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Genetic implications of fluid inclusions in skarn chimney ore, Las Animas Zn–Pb–Ag(–F) deposit, Zimapán, Mexico
Ore Geology Reviews 23 (2003) 91 – 96 www.elsevier.com/locate/oregeorev
Short communication
Genetic implications of fluid inclusions in skarn chimney ore, Las Animas Zn–Pb–Ag(–F) deposit, Zimapa´n, Mexico E. Gonza´lez-Partida*, A. Carrillo-Cha´vez, G. Levresse, J. Tritlla, A. Camprubı´ Centro de Geociencias, Programa de Geofluidos, UNAM, Campus Juriquilla A.P. 15, Juriquilla 76230, Qro., Mexico Received 17 December 2001; accepted 5 March 2003
Keywords: Fluid inclusion; Skarn chimney ore; Las Animas Zn – Pb – Ag( – F) deposit
1. Geological background Three tectonic and physiographic provinces are present in the study area: (1) the Sierra Madre Oriental (SMOR; Suter, 1987; Carrillo-Martı´nez and Suter, 1982) formed by the succession of anticlines and synclines with a consistent NNW– SSE trend; (2) the Basin and Range (BR), formed by horst and grabens oriented NE – SW and NW – SE; and (3) the Trans-Mexican Volcanic Belt (TMBV; Aranda-Go´mez et al., 2000), a continental volcanic arc on the southwest margin of the NorthAmerican plate resulting from the subduction of the Rivera and Cocos plates along the Acapulco trench (Fig. 1). The SMOR hosts a huge variety of ore deposits: Skarn Pb + Zn + Ag+(Hg – Sb) type ore deposits of Paleocene –Eocene age are found mostly at the El Pin˜o´n anticline; low sulfidation epithermal Au –Ag
* Corresponding author. Centro de Geosciancias-UNAM, Campus Juriquilla A.P. 15, Juriquilla 76230, Queretaro, Mexico. E-mail address: [email protected] (E. Gonza´lez-Partida).
and Pb– Zn – Ag veins formed between 49 and 26 Ma; cassiterite deposits related to the Oligocene volcanic events: and world-class fluorite deposits found in the contact between Cretaceous limestone and Tertiary rhyolites bodies located at the boundary between Guanajuato and San Luis Potosi States. The study area is mainly made up by a carbonate sequence of Jurassic to Cretaceous age, including the Jurassic Trancas Formation (Js; shale and limestone), the Tamaulipas Formation (Ks; deep basin limestone), the El Doctor Formation (Ks –Ki reef limestone; Ward, 1979), and the Soyatal and Mendez Formations ((Ks; limestone and calcareous shale). This sedimentary sequence is intruded by monzonite and quartz – monzonite bodies, generally oriented along the El Pen˜on anticline, with ages ranging from 62.2 to 40.5 Ma (Simmons and Mapes, 1956; Gayta´n-Rueda, 1975; Morrison, 1982). Tertiary rocks are also represented by the El Morro Formation (Ti), laying on angular unconformity on the Cretaceous formations (Fig. 1). The skarn ore deposits are exclusively developed at the contact between the carbonate sequence and quartz –monzonite intrusive bodies. The ore bodies include massive base metal sulfides, arsenopyrite and
0169-1368/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-1368(03)00015-5
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E. Gonza´lez-Partida et al. / Ore Geology Reviews 23 (2003) 91–96
Fig. 1. (A) General distribution map of some of the tectonic provinces in northern Mexico and location of the Las Animas area. (B) General geologic map of the Las Animas zone. (C) Schematic stratigraphic column of the Las Animas-Zimapan area.
others minor As-bearing mineral phases (Villasen˜or et al., 1996).
2. Description of the deposit The mineralized bodies at the Las Animas mine were formed in the contact between a monzonitic Tertiary igneous body and the Cretaceous limestone. The igneous intrusion produced an exoskarn body, with variable thickness (5 cm to 80 m), mainly composed by garnet bands (from 70% andradite to almost pure andradite) with variable amounts of diopside, wollastonite, idocrase and a Mg-Pyroxene (Villasen˜or et al., 1987). A late hydrothermal alteration, including epidote, chlorite, quartz, K-feldspar, fluorite
and calcite, overprinted the prograde skarn (Simmons and Mapes, 1956; Gonzalez and Jaimes, 1986). In Las Animas mine, three different mineralized structures are distinguished (Villasen˜or et al., 1995): (1) stratiform ore shoots; (2) mineralized veins; and (3) a big chimney-shaped structure, object of this paper. The chimney ore body displays a vertical zoning defined by mineral distribution. At the bottom of the structure (zone A), Cu sulfides are the dominant metallic species; the intermediate zone (B) is characterized by the presence of abundant Zn – Pb sulfides; in the middle – upper zone (C) Ag-sulfoantimoniures and fluorite dominate (Figs. 2 and 3); whereas the upper zone (D) is mainly devoid of sulfides and composed by fluorite and calcite. Quartz is found widespread in all the
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3.1. Fluid inclusions type Primary fluid inclusions were found in quartz crystals (zone A), sphalerite (zone B), fluorite (zone C) and late calcites (zone D). Primary, two-phase (L + V) fluid inclusions in zones A and B, found mainly in quartz and sphalerite, have a consistent visual estimation of the liquid to vapor ratio of 0.70– 0.55. Primary fluid inclusions found in some early fluorite crystals from the C-zone are both threephase (S + L + V) with a NaCl daughter crystals, and two-phase (L + V) and display heterogeneous L + V volumetric ratios within the same crystallographic plane (heterogeneous trapping), that can be interpreted as produced after boiling. Primary fluid inclusions in late calcite from the C-zone show a rectangular shape, are exclusively two-phase (L + V type) and liquid dominant (L/V ratio between 0.9 and 0.95). 3.2. Microthermometry
Fig. 2. Mineral assemblages succession at the Las Animas mine and distribution of temperature based on the results of fluid inclusions thermometry. The thickness of the lines indicates the abundance of the minerals.
zones, and is the dominant gangue mineral at deeper and intermediate zones. A complete mineral description can be found in (Villasen˜or et al., 1995, 1996).
3. Fluid inclusions Microthermometric analyses were performed on a Chaixmeca heating– freezing stage, calibrated with synthetic fluid inclusions. Accuracy in measurements is about F 0.2 jC for freezing runs and F 3 jC for heating runs. Salinities were calculated using the MacFlincor package (v. 0.92; Brown, 1989).
Early fluids from the A-zone have ice melting temperatures (Tmi) ranging from 4 to 9.5 jC, with corresponding calculated salinities from 6.5 to 13.4 wt.% NaCl equivalent. Homogenization temperatures (Th) for these early inclusions range between 300 and 430 jC. The ice melting temperatures in fluid inclusions contained in sphalerites from the B-zone range from 4 to 8.9 jC. Calculated salinities are 6.5 and 12.7 wt.% NaCl equivalent, values that are similar to those found in the early A-zone fluids, while homogenization temperatures are to some clearly below, ranging between 230 and 340 jC. The fluid inclusions found in fluorites from Czone present an uncommon composition and reflect a complicated history. Two different groups have been detected during the freezing runs. A first group of two-phase inclusions (L + V) present an intergrowth of hydrohalite, antarcticite (CaCl26H2O; pale-brown rounded crystals) and ice crystals at 150 jC. The eutectic temperature for these fluid inclusions is located around 52 jC, indicating the presence of CaCl2. After progressive heating, the hydrohalite melting (Tmi) occurs at temperatures between 31 and 28 jC, whereas ice melts (Tmi2) at temperatures between 7.8 and 11.3 jC. From both
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Fig. 3. Schematic cross-section of the ore body at Las Animas mine (chimney) and distribution of mineralization zones with the type of fluids inclusions observed within each zone.
hydrohalite and ice melting temperatures, and using the abacus of Oakes et al. (1990), calculated salinities span from 11 to 14.5 wt.% NaCl, with a XNaCl between 0.34 and 0.48 wt.%. Homogenization temperatures for these fluid inclusions are from 110 to 240 jC. A second group of two-phase fluid inclusions present a contrasted behavior, with a slightly higher eutectic temperature around 50 jC, the hydrohalite melting temperature located between 31 and 29 jC, and the final melting between of a high contrast phase (not clathrate) between + 5 and + 10 jC. This last melting phase is puzzling, but it can correspond to the fusion of antarcticite, which indicates a high concentration of CaCl2 in the fluids (Crawford, 1981). Due to absence of data on the CaCl26H2O + L field in the NaCl – CaCl2 – H2O theoretical system we calculated the salinities approaching the behavior to the pure CaCl2 – H2O system
(Crawford, 1981). As a consequence, calculated salinities for these fluids range from 13.5 to 20.5 wt.% CaCl2 equivalent. Homogenization temperatures by bubble shrinkage occur between 200 and 240 jC. Hypersaline, three-phase fluid inclusions in fluorite (zone C) have Th values between 150 and 300 jC and solubility temperatures (TsNaCl) for the NaCl cubes ranging from 160 to 320 jC, which corresponds to salinities between 29 and 38 wt.% NaCl. Two-phase fluid inclusions found in late calcite (shallowest samples, zone D) present final ice melting temperatures (Tmi) between 12 and 6 jC, with a predominance of the Tmi = 10 jC values. The corresponding calculated salinities range between 16 and 9 wt.% NaCl equivalent while their homogenization temperatures tightly cluster between 100 and 115 jC.
E. Gonza´lez-Partida et al. / Ore Geology Reviews 23 (2003) 91–96
4. Discussion Fluid inclusions found in A zone are arranged as two different salinity populations with similar Th, suggesting a dilution by external (meteoric?) fluids of a high-temperature, moderate salinity fluid linked with the deposition of Cu-rich sulfides. The more diluted fluids from zone A (6.5 wt.% NaCl equivalent) are similar in total salinity to the fluids found trapped in fluid inclusions within sphalerites from the B-zone, even though the latter present lower Th drawing a cooling fluid path from Cu-rich to Zn-rich ores. The different populations of primary fluid inclusions in the C zone that can be seen in Fig. 4 are probably consequence of different pulses of hydrothermal fluids whose salinity have been modified. Both the disposition in the Th vs. salinity plot of these and the clear evidences of inhomogeneous trapping found during the petrographic work indicate that during the precipitation of fluorite, boiling occurred from fluids of moderate salinity (11 – 15 wt.% NaCl equivalent) with the local generation of
95
hypersaline fluids (up to 35 wt.% NaCl equivalent). This evidences of boiling can be linked with the movement of local fractures during ore formation that triggered the hydrothermal fracturing of the deposit, generating the brecciated chimney structure. Calcite from zone C recorded a late, low temperature fluid pulse with similar salinity than fluids trapped in zones A, B and, partially, C. This late fluid composition indicates that the fluid reservoir cooled down Las Animas ore deposit formation. Similar results have been reported at the Zimapan mine, south of Las Animas deposit, by Lindgren and Whitehead (1914) which observed the presence of hypersaline fluid inclusions. Lately, Simmons (1951) reported homogenization temperatures ranging between 400 and 550 jC with corresponding salinities exceeding 26 wt.% NaCl equivalent. The presence of CaCl2-dominated fluids in the Las Animas chimney is a very unusual feature. It has been generally accepted that Zn – Pb ( F Ag, F Cu) skarn deposits present complex fluids, with predominance of NaCl brines (Einaudi et al., 1981; Meinert, 1992).
Fig. 4. Homogenization temperature (Th) vs. salinity (wt.% NaCl equivalent) plot of fluid inclusions within different minerals. Note that the wt.% for NaCl + CaCl2 has been added in terms of NaCl, so the salinity is estimated. The different zones (A – D) are widely discussed in the text. The arrow indicates the possible evolution trend of the mineralizing fluids.
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5. Summary and conclusions The chimney of Las Animas, Zimapan mining district presents a vertical zoning with an early chalcopyrite-rich zone, a galena-sphalerite intermediate zone, and a late silver-antimoniures zone. These zones are also characterized by fluids with different properties, indicating a general decrease in temperature from earlier to later zones. Besides, the silver-antimoniures zone present both textural and microthermometric evidences of boiling, with the local presence of unusually Ca-rich fluids.
References Aranda-Go´mez, J.J., Christopher, D., Luhr, H.F., 2000. Evolucio´n tectonomagma´tica post-paleoce´nica de la Sierra Madre Occidental y de la porcio´n meridional de la provincia tecto´nica de Cuencas y Sierras, Me´xico. Boletı´n de la Sociedad Geolo´gica Mexicana 3, 59 – 71. Brown, P.E., 1989. FLINCOR: a microcomputer program for the reduction and investigation of fluid inclusions data. The American Mineralogist 74, 1390 – 1393. Carrillo-Martı´nez, M., Suter, M., 1982. Tecto´nica de los alrededores de Zimapa´n, Hidalgo y Quere´taro: Convencio´n Nacional de la Sociedad Geolo´gica Mexicana. Libro guı´a, excursio´n geolo´gica a la regio´n de Zimapan y a´reas circundantes, 1 – 20. Crawford, M.L., 1981. Phase equilibria in aqueous fluid inclusions. In: Hollister, S., Crawford, M.L. (Eds.), Short Course in Fluid Inclusions Applications to Petrology, Chapter 4. University of Calgary, Canada, pp. 75 – 100. Einaudi, M.T., Meinert, L.D., Newberry, R.J., 1981. Skarn Deposits, 75th Anniversary. Economic Geology, 317 – 391. Gayta´n-Rueda, J.E., 1975. Exploration and development at the La Negra Mine, Maconi, Quere´taro Me´xico. Unpublished MSc thesis, Univ. Tucson, Arizona. 98 pp.
Gonzalez, C.V., Jaimes M.L., 1986. Modelo neotectonico de la mineralizacio´n de Tugsteno del skarn de plomo-zinc, Hidalgo, con base a datos isoto´picos de RB/Sr, C y O. Unpublished MSc thesis, Univ. Nal. Auto. de Mexico. 82 pp. Lindgren, W., Whitehead, W.L., 1914. A deposit of Jamesonite near Zimapan, Mexico. Economic Geology 9, 435 – 462. Meinert, L.D., 1992. Skarns and skarn deposits. Geosciences, Canada 19, 145 – 162. Morrison, G., 1982. Geology of the La Negra Mine Vicinity, State of Quere´taro. Unpublished PhD thesis. Colorado, Mines School. 119 pp. Oakes, C.S., Bodnar, R.J., Simonson, J.M., 1990. The system NaCl – CaCl2 – H2O: I. The Ice Liquidus at 1 atm total pressure. Geochimica et Cosmochimica Acta 54, 603 – 610. Simmons, F.S., 1951. Geology and ore deposits of the Zimapa´n mining district, Me´xico. Unpublished PhD thesis. Stanford University, p. 237. Simmons, F.S., Mapes, V.E., 1956. Geology and ore deposits of the Zimapa´n mining district. State of Hidalgo, Me´xico. U.S. Geological Survey, Professional Paper 284, 128. Suter, M., 1987. Structural transverse across the Sierra Madre Oriental fold-thrust belt in east-central Mexico. GSA Bulletin 98, 249 – 264. Villasen˜or, C.G., Go´mez, C., Medina, J.C., Condleffe, E., Lozano, S.R., 1987. Bulangerita de la chimenea Las Animas, Zimapa´n. Estado de Hidalgo, Bol. de Mineralogı´a y Metalogenia 3, 1 – 30. Villasen˜or, C.G., Gomez, C., Avedan˜o, S., Reyes-Salas, M., IslasCorte´z, V., 1995. Minerales argentiferos de Zimapa´n, Hidalgo, Me´xico. Actas INAGEQ, 123 – 127. Villasen˜or, C.G., Peterson, E., Avedan˜o, S., Go´mez-Caballero, A., Sousa, J., Reyes-Salas, M., 1996. Minerales del grupo de la tetrahedrita en las minas lomo de toro y las a´nimas, Zimapa´n Hidalgo. Actas INAGEQ 2, 124 – 129. Ward, J.A., 1979. Stratigraphy, depositional environments and diagenesis of the El Doctor platfoms, Quere´taro, Me´xico. Unpublished PhD thesis, New York State University (Binghampton), p. 183.
Short communication
Genetic implications of fluid inclusions in skarn chimney ore, Las Animas Zn–Pb–Ag(–F) deposit, Zimapa´n, Mexico E. Gonza´lez-Partida*, A. Carrillo-Cha´vez, G. Levresse, J. Tritlla, A. Camprubı´ Centro de Geociencias, Programa de Geofluidos, UNAM, Campus Juriquilla A.P. 15, Juriquilla 76230, Qro., Mexico Received 17 December 2001; accepted 5 March 2003
Keywords: Fluid inclusion; Skarn chimney ore; Las Animas Zn – Pb – Ag( – F) deposit
1. Geological background Three tectonic and physiographic provinces are present in the study area: (1) the Sierra Madre Oriental (SMOR; Suter, 1987; Carrillo-Martı´nez and Suter, 1982) formed by the succession of anticlines and synclines with a consistent NNW– SSE trend; (2) the Basin and Range (BR), formed by horst and grabens oriented NE – SW and NW – SE; and (3) the Trans-Mexican Volcanic Belt (TMBV; Aranda-Go´mez et al., 2000), a continental volcanic arc on the southwest margin of the NorthAmerican plate resulting from the subduction of the Rivera and Cocos plates along the Acapulco trench (Fig. 1). The SMOR hosts a huge variety of ore deposits: Skarn Pb + Zn + Ag+(Hg – Sb) type ore deposits of Paleocene –Eocene age are found mostly at the El Pin˜o´n anticline; low sulfidation epithermal Au –Ag
* Corresponding author. Centro de Geosciancias-UNAM, Campus Juriquilla A.P. 15, Juriquilla 76230, Queretaro, Mexico. E-mail address: [email protected] (E. Gonza´lez-Partida).
and Pb– Zn – Ag veins formed between 49 and 26 Ma; cassiterite deposits related to the Oligocene volcanic events: and world-class fluorite deposits found in the contact between Cretaceous limestone and Tertiary rhyolites bodies located at the boundary between Guanajuato and San Luis Potosi States. The study area is mainly made up by a carbonate sequence of Jurassic to Cretaceous age, including the Jurassic Trancas Formation (Js; shale and limestone), the Tamaulipas Formation (Ks; deep basin limestone), the El Doctor Formation (Ks –Ki reef limestone; Ward, 1979), and the Soyatal and Mendez Formations ((Ks; limestone and calcareous shale). This sedimentary sequence is intruded by monzonite and quartz – monzonite bodies, generally oriented along the El Pen˜on anticline, with ages ranging from 62.2 to 40.5 Ma (Simmons and Mapes, 1956; Gayta´n-Rueda, 1975; Morrison, 1982). Tertiary rocks are also represented by the El Morro Formation (Ti), laying on angular unconformity on the Cretaceous formations (Fig. 1). The skarn ore deposits are exclusively developed at the contact between the carbonate sequence and quartz –monzonite intrusive bodies. The ore bodies include massive base metal sulfides, arsenopyrite and
0169-1368/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-1368(03)00015-5
92
E. Gonza´lez-Partida et al. / Ore Geology Reviews 23 (2003) 91–96
Fig. 1. (A) General distribution map of some of the tectonic provinces in northern Mexico and location of the Las Animas area. (B) General geologic map of the Las Animas zone. (C) Schematic stratigraphic column of the Las Animas-Zimapan area.
others minor As-bearing mineral phases (Villasen˜or et al., 1996).
2. Description of the deposit The mineralized bodies at the Las Animas mine were formed in the contact between a monzonitic Tertiary igneous body and the Cretaceous limestone. The igneous intrusion produced an exoskarn body, with variable thickness (5 cm to 80 m), mainly composed by garnet bands (from 70% andradite to almost pure andradite) with variable amounts of diopside, wollastonite, idocrase and a Mg-Pyroxene (Villasen˜or et al., 1987). A late hydrothermal alteration, including epidote, chlorite, quartz, K-feldspar, fluorite
and calcite, overprinted the prograde skarn (Simmons and Mapes, 1956; Gonzalez and Jaimes, 1986). In Las Animas mine, three different mineralized structures are distinguished (Villasen˜or et al., 1995): (1) stratiform ore shoots; (2) mineralized veins; and (3) a big chimney-shaped structure, object of this paper. The chimney ore body displays a vertical zoning defined by mineral distribution. At the bottom of the structure (zone A), Cu sulfides are the dominant metallic species; the intermediate zone (B) is characterized by the presence of abundant Zn – Pb sulfides; in the middle – upper zone (C) Ag-sulfoantimoniures and fluorite dominate (Figs. 2 and 3); whereas the upper zone (D) is mainly devoid of sulfides and composed by fluorite and calcite. Quartz is found widespread in all the
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93
3.1. Fluid inclusions type Primary fluid inclusions were found in quartz crystals (zone A), sphalerite (zone B), fluorite (zone C) and late calcites (zone D). Primary, two-phase (L + V) fluid inclusions in zones A and B, found mainly in quartz and sphalerite, have a consistent visual estimation of the liquid to vapor ratio of 0.70– 0.55. Primary fluid inclusions found in some early fluorite crystals from the C-zone are both threephase (S + L + V) with a NaCl daughter crystals, and two-phase (L + V) and display heterogeneous L + V volumetric ratios within the same crystallographic plane (heterogeneous trapping), that can be interpreted as produced after boiling. Primary fluid inclusions in late calcite from the C-zone show a rectangular shape, are exclusively two-phase (L + V type) and liquid dominant (L/V ratio between 0.9 and 0.95). 3.2. Microthermometry
Fig. 2. Mineral assemblages succession at the Las Animas mine and distribution of temperature based on the results of fluid inclusions thermometry. The thickness of the lines indicates the abundance of the minerals.
zones, and is the dominant gangue mineral at deeper and intermediate zones. A complete mineral description can be found in (Villasen˜or et al., 1995, 1996).
3. Fluid inclusions Microthermometric analyses were performed on a Chaixmeca heating– freezing stage, calibrated with synthetic fluid inclusions. Accuracy in measurements is about F 0.2 jC for freezing runs and F 3 jC for heating runs. Salinities were calculated using the MacFlincor package (v. 0.92; Brown, 1989).
Early fluids from the A-zone have ice melting temperatures (Tmi) ranging from 4 to 9.5 jC, with corresponding calculated salinities from 6.5 to 13.4 wt.% NaCl equivalent. Homogenization temperatures (Th) for these early inclusions range between 300 and 430 jC. The ice melting temperatures in fluid inclusions contained in sphalerites from the B-zone range from 4 to 8.9 jC. Calculated salinities are 6.5 and 12.7 wt.% NaCl equivalent, values that are similar to those found in the early A-zone fluids, while homogenization temperatures are to some clearly below, ranging between 230 and 340 jC. The fluid inclusions found in fluorites from Czone present an uncommon composition and reflect a complicated history. Two different groups have been detected during the freezing runs. A first group of two-phase inclusions (L + V) present an intergrowth of hydrohalite, antarcticite (CaCl26H2O; pale-brown rounded crystals) and ice crystals at 150 jC. The eutectic temperature for these fluid inclusions is located around 52 jC, indicating the presence of CaCl2. After progressive heating, the hydrohalite melting (Tmi) occurs at temperatures between 31 and 28 jC, whereas ice melts (Tmi2) at temperatures between 7.8 and 11.3 jC. From both
94
E. Gonza´lez-Partida et al. / Ore Geology Reviews 23 (2003) 91–96
Fig. 3. Schematic cross-section of the ore body at Las Animas mine (chimney) and distribution of mineralization zones with the type of fluids inclusions observed within each zone.
hydrohalite and ice melting temperatures, and using the abacus of Oakes et al. (1990), calculated salinities span from 11 to 14.5 wt.% NaCl, with a XNaCl between 0.34 and 0.48 wt.%. Homogenization temperatures for these fluid inclusions are from 110 to 240 jC. A second group of two-phase fluid inclusions present a contrasted behavior, with a slightly higher eutectic temperature around 50 jC, the hydrohalite melting temperature located between 31 and 29 jC, and the final melting between of a high contrast phase (not clathrate) between + 5 and + 10 jC. This last melting phase is puzzling, but it can correspond to the fusion of antarcticite, which indicates a high concentration of CaCl2 in the fluids (Crawford, 1981). Due to absence of data on the CaCl26H2O + L field in the NaCl – CaCl2 – H2O theoretical system we calculated the salinities approaching the behavior to the pure CaCl2 – H2O system
(Crawford, 1981). As a consequence, calculated salinities for these fluids range from 13.5 to 20.5 wt.% CaCl2 equivalent. Homogenization temperatures by bubble shrinkage occur between 200 and 240 jC. Hypersaline, three-phase fluid inclusions in fluorite (zone C) have Th values between 150 and 300 jC and solubility temperatures (TsNaCl) for the NaCl cubes ranging from 160 to 320 jC, which corresponds to salinities between 29 and 38 wt.% NaCl. Two-phase fluid inclusions found in late calcite (shallowest samples, zone D) present final ice melting temperatures (Tmi) between 12 and 6 jC, with a predominance of the Tmi = 10 jC values. The corresponding calculated salinities range between 16 and 9 wt.% NaCl equivalent while their homogenization temperatures tightly cluster between 100 and 115 jC.
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4. Discussion Fluid inclusions found in A zone are arranged as two different salinity populations with similar Th, suggesting a dilution by external (meteoric?) fluids of a high-temperature, moderate salinity fluid linked with the deposition of Cu-rich sulfides. The more diluted fluids from zone A (6.5 wt.% NaCl equivalent) are similar in total salinity to the fluids found trapped in fluid inclusions within sphalerites from the B-zone, even though the latter present lower Th drawing a cooling fluid path from Cu-rich to Zn-rich ores. The different populations of primary fluid inclusions in the C zone that can be seen in Fig. 4 are probably consequence of different pulses of hydrothermal fluids whose salinity have been modified. Both the disposition in the Th vs. salinity plot of these and the clear evidences of inhomogeneous trapping found during the petrographic work indicate that during the precipitation of fluorite, boiling occurred from fluids of moderate salinity (11 – 15 wt.% NaCl equivalent) with the local generation of
95
hypersaline fluids (up to 35 wt.% NaCl equivalent). This evidences of boiling can be linked with the movement of local fractures during ore formation that triggered the hydrothermal fracturing of the deposit, generating the brecciated chimney structure. Calcite from zone C recorded a late, low temperature fluid pulse with similar salinity than fluids trapped in zones A, B and, partially, C. This late fluid composition indicates that the fluid reservoir cooled down Las Animas ore deposit formation. Similar results have been reported at the Zimapan mine, south of Las Animas deposit, by Lindgren and Whitehead (1914) which observed the presence of hypersaline fluid inclusions. Lately, Simmons (1951) reported homogenization temperatures ranging between 400 and 550 jC with corresponding salinities exceeding 26 wt.% NaCl equivalent. The presence of CaCl2-dominated fluids in the Las Animas chimney is a very unusual feature. It has been generally accepted that Zn – Pb ( F Ag, F Cu) skarn deposits present complex fluids, with predominance of NaCl brines (Einaudi et al., 1981; Meinert, 1992).
Fig. 4. Homogenization temperature (Th) vs. salinity (wt.% NaCl equivalent) plot of fluid inclusions within different minerals. Note that the wt.% for NaCl + CaCl2 has been added in terms of NaCl, so the salinity is estimated. The different zones (A – D) are widely discussed in the text. The arrow indicates the possible evolution trend of the mineralizing fluids.
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5. Summary and conclusions The chimney of Las Animas, Zimapan mining district presents a vertical zoning with an early chalcopyrite-rich zone, a galena-sphalerite intermediate zone, and a late silver-antimoniures zone. These zones are also characterized by fluids with different properties, indicating a general decrease in temperature from earlier to later zones. Besides, the silver-antimoniures zone present both textural and microthermometric evidences of boiling, with the local presence of unusually Ca-rich fluids.
References Aranda-Go´mez, J.J., Christopher, D., Luhr, H.F., 2000. Evolucio´n tectonomagma´tica post-paleoce´nica de la Sierra Madre Occidental y de la porcio´n meridional de la provincia tecto´nica de Cuencas y Sierras, Me´xico. Boletı´n de la Sociedad Geolo´gica Mexicana 3, 59 – 71. Brown, P.E., 1989. FLINCOR: a microcomputer program for the reduction and investigation of fluid inclusions data. The American Mineralogist 74, 1390 – 1393. Carrillo-Martı´nez, M., Suter, M., 1982. Tecto´nica de los alrededores de Zimapa´n, Hidalgo y Quere´taro: Convencio´n Nacional de la Sociedad Geolo´gica Mexicana. Libro guı´a, excursio´n geolo´gica a la regio´n de Zimapan y a´reas circundantes, 1 – 20. Crawford, M.L., 1981. Phase equilibria in aqueous fluid inclusions. In: Hollister, S., Crawford, M.L. (Eds.), Short Course in Fluid Inclusions Applications to Petrology, Chapter 4. University of Calgary, Canada, pp. 75 – 100. Einaudi, M.T., Meinert, L.D., Newberry, R.J., 1981. Skarn Deposits, 75th Anniversary. Economic Geology, 317 – 391. Gayta´n-Rueda, J.E., 1975. Exploration and development at the La Negra Mine, Maconi, Quere´taro Me´xico. Unpublished MSc thesis, Univ. Tucson, Arizona. 98 pp.
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