HS assemblage at Pefka. Selenium-bearing galena and a large suite of tellurides including calaverite, sylvanite, petzite, hessite, kostovite, empressite, tellurantimony, and coloradoite, in addition to native tellurium, account for the marked tellurium and selenium enrichment in the ores from Pefka. Tellurides and native gold at Pefka accompany the precipitation of Te-bearing minerals of the tetrahedrite group, such as “stibiogoldfieldite” and “arsenogoldfieldite”, and Cu-excess varieties of tetrahedrite and tennantite. However, the bulk of telluride deposition is associated with normal, fully substituted tetrahedrite-tennantite varieties.<br/><br/>

The St Philippos deposit is associated with a brecciated fault zone hosted by Eocene sandstones and Oligocene quartz-feldspar porphyry dikes. It is enriched in a large suite of incompatible elements, including Bi (>2000 ppm), Sn (>100 ppm), U (up to 200 ppm), Pb (>1 wt%), Zn (>1 wt%), Mo (up to 62 ppm), Ge (>100 ppm), Ga (up to 466 ppm) and In (up to 222 ppm), contrasting with the element suite defining the nearby Pefka deposit. The main carrier of In, Ga, and Ge is sphalerite (and wurtzite) with In-rich zones in sphalerite containing up to 6.1. wt% In. Germanium and Ga in sphalerite reach concentrations of up to 0.27 and 0.32 wt%, respectively. Sphalerite from the St Philippos deposit is extremely Fe-poor (<0.04 wt%), and is associated with an unusual suite of Sn-bearing sulfosalts (kësterite-stannite, Mn-bearing kësterite, unnamed Cu2MnSnS4), and enargite, marking an early HS event. Kësterite also hosts indium (up to 0.6 wt% In). Mn-bearing varieties of tennantite host inclusions of minor tellurides (e.g., hessite, altaite, and tsumoite) and formed later in the paragenetic sequence under transitional HS-IS and IS conditions.<br/><br/>

Both deposits are characterized by early high-temperature (>300 °C) and HS fluid conditions, followed by IS assemblages as temperatures waned. Rhyolitic oxidized magmas are considered to be the sources of metals in the St Philippos deposit; however, their anomalous W, Sn, U, and Bi contents suggest a contamination by crustal rocks. The Cu-Au-Te signature of the Pefka deposit is compatible with a genetic relationship to less fractionated andesitic magmas, although a possible contribution of In from rhyolitic magmas could explain the high In contents of the ore. However, other factors, as for example different metal-deposition mechanisms resulting in metal zonation around causative porphyry centers at depth, may also account for the observed metal endowment in these two deposits. The Sn-Te-In-(Ge-Ga) element association at Pefka and St Philippos is unusual in that it has been previously reported from only a few other places in the world (e.g., Capillitas deposit, Argentina, and the Kawazu deposit, Japan). We conclude based on this exotic mineralization-style that the northeastern part of Greece represents an area of great potential for the exploitation of critical metals and metalloids.