Isotope and Fluid Inclusion Studies of Geological and Hydrothermal Processes, Northern Peru Article

Macfarlane, AW, Prol-Ledesma, RM, Conrad, ME. (1994). Isotope and Fluid Inclusion Studies of Geological and Hydrothermal Processes, Northern Peru . INTERNATIONAL GEOLOGY REVIEW, 36(7), 645-677. 10.1080/00206819409465480

cited authors

  • Macfarlane, AW; Prol-Ledesma, RM; Conrad, ME

authors

abstract

  • Mineralization in the Hualgayoc district of northern Peru occurs in altered Miocene felsic intrusions and in mid-Cretaceous platform sedimentary rocks of the Goyllarisquizga, Inca, and Chulec formations. The ores occur both as stratiform and stratabound pyritiferous base-metal deposits (mantos), and as steeply dipping, sedimentary and intrusive rock-hosted base-metal veins. Igneous rocks in the district are affected by propylytic, sericitic-argillic, sericitic, potassic, and acid-sulfate alteration. K-Ar and Rb-Sr dating and geological evidence indicate multiple stages of intrusive activity and hydrothermal alteration, including close spatial emplacement of two or more separate Miocene magmatic-hydrothermal systems. K-Ar dates on sericite, hydrothermal biotite, and alunite indicate that the most important hydrothermal episodes in the district took place ≈13.24 and 12.4 Ma. Other K-Ar dates on altered rocks in the district may reflect various amounts of resetting by the emplacement of the 9.05±0.2 Ma Hualgayoc rhyodacite. A five-point Rb-Sr isochron for the San Miguel intrusion at Cerro Coymolache yields an age of 45 ± 3.4 Ma, which indicates much earlier magmatic activity in this area than recognized previously. Fluid inclusion and paragenetic studies reveal a clear temporal evolution of fluid temperature and chemistry in the San Agustín area at Hualgayoc. Ten distinct paragenetic stages are identified. Early, hot, and relatively saline fluids (300–350° C, up to 16 wt% NaCl equivalent) migrated along brittle fractures (and perhaps directly from the associated plutons) and formed manto deposits by metasomatic replacement of permeable, reactive beds. Gradually, ore formation shifted to precipitation of vein minerals in the brittle fractures as the mantos became less permeable and were sealed off. Vein formation continued from progressively cooler and more diluted fluids (down to ≈150° C and 4.3 wt% NaCl equivalent) as the system waned. No evidence for phase separation is observed in the fluids until the very last paragenetic stage, which contributed no economic mineralization. The shift of δ34S values of hydrothermal sulfide reported by Macfarlane and Shimizu (1991) is best explained by oxidation of the ore fluids as the system evolved.δ18O and 87Sr/86Sr values of igneous and sedimentary rocks and of gangue minerals in the district indicate that the hydrothermal fluids were dominantly meteoric throughout the oreforming event, and were well-equilibrated with the wall rocks, especially toward the end of the paragenetic sequence. No magmatic fluid is required to explain either isotopic data set. In light of the determination by Macfarlane and Petersen (1990) that the hydrothermal metals at Hualgayoc are overwhelmingly magmatic, our results show that the sources of hydrothermal fluids may have no bearing on the sources of metals precipitated at the same time in the same system. © 1994 by V. H. Winston & Son, Inc. All rights reserved.

publication date

  • January 1, 1994

published in

Digital Object Identifier (DOI)

start page

  • 645

end page

  • 677

volume

  • 36

issue

  • 7