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Trace element analyses of melt inclusions as probes for the evolution of Bolivian tin porphyry deposits
Nuclear Instruments and Methods in Physics Research B 158 (1999) 621±627
www.elsevier.nl/locate/nimb
Trace element analyses of melt inclusions as probes for the evolution of Bolivian tin porphyry deposits A. Wallianos b
a,*
, A. Dietrich b, B. Lehmann b, M. Mosbah c, K. Traxel
d
a Max-Planck-Institut f ur Kernphysik, P.O. Box 103980, 69029 Heidelberg, Germany Institut f ur Mineralogie und Mineralogische Rohstoe, TU Clausthal, Clausthal, Germany c Laboratoire Pierre S ue,CEA-CNRS, CE-Saclay, Gif-sur-Yvette, France d Physikalisches Institut, Universit at Heidelberg, Heidelberg, Germany
Abstract Micron-sized melt inclusions (MI), trapped in host quartz crystals from rocks of magmatic systems, probe the evolution of ore deposits. PIXE in combination with NRA of light element concentrations provide a nearly complete database for geochemical characterisation. Tin deposits are usually associated with highly fractionated granitic magmatism. The tin porphyry deposits of Bolivia however have only a moderately fractionated bulk rock geochemistry. The investigation of MI revealed high degrees of fractionation with compositions similar to tin granites. It is supposed that the bulk rock geochemistry must be interpreted as the product of magma mixing. Unexposed granitic portions, represented by MI, provide magmatic vapour phases for hydrothermal alteration and mineralisation. Ó 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 81.70; 61.16; 25.40.Ny; 91.65.Nd Keywords: Geology; PIXE; NRA; Trace element analyses
1. Introduction In the so called Andean tin belt, extending about 1000 km from southern Peru to northern Argentina, dierent types of tin deposits show up (Fig. 1). The tin porphyry systems are subvolcanic intrusions of 21 to 14 Ma age, superimposed by hydrothermal overprint immediately after emplacement. Strong tourmalinization and
* Corresponding author. Tel: +49-6221-516210; fax: +496221-516540; e-mail: [email protected]
pervasive sericitization with disseminated Sn-mineralization are followed by complex vein mineralization. Most of the metals forming that type of deposits are supposed to be derived from the magma. The conceptual model for the metallogeny of Sn is usually based on intracrustal melting followed by an advanced degree of magmatic fractionation in high-silica systems [1]. As a worldwide exceptional situation, the Bolivian tin porphyry deposits display tin mineralization in association with only little developed rhyodacitic to dacitic bulk rock compositions.
0168-583X/99/$ - see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 8 0 - 8
622
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627
Fig. 1. Survey of the tin belt at the Eastern Cordillera in Bolivia [2]. MI's, from the tin deposits Llallagua, Potosi and Chorolque were analysed. Reference data from Macusani [3±5], Quimsa Cruz [6], Morococala [7] and Los Frailes [8] will be used in Section 3.
We investigated the worldclass tin porphyry deposits of Llallagua (Sn), Cerro Rico de Potosi (Ag, Sn) and Chorolque (Sn) situated in the Eastern Cordillera of Bolivia (Fig. 1). The igneous rocks are composed of ®ne- to coarse-grained phenocrysts of feldspars, biotite and quartz set in a ®ne grained matrix, which shares about 50% of the rock. Silicate melt inclusions (MI) are entrapped liquid melt
phase and occur in quartz phenocrysts, sheltered from outside in¯uences of pervasive hydrothermal overprint. For that reason MI can serve as probes of the geochemical evolution of these deposits. Their sizes range from a few up to 100 lm, with round to negative crystal shapes (Fig. 2). Most MI consist of colourless glass with a shrinkage bubble, some of them are opaque and recrystallized.
Fig. 2. Several examples of MI's with diameter size range from 20 to 100 lm.
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627
623
Fig. 3. PIXE spectrum of a MI aquired with a 333 lm carbon-absorber in front of the SiLi-Detector.
Trace element compositions are more sensitive than major element concentrations to characterize magmatic rocks and dierentiation processes like fractional crystallization or partial melting [9]. We used l-PIXE for trace element analysis as well as l-NRA for boron measurements in order to characterize these magmatic melt inclusions. 2. Methods The microanalysis with EMPA 1, SIMS 2, PIXE 3 and NRA 4 of MIs was done on handpolished sections of quartz phenocrysts with the MIs exposed at the surface. Recrystallized MIs were rehomogenized in a tube furnace before polishing. PIXE-analyses for major, minor and trace elements were done at the Heidelberg microprobe [10]. The proton energy was 2.3 MeV with beam currents below 100 pA and a lateral resolution of 1 to 5 lm. An 80 mm2 Si(Li) detector of 5 mm thickness and a solid angle of 290 mSr was used for X-Ray detection. For trace element analyses at least one lC proton charge was accumulated (Fig. 3). Calibration was done using thick pure element targets and veri®ed by a number of standard
1 2 3 4
Electron microprobe analysis. Sputtered ion mass spectrometry. Proton induced X-ray emission. Nuclear reaction analysis.
glasses (e.g. basalt BCR1). Precise beam current determination leads to an overall accuracy for absolute quanti®cation of 5% [11]. The boron NRA-analysis was done at the Saclay nuclear microprobe using the nuclear reaction of 11 B(p,a)8 Be at the resonance energy of 660 keV. With beam currents of about 500 pA and a total accumulated proton charge of 1 lC typical limits of detection (LOD) of 5 ppm were achieved [12]. The a particles were detected at 160° backward angle with a 100 mm2 surface barrier detector and 100 lm depleted depth. An 11 lm Mylar absorber protected the detector against backscattered protons as well as secondary electrons. The incident protons induced the following additional reactions: 7
4
Li p; a He
18
O p; a15 N
19
F p; a O
16
The 18 O-spectra could not be used for quanti®cation due to its overlap with the 11 B-spectra. An a-spectrum from a Ta2 O5 target doped with 18 O did allow to de®ne a lower energy cut-o for the boron-reaction a-spectrum used for quanti®cation (Fig. 4). As the proton energy was not in optimum for the lithium or ¯uorine reaction, these elements could be detected with only poor LOD of 100 ppm and 2000 ppm, respectively. Calibration was done by the NBS610 standard.
624
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627
Fig. 4. a-spectra of a Ta2 O5 target doped with O18 , the NBS610-standard and of a MI.
The boron and lithium data were cross-checked by SIMS using the Cameca IMS 4f ion microprobe at Woods Hole [13]. Major elements were also analysed by a Cameca SX100 electron microprobe at TU Clausthal [14]. 3. Results Immobile element data of bulk rock samples [15] display a rhyodacitic to dacitic composition with only moderate degree of fractionation. Analyses of surface and underground samples of 2±5 kg weight by XRF, ICP-MS, and INAA supplied element concentrations of 0.5±0.9 wt% TiO2 , 100± 300 ppm Zr and 1.1±4.6 ppm Ta. On the variation diagram of Zr or Ta vs TiO2 (Fig. 5 (a) and (b)) the bulk rock data plot in between the bulk crust and upper crust reference points. The scatter distribution of the bulk rock tin data (Fig. 5(c)) re¯ects the pervasive hydrothermal overprint. Contrary to this the PIXE analyses of MIs revealed their highly fractionated character similar to tin granites. We found 0.03±0.12 wt% Ti02 , 15± 85 ppm Zr, 5±17 ppm Ta and 5±43 ppm Sn. The ratios of Rb/Sr, Nb/Ta, and Zr/Hf reached up to 20, 2±6 and 3±11, respectively. The MI compositions align with the general fractionation trend of felsic rocks from the tin belt (Fig. 5(a)±(c)). The MI data plot near the Macusani and Quimsa Cruz reference data, which represent highly fractionated material.
Fig. 5. Variation plots of Zr, Ta and Sn vs. TiO2 . Bulk rock-, MI- as well as reference-data are shown. A positive correlation can be seen for compatible components like Zr and Ti. On the other hand the incompatible elements Ta and Sn show enrichment with increasing fractionation.
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627
625
Table 1 Boron and Lithium data from NRA and SIMS analyses agree within their experimental error. Data of MIÂs from Llallagua (L), Chorolque (C) and Potosi (P) Element
B
Li
Method
NRA
SIMS
NRA
SIMS
C36-4 P94b-2 P95-3 P97-3 P97-6 P97-4 L24a-3 L24a-2 P95-4 P97-2 P95-1 C44-2
233 29 162 23 265 34 643 76 346 46 179 28 35 5 105 17 n.a. n.a. 357 64 275 13
220 17 n.a. 302 18 544 27 330 21 212 17 47 6 n.a. 71 8 275 19 n.a. n.a.
248 71 71 42 189 69 35 35 n.a. n.a. 623 80 520 122 n.a. n.a. 264 112 n.a.
198 8 n.a. 59 3 13 5 13 5 13 5 559 11 n.a. 106 4 13 5 n.a. n.a.
Hydrothermal alteration of tin porphyry deposits results in high B-concentrations of bulk rock samples with strong tourmalinisation. Populations of strong and weak tourmalised samples have geometric means of 6500 ppm and 285 ppm B, respectively [16]. From 12 measured quartzhosted MI a geometric mean of 225 ppm boron can be derived (Table 1). 4. Conclusions The highly evolved nature of the MI with respect to bulk rock composition cannot be explained by equilibrum crystal±liquid fractionation alone. As the total amount of phenocrysts within the tin porphyry rocks reaches 40±50%, an only twofold enrichment of perfect incompatible elements is allowed. For the incompatible tantalum we found more than fourfold enrichment. Based on the analytical results, we propose the existence of a highly evolved granitic system at the time of quartz-crystallization. The conversion of a former granitic melt to the present intermediate rhyodacitic bulk rock composition is supposed to have been initiated by a magma mixing process with a primitive andesitic to basaltic melt.
Beside petrological considerations we suggest hydrothermal ¯uids to be supplied by unexposed highly evolved granitic portions, represented by MI. According to the worldwide observed phenomenon, the tin mineralisation in Bolivian tin porphyries is then associated to highly fractionated granitic melts too. The high boron content of MI suggest a magmatic boron input into the hydrothermal systems. Within the variation diagram of B vs incompatible elements (As, Rb or Cs) a positive correlation can be observed. Diagrams of B vs compatible components (Ti or Zr) correlate negatively (Figs. 6(a)±(c)). The incompatible boron seems to join the magmatic evolution of these components. This observations argue against postmagmatic hydrothermal boron leaching from pelitic country rock. The high B, As, Cs and Li contents in MI point to an involvement of pelitic source lithologies not depleted in these ¯uid-mobile elements, i.e. ®rst cycle metamorphic rocks. Magmatic fractionation enriched (depleted) the trace-element abundances within one-log-unit range. We gave some hints how trace element analysis of micron sized melt inclusions can probe the evolution of strongly altered porphyry systems. The nuclear microprobe can give new insights in
626
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627
magmatic processes which provide ore deposit evolution. More detailed data presentation and geochemical discussion is given in [15±17] and will be presented in [14].
Acknowledgements This study was sponsored by Deutsche Forschungs Gemeinschaft by grant Le578/9-1.
References
Fig. 6. Variation plots of B vs. As, Cs and Ti. MI and reference data are shown. The correlation of compatible (Ti) to incompatible (B, As and Cs) elements point to the general fractionation trend of felsic rocks from the tin belt.
[1] B. Lehmann, Metallogeny of Tin, Springer, Berlin, 1990. [2] D.C. Noble, T.A. Vogel, P.S. Peterson, G.P. Landis, N.K. Grant, P.A. Jezek, E.H. McKee, Geology 12 (1984) 35. [3] B. Lehmann, Petrochemical factors governing the metallogeny of the Bolivian tin belt, in: K.J. Reutter, E. Scheuber, P.J. Wigger (Eds.), Tectonics of the Southern Central Andes, Structure and Evolution of an Active Continental Margin, Springer, Berlin, 1994. [4] M. Pichavant, J.V. Herrera, S. Boulmier, L. Brique, J.-L. Joron, M. Juteau, L. Marin, A. Michard, S.M.F. Sheppard, M. Treuil, M.Vernet, The Macusani glasses, SE Peru: Evidence of chemical fractionation in peraluminous magmas, in: B.O. Mysen (Ed.), Magmatic Processes: Physiochemical Principles, vol. 1, Geochemical Society Special Publications, 1987, p. 359. [5] M. Pichavant, D.J. Kontak, L. Brique, J.V. Herrera, A.H. Clark, Contrib. Mineral. Petrol. 100 (1988) 300. [6] B. Lehmann, Schichtgebundene Sn-Lagerst atten in der Cordillera Real/Bolivien, Berl. Geowiss. Abh. A 14 (1979) 1. [7] G.B. Morgan VI, D. London, R.G. Luedke, J. Petrology 39 (1998) 601. [8] A. Dietrich, Geologie, Petrographie und Geochemie des vulkanogenen Epithermal-Systems am Cerro Chakachivi, Meseta de Los Frailes/Bolivien, Diploma Thesis at the TU Clausthal, 1994. [9] M. Wilson, Igneous Petrogenesis, a Global Tectonic Approach, Chapman & Hall, London, 1994. [10] K. Traxel, P. Arndt, J. Bohsung, K.-U. Braun-Dullaeus, M. Maetz, D. Reimold, H. Schiebler, A. Wallianos, Nucl. Instr. and Meth. B 104 (1995) 19. [11] A. Wallianos, P. Arndt, M. Maetz, T. Schneider, K. Traxel, Nucl. Instr. and Meth. B 130 (1997) 144. [12] M. Mosbah, R. Clocchiatti, N. Metrich, D. Piccot, S. Rio, J. Tirira, Nucl. Instr. and Meth. B 104 (1995) 271. [13] J.D. Webster, R. Thomas, D. Rehde, H.J. F orster, R. Seltmann, Geochim. Cosmochim. Acta 61 (1997) 2589. [14] A. Dietrich, Ph.D. Thesis at the TU Clausthal, to be ®nished in July 1999.
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627 [15] A. Dietrich, B. Lehmann, K. Traxel, A. Wallianos, The Lower Miocene tin porphyry system of Llallagua, Bolivia: bulk rock and melt inclusion geochemistry, Mineral Deposits, Papunen (ed.), 1997, p. 625. [16] B. Lehmann, A. Dietrich, J. Heinhorst, N. Metrich, M. Mosbah, C. Palacios. H.-J. Schneider, A. Wallianos, J.
627
Webster, L. Winkelmann, Mineral Deposita, (1998), in review. [17] A. Dietrich, B. Lehmann, A. Wallianos, K. Traxel, C. Palacios, Naturwissenschaften 86 (1999) 40.
www.elsevier.nl/locate/nimb
Trace element analyses of melt inclusions as probes for the evolution of Bolivian tin porphyry deposits A. Wallianos b
a,*
, A. Dietrich b, B. Lehmann b, M. Mosbah c, K. Traxel
d
a Max-Planck-Institut f ur Kernphysik, P.O. Box 103980, 69029 Heidelberg, Germany Institut f ur Mineralogie und Mineralogische Rohstoe, TU Clausthal, Clausthal, Germany c Laboratoire Pierre S ue,CEA-CNRS, CE-Saclay, Gif-sur-Yvette, France d Physikalisches Institut, Universit at Heidelberg, Heidelberg, Germany
Abstract Micron-sized melt inclusions (MI), trapped in host quartz crystals from rocks of magmatic systems, probe the evolution of ore deposits. PIXE in combination with NRA of light element concentrations provide a nearly complete database for geochemical characterisation. Tin deposits are usually associated with highly fractionated granitic magmatism. The tin porphyry deposits of Bolivia however have only a moderately fractionated bulk rock geochemistry. The investigation of MI revealed high degrees of fractionation with compositions similar to tin granites. It is supposed that the bulk rock geochemistry must be interpreted as the product of magma mixing. Unexposed granitic portions, represented by MI, provide magmatic vapour phases for hydrothermal alteration and mineralisation. Ó 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 81.70; 61.16; 25.40.Ny; 91.65.Nd Keywords: Geology; PIXE; NRA; Trace element analyses
1. Introduction In the so called Andean tin belt, extending about 1000 km from southern Peru to northern Argentina, dierent types of tin deposits show up (Fig. 1). The tin porphyry systems are subvolcanic intrusions of 21 to 14 Ma age, superimposed by hydrothermal overprint immediately after emplacement. Strong tourmalinization and
* Corresponding author. Tel: +49-6221-516210; fax: +496221-516540; e-mail: [email protected]
pervasive sericitization with disseminated Sn-mineralization are followed by complex vein mineralization. Most of the metals forming that type of deposits are supposed to be derived from the magma. The conceptual model for the metallogeny of Sn is usually based on intracrustal melting followed by an advanced degree of magmatic fractionation in high-silica systems [1]. As a worldwide exceptional situation, the Bolivian tin porphyry deposits display tin mineralization in association with only little developed rhyodacitic to dacitic bulk rock compositions.
0168-583X/99/$ - see front matter Ó 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 8 0 - 8
622
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627
Fig. 1. Survey of the tin belt at the Eastern Cordillera in Bolivia [2]. MI's, from the tin deposits Llallagua, Potosi and Chorolque were analysed. Reference data from Macusani [3±5], Quimsa Cruz [6], Morococala [7] and Los Frailes [8] will be used in Section 3.
We investigated the worldclass tin porphyry deposits of Llallagua (Sn), Cerro Rico de Potosi (Ag, Sn) and Chorolque (Sn) situated in the Eastern Cordillera of Bolivia (Fig. 1). The igneous rocks are composed of ®ne- to coarse-grained phenocrysts of feldspars, biotite and quartz set in a ®ne grained matrix, which shares about 50% of the rock. Silicate melt inclusions (MI) are entrapped liquid melt
phase and occur in quartz phenocrysts, sheltered from outside in¯uences of pervasive hydrothermal overprint. For that reason MI can serve as probes of the geochemical evolution of these deposits. Their sizes range from a few up to 100 lm, with round to negative crystal shapes (Fig. 2). Most MI consist of colourless glass with a shrinkage bubble, some of them are opaque and recrystallized.
Fig. 2. Several examples of MI's with diameter size range from 20 to 100 lm.
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627
623
Fig. 3. PIXE spectrum of a MI aquired with a 333 lm carbon-absorber in front of the SiLi-Detector.
Trace element compositions are more sensitive than major element concentrations to characterize magmatic rocks and dierentiation processes like fractional crystallization or partial melting [9]. We used l-PIXE for trace element analysis as well as l-NRA for boron measurements in order to characterize these magmatic melt inclusions. 2. Methods The microanalysis with EMPA 1, SIMS 2, PIXE 3 and NRA 4 of MIs was done on handpolished sections of quartz phenocrysts with the MIs exposed at the surface. Recrystallized MIs were rehomogenized in a tube furnace before polishing. PIXE-analyses for major, minor and trace elements were done at the Heidelberg microprobe [10]. The proton energy was 2.3 MeV with beam currents below 100 pA and a lateral resolution of 1 to 5 lm. An 80 mm2 Si(Li) detector of 5 mm thickness and a solid angle of 290 mSr was used for X-Ray detection. For trace element analyses at least one lC proton charge was accumulated (Fig. 3). Calibration was done using thick pure element targets and veri®ed by a number of standard
1 2 3 4
Electron microprobe analysis. Sputtered ion mass spectrometry. Proton induced X-ray emission. Nuclear reaction analysis.
glasses (e.g. basalt BCR1). Precise beam current determination leads to an overall accuracy for absolute quanti®cation of 5% [11]. The boron NRA-analysis was done at the Saclay nuclear microprobe using the nuclear reaction of 11 B(p,a)8 Be at the resonance energy of 660 keV. With beam currents of about 500 pA and a total accumulated proton charge of 1 lC typical limits of detection (LOD) of 5 ppm were achieved [12]. The a particles were detected at 160° backward angle with a 100 mm2 surface barrier detector and 100 lm depleted depth. An 11 lm Mylar absorber protected the detector against backscattered protons as well as secondary electrons. The incident protons induced the following additional reactions: 7
4
Li p; a He
18
O p; a15 N
19
F p; a O
16
The 18 O-spectra could not be used for quanti®cation due to its overlap with the 11 B-spectra. An a-spectrum from a Ta2 O5 target doped with 18 O did allow to de®ne a lower energy cut-o for the boron-reaction a-spectrum used for quanti®cation (Fig. 4). As the proton energy was not in optimum for the lithium or ¯uorine reaction, these elements could be detected with only poor LOD of 100 ppm and 2000 ppm, respectively. Calibration was done by the NBS610 standard.
624
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627
Fig. 4. a-spectra of a Ta2 O5 target doped with O18 , the NBS610-standard and of a MI.
The boron and lithium data were cross-checked by SIMS using the Cameca IMS 4f ion microprobe at Woods Hole [13]. Major elements were also analysed by a Cameca SX100 electron microprobe at TU Clausthal [14]. 3. Results Immobile element data of bulk rock samples [15] display a rhyodacitic to dacitic composition with only moderate degree of fractionation. Analyses of surface and underground samples of 2±5 kg weight by XRF, ICP-MS, and INAA supplied element concentrations of 0.5±0.9 wt% TiO2 , 100± 300 ppm Zr and 1.1±4.6 ppm Ta. On the variation diagram of Zr or Ta vs TiO2 (Fig. 5 (a) and (b)) the bulk rock data plot in between the bulk crust and upper crust reference points. The scatter distribution of the bulk rock tin data (Fig. 5(c)) re¯ects the pervasive hydrothermal overprint. Contrary to this the PIXE analyses of MIs revealed their highly fractionated character similar to tin granites. We found 0.03±0.12 wt% Ti02 , 15± 85 ppm Zr, 5±17 ppm Ta and 5±43 ppm Sn. The ratios of Rb/Sr, Nb/Ta, and Zr/Hf reached up to 20, 2±6 and 3±11, respectively. The MI compositions align with the general fractionation trend of felsic rocks from the tin belt (Fig. 5(a)±(c)). The MI data plot near the Macusani and Quimsa Cruz reference data, which represent highly fractionated material.
Fig. 5. Variation plots of Zr, Ta and Sn vs. TiO2 . Bulk rock-, MI- as well as reference-data are shown. A positive correlation can be seen for compatible components like Zr and Ti. On the other hand the incompatible elements Ta and Sn show enrichment with increasing fractionation.
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627
625
Table 1 Boron and Lithium data from NRA and SIMS analyses agree within their experimental error. Data of MIÂs from Llallagua (L), Chorolque (C) and Potosi (P) Element
B
Li
Method
NRA
SIMS
NRA
SIMS
C36-4 P94b-2 P95-3 P97-3 P97-6 P97-4 L24a-3 L24a-2 P95-4 P97-2 P95-1 C44-2
233 29 162 23 265 34 643 76 346 46 179 28 35 5 105 17 n.a. n.a. 357 64 275 13
220 17 n.a. 302 18 544 27 330 21 212 17 47 6 n.a. 71 8 275 19 n.a. n.a.
248 71 71 42 189 69 35 35 n.a. n.a. 623 80 520 122 n.a. n.a. 264 112 n.a.
198 8 n.a. 59 3 13 5 13 5 13 5 559 11 n.a. 106 4 13 5 n.a. n.a.
Hydrothermal alteration of tin porphyry deposits results in high B-concentrations of bulk rock samples with strong tourmalinisation. Populations of strong and weak tourmalised samples have geometric means of 6500 ppm and 285 ppm B, respectively [16]. From 12 measured quartzhosted MI a geometric mean of 225 ppm boron can be derived (Table 1). 4. Conclusions The highly evolved nature of the MI with respect to bulk rock composition cannot be explained by equilibrum crystal±liquid fractionation alone. As the total amount of phenocrysts within the tin porphyry rocks reaches 40±50%, an only twofold enrichment of perfect incompatible elements is allowed. For the incompatible tantalum we found more than fourfold enrichment. Based on the analytical results, we propose the existence of a highly evolved granitic system at the time of quartz-crystallization. The conversion of a former granitic melt to the present intermediate rhyodacitic bulk rock composition is supposed to have been initiated by a magma mixing process with a primitive andesitic to basaltic melt.
Beside petrological considerations we suggest hydrothermal ¯uids to be supplied by unexposed highly evolved granitic portions, represented by MI. According to the worldwide observed phenomenon, the tin mineralisation in Bolivian tin porphyries is then associated to highly fractionated granitic melts too. The high boron content of MI suggest a magmatic boron input into the hydrothermal systems. Within the variation diagram of B vs incompatible elements (As, Rb or Cs) a positive correlation can be observed. Diagrams of B vs compatible components (Ti or Zr) correlate negatively (Figs. 6(a)±(c)). The incompatible boron seems to join the magmatic evolution of these components. This observations argue against postmagmatic hydrothermal boron leaching from pelitic country rock. The high B, As, Cs and Li contents in MI point to an involvement of pelitic source lithologies not depleted in these ¯uid-mobile elements, i.e. ®rst cycle metamorphic rocks. Magmatic fractionation enriched (depleted) the trace-element abundances within one-log-unit range. We gave some hints how trace element analysis of micron sized melt inclusions can probe the evolution of strongly altered porphyry systems. The nuclear microprobe can give new insights in
626
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627
magmatic processes which provide ore deposit evolution. More detailed data presentation and geochemical discussion is given in [15±17] and will be presented in [14].
Acknowledgements This study was sponsored by Deutsche Forschungs Gemeinschaft by grant Le578/9-1.
References
Fig. 6. Variation plots of B vs. As, Cs and Ti. MI and reference data are shown. The correlation of compatible (Ti) to incompatible (B, As and Cs) elements point to the general fractionation trend of felsic rocks from the tin belt.
[1] B. Lehmann, Metallogeny of Tin, Springer, Berlin, 1990. [2] D.C. Noble, T.A. Vogel, P.S. Peterson, G.P. Landis, N.K. Grant, P.A. Jezek, E.H. McKee, Geology 12 (1984) 35. [3] B. Lehmann, Petrochemical factors governing the metallogeny of the Bolivian tin belt, in: K.J. Reutter, E. Scheuber, P.J. Wigger (Eds.), Tectonics of the Southern Central Andes, Structure and Evolution of an Active Continental Margin, Springer, Berlin, 1994. [4] M. Pichavant, J.V. Herrera, S. Boulmier, L. Brique, J.-L. Joron, M. Juteau, L. Marin, A. Michard, S.M.F. Sheppard, M. Treuil, M.Vernet, The Macusani glasses, SE Peru: Evidence of chemical fractionation in peraluminous magmas, in: B.O. Mysen (Ed.), Magmatic Processes: Physiochemical Principles, vol. 1, Geochemical Society Special Publications, 1987, p. 359. [5] M. Pichavant, D.J. Kontak, L. Brique, J.V. Herrera, A.H. Clark, Contrib. Mineral. Petrol. 100 (1988) 300. [6] B. Lehmann, Schichtgebundene Sn-Lagerst atten in der Cordillera Real/Bolivien, Berl. Geowiss. Abh. A 14 (1979) 1. [7] G.B. Morgan VI, D. London, R.G. Luedke, J. Petrology 39 (1998) 601. [8] A. Dietrich, Geologie, Petrographie und Geochemie des vulkanogenen Epithermal-Systems am Cerro Chakachivi, Meseta de Los Frailes/Bolivien, Diploma Thesis at the TU Clausthal, 1994. [9] M. Wilson, Igneous Petrogenesis, a Global Tectonic Approach, Chapman & Hall, London, 1994. [10] K. Traxel, P. Arndt, J. Bohsung, K.-U. Braun-Dullaeus, M. Maetz, D. Reimold, H. Schiebler, A. Wallianos, Nucl. Instr. and Meth. B 104 (1995) 19. [11] A. Wallianos, P. Arndt, M. Maetz, T. Schneider, K. Traxel, Nucl. Instr. and Meth. B 130 (1997) 144. [12] M. Mosbah, R. Clocchiatti, N. Metrich, D. Piccot, S. Rio, J. Tirira, Nucl. Instr. and Meth. B 104 (1995) 271. [13] J.D. Webster, R. Thomas, D. Rehde, H.J. F orster, R. Seltmann, Geochim. Cosmochim. Acta 61 (1997) 2589. [14] A. Dietrich, Ph.D. Thesis at the TU Clausthal, to be ®nished in July 1999.
A. Wallianos et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 621±627 [15] A. Dietrich, B. Lehmann, K. Traxel, A. Wallianos, The Lower Miocene tin porphyry system of Llallagua, Bolivia: bulk rock and melt inclusion geochemistry, Mineral Deposits, Papunen (ed.), 1997, p. 625. [16] B. Lehmann, A. Dietrich, J. Heinhorst, N. Metrich, M. Mosbah, C. Palacios. H.-J. Schneider, A. Wallianos, J.
627
Webster, L. Winkelmann, Mineral Deposita, (1998), in review. [17] A. Dietrich, B. Lehmann, A. Wallianos, K. Traxel, C. Palacios, Naturwissenschaften 86 (1999) 40.