Mineralogy and geochemistry of the Gualilán skarn deposit in the Precordillera of western Argentina

Abstract

The Gualilán Au-rich Zn skarn is located in western Argentina. Skarn resulted from the interaction of hydrothermal fluids associated with Middle Miocene calc-alkaline magmatism and Ordovician limestones. The magmatism is associated with a tectonic environment of transitional post-subduction in the Tertiary back-arc basin of the Precordillera thrust-belt. The Gualilán skarn formed in a hypabyssal environment distal to the igneous source; it is structurally and lithologically controlled. Mineralization occurred in three main stages that are separated by faulting: (I) prograde skarn; (II) retrograde alteration of the skarn; and (III) lead-rich veins. Cogenetic subvolcanic dacite porphyries were emplaced at the end of Stage I and were subsequently hydrothermally altered during Stages II and III. The δ 34S (−1.0‰ to 1.9‰) indicates a magmatic source for the sulfur of Stages I and II sulfides. During Stage I, metasomatism led to the formation of a calcic exoskarn consisting mainly of pyroxene (Hd 38–98Di 0–43Jo 1–54) and minor garnet (Ad 90–98Sp 0–3Gr 0–7) by selectively replacing a cherty unit in the Ordovician limestones. The exoskarn formed from H 2O-rich fluids ( X CO 2 <0.1) with an upper temperature limit between 420°C and 390°C and an upper limit for log f O 2 between −28 and −29. The higher organic matter content in the cherty limestone compared to the other layers probably provided the optimal reducing environment to allow pyroxene formation at temperatures below 420°C. Magnetite, minor pyrrhotite and chalcopyrite were formed at the end of Stage I. Mineral assemblages indicate that the log f S 2 in the system during Stage I was fairly constant at about −10. During Stage II, the alteration of pyroxene (to amphibole, ilvaite, chlorite, epidote, smectite, calcite and quartz) and of the dacite porphyries (to sericitic, argillic and propylitic assemblages), accompanied the main deposition of sulfides (pyrite, sphalerite and minor galena). Fluid inclusion data in quartz from the assemblages pyrite–sphalerite–quartz and ilvaite–quartz–calcite (decreasing filling temperatures and salinities from 294°C to 245°C and from 6.4 to 0.7 wt.% NaCl eq., respectively) suggest that mixing of fluids was an important mechanism in the sulfide deposition during Stage II. Stage III veins (galena, electrum, argentite, hessite, minor pyrite and sphalerite, quartz and calcite) cross-cut both the skarn and the dacite porphyry dikes. They formed from late mineralizing fluids that had temperatures ranging from 340°C to 300°C, and salinities varying from 4 to 0.4 wt.% NaCl eq., suggesting, therefore, that boiling was a permissible mechanism of gold deposition.