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

The Gualilan Au-rich Zn skarn is located in western Argentina. Skarn result ed from the interaction of hydrothermal fluids associated with Middle Mioce ne calc-alkaline magmatism and Ordovician limestones. The magmatism is asso ciated with a tectonic environment of transitional post-subduction in the T ertiary back-are basin of the Precordillera thrust-belt. The Gualilain skar n formed in a hypabyssal environment distal to the igneous source; it is st ructurally and lithologically controlled, Mineralization occurred in three main stages that are separated by faulting: (I) prograde skarn; (II) retrog rade alteration of the sk,rn; and (III) lead-rich veins. Cogenetic subvolca nic dacite porphyries were emplaced at the end of Stage I and were subseque ntly hydrothermally altered during Stages II and III. The delta(34)S (-1.0 parts per thousand to 1.9 parts per thousand) indicates a magmatic source f or 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 -98)Di(0-43)Jo(1-54)) and minor garnet (Ad(90-98)Sp(0-3)Gr(0-7)) by Selecti vely replacing a cherty unit in the Ordovician limestones, The exoskarn for med from H2O-rich fluids (X-CO2 < 0.1) With an upper temperature limit betw een 420 degrees C and 390 degrees C and an upper limit for log f(O2) betwee n -28 and -29. The higher organic matter content in the cherty limestone co mpared to the other layers probably provided the optimal reducing environme nt to allow pyroxene formation at temperatures below 420 degrees C, Magneti te, minor pyrrhotite and chalcopyrite were formed at the end of Stage I, Mi neral assemblages indicate that the log f(S2) in the system during Stage I was fairly constant at about -10. During Stage II, the alteration of pyroxe ne (to amphibole, ilvaite, chlorite, epidote, smectite, calcite and quartz) and of the dacite porphyries (to sericitic, argillic and propylitic assemb lages), 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 temperatur es and salinities from 294 degrees C to 245 degrees C and from 6.4 to 0.7 w t.% NaCl eq., respectively) suggest that mixing of fluids was an important mechanism in the sulfide deposition during Stage II. Stage III veins (galen a, electrum, argentite, hessite, minor pyrite and sphalerite, quartz and ca lcite) cross-cut both the skarn and the dacite porphyry dikes, They formed from late mineralizing fluids that had temperatures ranging from 340 degree s C to 300 degrees C, and salinities varying from 4 to 0.4 wt.% NaCl eq,, s uggesting, therefore, that boiling was a permissible mechanism of gold depo sition. (C) 2000 Elsevier Science B.V. All rights reserved.