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Fluid characteristics of the world-class, carbonate-hosted Las Cuevas fluorite deposit (San Luis Potosí, Mexico)
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ELSEVIER
C~}DIRECTe
JOURNAL OF
GEOCHEMICAL EXPLORATION
Journal of Geochemical Exploration 78-79 (2003) 537-543 www.elsevier.com/locate/jgeoexp
Abstract
Fluid characteristics of the world-class, carbonate-hosted Las Cuevas fluorite deposit (San Luis Potosi, Mexico) G. Levresse a, E. Gonzalez-Partida a'*, J. Tritlla a, A. Camprubl a, E. Cienfuegos-Alvarado b, P. Morales-Puente b aCentro de Geociencias, Campus Juriquilla, UNAM, Cart. 57 Qro-SLP Ion 15,5, Santiago de Querdtaro 76230, Quer&aro, Mexico bLaboratorio Universitario de Geoqulmica Isotrpica, Instituto de Geologla, UNAM, 04510 M~xico D.F., Mexico
Abstract Las Cuevas is a world-class high-grade fluorite district that accounts for over 7% of the world total fluorite production. This district is mainly hosted in the Cretaceous limestones of the E1 Doctor Formation, and is in fault contact with Tertiary rhyolites. This preliminary study is focused on the "G" orebody, a mass of fme-grained fluorite, with abundant cavities lined up by fluorite, sometimes stalactitic, late calcite and clays. Fluid inclusions in cavity filling minerals have salinities up to 0.18 wt.% NaC1 eq. with homogenization temperatures ranging from 60 to 110 °C, with the Th decreasing consistently from early cavity filling fluorite to late calcite. 61So and •13C values suggest that both an organic matter maturation and a decarbonation process might have occurred during the formation of the deposit. All the characteristics of the deposit suggest an MVT-related origin rather than a magmatic-hydrothermal one as previously proposed by other authors. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Fluorite; Hydrothermal solutions; Oxygen-carbon isotopes; MVT; Mexico
1. Introduction
2. Geographical and geological setting
In this paper, new data are presented on the fluids that originated the G orebody of the Las Cuevas world-class fluorite deposit. After these data, contrary to previous interpretations, the fluorite deposition would be related to the circulation of heated meteoric waters flowing through fractures confining the ore-
Las Cuevas district is located in the Sierra de Alvarez, about 40 km east of the city of San Luis Potosi, at the southwestern limit of the Valles-San Luis Potosi Jurassic-Cretaceous carbonate platform (Carrillo-Bravo, 1971; Carrillo-Martinez, 1981; Suter, 1984). All the seven known, massive and almost monomineralic fluorite bodies in this district are exclusively carbonate-hosted, clearly epigenetic, and systematically located along the tectonic contact between Tertiary rhyolites and the Cretaceous limestones of the E1 Doctor Formation (Carrillo-Martinez, 1981). The presently mined " G " orebody is the largest known so far and the main focus of this paper (Fig. 1).
body.
* Corresponding author. Tel.: +52-442-2381116; fax: +52-4422381110. E-mail address." [email protected] (E. Gonzalez-Partida).
0375-6742/03/$ - see front matter © 2003 Elsevier Science B.V. All rights reserved. doi: 10.1016/S0375-6742(03)00145-6
Abstract
538
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Fig. 1. (a) G e o l o g i c a l sketch o f the Las C u e v a s district w i t h the location o f fluorite bodies at level 180, p r o j e c t e d to the t o p o g r a p h i c a l surface.
Ruiz et al. (1980) carried out the only study known to date on this deposit. These authors, mainly based on partial petrographic and microthermometric analysis on the presently exhausted and inaccessible " E " orebody, proposed a magmatic-hydrothermal model for these deposits, closely associated with the emplacement of the Tertiary rhyolites.
3. Description of the "G" orebody The " G " orebody (50 Mt at >98% CaF2) occurs at the fault contact between the limestones of the E1 Doctor Formation and a Tertiary rhyolite, with breccia development at the contact (Fig. 2a). As the G fluorite body does not outcrop, this N W - S E fault contact has been seen only in the underground mine workings, even though this fault can be recognized in surface affecting mainly the rhyolite body (Fig. 1). The
transition from the massive fluorite orebody to the enclosing limestone is gradual and consists on the progressive replacement of the massive fluorite body through a network of fluorite-calcite veins crosscutting a thin envelope of recrystallized limestone developed around the orebody, grading into a fresh, fossilbearing limestone (Fig. 2d). Both the size of the veins and the degree of recrystallization of the limestone gradually decrease from the ore grading into the fresh limestone. The mineral composition of the orebody is very simple, as fluorite accounts for up to 95% of the deposit. Texturally, fluorite is mostly fine-grained, massive to finely laminated, with variable colors grading from white to yellow, green and, locally, bright red probably due to the presence of minute iron oxide inclusions. It is impossible to suggest a clear paragenetic succession within the main fluorite stage due to the huge internal variations in color and
Abstract
539
Fig. 2. (a) Highlyalteredrhyoliteclasts withinthe rhyolite-fluoritebrecciacontact;(b) late cavitylinedby botryoidalfluoriteand late calcite; (c) fluoritestalactites; (d) reerystallizedlimestonecrosscutby a networkof fluorite-calciteveins in the orebodyouter limit.
texture. In addition, the crosscutting relationships found are extremely complicated and variable at any scale. This inhomogeneity probably reflects the original inhomogeneities of the enclosing rock prior to its substitution by fluorite and/or possible fluctuations of the hydrothermal fluid chemistry. After the deposition of the main stage consisting of massive, fine-grained fluorite, the remaining cavities are lined up by botryoidal masses of gray to yellow fluorite, made up by radiated, skeletal crystals up to several centimeters long (Fig. 2b). Occasionally, a thin crust of purple, minute cubic fluorite crystals has been observed covering the botryoidal concretions, representing the last fluorite generation in the mineral sequence. Also, late stalagmites and/or stalactites of bright yellow fluorite partially fill the remaining space of the cavities (Fig. 2c). Minute quartz crystals have been rarely observed after the latest fluorite generations and are locally followed by calcite. The latter occurs as late, millimeter- to decimeter-sized idiomorphic crystals, precipitated within the cavities. Finally, a brown-red plastic clay, probably a residue derived
from the dissolution of the original limestone, usually fills up the remaining porosity. A rhyolite-fluorite breccia represents the boundary between the barren Tertiary rhyolite and the massive orebody. This breccia consists mainly on subangular altered (silicifled) rhyolite fragments with completely kaolinitized feldspars. These fragments are angular to subangular, centimeter- to decimeter-sized, and are partially embedded into a clay matrix. In the vicinity of the fluorite body, fluorite veins and veinlets crosscut this breccia indicating that the fluorite postdates the main fragile event. The orebodies close to the rhyolite breccias are also brecciated and cemented by fluorite, forming the so-called "fluorite-fluorite breccias". The observation of different breccia generations and cements indicates that these are linked to a polyphasic fault zone. No fluorite clasts are found within the rhyolite breccia but, in turn, this is crosscut by fluorite veinlets, so the fault zone originated before the emplacement of the fluorite body. Moreover, the
540
Abstract
intense kaolinitization and silicification suggest the presence of a hot mineralizing fluid that was focused and flowed through the fault zone, and was responsible for the alteration that affects the rhyolite. The presence of this polyphasic fault zone suggests that a seismic-pumping mechanism, in the sense of Sibson et al. (1975) and Sibson (1994), acted as the main flow mechanism for the hydrothermal fluids, generating the several breccia generations observed within the fluorite body at the vicinity of the fault zone.
4. Fluid inclusions Microthermometric analyses were performed on a Chaixmeca heating-freezing stage, calibrated with synthetic fluid inclusions and the appropriate chemicals from the Merck. Accuracy in measurements is about + 0.2 °C for freezing runs and + 3 °C for heating runs. Salinities were calculated using the MacFlincor package (v. 0.92; Brown, 1989). The main ore, formed by fine-grained massive fluorite, is devoid of measurable fluid inclusions. Only late botryoidal fluorite and calcite, found in the remnant vugs, contain fluid inclusions suitable for microthermometric analyses. Moreover, minute oil-bearing fluid inclusions were also found within the botryoidal fluorite but, due to its minute size, they were not appropriate for microthermometric analyses. Twenty samples of botryoidal fluorite (brown and purple) and late calcite from levels 180, 220 m, and the top of the " G " orebody, along with the only quartz sample found, were selected for this purpose. Primary, two-phase (liquid + vapor bubble) fluid inclusions are scarce and minute (< 10 mm). The visual estimation of the liquid to vapor ratio is between 0.9 and 0.95. Numerous primary fluid inclusions decrepitate at low temperature. Despite this, ice melting temperatures (Tmi) were determined in some inclusions to be between - 0 . 5 and 0.0 °C, with more than 90% of the data clustering around - 0.1 °C (either for quartz, fluorite and calcite), corresponding to a salinity of 0.18 wt.% NaC1 eq. Homogenization temperatures (Th) range from 60 to 110 °C, decreasing from brown fluorite (75-110 °C), to quartz (101-107 °C), to purple fluorite (65-77 °C) and to late calcite (60-98 °C), indicating a fluid cooling path and the cease of the hydrothermal circulation
and ore formation (Fig. 3a). Moreover, our Th data show no vertical or horizontal temperature zoning, in agreement with the results ofRuiz et al. (1980) in the " E " orebody.
5. Oxygen and carbon isotopes Carbon and oxygen isotope ratios were measured on the COe extracted from calcite and limestone using the method of McCrea (1950). The isotopic composition was determined using a Finnigan MAT 250 mass spectrometer. Isotope data are reported using the delta per mil notation relative to the PDB (carbon) and VSMOW (oxygen) standards. The standard deviation for each analysis is + 0.1%o for both carbon and oxygen. Seven apparently unaltered cretaceous limestones from the E1 Doctor Formation were collected from the open pit to the level 220 m to determine the isotopic composition of the enclosing rock. Five of them, sampled far from the mineralized bodies, gave 8180 values between +19.6%o and +23.3%0, and 813C values from - 1.61%0 to + 1.10%o. Two of the limestones, sampled close to the fluorite orebody, proved to be recrystallized into some degree after petrographic examination, and have lower values on both 8~So (+16.5%o and +16.6%o) and 813C ( - 2 . 4 % 0 and - 2 . 2 %0) than the fresh limestones. Late calcite 8180 and 813C compositions (15 samples) span from + 13.7 %o to + 17.8 %0 and from - 5.7 %o to - 1.9 %o, respectively (Fig. 3b). As can be seen in Fig. 3b, the isotopic composition of late calcite differs notably from the composition of the fresh limestones, but is very similar to the composition of the recrystallized limestones. The 8180 shift to lower values from limestone to calcite is a temperature effect probably due to the interaction of the regional limestones with a hot aqueous fluid. The calculated isotopic composition of water in equilibrium with recrystallized limestone at temperatures around 60 °C gave similar 8180 and 813C values to the calculated composition of water in isotopic equilibrium with calcite at the same temperature. It is also noteworthy that the isotopic composition of late calcite shows a negative correlation between 8180 and 813C. This trend suggests that ore formation occurred in relationship with both the maturation of organic matter and the degassing of COa.
541
Abstract
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100 Th Fluorite Th Calcite 80
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542
Abstract
6. Discussion and conclusions The low salinity obtained from the ore-forming fluids by microthermometric analyses suggests that the main fluid involved in the genesis of the Las Cuevas fluorite deposit was heated meteoric water. The lack of important temperature variations during the last stage of mineralization o f the deposit can be due to two possible situations: (1) the studied fluids were trapped exclusively during the waning stage of the hydrothermal system, so the solution was already equilibrated with the host rock temperature and cooled down; (2) the fluid flow was not canalized by any major discontinuity, but by intergranular flow promoted by the recrystallization of the limestone, so its temperature equilibrated very effectively with the host rock. It has also to be pointed out that no fluid inclusions were measured within the main finegrained, massive fluorite, so no direct assumptions can be made on the nature and the PT conditions of the hydrothermal fluid at the main stage. The negative correlation between 6180and 613C in late calcites suggests that the precipitation of carbonates was controlled by (1) the reaction between acidic mineralizing fluids and the host rocks, (2) the maturation of organic matter contained in the enclosing rock, and (3) the consequent release of CO2 that would cause the basification of the solution, triggering the precipitation of carbonates. As CO2 tends to fractionate the heavier carbon isotope (Bowers, 1991; Tritlla et al., 2001b), the fluid would be progressively depleted in 13C, as recorded in calcite. The evolution of the oxygen isotopic composition to higher values can be due to the cooling of the solution during the latest stages of the system. In consequence, we suggest that there was no direct magrnatic contribution to this deposit, although fluorine was probably leached from magmatic rocks by deep-circulating meteoric fluids, heated up by the geothermal gradient, and mobilized by a seismic pumping mechanism generated by the fault movement. The source rocks for fluorine could be the hypabissal equivalents o f the topaz-rich rhyolites that are widespread in the area. The hydrothermal fluids alterated the rock, scavenged fluorine and ascended through deeply rooted faults. When these faults developed at the contact between a rhyolite body and highly reactive limestones, the fluid flowed through
the breccia alterating the rhyolite clasts and replacing the limestone by fluorite, acting the carbonate-rich sequence as a trap. When this fault crosscuts the rhyolite exclusively, there is no evidence of mineralization. The front of the orebody is characterized by a halo of recrystallized limestone as a thermal effect caused by the flow of the hydrothermal fluids through a network of fractures, forming calcite veins. The recrystallization o f the enclosing limestone prior to its substitution by fluorite is a common effect found in other carbonate-hosted fluorite deposits in Mexico (see Tritlla et al., 2001a). Furthermore, the low homogenization temperatures and the presence of oil-bearing fluid inclusions make this deposit closer to a M V T s.1. than to a magmatic-hydrothermal derived deposit, in accordance with the conclusions drawn by other authors in similar deposits in Mexico (Gonzfilez-Partida et al., 2002).
Acknowledgements This study was partially financed by the CONACyT project G35442T. Permissions and assistance during fleldwork were kindly provided by Ing. Guidebaldo Olalde and Compafiia Minera Las Cuevas, S.A. de C.V.
References Bowers, T.S., 1991. The deposition of gold and other metals: pressure-induced fluid immiscibility and associated stable isotope signatures. Geochim. Cosmochim. Acta 55, 2417-2434. Brown, P.E., 1989. FLINCOR: a microcomputer program for the reduction and investigation of fluid inclusions data. Am. Mineral. 74, 1390 1393. Carrillo-Bravo,J., 1971. La plataforma Valles-SanLuis Potosi. Bol. Asoc. Mex. Geol. Pet. 23 (1), 61-102. Carrillo-Martinez, M., 1981. Contribncidn al estudio geoldgico del macizo calcfireoE1 Doctor, Qro. Rev. Inst. Geol., UNAM 5 (1), 25-29. Gonzfilez-Partida, E., Carrillo-Chitvez,A., Grimmer, J.O.W., Pironon, J., 2002. Petroleum-rich fluid inclusions in fluorite, Purisima Mine, Coahuila, Mexico. Int. Geol. Rev. 44, 755-764. McCrea, J.M., 1950. On the isotopic of carbonates and palotemperature scale. J. Chem. Phys. 18, 849-857. Ruiz, J., Kesler, S.E., Jones, L.M., Sutter, J.F., 1980. Geology and geochemistry of the Las Cuevas fluorite deposit, San Luis Potosi, Mexico. Econ. Geol. 75, 1200-1209. Sibson, R.H., 1994. Crustal stress, faulting, and fluid flow. In: Parnell, J. (Ed.), Geofluids: Origin, Migration, and
Abstract
Evolution of Fluids in Sedimentary Basins. Geol. Soc. Sp. Pub., Geol. Soc. of London, vol. 78, pp. 69 84. Sibson, R.H., Moore, J.M., Rankin, A.H., 1975. Seismic pumping--a hydrothermal fluid transport mechanism. J. Geol. Soc. (Lond.) 131,653 659. Surer, M., 1984. Cordilleran deformation along the eastern edge of the Valles-San Luis Potosi carbonate platform, Sierra Madre Oriental fold-thrust belt, east-central Mexico. Geol. Soc. Amer. Bull. 95, 1387-1397.
543
Tritlla, J., Camprnbi, A., Corona-Esqnivel, R., 2001a. The Taxco fluorite deposit (Mexico): a new pseudo-chromatographic mechanism for rhythmite formation. In: Piestrzynski, A., et al. (Eds.), Mineral Deposits at the Beginning of the 21st Century. Swets & Zeitlinger Publishers, Lisse, pp. 979 982. Tritlla, J., Cardellach, E., Sharp, Z.D., 2001b. Origin of vein hydrothermal carbonates in Triassic limestones of the Espadfin Ranges (Iberian Chain, E Spain). Chem. Geol. 172, 291-305.
ELSEVIER
C~}DIRECTe
JOURNAL OF
GEOCHEMICAL EXPLORATION
Journal of Geochemical Exploration 78-79 (2003) 537-543 www.elsevier.com/locate/jgeoexp
Abstract
Fluid characteristics of the world-class, carbonate-hosted Las Cuevas fluorite deposit (San Luis Potosi, Mexico) G. Levresse a, E. Gonzalez-Partida a'*, J. Tritlla a, A. Camprubl a, E. Cienfuegos-Alvarado b, P. Morales-Puente b aCentro de Geociencias, Campus Juriquilla, UNAM, Cart. 57 Qro-SLP Ion 15,5, Santiago de Querdtaro 76230, Quer&aro, Mexico bLaboratorio Universitario de Geoqulmica Isotrpica, Instituto de Geologla, UNAM, 04510 M~xico D.F., Mexico
Abstract Las Cuevas is a world-class high-grade fluorite district that accounts for over 7% of the world total fluorite production. This district is mainly hosted in the Cretaceous limestones of the E1 Doctor Formation, and is in fault contact with Tertiary rhyolites. This preliminary study is focused on the "G" orebody, a mass of fme-grained fluorite, with abundant cavities lined up by fluorite, sometimes stalactitic, late calcite and clays. Fluid inclusions in cavity filling minerals have salinities up to 0.18 wt.% NaC1 eq. with homogenization temperatures ranging from 60 to 110 °C, with the Th decreasing consistently from early cavity filling fluorite to late calcite. 61So and •13C values suggest that both an organic matter maturation and a decarbonation process might have occurred during the formation of the deposit. All the characteristics of the deposit suggest an MVT-related origin rather than a magmatic-hydrothermal one as previously proposed by other authors. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Fluorite; Hydrothermal solutions; Oxygen-carbon isotopes; MVT; Mexico
1. Introduction
2. Geographical and geological setting
In this paper, new data are presented on the fluids that originated the G orebody of the Las Cuevas world-class fluorite deposit. After these data, contrary to previous interpretations, the fluorite deposition would be related to the circulation of heated meteoric waters flowing through fractures confining the ore-
Las Cuevas district is located in the Sierra de Alvarez, about 40 km east of the city of San Luis Potosi, at the southwestern limit of the Valles-San Luis Potosi Jurassic-Cretaceous carbonate platform (Carrillo-Bravo, 1971; Carrillo-Martinez, 1981; Suter, 1984). All the seven known, massive and almost monomineralic fluorite bodies in this district are exclusively carbonate-hosted, clearly epigenetic, and systematically located along the tectonic contact between Tertiary rhyolites and the Cretaceous limestones of the E1 Doctor Formation (Carrillo-Martinez, 1981). The presently mined " G " orebody is the largest known so far and the main focus of this paper (Fig. 1).
body.
* Corresponding author. Tel.: +52-442-2381116; fax: +52-4422381110. E-mail address." [email protected] (E. Gonzalez-Partida).
0375-6742/03/$ - see front matter © 2003 Elsevier Science B.V. All rights reserved. doi: 10.1016/S0375-6742(03)00145-6
Abstract
538
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Fig. 1. (a) G e o l o g i c a l sketch o f the Las C u e v a s district w i t h the location o f fluorite bodies at level 180, p r o j e c t e d to the t o p o g r a p h i c a l surface.
Ruiz et al. (1980) carried out the only study known to date on this deposit. These authors, mainly based on partial petrographic and microthermometric analysis on the presently exhausted and inaccessible " E " orebody, proposed a magmatic-hydrothermal model for these deposits, closely associated with the emplacement of the Tertiary rhyolites.
3. Description of the "G" orebody The " G " orebody (50 Mt at >98% CaF2) occurs at the fault contact between the limestones of the E1 Doctor Formation and a Tertiary rhyolite, with breccia development at the contact (Fig. 2a). As the G fluorite body does not outcrop, this N W - S E fault contact has been seen only in the underground mine workings, even though this fault can be recognized in surface affecting mainly the rhyolite body (Fig. 1). The
transition from the massive fluorite orebody to the enclosing limestone is gradual and consists on the progressive replacement of the massive fluorite body through a network of fluorite-calcite veins crosscutting a thin envelope of recrystallized limestone developed around the orebody, grading into a fresh, fossilbearing limestone (Fig. 2d). Both the size of the veins and the degree of recrystallization of the limestone gradually decrease from the ore grading into the fresh limestone. The mineral composition of the orebody is very simple, as fluorite accounts for up to 95% of the deposit. Texturally, fluorite is mostly fine-grained, massive to finely laminated, with variable colors grading from white to yellow, green and, locally, bright red probably due to the presence of minute iron oxide inclusions. It is impossible to suggest a clear paragenetic succession within the main fluorite stage due to the huge internal variations in color and
Abstract
539
Fig. 2. (a) Highlyalteredrhyoliteclasts withinthe rhyolite-fluoritebrecciacontact;(b) late cavitylinedby botryoidalfluoriteand late calcite; (c) fluoritestalactites; (d) reerystallizedlimestonecrosscutby a networkof fluorite-calciteveins in the orebodyouter limit.
texture. In addition, the crosscutting relationships found are extremely complicated and variable at any scale. This inhomogeneity probably reflects the original inhomogeneities of the enclosing rock prior to its substitution by fluorite and/or possible fluctuations of the hydrothermal fluid chemistry. After the deposition of the main stage consisting of massive, fine-grained fluorite, the remaining cavities are lined up by botryoidal masses of gray to yellow fluorite, made up by radiated, skeletal crystals up to several centimeters long (Fig. 2b). Occasionally, a thin crust of purple, minute cubic fluorite crystals has been observed covering the botryoidal concretions, representing the last fluorite generation in the mineral sequence. Also, late stalagmites and/or stalactites of bright yellow fluorite partially fill the remaining space of the cavities (Fig. 2c). Minute quartz crystals have been rarely observed after the latest fluorite generations and are locally followed by calcite. The latter occurs as late, millimeter- to decimeter-sized idiomorphic crystals, precipitated within the cavities. Finally, a brown-red plastic clay, probably a residue derived
from the dissolution of the original limestone, usually fills up the remaining porosity. A rhyolite-fluorite breccia represents the boundary between the barren Tertiary rhyolite and the massive orebody. This breccia consists mainly on subangular altered (silicifled) rhyolite fragments with completely kaolinitized feldspars. These fragments are angular to subangular, centimeter- to decimeter-sized, and are partially embedded into a clay matrix. In the vicinity of the fluorite body, fluorite veins and veinlets crosscut this breccia indicating that the fluorite postdates the main fragile event. The orebodies close to the rhyolite breccias are also brecciated and cemented by fluorite, forming the so-called "fluorite-fluorite breccias". The observation of different breccia generations and cements indicates that these are linked to a polyphasic fault zone. No fluorite clasts are found within the rhyolite breccia but, in turn, this is crosscut by fluorite veinlets, so the fault zone originated before the emplacement of the fluorite body. Moreover, the
540
Abstract
intense kaolinitization and silicification suggest the presence of a hot mineralizing fluid that was focused and flowed through the fault zone, and was responsible for the alteration that affects the rhyolite. The presence of this polyphasic fault zone suggests that a seismic-pumping mechanism, in the sense of Sibson et al. (1975) and Sibson (1994), acted as the main flow mechanism for the hydrothermal fluids, generating the several breccia generations observed within the fluorite body at the vicinity of the fault zone.
4. Fluid inclusions Microthermometric analyses were performed on a Chaixmeca heating-freezing stage, calibrated with synthetic fluid inclusions and the appropriate chemicals from the Merck. Accuracy in measurements is about + 0.2 °C for freezing runs and + 3 °C for heating runs. Salinities were calculated using the MacFlincor package (v. 0.92; Brown, 1989). The main ore, formed by fine-grained massive fluorite, is devoid of measurable fluid inclusions. Only late botryoidal fluorite and calcite, found in the remnant vugs, contain fluid inclusions suitable for microthermometric analyses. Moreover, minute oil-bearing fluid inclusions were also found within the botryoidal fluorite but, due to its minute size, they were not appropriate for microthermometric analyses. Twenty samples of botryoidal fluorite (brown and purple) and late calcite from levels 180, 220 m, and the top of the " G " orebody, along with the only quartz sample found, were selected for this purpose. Primary, two-phase (liquid + vapor bubble) fluid inclusions are scarce and minute (< 10 mm). The visual estimation of the liquid to vapor ratio is between 0.9 and 0.95. Numerous primary fluid inclusions decrepitate at low temperature. Despite this, ice melting temperatures (Tmi) were determined in some inclusions to be between - 0 . 5 and 0.0 °C, with more than 90% of the data clustering around - 0.1 °C (either for quartz, fluorite and calcite), corresponding to a salinity of 0.18 wt.% NaC1 eq. Homogenization temperatures (Th) range from 60 to 110 °C, decreasing from brown fluorite (75-110 °C), to quartz (101-107 °C), to purple fluorite (65-77 °C) and to late calcite (60-98 °C), indicating a fluid cooling path and the cease of the hydrothermal circulation
and ore formation (Fig. 3a). Moreover, our Th data show no vertical or horizontal temperature zoning, in agreement with the results ofRuiz et al. (1980) in the " E " orebody.
5. Oxygen and carbon isotopes Carbon and oxygen isotope ratios were measured on the COe extracted from calcite and limestone using the method of McCrea (1950). The isotopic composition was determined using a Finnigan MAT 250 mass spectrometer. Isotope data are reported using the delta per mil notation relative to the PDB (carbon) and VSMOW (oxygen) standards. The standard deviation for each analysis is + 0.1%o for both carbon and oxygen. Seven apparently unaltered cretaceous limestones from the E1 Doctor Formation were collected from the open pit to the level 220 m to determine the isotopic composition of the enclosing rock. Five of them, sampled far from the mineralized bodies, gave 8180 values between +19.6%o and +23.3%0, and 813C values from - 1.61%0 to + 1.10%o. Two of the limestones, sampled close to the fluorite orebody, proved to be recrystallized into some degree after petrographic examination, and have lower values on both 8~So (+16.5%o and +16.6%o) and 813C ( - 2 . 4 % 0 and - 2 . 2 %0) than the fresh limestones. Late calcite 8180 and 813C compositions (15 samples) span from + 13.7 %o to + 17.8 %0 and from - 5.7 %o to - 1.9 %o, respectively (Fig. 3b). As can be seen in Fig. 3b, the isotopic composition of late calcite differs notably from the composition of the fresh limestones, but is very similar to the composition of the recrystallized limestones. The 8180 shift to lower values from limestone to calcite is a temperature effect probably due to the interaction of the regional limestones with a hot aqueous fluid. The calculated isotopic composition of water in equilibrium with recrystallized limestone at temperatures around 60 °C gave similar 8180 and 813C values to the calculated composition of water in isotopic equilibrium with calcite at the same temperature. It is also noteworthy that the isotopic composition of late calcite shows a negative correlation between 8180 and 813C. This trend suggests that ore formation occurred in relationship with both the maturation of organic matter and the degassing of COa.
541
Abstract
Ca)
100 Th Fluorite Th Calcite 80
6O
40
20
0 L 0 20
(b)
40
60
80 100 120 140 160 180 200 220 240 260 280 300 Th°C
4 Regional limestone
[] 0 ~.
Surface Level180 Level220
Late ore calcite • • •
~.
Surface Level 180 Level 220
-2
-4 n
• n n
-6
-8
12
I
I
I
I
I
14
16
18
20
22
24
~SOv.sMOW Fig. 3. (a) Histogram of homogenization temperatures for late fluorite, calcite and quartz; (b) 6180 vs. 613C plot, see text for further explanation.
542
Abstract
6. Discussion and conclusions The low salinity obtained from the ore-forming fluids by microthermometric analyses suggests that the main fluid involved in the genesis of the Las Cuevas fluorite deposit was heated meteoric water. The lack of important temperature variations during the last stage of mineralization o f the deposit can be due to two possible situations: (1) the studied fluids were trapped exclusively during the waning stage of the hydrothermal system, so the solution was already equilibrated with the host rock temperature and cooled down; (2) the fluid flow was not canalized by any major discontinuity, but by intergranular flow promoted by the recrystallization of the limestone, so its temperature equilibrated very effectively with the host rock. It has also to be pointed out that no fluid inclusions were measured within the main finegrained, massive fluorite, so no direct assumptions can be made on the nature and the PT conditions of the hydrothermal fluid at the main stage. The negative correlation between 6180and 613C in late calcites suggests that the precipitation of carbonates was controlled by (1) the reaction between acidic mineralizing fluids and the host rocks, (2) the maturation of organic matter contained in the enclosing rock, and (3) the consequent release of CO2 that would cause the basification of the solution, triggering the precipitation of carbonates. As CO2 tends to fractionate the heavier carbon isotope (Bowers, 1991; Tritlla et al., 2001b), the fluid would be progressively depleted in 13C, as recorded in calcite. The evolution of the oxygen isotopic composition to higher values can be due to the cooling of the solution during the latest stages of the system. In consequence, we suggest that there was no direct magrnatic contribution to this deposit, although fluorine was probably leached from magmatic rocks by deep-circulating meteoric fluids, heated up by the geothermal gradient, and mobilized by a seismic pumping mechanism generated by the fault movement. The source rocks for fluorine could be the hypabissal equivalents o f the topaz-rich rhyolites that are widespread in the area. The hydrothermal fluids alterated the rock, scavenged fluorine and ascended through deeply rooted faults. When these faults developed at the contact between a rhyolite body and highly reactive limestones, the fluid flowed through
the breccia alterating the rhyolite clasts and replacing the limestone by fluorite, acting the carbonate-rich sequence as a trap. When this fault crosscuts the rhyolite exclusively, there is no evidence of mineralization. The front of the orebody is characterized by a halo of recrystallized limestone as a thermal effect caused by the flow of the hydrothermal fluids through a network of fractures, forming calcite veins. The recrystallization o f the enclosing limestone prior to its substitution by fluorite is a common effect found in other carbonate-hosted fluorite deposits in Mexico (see Tritlla et al., 2001a). Furthermore, the low homogenization temperatures and the presence of oil-bearing fluid inclusions make this deposit closer to a M V T s.1. than to a magmatic-hydrothermal derived deposit, in accordance with the conclusions drawn by other authors in similar deposits in Mexico (Gonzfilez-Partida et al., 2002).
Acknowledgements This study was partially financed by the CONACyT project G35442T. Permissions and assistance during fleldwork were kindly provided by Ing. Guidebaldo Olalde and Compafiia Minera Las Cuevas, S.A. de C.V.
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