Raúl Lira

Fluid inclusion studies of the Rodeo de Los Molles REE and Th deposit, Las Chacras Batholith, Central Argentina

Abstract The Rodeo de Los Molles rare earth element (REE) and thorium deposit is located in granitic rocks of the Las Chacras-Piedras Coloradas Batholith, in the southern block of the Eastern Pampean Ranges, Central Argentina. Mineralization occurs within an elongate (2 km × 0.6 km) body of alkalifeldspar granite (alaskite) localized along the northeastern edge of a composite batholith. The surrounding lithology is predominantly a biotite monzogranite. Both the alaskite and localized areas of quartz alkalifeldspar syenite within the alaskite have been produced by hydrothermal alteration of a late-crystallizing phase of the monzogranite. REE minerals are primarily of the cerium group and include britholite and allanite, both partially replaced by bastnaesite or thorbastnaesite. These minerals occur as nodules with quartz, fluorite, aegirine-augite, sphene, and Fe-Ti oxides within aplitic to pegmatoidal quartz alkalifeldspar syenite. Uranothorite, along with a second generation of fluorite and minor amounts of MnBa oxides, occurs in the alaskite as nodules, or within quartz-lined miarolitic cavities, but is not found with the Ce-mineralization. Studies of fluid inclusions contained in quartz and fluorite indicate a complex history of open-system fluid migration and interaction with monzogranite host rocks. Fluids responsible for REE mineralization and quartz deposition, along with initial alteration of the monzogranite to alaskite and quartz alkalifeldspar syenite, were of relatively high temperature (Th of fluid inclusions in quartz = 356–535°C) and moderate salinity (15–25 eq. wt% NaCl). Mixed CO2H2O fluids (XCO2 = .13–.07) found as both primary and secondary inclusions within fluorite are representative of fluids involved in the replacement of britholite-allanite by bastnaesite and sphene, aegirine-augite, and plagioclase by calcite. Minimum pressures of mineral deposition estimated from H2OCO2NaCl phase relations range from 1 to 2 kbars. Secondary aqueous fluid inclusions in quartz define a trend of low salinity-high temperature to high salinity-low temperature, thought to be a result of hydration reactions occurring in alaskite and quartz alkali-feldspar syenite. The highest salinity fluids (35–37 eq. wt% NaCl) detected in the area are associated with the formation of uranothorite and late fluorite. Multiple periods of hydrothermal fluid introduction are consistent with recent geological data that indicate that the batholith is composed of several stock-like bodies. The location of the mineralized area near the top of the magma chamber, the presence of numerous miarolitic cavities, and the bulk composition of inclusion fluids (Na ≥ K > Ca) suggest that the fluids responsible for REE and Th mineralization were of magmatic origin.

Hydrothermal alteration and REE-Th mineralization at the Rodeo de Los Molles deposit, Las Chacras batholith, central Argentina

REE (rare-earth-element) and Th mineralization at the Rodeo de Los Molles deposit occurs within an elliptical body of hydrothermally altered rocks (fenite) located in a biotite monzogranite of the Las Chacras batholith. Ore assemblages are found as isolated patches of intergrown britholite, allanite, apatite, bastnaesite, fluorite, sphene, quartz, and aegirine-augite, as well as nodules of uranothorite and late-stage veins of calcite, fluorite, and bastnaesite. Composition-volume computations suggest that the fenite was produced by alteration of the biotite monzogranite by addition of K and Na, and loss of Ca and Sr. Petrographic evaluations indicate that microcline and plagioclase have been replaced by perthite, and biotite was converted to aggregates of clinochlore, anatase, kaolinite, and hematite. Relict biotite is characterized by lower Fe/(Fe+Mg) and Ti values with progressive alteration. Fluorine-rich phlogopite is present in mineralized areas, but textural evidence suggests that it was not produced via biotite alteration. Mass-balance constraints also show that Ca and Mg in ore zones may result from redistribution, rather than their being a result of external derivation. The δ18O values of quartz (8.6–11.1‰) and feldspar (7.8–10.6‰) suggest that feldspar continued to exchange oxygen isotopes with a fluid to lower temperatures than did quartz. Feldspars equilibrated with a fluid of δ18O≈8‰ at a fluid/rock ratio less than 1. The δ18O values of quartz and aegirine-augite that crystallized during REE mineralization also suggest equilibration with a fluid of δ18O≈8‰. The δD values of biotite (-83 to-120‰) are relatively low for igneous rocks and are thought to have resulted from exsolution of a D-enriched magmatic vapor. The δD values of both mineralized and barren fenites are consistent with equilibration with fluid of magmatic origin. Meteoric water was involved in the production of calcite and clinochlore alteration, and late-stage calcite-fluorite-bastnaesite veins. The δ13C values of calcite and bastnaesite (-7.8 to-13.5%) suggest that carbon was derived by leaching of carbon from igneous and/or enclosing metamorphic rock types, and that a majority of carbon ultimately was derived from sedimentary organic meterial.

METEORIC WATER INDUCED SELVAGE-STYLE GREISEN ALTERATION IN THE ACHALA BATHOLITH, CENTRAL ARGENTINA

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 18Ofds vs. δ 18Oqtz or δ 18Omus 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 δ18O ∼ −23 to −18‰. However, secondary muscovite and quartz are characterized by δ18O 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 18OH2O 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.