Geologic, Fluid Inclusion, and Stable Isotope Studies of the Pasto Buena Tungsten-Base Metal Ore Deposit, Northern Peru

Abstract

The Pasto Bueno tungsten-base metal ore deposit is situated at an elevation of nearly equal 4,000 meters in the north-central Andes of Peru. Mineralization occurs in near-vertical quartz vein systems that span several hundred meters on either side of the upper intrusive contact of a 9.5-m.y.-old quartz monzonite stock emplaced in a Jurassic-Cretaceous shale and quartzite sequence. The stock exhibits four pervasive and roughly zoned alteration assemblages from core to periphery: (1) alkalic, (2) phyllic-sericitic, (3) argillic, and (4) propylitic. Greisen assemblages of zinnwaldite, fluorite, pyrite, and minor topaz and tourmaline occur within the phyllic zone.The principal vein minerals are wolframite, tetrahedrite/tennantite, sphalerite, galena, and pyrite in a gangue of quartz, fluorite, sericite, and carbonate. Detailed studies of the hydrothermal mineral paragenesis established three major recognizable divisions: Greisen (60 to 70 percent of deposition), Vein (25 to 35 percent of deposition), and Vug ( 40 equivalent weight percent NaCl), high-temperature (400 degrees to 500 degrees C) solutions of magmatic derivation. The subsequent main Vein stage ore fluids attained a temperature range of 175 degrees to 290 degrees C and a salinity range of 2 to 17 equivalent weight percent NaCl. Boiling of the ore solutions is indicated only for the Greisen and early Vein stages of hydrothermal activity.The results of stable isotope studies on water in primary fluid inclusions indicate that the delta D (sub H 2 O) of the ore fluids varied from -29 per mil to -88 per mil (SMOW). Analyses of water contained in secondary inclusions indicate the delta D (sub H 2 O) of the fluids attained values as low as -145 per mil prior to the cessation of hydrothermal activity. The delta 18 O (sub H 2 O) of the hydrothermal fluids, as calculated from the delta 18 O quartz and carbonate data and the temperature data, range from +7.8 per mil to +0.0 per mil (SMOW). The delta D value of present-day meteoric water is --96 per mil.The patterns for delta D (sub H 2 O) and delta 18 O (sub H 2 O) values of the hydrothermal fluids indicate that mixing of a meteoric and possibly a metamorphic or other water component with water of magmatic derivation occurred during Vein stage deposition. Major variations in the deuterium content of the ore solutions are not reflected by fluctuations in the 18 O content, indicating that meteoric water circulated deep into the hydrothermal plumbing system. Wolframite deposition was associated with episodes of meteoric water influx that are reflected in the temperature, salinity, and delta D values of the water in fluid inclusions. Sulfide mineralization, on the other hand, was associated with water of magmatic derivation.The delta 13 C data for hydrothermal CO 2 range from --4.1 per mil to -11.9 per mil (PDB) and are interpreted to indicate that the carbon in the ore fluids was derived from both sedimentary and deep-seated sources. delta 34 S values of pyrite data exhibit a narrow range of values (-2.5 per mil to +3.9 per mil) with an average of +0.6 per mil, indicating that the sulfur was derived from a deep-seated or mantle source.Most of the components present in the Pasto Bueno ore deposits appear to be of magmatic origin. Clearly however, significant volumes of meteoric water and possibly other water were involved in some stages of the ore deposition process.