Most trace elements in the San Rafael tourmaline do not correlate with Fe/(Fe + Mg) ratios, suggesting that their incorporation was likely controlled by the melt/fluid composition and local fluid-rock interactions. The initial radiogenic Sr and Nd isotope compositions of the three aforementioned tourmaline generations (0.7160-0.7276 for Sr-87/Sr-86((t)) and 0.5119-0.5124 for Nd-143/Nd-144((i))) mostly overlap those of the San Rafael granites (Sr-87/Sr-86((i)) = 0.7131-0.7202 and Nd-143/Nd-144((i)) = 0.5121-0.5122) and support a dominantly magmatic origin of the hydrothermal fluids. These compositions also overlap the initial Nd isotope values of Bolivian tin porphyries. The initial Pb isotope compositions of tourmaline show larger variations, with Pb-206/Pb-204((i)), Pb-207/Pb-204((i)), and Pb-208/Pb-204((i)) ratios mostly falling in the range of 18.6 to 19.3, 15.6 to 16.0, and 38.6 to 39.7, respectively. These compositions partly overlap the initial Pb isotope values of the San Rafael granites (Pb-206/Pb-204((i)) = 18.6-18.8, Pb-207/Pb-204((i)) = 15.6-15.7, and Pb-208/Pb-204((i)) = 38.9-39.0) and are also similar to those of other Oligocene to Miocene Sn-W +/- Cu-Zn-Pb-Ag deposits in southeast Peru. Rare earth element patterns of tourmaline are characterized, from Tur 1 to Tur 3, by decreasing (Eu/Eu*)(N) ratios (from 20 to 2) that correlate with increasing Sn contents (from IOs to >1,000 ppm). These variations are interpreted to reflect evolution of the hydrothermal system from reducing toward relatively more oxidizing conditions, still in a low-sulfidation environment, as indicated by the pyrrhotite-arsenopyrite assemblage. The changing textural and compositional features of Tur 1 to Tur 3 reflect the evolution of the San Rafael magmatic-hydrothermal system and support the model of fluid mixing between reduced, Sn-rich magmatic fluids and cooler, oxidizing meteoric waters as the main process that caused cassiterite precipitation.
Tourmaline as a Tracer of Late-Magmatic to Hydrothermal Fluid Evolution: The World-Class San Rafael Tin (-Copper) Deposit, Peru
Dini Andrea;
2020
Abstract
Most trace elements in the San Rafael tourmaline do not correlate with Fe/(Fe + Mg) ratios, suggesting that their incorporation was likely controlled by the melt/fluid composition and local fluid-rock interactions. The initial radiogenic Sr and Nd isotope compositions of the three aforementioned tourmaline generations (0.7160-0.7276 for Sr-87/Sr-86((t)) and 0.5119-0.5124 for Nd-143/Nd-144((i))) mostly overlap those of the San Rafael granites (Sr-87/Sr-86((i)) = 0.7131-0.7202 and Nd-143/Nd-144((i)) = 0.5121-0.5122) and support a dominantly magmatic origin of the hydrothermal fluids. These compositions also overlap the initial Nd isotope values of Bolivian tin porphyries. The initial Pb isotope compositions of tourmaline show larger variations, with Pb-206/Pb-204((i)), Pb-207/Pb-204((i)), and Pb-208/Pb-204((i)) ratios mostly falling in the range of 18.6 to 19.3, 15.6 to 16.0, and 38.6 to 39.7, respectively. These compositions partly overlap the initial Pb isotope values of the San Rafael granites (Pb-206/Pb-204((i)) = 18.6-18.8, Pb-207/Pb-204((i)) = 15.6-15.7, and Pb-208/Pb-204((i)) = 38.9-39.0) and are also similar to those of other Oligocene to Miocene Sn-W +/- Cu-Zn-Pb-Ag deposits in southeast Peru. Rare earth element patterns of tourmaline are characterized, from Tur 1 to Tur 3, by decreasing (Eu/Eu*)(N) ratios (from 20 to 2) that correlate with increasing Sn contents (from IOs to >1,000 ppm). These variations are interpreted to reflect evolution of the hydrothermal system from reducing toward relatively more oxidizing conditions, still in a low-sulfidation environment, as indicated by the pyrrhotite-arsenopyrite assemblage. The changing textural and compositional features of Tur 1 to Tur 3 reflect the evolution of the San Rafael magmatic-hydrothermal system and support the model of fluid mixing between reduced, Sn-rich magmatic fluids and cooler, oxidizing meteoric waters as the main process that caused cassiterite precipitation.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
