Meteoric water induced selvage-style greisen alteration in the Achala Batholith, central Argentina

Abstract

Zoned, selvage-style greisen alteration is common along fracture zones in the ∼ 350-Ma Achala Batholith of central Argentina. Zonation proceeds from only slightly altered monzogranite, through zones of biotite alteration (Zone 1), partial alteration of feldspar (Zone 2), and intensely altered zones characterized by the complete destruction of feldspar, formation of secondary muscovite, and crystallization of secondary quartz in veins and vugs (Zone 3). Fluid inclusion studies indicate that fluids involved in the formation of secondary quartz were dilute (aye. = 1.5 eq wt% NaCl), and were trapped at temperatures less than ∼ 350°C. Oxygen and hydrogen isotopic data collected from primary quartz, feldspar, and muscovite, and secondary muscovite and quartz indicate a complex history of isotopic exchange and mineral precipitation. Feldspar δ 18O values range from ∼ 3.1 to + 11.6‰, whereas primary quartz δ 18O values are much more restricted at + 12.0 to + 12.7‰. Plots of δ 18O fds vs. δ 18O qtz or δ 18O mus show steep slopes characteristic of kinetically dominated exchange with a low- 18O fluid. Feldspar, quartz, and primary muscovite δ 18O values, as well as muscovite δ D values decrease toward Zone 3 and are consistent with a model of fluid advection through fractures and diffusive transport perpendicular to fracture margins. Isotopic data suggest that primary minerals initially exchanged with a fluid of δ D ∼ −175 to - 135‰ and δ 18O ∼ −23 to −18‰. However, secondary muscovite and quartz are characterized by δ 18O values of −11.6 to −5.0‰ and −16.0 to −7.1‰, respectively. Secondary muscovite δ D values (−117 to −83‰) are generally higher than those of exchanged, primary muscovite in Zones 2 and 3. Fluid inclusions extracted from late-stage quartz have δ D values of −55 to −54‰, consistent with computed values for fluid in equilibrium with secondary muscovite. Meteoric water with this hydrogen isotopic composition would be characterized by a δ 18O value of ∼ − 8‰, again consistent with δ 18O H 2O computed from late-stage secondary muscovite. The data suggest that early isotopic exchange was initiated by reaction with a low-D, low-180 meteoric water, but that fluid composition changed through time. Increases in fluid δ 18O and δ D values may be related to changes in climate and geographic location or elevation of the area, or may in part be related to interaction with evolved fluids that exchanged isotopes in other pottions of the flow system. Exchange durations computed from feldspar and quartz δ 18O values are < 65,000 yr, and indicate that fluid introduction into the fracture system was episodic in nature. Such a pattern of changes with time in the isotopic composition of meteoric mater may be characteristic of hydrothermal alteration of bark-arc granitoid rocks of the Eastern Sierras Pampeanas.