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Fluid inclusion study of the Plomositas–Los Arcos polymetallic epithermal vein tract, Plomosas district, Sinaloa, Mexico
Journal of Geochemical Exploration 89 (2006) 143 – 148 www.elsevier.com/locate/jgeoexp
Fluid inclusion study of the Plomositas–Los Arcos polymetallic epithermal vein tract, Plomosas district, Sinaloa, Mexico Eduardo González-Partida a , Antoni Camprubí a,⁎, Francisco González-Sánchez a,b , Jaime Sánchez-Torres c a
c
Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Carretera 57 km. 15.5, 76023 Santiago de Querétaro, Qro., Mexico b Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, Campus Juriquilla, Carretera 57 km. 15.5, 76023 Santiago de Querétaro, Qro., Mexico Industrial Minera México S.A. de C.V., Santos Degollado 1053, Colonia Nuevo Chapultepec Sur, 58280 Morelia, Mich., Mexico Received 27 July 2005; accepted 14 November 2005 Available online 20 March 2006
Abstract The most important deposit in the Plomosas–Rosario district, Sinaloa, is the vein tract named Plomositas–Plomosas–Los Arcos. These are NNW–SSE striking veins hosted in rocks of the Lower Volcanic Supergroup (LVS), and also in rocks at the bottom of the Upper Volcanic Supergroup (UVS). Both supergroups belong to the Sierra Madre Occidental. These veins evolved from an early intermediate sulfidation stage (1), rich in base metal sulfides, to a low sulfidation stage (2), rich in silver sulfides and sulfosalts. There is also a 45 m-wide stockwork with native silver and gold. Stage 1 is found in the deeper portion of the veins whereas stage 2 is found in the most shallow portion of the deposit. These stages record fluid inclusion salinities ranging from 7 to 12 wt.% NaCl equiv., and from 0.2 to 3.5 wt.% NaCl equiv., respectively. Homogenization temperatures range from 120 °C for surface samples to ∼200 °C at a depth of 320 m. The low homogenization temperatures recorded, and the dispersion of veins within host rocks as veinlets, suggest that this deposit formed at shallow depths and was probably blind. © 2006 Elsevier B.V. All rights reserved. Keywords: Plomositas; Plomosas; Mexico; Epithermal veins; Intermediate–low sulfidation; Fluid inclusions
1. Introduction The Plomosas–Rosario mining district is located in the southeastern part of the Sinaloa state in western Mexico, close to the state limits with Durango (Fig. 1). This district lies in the western branch of the broad north-northwest trending belt of polymetallic epithermal deposits (Damon et al., 1981; Clark et al., 1982) in a trend that comprises some of the oldest epithermal ⁎ Corresponding author. E-mail address: [email protected] (A. Camprubí). 0375-6742/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.11.052
deposits in Mexico, like Tayoltita, Orión and Topia, as well as some of the youngest, like Lluvia de Oro and El Indio–Huajicori (Camprubí et al., 2003). The vast majority of epithermal deposits in Mexico formed between ~40 and ~27 Ma, and are distributed along a NW–SE belt from Chihuahua to the México State. This range of ages corresponds to the first bimodal andesiticrhyolitic volcanic episode of the Lower Volcanic Series (LVS) of the Sierra Madre Occidental (Fredrickson, 1974; McDowell and Keizer, 1977). The LVS formed about 45–35 Ma, is about 1000 to 1500 m thick, and is largely formed by andesites with local rhyolitic volcanic
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centers, which are generally associated with important epithermal deposits such as Tayoltita in Durango and Pánuco in Sinaloa. Staude and Barton (2001) suggested that similar deposits may be buried under the Upper Volcanic Series (UVS) of the Sierra Madre Occidental. The UVS formed about 35 to 23 Ma, is ∼1000 m thick, and is characterized by rhyolitic ignimbrites, tuffs, and domes that formed in an back-arc extensional environment. These two main volcanic events ruled the formation of epithermal deposits, and thus the age distribution of the deposits in this area reflects periods of volcanic quiescence (Camprubí et al., 2003). Albinson (1988) defined a preferential range of ages between 35 and 30 Ma for the formation of epithermal deposits in the central part of the Mexican Altiplano, a period that corresponds with the climax of volcanic activity with dominant intermediate compositions. The district took its name, Plomosas, from its relative abundance of lead (in Spanish, plomo). Mill-head grades run 1.95% Pb, 1% Zn, 0.18% Cu, 360 ppm Ag, and 1 ppm Au, though average grades at the stockwork zone are 18 ppm Au. Metal zoning with depth does not show large variations, but Cu and Ag grades are higher in the deeper parts of the veins (level 750), Pb and Zn are dominant in intermediate depths, and Au–Ag are richest in the upper parts of veins (level 1050). The Plomosas– Rosario district contains four main groups of mineralized bodies, namely Plomositas–Plomosas–Los Arcos vein tract, the object of this study, and the Magistral– San Juan–El Trampolín, Loma Dorada–Yecora, and Valenzuela–La Unión vein tracts (Fig. 1). All of the veins in the district strike NNW–SSE, though the veins in the west of the district dip to the ENE, and the veins in the east of the district dip to the WSW. 2. Regional geology The epithermal veins of Plomosas–Rosario district are hosted by Tertiary volcanic rocks only (Fig. 1), but in neighboring areas a metamorphic pre-Laramidic basement crops out. The different volcanic units in the area belong either to the Lower (LVS) or the Upper Volcanic Supergroup (UVS) of the Sierra Madre Occidental. The oldest lithological unit in the area is the Plomosas Andesite, consisting of porphyric andesites and andesitic breccias. It is discordantly covered by
the Panteón Trachyte, mostly consisting of trachyte flows, that is the most broadly distributed lithological unit in the area. Both units are part of the LVS. The transition between the LVS and the UVS is marked by an immature conglomerate, locally named Aglomerado Rojo, that consists of fragments of trachyte as clasts and a sandy matrix, that underwent some degree of silicification. The Mojonera Rhyolite consists of white to pink rhyolite flows with porphyric texture and quartz phenocrysts, and covers the above units. East of the La Cruz mine occur thick rhyolitic tuffs characteristic of the UVS, that covered the epithermal deposits. In the study area there are also several rhyolitic dikes. In the study area there is evidence for post-Laramide extension in two stages: (1) extension ENE–WSW from 20 to 10 Ma, and (2) extension E–W to ESE–WNW from 10 Ma to the present. Basin-and-range type structures also comprise NW–SE dextral strike-slip faults due to E–W and ESE–WNW extension, which produced the NNE–SSW faults where the veins formed. Two main fault systems occur in the study area: (1) faults striking NNW–SSE, where the epithermal deposits formed, and (2) later faults striking E–W with minor displacements. 3. Vein stratigraphy and internal structure The Plomositas–Plomosas–Los Arcos vein tract strikes about 10° (NNE–SSW), dips at 30° to 60° to the ESE, and has a strike length of more than 1000 m (Fig. 1). Vein thickness is very irregular and variable, being up to 30 cm thick in surface exposures and 2 to 5 m thick underground. Between mining levels 950 and 1000 (upper part of the deposit), there is a Au-rich stockwork zone associated with the veins with an average thickness of ∼45 m. The main host rock for the veins is the Plomosas Andesite, though some veinlet arrays are hosted by the lower part of the UVS ignimbritic and rhyolitic unit. These veinlets are at the top of the deposit and may have formed as blind veins. The mineralogy of the orebodies has the following sequence: (1) an initial stage with quartz + barite, hematite + pyrite, sphalerite, galena, and chalcopyrite ± argentite, and (2) quartz, pyrite, sphalerite, galena, chalcopyrite, argentite, proustite–pyrargyrite, and calcite. Barite occurs preferentially at the uppermost part
Fig. 1. Location and geological map of the Plomosas–Rosario district, southern Sinaloa state, Mexico, with the stratigraphy of the local Tertiary volcanic rocks of the SMO, in the middle left, the structural model for the formation of epithermal veins by Starling (1996), in the middle right, and a representative geologic cross-section, below. Notice the distribution of argillic alteration assemblages, as in situ hypogene imprints, thus indicating that the exposed sections of veins actually correspond to the most shallow part of the deposits. SMO = Sierra Madre Occidental, TMVB = TransMexican Volcanic Belt.
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of the veins. Late veinlets exhibit quartz, native silver and gold. Supergene minerals are chalcocite, covellite, anglesite, smithsonite, pyromorphite, and iron and manganese oxides. The veins exhibit an outer halo of propylitic alteration that extends for tens of meters that is composed of quartz, clorite, calcite, stilbite, mordenite, and pyrite, and an inner halo of phyllic alteration adjacent to veins that is composed of quartz and illite–smectite. The metallic mineral associations of stages 1 and 2 suggest that these are equivalent to intermediate and low sulfidation styles, respectively, as defined by Sillitoe and Hedenquist (2003). The occurrence of both styles of mineralization within the same deposit or district has not been reported by Sillitoe and Hedenquist (2003), but is observable in several epithermal deposits in Mexico (Camprubí and Albinson, in press). 4. Fluid inclusions For our microthermometric study on fluid inclusions we selected 100 samples covering a depth range of 320 m (from the surface or level 1050 to level 730), and a strike length of about 1000 m. Suitable fluid inclusions were analyzed in barite, quartz, and sphalerite. Five additional quartz samples were analyzed from the Au-rich stockwork at the footwall of the Plomositas–Los Arcos vein tract. We analyzed inclusions hosted by minerals lacking evidence for recrystallization. Primary, secondary, and pseudosecondary inclusions were found, which are liquid-rich or vapor-rich with no daughter minerals. The fluid inclusions are mostly 5 to 8 μm in size with a maximum of 30 μm, and they have two phases at room temperature (Fig. 2), with the sole exception of some surface samples from a jasperoid zone. Twophase inclusions rich in vapor and liquid-rich inclusions were found in some samples, forming a fluid inclusion assemblage. All the samples with analyzed vapor-rich fluid inclusions lacked any evidence of post-entrapment changes. Coexisting vapor-rich and liquid-rich fluid inclusions with no textural evidence for such changes are evidence for boiling. The statistic distribution of salinities is bimodal, ranging from 0.2 to 3.5 wt.% NaCl equiv. and 7 to 12 wt.% NaCl equiv., and corresponds to the shallower low sulfidation stage (stage 2) and deeper intermediate sulfidation stage in the veins (stage 1), respectively. Temperatures of homogenization increase with depth, up to 200 °C, and in surface samples they generally range from 120 to 130 °C. The data show a positive
Fig. 2. Photomicrographs showing representative fluid inclusion associations in this study, in quartz (A) and sphalerite (B and C). L = liquid, V = vapor, P = primary fluid inclusions, PS = pseudosecondary fluid inclusions.
correlation between temperature of homogenization, salinity, and depth (Fig. 3), though extreme values correspond to different stages of mineralization. Additionally, some clathrate melting temperatures above 0 °C indicate the occurrence of CO2 in the shallower part of the deposit and in the Au-rich stockwork. 5. Conclusions The Plomositas–Plomosas–Los Arcos vein tract is an epithermal deposit due to a sinistral transtensional shear forming pinch-and-swell veins. It formed in two hydrothermal stages: (1) an early deep intermediate sulfidation stage, associated with relatively hot brines with salinities ranging from 7 to 12 wt.% NaCl equiv., that formed a base metal sulfide association, and
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Fig. 3. Above: Longitudinal section of the Plomositas–Plomosas–Los Arcos vein tract, with the location of sampling stations (black dots), constructed isotherms and the distribution of ice and clathrate melting temperatures from fluid inclusion microthermometric data. All temperatures are reported in degrees Celsius. Below: Correlation between homogenization temperatures (Th) and ice/clathrate melting temperatures (Tm) in fluid inclusions. The average, maximum, and minimum temperatures is shown for each sample.
(2) a late shallow low sulfidation stage, associated with cooler brines with salinities ranging from 0.2 to 3.5 wt.% NaCl equiv., that formed a precious metal mineral association, including an Au-rich stockwork zone. Boiling was the main precipitation mechanism, as manifested by the occurrence of both liquid-and vaporrich fluid inclusions within single fluid inclusion assemblages. Additionally, the occurrence of argillic alteration zones in the uppermost portions of the veins in the Plososas-Rosario district suggest that these veins formed in steam-heated environments due to the incursion of boiled-off vapors. This evidence, and the unusually low temperatures of homogenization in most fluid inclusions, suggests that the studied veins correspond to the upper part of the epithermal deposit, and that
there may be undiscovered resources at depth, probably associated with the early intermediate sulfidation stage. Acknowledgements This work was partially funded with the Conacyt research project 46473. Special thanks to Industrial Minera México S.A. de C.V. and the staff at Plomosas for their assistance. We also gratefully acknowledge the critical reviews of Daniel Marshall and an anonymous referee. References Albinson, T., 1988. Geologic reconstruction of paleosurfaces in the Sombrerete, Colorada, and Fresnillo District, Zacatecas State, Mexico. Economic Geology 83, 1647–1667.
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Camprubí, A., Albinson, T., in press. Epithermal deposits in Mexico — an update of current knowledge, and an empirical reclassification. Geological Society of America Special Paper. Camprubí, A., Ferrari, L., Cosca, M.A., Cardellach, E., Canals, À., 2003. Ages of epithermal deposits in Mexico: regional significance and links with the evolution of Tertiary volcanism. Economic Geology 98, 1029–1037. Clark, K.F., Foster, C.T., Damon, P.E., 1982. Cenozoic mineral deposits and subsuction-related magmatic arcs in Mexico. Geological Society of America Bulletin 93, 533–544. Damon, P.E., Shafiqullah, M., Clark, K.F., 1981. Evolución de los arcos magmáticos en México y su relación con la metalogénesis. Revista del Instituto de Geología U.N.A.M. 5, 131–139. Fredrickson, G., 1974. Geology of the Mazatlan area, Sinaloa. University of Texas at Austin, Austin, TX, United States, unpublished PhD thesis, 209 p.
McDowell, F.W., Keizer, R.P., 1977. Timing of mid-Tertiary volcanism in the Sierra Madre Occidental between Durango City and Mazatlan, Mexico. Geologial Society of America Bulletin 88, 1479–1487. Sillitoe, R.H., Hedenquist, J.W., 2003. Linkages between volcanotectonic settings, ore–fluid compositions, and epithermal precious metal deposits. Society of Economic Geologists. Special Publication Series, vol. 10, pp. 314–343. Starling, T., 1996. Structural remote sensing analysis of the Rosario Mine District, Sinaloa. Unpublished internal report, IMMSA, 70 p. Staude, J.-M., Barton, M.D., 2001. Jurassic to Holocene tectonics, magmatism, and metallogeny of northwestern Mexico. Geological Society of America Bulletin 113, 1357–1374.
Fluid inclusion study of the Plomositas–Los Arcos polymetallic epithermal vein tract, Plomosas district, Sinaloa, Mexico Eduardo González-Partida a , Antoni Camprubí a,⁎, Francisco González-Sánchez a,b , Jaime Sánchez-Torres c a
c
Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Carretera 57 km. 15.5, 76023 Santiago de Querétaro, Qro., Mexico b Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, Campus Juriquilla, Carretera 57 km. 15.5, 76023 Santiago de Querétaro, Qro., Mexico Industrial Minera México S.A. de C.V., Santos Degollado 1053, Colonia Nuevo Chapultepec Sur, 58280 Morelia, Mich., Mexico Received 27 July 2005; accepted 14 November 2005 Available online 20 March 2006
Abstract The most important deposit in the Plomosas–Rosario district, Sinaloa, is the vein tract named Plomositas–Plomosas–Los Arcos. These are NNW–SSE striking veins hosted in rocks of the Lower Volcanic Supergroup (LVS), and also in rocks at the bottom of the Upper Volcanic Supergroup (UVS). Both supergroups belong to the Sierra Madre Occidental. These veins evolved from an early intermediate sulfidation stage (1), rich in base metal sulfides, to a low sulfidation stage (2), rich in silver sulfides and sulfosalts. There is also a 45 m-wide stockwork with native silver and gold. Stage 1 is found in the deeper portion of the veins whereas stage 2 is found in the most shallow portion of the deposit. These stages record fluid inclusion salinities ranging from 7 to 12 wt.% NaCl equiv., and from 0.2 to 3.5 wt.% NaCl equiv., respectively. Homogenization temperatures range from 120 °C for surface samples to ∼200 °C at a depth of 320 m. The low homogenization temperatures recorded, and the dispersion of veins within host rocks as veinlets, suggest that this deposit formed at shallow depths and was probably blind. © 2006 Elsevier B.V. All rights reserved. Keywords: Plomositas; Plomosas; Mexico; Epithermal veins; Intermediate–low sulfidation; Fluid inclusions
1. Introduction The Plomosas–Rosario mining district is located in the southeastern part of the Sinaloa state in western Mexico, close to the state limits with Durango (Fig. 1). This district lies in the western branch of the broad north-northwest trending belt of polymetallic epithermal deposits (Damon et al., 1981; Clark et al., 1982) in a trend that comprises some of the oldest epithermal ⁎ Corresponding author. E-mail address: [email protected] (A. Camprubí). 0375-6742/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.11.052
deposits in Mexico, like Tayoltita, Orión and Topia, as well as some of the youngest, like Lluvia de Oro and El Indio–Huajicori (Camprubí et al., 2003). The vast majority of epithermal deposits in Mexico formed between ~40 and ~27 Ma, and are distributed along a NW–SE belt from Chihuahua to the México State. This range of ages corresponds to the first bimodal andesiticrhyolitic volcanic episode of the Lower Volcanic Series (LVS) of the Sierra Madre Occidental (Fredrickson, 1974; McDowell and Keizer, 1977). The LVS formed about 45–35 Ma, is about 1000 to 1500 m thick, and is largely formed by andesites with local rhyolitic volcanic
144
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centers, which are generally associated with important epithermal deposits such as Tayoltita in Durango and Pánuco in Sinaloa. Staude and Barton (2001) suggested that similar deposits may be buried under the Upper Volcanic Series (UVS) of the Sierra Madre Occidental. The UVS formed about 35 to 23 Ma, is ∼1000 m thick, and is characterized by rhyolitic ignimbrites, tuffs, and domes that formed in an back-arc extensional environment. These two main volcanic events ruled the formation of epithermal deposits, and thus the age distribution of the deposits in this area reflects periods of volcanic quiescence (Camprubí et al., 2003). Albinson (1988) defined a preferential range of ages between 35 and 30 Ma for the formation of epithermal deposits in the central part of the Mexican Altiplano, a period that corresponds with the climax of volcanic activity with dominant intermediate compositions. The district took its name, Plomosas, from its relative abundance of lead (in Spanish, plomo). Mill-head grades run 1.95% Pb, 1% Zn, 0.18% Cu, 360 ppm Ag, and 1 ppm Au, though average grades at the stockwork zone are 18 ppm Au. Metal zoning with depth does not show large variations, but Cu and Ag grades are higher in the deeper parts of the veins (level 750), Pb and Zn are dominant in intermediate depths, and Au–Ag are richest in the upper parts of veins (level 1050). The Plomosas– Rosario district contains four main groups of mineralized bodies, namely Plomositas–Plomosas–Los Arcos vein tract, the object of this study, and the Magistral– San Juan–El Trampolín, Loma Dorada–Yecora, and Valenzuela–La Unión vein tracts (Fig. 1). All of the veins in the district strike NNW–SSE, though the veins in the west of the district dip to the ENE, and the veins in the east of the district dip to the WSW. 2. Regional geology The epithermal veins of Plomosas–Rosario district are hosted by Tertiary volcanic rocks only (Fig. 1), but in neighboring areas a metamorphic pre-Laramidic basement crops out. The different volcanic units in the area belong either to the Lower (LVS) or the Upper Volcanic Supergroup (UVS) of the Sierra Madre Occidental. The oldest lithological unit in the area is the Plomosas Andesite, consisting of porphyric andesites and andesitic breccias. It is discordantly covered by
the Panteón Trachyte, mostly consisting of trachyte flows, that is the most broadly distributed lithological unit in the area. Both units are part of the LVS. The transition between the LVS and the UVS is marked by an immature conglomerate, locally named Aglomerado Rojo, that consists of fragments of trachyte as clasts and a sandy matrix, that underwent some degree of silicification. The Mojonera Rhyolite consists of white to pink rhyolite flows with porphyric texture and quartz phenocrysts, and covers the above units. East of the La Cruz mine occur thick rhyolitic tuffs characteristic of the UVS, that covered the epithermal deposits. In the study area there are also several rhyolitic dikes. In the study area there is evidence for post-Laramide extension in two stages: (1) extension ENE–WSW from 20 to 10 Ma, and (2) extension E–W to ESE–WNW from 10 Ma to the present. Basin-and-range type structures also comprise NW–SE dextral strike-slip faults due to E–W and ESE–WNW extension, which produced the NNE–SSW faults where the veins formed. Two main fault systems occur in the study area: (1) faults striking NNW–SSE, where the epithermal deposits formed, and (2) later faults striking E–W with minor displacements. 3. Vein stratigraphy and internal structure The Plomositas–Plomosas–Los Arcos vein tract strikes about 10° (NNE–SSW), dips at 30° to 60° to the ESE, and has a strike length of more than 1000 m (Fig. 1). Vein thickness is very irregular and variable, being up to 30 cm thick in surface exposures and 2 to 5 m thick underground. Between mining levels 950 and 1000 (upper part of the deposit), there is a Au-rich stockwork zone associated with the veins with an average thickness of ∼45 m. The main host rock for the veins is the Plomosas Andesite, though some veinlet arrays are hosted by the lower part of the UVS ignimbritic and rhyolitic unit. These veinlets are at the top of the deposit and may have formed as blind veins. The mineralogy of the orebodies has the following sequence: (1) an initial stage with quartz + barite, hematite + pyrite, sphalerite, galena, and chalcopyrite ± argentite, and (2) quartz, pyrite, sphalerite, galena, chalcopyrite, argentite, proustite–pyrargyrite, and calcite. Barite occurs preferentially at the uppermost part
Fig. 1. Location and geological map of the Plomosas–Rosario district, southern Sinaloa state, Mexico, with the stratigraphy of the local Tertiary volcanic rocks of the SMO, in the middle left, the structural model for the formation of epithermal veins by Starling (1996), in the middle right, and a representative geologic cross-section, below. Notice the distribution of argillic alteration assemblages, as in situ hypogene imprints, thus indicating that the exposed sections of veins actually correspond to the most shallow part of the deposits. SMO = Sierra Madre Occidental, TMVB = TransMexican Volcanic Belt.
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of the veins. Late veinlets exhibit quartz, native silver and gold. Supergene minerals are chalcocite, covellite, anglesite, smithsonite, pyromorphite, and iron and manganese oxides. The veins exhibit an outer halo of propylitic alteration that extends for tens of meters that is composed of quartz, clorite, calcite, stilbite, mordenite, and pyrite, and an inner halo of phyllic alteration adjacent to veins that is composed of quartz and illite–smectite. The metallic mineral associations of stages 1 and 2 suggest that these are equivalent to intermediate and low sulfidation styles, respectively, as defined by Sillitoe and Hedenquist (2003). The occurrence of both styles of mineralization within the same deposit or district has not been reported by Sillitoe and Hedenquist (2003), but is observable in several epithermal deposits in Mexico (Camprubí and Albinson, in press). 4. Fluid inclusions For our microthermometric study on fluid inclusions we selected 100 samples covering a depth range of 320 m (from the surface or level 1050 to level 730), and a strike length of about 1000 m. Suitable fluid inclusions were analyzed in barite, quartz, and sphalerite. Five additional quartz samples were analyzed from the Au-rich stockwork at the footwall of the Plomositas–Los Arcos vein tract. We analyzed inclusions hosted by minerals lacking evidence for recrystallization. Primary, secondary, and pseudosecondary inclusions were found, which are liquid-rich or vapor-rich with no daughter minerals. The fluid inclusions are mostly 5 to 8 μm in size with a maximum of 30 μm, and they have two phases at room temperature (Fig. 2), with the sole exception of some surface samples from a jasperoid zone. Twophase inclusions rich in vapor and liquid-rich inclusions were found in some samples, forming a fluid inclusion assemblage. All the samples with analyzed vapor-rich fluid inclusions lacked any evidence of post-entrapment changes. Coexisting vapor-rich and liquid-rich fluid inclusions with no textural evidence for such changes are evidence for boiling. The statistic distribution of salinities is bimodal, ranging from 0.2 to 3.5 wt.% NaCl equiv. and 7 to 12 wt.% NaCl equiv., and corresponds to the shallower low sulfidation stage (stage 2) and deeper intermediate sulfidation stage in the veins (stage 1), respectively. Temperatures of homogenization increase with depth, up to 200 °C, and in surface samples they generally range from 120 to 130 °C. The data show a positive
Fig. 2. Photomicrographs showing representative fluid inclusion associations in this study, in quartz (A) and sphalerite (B and C). L = liquid, V = vapor, P = primary fluid inclusions, PS = pseudosecondary fluid inclusions.
correlation between temperature of homogenization, salinity, and depth (Fig. 3), though extreme values correspond to different stages of mineralization. Additionally, some clathrate melting temperatures above 0 °C indicate the occurrence of CO2 in the shallower part of the deposit and in the Au-rich stockwork. 5. Conclusions The Plomositas–Plomosas–Los Arcos vein tract is an epithermal deposit due to a sinistral transtensional shear forming pinch-and-swell veins. It formed in two hydrothermal stages: (1) an early deep intermediate sulfidation stage, associated with relatively hot brines with salinities ranging from 7 to 12 wt.% NaCl equiv., that formed a base metal sulfide association, and
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147
Fig. 3. Above: Longitudinal section of the Plomositas–Plomosas–Los Arcos vein tract, with the location of sampling stations (black dots), constructed isotherms and the distribution of ice and clathrate melting temperatures from fluid inclusion microthermometric data. All temperatures are reported in degrees Celsius. Below: Correlation between homogenization temperatures (Th) and ice/clathrate melting temperatures (Tm) in fluid inclusions. The average, maximum, and minimum temperatures is shown for each sample.
(2) a late shallow low sulfidation stage, associated with cooler brines with salinities ranging from 0.2 to 3.5 wt.% NaCl equiv., that formed a precious metal mineral association, including an Au-rich stockwork zone. Boiling was the main precipitation mechanism, as manifested by the occurrence of both liquid-and vaporrich fluid inclusions within single fluid inclusion assemblages. Additionally, the occurrence of argillic alteration zones in the uppermost portions of the veins in the Plososas-Rosario district suggest that these veins formed in steam-heated environments due to the incursion of boiled-off vapors. This evidence, and the unusually low temperatures of homogenization in most fluid inclusions, suggests that the studied veins correspond to the upper part of the epithermal deposit, and that
there may be undiscovered resources at depth, probably associated with the early intermediate sulfidation stage. Acknowledgements This work was partially funded with the Conacyt research project 46473. Special thanks to Industrial Minera México S.A. de C.V. and the staff at Plomosas for their assistance. We also gratefully acknowledge the critical reviews of Daniel Marshall and an anonymous referee. References Albinson, T., 1988. Geologic reconstruction of paleosurfaces in the Sombrerete, Colorada, and Fresnillo District, Zacatecas State, Mexico. Economic Geology 83, 1647–1667.
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Camprubí, A., Albinson, T., in press. Epithermal deposits in Mexico — an update of current knowledge, and an empirical reclassification. Geological Society of America Special Paper. Camprubí, A., Ferrari, L., Cosca, M.A., Cardellach, E., Canals, À., 2003. Ages of epithermal deposits in Mexico: regional significance and links with the evolution of Tertiary volcanism. Economic Geology 98, 1029–1037. Clark, K.F., Foster, C.T., Damon, P.E., 1982. Cenozoic mineral deposits and subsuction-related magmatic arcs in Mexico. Geological Society of America Bulletin 93, 533–544. Damon, P.E., Shafiqullah, M., Clark, K.F., 1981. Evolución de los arcos magmáticos en México y su relación con la metalogénesis. Revista del Instituto de Geología U.N.A.M. 5, 131–139. Fredrickson, G., 1974. Geology of the Mazatlan area, Sinaloa. University of Texas at Austin, Austin, TX, United States, unpublished PhD thesis, 209 p.
McDowell, F.W., Keizer, R.P., 1977. Timing of mid-Tertiary volcanism in the Sierra Madre Occidental between Durango City and Mazatlan, Mexico. Geologial Society of America Bulletin 88, 1479–1487. Sillitoe, R.H., Hedenquist, J.W., 2003. Linkages between volcanotectonic settings, ore–fluid compositions, and epithermal precious metal deposits. Society of Economic Geologists. Special Publication Series, vol. 10, pp. 314–343. Starling, T., 1996. Structural remote sensing analysis of the Rosario Mine District, Sinaloa. Unpublished internal report, IMMSA, 70 p. Staude, J.-M., Barton, M.D., 2001. Jurassic to Holocene tectonics, magmatism, and metallogeny of northwestern Mexico. Geological Society of America Bulletin 113, 1357–1374.