Origin and evolution of fluids associated with the ...

11 th SGA Biennial Meeting

Let’s Talk Ore Deposits 26-29th September 2011 Antofagasta, Chile

Origin and evolution of fluids associated with the Cerro Quema Au-Cu deposit (Azuero Peninsula, Panama): evidence from microthermometry, O, H and S isotopes Isaac Corral, Esteve Cardellach, Mercè Corbella Departament de Geologia, Universitat Autònoma de Barcelona, 08193 Barcelona. España Àngels Canals Facultat de Geologia, Universitat de Barcelona, 08028 Barcelona. España Craig A. Johnson U.S. Geological Survey, Denver, CO 80225. USA Abstract. The Cerro Quema Au-Cu deposit is located in the Azuero Peninsula, Panama. Although the geology of the area is not well known recent studies have only focused on the economic potential of gold. In the present work, mineralogical, fluid inclusion and stable isotope data are presented in order to understand the origin and evolution of the hydrothermal system of this Au-Cu deposit. Secondary fluid inclusions in magmatic quartz phenocrysts have an average homogenization temperature (Th) of 205ºC and ice melting temperatures 18 (Tmi) are around -1.2ºC. � O values of the fluid in equilibrium with vuggy silica at 250ºC range from +1.5 to 18 +9.1‰. � O values of the fluid in equilibrium with 34 kaolinite range from -5.70 to 14.07‰. The high � S values of alunite (�+17‰) together with the negative values of coexisting pyrite and enargite (�-8‰) are compatible with a magmatic hydrothermal origin of alunite. Ore deposition took place from fluids of low salinity (�2% wt NaCl eq.) and moderate temperatures (�250°C). O and D data of silicates and sulfates suggest a contribution of surface fluids during hydrothermal alteration and ore deposition. Keywords: Cerro Quema, inclusions, H-O-S isotopes

Au-Cu

deposit,

Panama is a microplate situated in South Central America and constitutes the youngest segment of the land bridge between North and South American plates. During the Late Cretaceous this region was characterized by the subduction of the Farallon plate beneath the Caribbean plate. Subsequently arcmagmatism developed on top of the Caribbean plate. The Cerro Quema deposit is hosted by the fore-arc basin rocks of this volcanic arc (Corral et al., in press). The study area is constituted by volcanic and volcaniclastic sediments interbedded with hemipelagic limestones, submarine dacite lava domes and by crosscutting basaltic-andesitic dikes, belonging to the Río Quema Formation (RQF; Corral et al., in press), representing a fore-arc infill sequence. Present work is part of a project that includes the geology of the area and the mineralogy and geochemistry of the ore bodies. In this article, mineralogical, fluid inclusion and stable isotope data from ore and alteration minerals- are presented in order to understand the origin and evolution of the hydrothermal system of Cerro Quema Au-Cu deposit.

fluid

2 Methodology

1 Introduction

A mineralogical study of the hydrothermal alteration has been carried on surface and drill core samples using optical microscopy, SEM-EDS and XRD. Stable isotope analyses have been performed at the USGS laboratories in Denver (USA). In order to determine the sulfur isotopic composition of sulfates and sulphides, the samples were combined with V2O5 and combusted in an elemental analyser. The resulting SO2 flowed directly to a Thermo Delta mass spectrometer for sulfur isotope measurement (method of Giesemann et al., 1994). Silicates were reacted with BrF5 (Clayton and Mayeda, 1963) and the resulting CO2 gas was analyzed using a FinniganMAT252. Oxygen isotope analyses of alunite and barite were performed by online high-temperature carbon reduction. Microthermometric measurements of fluid inclusions were carried out on a Linkam ThMSG600 heating-freezing stage.

The Cerro Quema Au-Cu deposit is located in the Azuero Peninsula, SW Panama (Fig. 1). The deposit is constituted by several mineable bodies named (from East to West) Cerro Quema, Cerro Idaida, Cerro Quemita and La Pava. The estimated total resources are 106 metric tons with an average gold grade of 1.26g/t (Torrey and Keenan, 1994).

3 Hydrothermal alteration The Cerro Quema Au-Cu deposit is hosted by dacite lava domes belonging to the Río Quema Formation. It is a

Figure 1. Location of the Cerro Quema Au-Cu deposit, adapted from Corral et al., (in press).

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11 th SGA Biennial Meeting

Let’s Talk Ore Deposits 26-29th September 2011 Antofagasta, Chile

244 to 295ºC.

volcanosedimentary sequence consisting of volcanic and volcaniclastic sediments interbedded with hemipelagic limestones, which are intruded by submarine dacite lava domes and crosscut by basaltic-andesitic dikes (Corral et al., in press). The mineral deposit is characterized by the presence of a widespread hydrothermal alteration which developed concentric alteration halos in the host rock. The alteration is clearly fault controlled and follows E-W trending regional faults (Corral et al., in press). Results of the mineralogical study show an alteration pattern composed of three zones: An inner zone; characterized at the surface by vuggy silica with hematite, goethite, barite, pyrite and rutile. At depth it contains quartz, alunite-natroalunite, aluminium phosphatesulfate minerals (APS), dickite, barite, pyrite, enargite and rutile. This mineral paragenesis corresponds to advanced argillic alteration. - An outer rim; composed of quartz, kaolinite, illite, smectite and interlayered illite-smectite in both, surface exposures and at depth, corresponding to an argillic alteration zone. - A propylitic zone has only been observed in few drill core samples, apparently unrelated to the previous alteration zones. It is characterized by pyrite, chlorite, epidote, calcite and siderite. The ore minerals consist of disseminated auriferous pyrite, enargite and a poorly developed stockwork of quartz, pyrite, chalcopyrite and barite with traces of galena and sphalerite. Gold occurs as disseminated microscopic grains of native gold and as “invisible gold” within the crystalline structure of pyrite, mainly associated to the advanced argillic alteration zone (Corral et al., in press).

5 Fluid inclusion microthermometry Microthermometric data has been obtained from secondary fluid inclusions in primary quartz phenocrysts from the volcanic host rock affected by the advanced argillic alteration. Due to the small size (mostly less than 5�m) only a few measurements could be made. Fluid inclusions are biphase (L+V) at room temperature and have an average homogenization temperature (Th) of 205ºC (n=35; �= 18ºC) and an average melting ice temperature (Tmi) of -1.3ºC (n= 27; �= 1.3ºC).

6 Discussion Assuming that secondary fluid inclusions in quartz phenocrysts formed during the ore deposition-alteration stages, the Th/Tmi data indicate that the hydrothermal system was dominated by low salinity (up to 5% wt NaCl eq.) and moderate temperature fluids. With a mean deposition temperature of 250°C (pyrite-alunite isotope geothermometry), the �18O of the fluid in equilibrium with vuggy silica, calculated from the equation of Mathsuhisa et al., (1979), ranges from +1.5 to + 9.1‰ (Fig. 2), suggesting the presence of originally surface waters in the system during vuggy quartz precipitation.

4 Stable isotope geochemistry Stable isotopes ratios have been analyzed on the characteristic minerals of the hydrothermal alteration such as quartz, kaolinite, dickite, alunite, barite, pyrite and enargite. Samples were collected from surface and drill cores from the main mineralized bodies. The results are shown in Table 1, Figure 2 and Figure 3. 18

Quartz Kaolinite Alunite Barite

� O +10.4 to +18.0 (n=28) +14.0 to +19.2 (n=25) -1.6 to +9.8 (n=11) +2.7 to +16.2 (n=7)

�D – -30 to -103 (n=27)

Figure 2. Oxygen and hydrogen isotope values (expressed in ‰) from different minerals from the alteration zone and their calculated waters. PDMW: Present Day Meteoric Water, data from Caballero (2010). ATMW: Arc-type Magmatic waters (Giggenbach, 1992; Taylor, 1986). PMW: Primary Magmatic Waters.

– –

Table 1. Oxygen and hydrogen isotope data (expressed in ‰) from different minerals from the alteration zone.

For kaolinite-dickite precipitation we have estimated different temperatures based on the mineral assamblages and/or isotopic arguments. Calculations have been carried out using the equations of Gilg and Sheppard, (1996) and Sheppard and Gilg, (1996). For a temperature of 250ºC calculated �D and �18O of hydrothermal fluids during dickite formation range from -14 to -28‰ and

Taking into account the coexistence of alunite and pyrite and assuming S-isotopic equilibrium, the �34S values of sulfides and sulfates (Fig. 3) could be used as a geothermometer with the equation of Ohmoto and Rye (1979). These isotopic pairs give a temperature between

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11 th SGA Biennial Meeting

Let’s Talk Ore Deposits 26-29th September 2011 Antofagasta, Chile

data of silicates and sulfates (quartz, kaolinite, alunitenatroalunite and barite) suggest an important contribution of surface fluids during sulfate precipitation and hydrothermal alteration.

from 10.6 to 14.0‰ respectively. For a temperature of 200ºC, �D and �18O of hydrothermal fluids during kaolinite formation range from -17 to -39‰ and from 9.0 to 13.6‰ respectively. Finally for a temperature of 25ºC, �D and �18O range respectively from -24 to -70‰ and from -5.3 to -9.7‰. Calculated �18O values suggest an isotopic exchange of mineralizing fluids with the host and the enclosing rocks, especially volcaniclastic sediments and limestones, during kaolinite-dickite formation. The high �34S values of alunite (�+17‰) together with the negative values of coexisting pyrite and enargite are compatible with a magmatic hydrothermal origin of alunite (Rye et al., 1992). The �34S values of barite, similar to alunite (Fig. 3), suggest a related sulfur source. �18O of alunite shows a wide range of values (-1.6 to 9.8‰), and fluids in equilibrium with alunite (using the equation of Stoffregen et al, 1968) at a temperature of 250°C, range from -9.9 to +1.4‰. However, calculated �18O of fluids during barite precipitation (using the equation of Lloyd, 1968; at 250ºC), range from -3.5 to +5.3‰. These data suggest a mixing in different proportions between surface and magmatic fluids during alunite and barite precipitation. The �34S values of pyrite and enargite coexisting with alunite, suggest an isotopic equilibrium between H2S and SO42- in the fluids. If this is the case, coexisting pyritealunite pairs give an average equilibrium temperatures of 250ºC, which are slightly higher than the Th values measured from the fluid inclusions. This difference might be due to the pressure effect on the trapping temperatures of the fluids during the mineralizing event.

Acknowledgements This study was supported by the Spanish Ministry of Science and Education project CGL2007-62690/BTE and a pre-doctoral grant from the Catalan Dept. of Universities. The corresponding author would like to express his gratitude to the SEG Foundation and the SEG Canada Foundation for the 2009 and 2010 student research grants (Hugh E. McKinstry) awarded to him. We thank Bellhaven Copper and Gold Inc. for access to mine samples and drill cores used in this study. Help received in mineral separation, sample preparation and mineral analysis by José Berrenchea (Universidad Complutense de Madrid), George Breit and Cayce A. Gulbransen (U.S. Geological Survey) is greatly appreciated.

References Caballero, A., 2010, Exploración de aguas subterráneas en el arco seco de Panamá (Sector de Las Tablas) mediante métodos geofísicos. Ph.D thesis, Universitat de Barcelona, pp. 271 Clayton, R.N. and Mayeda, T.K., 1963, The use of bromine pentafluoride in the extraction of oxygen from oxides and silicates for isotopic analysis: Geochimica et Cosmochimica Acta, v. 27, p.43-52. Corral, I., Griera, A., Gómez-Gras, D., Corbella, M., Canals, À., Pineda-Falconett, M. and Cardellach, E., in press, Geology of the Cerro quema Au-Cu deposit (Azuero Peninsula, Panama): Geologica Acta. Giesemann, A., Jager, H.J., Norman, A.L., Krouse, H.R., and Brand, W.A., 1994, On-line sulfur isotope determination using an elemental analyzer coupled to a mass spectrometer: Analytical Chemistry, v. 66, p. 2816-2819. Giggenbach, W. F., 1992, Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin: Earth and planetary Science Letters, v. 113, p. 495-510. Gilg, H. A., and Sheppard, S. M. F., 1996, Hydrogen isotope fractionation between kaolinite and water revisited: Geochimica et Cosmochimica Acta, v. 60, p. 529-533. Lloyd, R. M., 1968, Oxygen isotope behavior in the sulfate-water system: Journal of Geophysical Research, v. 73, p. 6099-6110. Mathsuhisa, Y., Goldsmith, H.J.R., Clayton, R.N., 1979, Oxygene isotopic fractionation in the system quartz-albite-anorthitewater. Geochimica et Cosmochimica Acta, v 43, p. 1131-1140. Ohmoto, H.and Rye, R. O., 1979, Isotopes of sulfu and carbon. In “Geochemistry of hydrothermal ore deposits”. John Wiley & Sons, New York, p. 509-567. Rye, R. O., Bethke, P. M., and Wasserman, M. D., 1992, The stable isotope geochemistry of acid sulfate alteration: Economic Geology, v. 87, p. 225-262. Sheppard, S. M. F., and Gilg, H. A., 1996, Stable isotope geochemistry of clay minerals; The story of sloppy, sticky, lumpy and tough, Cairns-Smith (1971): Clay Minerals, v. 31, p. 1-24. Stoffregen, R. E., 1987, Genesis of acid-sulfate alteration and AuCu-Ag mineralization at Summitville, Colorado: Economic Geology, v. 82, p. 1575-1591. Taylor, B.E., 1986, Magmatic volatiles: Isotopic variations in C, H and S: Reviews in Mineralogy, v. 16, p. 185-225. Torrey, C., and Keenan, J., 1994, Cerro Quema Project, Panama, Prospecting in tropical and arid terrains: Toronto, Ontario, Canada.

Figure 3. Sulfur isotope data (expressed in ‰) from different minerals from the alteration zone.

6 Conclusions The mineralogy and geochemical data of the Cerro Quema Au-Cu deposit demonstrate that it formed due to a high sulfidation hydrothermal system. Mineralization is related to E-W trending faults that affected dacite lava domes and andesites of the Rio Quema Formation. Ore mineralogy is characterized by native gold, pyrite, chalcopyrite, enargite, and minor amounts of sphalerite and galena. Hydrothermal alteration is observed as advanced argillic, argillic, and minor propylitic halos. Ore deposition took place from fluids of low salinity (�2% wt NaCl eq.) and moderate temperatures (�250°C). Sulfur isotope data of sulfides and sulfates indicate a sulfur source of magmatic origin. O and D

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