Research Article| June 01, 2015 Porphyry to Epithermal Transition in the Altar Cu-(Au-Mo) Deposit, Argentina, Studied by Cathodoluminescence, LA-ICP-MS, and Fluid Inclusion Analysis Laura Maydagán; Laura Maydagán † 1Centro Patagónico de Estudios Metalogenéticos-CONICET-INGEOSUR, Departamento de Geología, Universidad Nacional del Sur, San Juan 670, 8000 Bahía Blanca, Argentina †Corresponding author: e-mail, lauramaydagan@yahoo.com.ar Search for other works by this author on: GSW Google Scholar Marta Franchini; Marta Franchini 2Centro Patagónico de Estudios Metalogenéticos-CONICET, Instituto de Investigación en Paleobiología y Geología, Universidad Nacional de Río Negro, Av. Roca 1242, 8332 Roca, Argentina3Departamento de Geología y Petróleo, Facultad de Ingeniería, Universidad Nacional del Comahue, Buenos Aires 1400, 8300 Neuquén, Argentina Search for other works by this author on: GSW Google Scholar Brian Rusk; Brian Rusk 4Department of Geology, Western Washington University, Bellingham, Washington, USA Search for other works by this author on: GSW Google Scholar David R. Lentz; David R. Lentz 5Department of Earth Sciences, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada Search for other works by this author on: GSW Google Scholar Christopher McFarlane; Christopher McFarlane 5Department of Earth Sciences, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada Search for other works by this author on: GSW Google Scholar Agnes Impiccini; Agnes Impiccini 3Departamento de Geología y Petróleo, Facultad de Ingeniería, Universidad Nacional del Comahue, Buenos Aires 1400, 8300 Neuquén, Argentina Search for other works by this author on: GSW Google Scholar Francisco Javier Ríos; Francisco Javier Ríos 6Centro de Desenvolvimiento da Tecnologia Nuclear, CNEN, CxPs 941- Belo Horizonte, Brazil Search for other works by this author on: GSW Google Scholar Roger Rey Roger Rey 7Minera Peregrine Argentina S.A, Santa Fe (Oeste) 117, Piso 5, Edificio Derby, Ciudad San Juan, Argentina Search for other works by this author on: GSW Google Scholar Economic Geology (2015) 110 (4): 889–923. https://doi.org/10.2113/econgeo.110.4.889 Article history received: 06 Mar 2013 accepted: 14 Oct 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Laura Maydagán, Marta Franchini, Brian Rusk, David R. Lentz, Christopher McFarlane, Agnes Impiccini, Francisco Javier Ríos, Roger Rey; Porphyry to Epithermal Transition in the Altar Cu-(Au-Mo) Deposit, Argentina, Studied by Cathodoluminescence, LA-ICP-MS, and Fluid Inclusion Analysis. Economic Geology 2015;; 110 (4): 889–923. doi: https://doi.org/10.2113/econgeo.110.4.889 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The middle to late Miocene Altar porphyry Cu-(Au-Mo) deposit, located in the Andean Main Cordillera of San Juan Province (Argentina), is characterized by the superposition of multiple vein generations consisting of both porphyry-type and high sulfidation epithermal-style alteration and mineralization. We constrain the physical and chemical evolution of the hydrothermal fluids that formed this deposit based on description and distribution of vein types, scanning electron microscopy, cathodoluminescence (CL) imaging, trace elements in quartz veins, and fluid inclusion microthermometry.Quartz CL textures and trace elements (chiefly Li, Al, Ti, and Ge) differentiate among quartz generations precipitated during different mineralization and alteration events. Early quartz ± chalcopyrite ± pyrite veins and quartz ± molybdenite veins (A and B veins) show considerable complexity and were commonly reopened, and some underwent quartz dissolution. Early quartz ± chalcopyrite ± pyrite veins (A veins) are dominated by equigranular bright CL quartz with homogeneous texture. Most of these veins contain higher Ti concentrations than any other vein type (average: 100 ppm) and have low to intermediate Al concentrations (65–448 ppm). Quartz ± molybdenite (B veins) and chlorite + rutile ± hematite (C veins) veins contain quartz of intermediate CL intensity that commonly shows growth zones with oscillatory CL intensity. Quartz from these veins has intermediate Ti concentrations (~20 ppm) and Al concentrations similar to those of A veins. Quartz from later quartz + pyrite veins with quartz + muscovite ± tourmaline halos (D veins) has significantly lower CL intensity, low Ti (<15 ppm) and elevated Al concentrations (up to 1,000 ppm), and typically contains euhedral growth zones. Late veins rich in sulfides and sulfosalts show CL textures typical of epithermal deposits (dark CL quartz, crustiform banding, and euhedral growth zones). Quartz from these veins typically contains less than 5 ppm Ti, and Al, Li, and Ge concentrations are elevated relative to other vein types. Based on experimentally established relationships between Ti concentration in quartz and temperature, the decrease in Ti content in successively later quartz generations indicates that the temperature of the hydrothermal fluids decreased through time during the evolution of the system.Vein formation at Altar occurred at progressively lower pressure, shallower paleodepth, and lower temperature. Under lithostatic pressures, the magma supplied low-salinity aqueous fluids at depths of ~6 to 6.8 km (pressures of 1.6–1.8 kbar) and temperatures of 670° to 730°C (first quartz generation of early quartz ± chalcopyrite ± pyrite veins). This parental fluid episodically depressurized and cooled at temperatures and pressures below the brine-vapor solvus. Quartz ± molybdenite veins precipitated from fluids at temperatures of 510° to 540°C and pressures of 800 to 1,000 bars, corresponding to depths of 3 to 3.7 km under lithostatic pressures. Further cooling of hydrothermal fluids to temperatures between 425° and 370°C under hydrostatic pressures of 200 to 350 bars produced pyrite-quartz veins and pervasive quartz + muscovite ± tourmaline and illite alteration that overprinted the early hydrothermal assemblages. Late veins rich in sulfides and sulfosalts that overlapped the deep and intermediate high-temperature veins formed from fluids at temperatures of 250° to 280°C and pressures of 20 to 150 bars. The epithermal siliceous ledges formed from low-temperature fluids (<230°C) at hydrostatic pressures of <100 bars corresponding to depths of <<1 km. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
The Butte porphyry Cu-Mo deposit is cut by the Butte Main Stage, a system of veins that constitute one of the world’s largest Cordilleran-style base-metal lode deposits. The vein system is zoned from a central Cu-rich zone containing covellite, chalcocite, digenite, and enargite to an intermediate zone containing both Cu and Zn sulfides, to a peripheral zone dominated by sphalerite, galena, and rhodochrosite.
We examined fluid inclusions in ~50 veins from throughout the lateral and vertical extent of the deposit and conducted microthermometry on 13 of these samples. Fluid inclusions in Main Stage veins are similar in appearance throughout the central, intermediate, and peripheral zones such that only one type of fluid inclusion dominates all samples observed. At room temperature the fluid inclusions are liquid-rich, with 20 volume % bubble (B20 inclusions). Most inclusions analyzed contain between 1 and 4 wt. % NaCl equivalent, and between 0.2 and 1 mol % CO2 . Most inclusions homogenize to liquid between 250°C and 300°C. Even though there is considerable overlap, there is a weak trend from higher to lower homogenization temperatures and salinities from the central zone to the peripheral zone.
Vapor-rich inclusions are rare and were identified in only one Main Stage vein, thus, we infer that nearly all inclusions were trapped in the liquid field at pressures
above the boiling curve. Maximum estimated depth of formation for Main Stage veins is 6 km. At such pressures, an isochoric temperature adjustment of up to about 50°C is
required, indicating that most Main Stage veins formed at temperatures between about 250°C and 350°C.
We suggest that Main Stage veins formed where single-phase B60 fluids, which formed pre-Main Stage pyrite-quartz veins with sericitic alteration, decompressed and mixed with meteoric water in a hydrostatic pressure regime. Pre-Main Stage brines were not likely involved in Main Stage vein formation, and the role of pre-Main Stage vapor in the formation of Main Stage veins is not known.
Models of the evolution of the hydrothermal systems that form porphyry Cu (Mo-Au) deposits are compromised because aqueous magma-derived fluids in the ore zones of most deposits have changed from their original magmatic compositions as a result of cooling, depressurising, mineral precipitation, brine-vapour unmixing and fluid-rock
reactions. However, in deep quartz-rich, sulfide-poor veins from numerous porphyry type deposits, we have identified parental fluids trapped in inclusions at near magmatic
temperatures and pressures above the brine-vapour unmixing solvus. We have analysed these inclusions for bulk salinity, density, solute chemistry, helium isotopic ratios and elemental composition, These parental inclusions contain 35 - 70 volume per cent bubble, are low to moderate salinity, contain up to ten mol per cent CO2, and commonly contain a chalcopyrite daughter crystal. Our results indicate that these Cu-rich fluids transport Cu from a plutonic complex below upward into a hydrothermal system, where decompression, cooling, unmixing and water-rock
reaction drive ore-mineral precipitation. Na/CI ratios greater than one indicate that in addition to chlorine, sulfur and/or carbonate must play a key role in Cu transportation. Helium isotope ratios indicate that between - 15 and 100 per cent of helium in these fluids is mantle-derived. We suggest that in addition to He, volatiles from mafic magmas in the mantle are also likely to supply CO2 Cu and S to the fluids that form porphyry copper deposits.
ABSTRACT We examined cathodoluminescence (CL) colors of quartz by using red (590-780 nm), green (515-590 nm), and blue (380-515 nm) optical filters interfaced with a cathodoluminescence (CL) detector attached to a scanning electron microscope (SEM). SEM/CL images taken through these filters were captured digitally and transferred to a computer. Luminescence intensities (luminosities) of the images were measured by using available commercial software. Measured luminosities of these CL images are directly related to relative intensities of red, green, and blue CL emissions. Luminosity data were then used to construct plots that display relative luminosities of the CL images acquired through the red, green, and blue filters. An unfiltered CL image of each quartz grain, generated by photons with wavelengths ranging from 200-700 nm, was also acquired. By subtracting the numerical luminosity values of the images acquired through the red, green, and blue filters from the luminosity value of the unfiltered image, the contribution to total luminosity provided by CL emission in the near ultraviolet (UV) was calculated. The CL colors of quartz from a variety of volcanic, plutonic, and metamorphic rocks and hydrothermal deposits were examined. Volcanic quartz phenocrysts have the most restricted CL color range, with strongest emission intensity in the blue wavelength band. CL colors of plutonic quartz overlap those of volcanic phenocrysts but extend over a broader range to include quartz that displays higher intensity of red emission. CL emission in hydrothermal (vein) quartz is similar to that in plutonic quartz, although some hydrothermal quartz exhibits stronger green-CL emission than does plutonic quartz. The CL colors of metamorphic quartz exhibit the widest variation, overlapping the color fields of both volcanic and plutonic quartz and extending further into the red. CL emission in the near UV makes a significant contribution ( 5-85 percent) to the total luminosity of SEM/CL images, particularly images of plutonic quartz. Because of overlap in the CL color ranges of volcanic, plutonic, metamorphic, and hydrothermal quartz, unambiguous identification of quartz provenance on the basis of CL color alone is problematic. It is difficult to distinguish between volcanic and some plutonic quartz, and between some plutonic and hydrothermal quartz, or to distinguish magmatic quartz from metamorphic quartz that exhibits blue CL color. Only metamorphic quartz that exhibits moderately strong red emission appears distinguishable (on the basis of color) from quartz of other origins. Our work thus suggests that CL color is not a reliable indicator of quartz provenance.
This paper evaluates controls on cassiterite crystallization under hydrothermal conditions based on the textural setting and geochemistry of cassiterite from six different mineralization environments from the world-class Gejiu tin district, southwest China. The cassiterite samples feature diverse internal textures, as revealed by cathodoluminescence (CL) imaging, and contain a range of trivalent (Ga, Sc, Fe, Sb), quadrivalent (W, U, Ti, Zr, Hf), and pentavalent (Nb, Ta, V) trace elements, with Fe, Ti, and W being the most abundant trace elements. Cassiterite Ti/Zr ratios tend to decrease with distance away from the causative granite intrusion, and so has potential to be used as a broad tool for vectoring toward a mineralized intrusive system.
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