The Tatatila–Las Minas IOCG skarn (Veracruz, Mexico): Mineralogical, fluid inclusion and stable isotope constraints

Abstract

The Miocene skarn deposits at Tatatila–Las Minas are found in central-eastern Veracruz state, east Mexico, near the Palma Sola massif. These deposits are geologically associated with the early stages of the Trans-Mexican Volcanic Belt (TMVB) intruded Mesozoic carbonate rocks of the Sierra Madre Oriental. Skarn associations are distributed in the classic evolution from prograde to retrograde mineralization stage. Prograde associations consist mainly of grossular-andradite, clinopyroxene, quartz, wollastonite, clinopyroxene, potassium feldspar, quartz, epidote, chromian muscovite, and are rich in magnetite, whereas retrograde associations are richer in hematite than magnetite, chlorite, fuchsite, hornblende and additionally contain chalcopyrite, pyrite, bornite, and native gold. The systematic study of fluid inclusions shows a broad variety of petrographic and microthermometric features. Fluid inclusion associations in early prograde minerals show evidence for boiling and effervescence, which are hereby interpreted as deep boiling in the -magmatic environment (or “first boiling”) with no evidence for supercritic fluids. Such a process would account for the generation of two distinct brines at ≤650 °C (a) up to ∼20 wt% NaCl equiv. and (b) up to ∼70 wt% NaCl. Both brines have cooled down independently until the brittle-ductile transition (∼400 °C) bypassed it and were able to interact with other fluids, and perhaps between with each other. Later on, deep isothermal mixing would have occurred between both brines and deeply evolved meteoric water (thermally equilibrated with host rocks). REE geochemistry of garnet suggests that mineralizing fluids at prograde stage evolved from near neutral pH and oxidizing conditions to mildly acidic and reducing conditions, perhaps reflecting the entrainment of shallow fluids. Both brines or their mixed products have experienced further dilution leading to the precipitation of retrograde associations. The most noticeable features in the retrograde stages, however, are conductive cooling and shallow isothermal mixing, this time between the products of the two pathways that originated as two distinct types of magmatic brines. The occurrence of magmatic fluids is corroborated by C and O isotope geochemistry, in which such source presents different types of water/rock interaction with sedimentary C and O from local limestones (δ13CVPDB between 0.9 and 3.7‰ and δ18OVPDB between −7.5 and −3.7‰), thus drawing an isotopic zonation that parallels the marble front associated with skarns (endoskarn calcite, δ13CVPDB between −8.5 and −3.5‰, and δ18OVPDB between −22.4 and −15.0‰). δ34SVCDT values from pyrite, chalcopyrite and galena in retrograde associations (between −3 and 4.2‰) also indicate a dominant magmatic source for sulfur, with a minor sedimentary/metasedimentary contribution. Low H2S concentrations enhanced magnetite deposition in prograde stages, and the progressive increase in H2S concentrations, coupled with a decrease in temperature by conductive cooling, led to sulfide and gold precipitation during retrograde stages. These deposits are hereby classified as IOCG skarn that were developed in association with the regional adakitic stage of the TMVB.