U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA

10° CONGRESO GEOLÓGICO CHILENO 2003

TOURMALINE AND TOURMALINE BRECCIA PIPES FROM THE SUPERGIANT PORPHYRY COPPER DEPOSITS OF THE EL TENIENTE

BELT, CENTRAL CHILE

KING, R.W. RABBIA, O.M. HERNANDEZ, L.B. and LOPEZ-ESCOBAR, L.

Instituto GEA, Universidad de Concepción, C-160, Concepción, Chile INTRODUCTION Tourmaline is a complex borosilicate mineral that is the chief repository of boron in a variety of rock types. Tourmaline has also been observed to be associated with diverse types of ore mineralization and some authors have even proposed its use in mineral exploration. Due to its resistant nature to alteration, metamorphism and weathering, studies of tourmaline chemistry have yielded valuable information of its ambient of crystallization. Tourmaline possesses several fundamental characteristics that that make it ideal for geochemical studies and exploration, including: 1. Tourmaline has a complex chemical structure which can accommodate a wide

spectrum of major elements. This is reflected in the large number of naturally occurring (11 IMA approved) species and (5 approved) synthetic species.

2. The "loose" crystal structure of tourmaline allows minor, trace and rare earth elements

present in the environment of crystallization to be adsorbed and/or substituted into the crystal lattice. A multitude of these elements in various concentrations have been reported in the literature (see Henry and Dutrow 1996; King 1990 for reviews). The assumption that tourmaline is an effective trace element scavenger is substantiated by the number of these different elements found in tourmaline.

3. Tourmaline is characterized with a robustness that typically survives weathering and

most alteration events. Thus tourmaline will scavenge a wide assortment of elements from its environment of crystallization and will retain these elements during subsequent alteration, metamorphism and weathering. In some deposits tourmaline composition reflects the primary chemical and isotopic features of the ambient mineralizing conditions at the time of its crystallization (King, 1990). In central Chile, tourmaline is also commonly found associated with copper mineralization in the giant porphyry copper deposits of the El Teniente Belt and is in fact quite abundant. In this presentation, we report a study on the mineral chemistry of tourmaline. Emphasis will be placed on distribution, texture and compositional characteristics. Information that can be deciphered on

Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad.

its ambient of formation will also be discussed. The goal has been to define the chemical variability of tourmaline in the different settings and use a commonly available instrument like the electron microprobe to identify ionic substitutions and identify the types and degree of zonation. Work is currently underway to characterize the isotopic and trace element characteristics of the tourmaline compositions. GEOLOGICAL SETTING The El Teniente Belt is located in central Chile between latitudes 32 °S and 34 °S (Fig.1 ). The belt is distinguished by the presence of three very large to what some have even classed as “behemothian” porphyry Cu deposits, including, Los Pelambres, Rio Blanco–Los Bronces and El Teniente (Fig. 1). Apart from their phenomenal sizes, other characteristic features include their young ages (9 - 4.7 Ma) and the presence of numerous metal-rich breccias.

The formation of the deposits has been closely linked in time to changes in the tectonic and volcanic configuration of the area. The progressive flattening of the subduction angle below the flat-slab segment resulted in an eastward migration of the volcanic arc and corresponding waning of magmatic activity in the region. In conjunction, the progressive flattening caused substantial crustal deformation, uplift and erosion. A description of the geology of each of the deposits is outside the scope of this presentation and the reader is referred to works such as Skewes, (1992) and Skewes and Stern, (1995) and references therein for excellent reviews of the local geology.

TOURMALINE OCCURRENCE

Tourmaline is widespread in all the porphyry copper deposits of the El Teniente Belt. It generally occurs as a constituent in veins and wallrocks associated with stockwork mineralization and alteration (Fig. 2A).

One of the distinguishing features of the deposits within this belt is the occurrence of breccia pipes or bodies –the Rio Blanco-Los Bronces being a prime example with its over one dozen breccia units. Within these breccias, the main matrix mineral is often tourmaline, constituting as much as 80% of the rock (Fig. 2B). Texturally, the tourmaline occurs as scattered isolated needles, as radial groups of crystals (starburst/sunburst) (Fig. 2A), as massive granular aggregates and as felted aggregates of fine-grained fibrous needles. Within the matrix of the breccias, the tourmaline is generally macroscopically massive, composed fine interlocking grains intermixed with quartz, anhydrite and sulphides (Fig. 2C). In proximity to vugs and intensely altered clast larger acicular needles can be discerned. In hand specimen, the color of the tourmaline is matte black and based on this color, numerous workers have falsely labeled the tourmaline as schorl. In thin section, the tourmalines display moderately to extremely intense pleochroism with coloration varying broadly in shades of light to dark browns, greens and blues (Fig. 2D). Based on optical microscope observation and backscattered electron imaging (BEI) using the electron microprobe, zonation is pronounced in the majority of grains from all three deposits studied. The style and intensity of zonation is variable and two types of zonation have been observed. A core-rim oscillatory type (Fig. 2E) and a much more common, sector or patchy zonation (Fig. 2F) TOURMALINE CHEMISTRY The mineral tourmaline, as mentioned above, is a chemically complex borosilicate with a large number of possible substitutions. Using the commonly accepted site reference scheme, the tourmaline formula can be expressed as:

XY3Z6(BO3)3Si6O18 (OH,F)4 where: X representing Na, Ca, K or may be vacant

Y representing Fe2+, Mg2+, Mn2+ or Fe3+, Cr3+. V3+, Ti4+ or Al (where X is vacant) or Li (when in coupled substitution with Al) Z representing Al, Fe3+, Cr3+, V3+ or Fe2+, Mg2+ (when in coupled substitution with X) or 1.33Ti4+ (as expressed by London & Manning 1995).

Overall, the tourmalines from all three deposits possesses characteristic major and trace element compositions and are analogous to each other. SiO2 is quite uniform varying between 34.68 to

36.66 wt%, approximately 6 atoms per formula unit (apfu). Boron could not be determined with the microprobe and thus was assumed to be stoichiometric at 3 atoms per formula unit, which

translates to a recalculated B2O3 value between 9.90 to 10.67 wt%. The elements Al2O3 (22.79 to 34.31 wt%), FeO (5.80 to 17.80wt%) and MgO (6.10 to 9.09 wt%) show the most significant

compositional ranges. Abundances of K2O (0.01 to 0.09 wt%) and CaO (0.05 to 1.40 wt%) are systematically low, while Na2O, the main cation filling the X structural site varies from 1.22 to 2.73 wt%. Contents of MnO and Cr2O3 are insignificant and commonly below detection limits.

TiO2 varies widely between 0.07 to 1.75 wt%. Table 1 list representative electron microprobe results from all three deposits. In this study the tourmaline occurrences were grouped as tourmaline occurring in 1) veins, 2) as fracture infilling, 3) disseminated in alteration haloes in host rocks and 4) as matrix mineral in breccias. In general, the tourmaline from all three deposits can be considered as alkali group tourmaline with minor X-site vacancy and substitution of Ca for Na (Fig. 3A ). Samples from Los Pelambres cluster tightly while there is more scatter in the tourmalines from Andina and especially El Teniente. An important observation apparent in this diagram (and in fact all the following diagrams) is that there are no major compositional discrepancies between the different tourmaline settings listed above. Tourmaline occurring in veins plot within the same cluster as tourmaline disseminated in wall rocks or occurring as matrix mineral in breccias. The most notable compositional variation observed in the tourmalines involves Fe, and Al and to a lesser extent Mg. The Al-Fe-Mg ternary diagram (Fig. 3B) demonstrates that all the tourmalines

plot in roughly the same fields and based on this diagram can be classified as Mg-rich members of the schorl-dravite solid solution series. Furthermore, it can be observed on the ternary diagram and corroborated with the Fe vs Mg diagram (Fig. 4A), that Mg is relatively uniform, varying between 1.4 to 2.4 afpu while the elements Al and Fe display the most variation. Figure 4B illustrates possible cation substitutions in tourmaline which can lead to the observed compositional varieties and is based on a similar diagram used by London and Manning 1995. The composition of an ideal schorl-dravite, plots in the centre of the diagram where X +Y = 4 and Al or Z=6. The vectors radiating from the ideal schorl-dravite represents uvite (Ca(Fe,Mg)AlNa-1), proton-deficient ({(R3)O}{R2)OH)}-1) and alkali deficient ({{R1}Al}{Na(Fe,Mg)}-1) substitution vectors. Tourmalines from the three deposits are more or less centered around the ideal schorl-dravite point with some deviations. Samples that are Al-poor fall along the uvite vector and tourmalines that are Al-rich generally plot between the proton and alkali deficient vectors. The majority of

tourmalines analyzed from Los Pelambres are Al-rich and cluster tightly along the alkali-deficient vector. Tourmaline from El Teniente on the other hand displays a bimodal distribution with one population being Al-rich and plotting between the alkali- and proton- deficient vectors. An equally as large population is slightly more deficient in Al falling along the uvite vector. The majority of samples from Andina are Al-poor and plot along the uvite vector. Although many of the Al-poor tourmalines from El Teniente and Andina fall along the uvite vector, in reality these tourmalines are not uvites. Such diagrams can be misleading at times and care must be exercised in their use. Returning to the Al-Fe-Mg ternary diagram( Fig 3B), it is obvious that none of the Al-poor tourmalines plot near the uvite end-member. In fact the trend is to deviate from this ideal end-member. In addition, the microprobe compositions of the tourmalines analyzed have low Ca concentrations. The variation of Al in the Al-poor tourmalines is not compensated by a uvite component, but in fact by a Fe-Al substitution. Figure 4C plots Fe(tot) vs Al yielding an inverse linear correlation along the Fe2+Fe3+MgAl vector. The graph demonstrates that for Al below 6 apfu, Fe substitutes directly for Al in the Z site to fill that structural site. In addition, in these tourmalines, Mg is relatively uniform filling the majority of the Y site. Fe occupies a portion of both the Y and Z sites which in turn requires the presence of both Fe2+ and Fe3+. The presence of Fe in both the ferrous and ferric states is also corroborated by the intense pleochroism displayed in these Al-poor tourmalines, especially when compared to the more normal schorl-dravites and Al-rich tourmalines. Mattson and Rossman 1987 noted that tourmaline containing (Fe2+ & Fe3+) exhibit very strong optical absorption and pleochroism due to inter valence charge transfer. They also noted that Fe-bearing tourmaline with weak absorption is likely to be homovalent having almost exclusively Fe2+ or Fe3+ but not a mixture of both. All of the tourmalines observed in this study showed optical and chemical zonation to some degree, especially when viewed as backscattered electron images with the electron microprobe. The majority of optical zonation occurs as sector or patchy type. Lesser occurrences of fine scale oscillatory zonation have also been observed in all three deposits. An attempt to relate variations in chemistry with optical zonation resulted in the characterization of three modes. One style of zonation consists of a variation of Fe and Mg in the different zones and where the concentration of Al remains uniform. The second consist in variations of Fe and Al while the concentration of Mg is stable. A third mode that has been documented, is the lack of chemical variation related to optical zonation –at least on the scale and detection of the elements detected with the microprobe. This style or lack of zonation was in particular pronounced in the more strongly optically zoned and pleochroic tourmalines and may be related to inter-valence charge transfer with variation of Fe2+ and Fe3+ in the different sectors. In addition, many of these optically zoned tourmalines do not appear as strongly zoned when viewed in backscatter image mode. Oscillatory and patchy zonation has commonly been observed in hydrothermal tourmalines and is generally related to changes of P, T or the composition of the fluid on a small scale. For example the precipitation of a sulphide mineral adjacent to a crystallizing tourmaline crystal could greatly influence the Fe and Mg distribution of that tourmaline. Likewise, sector or patchy zonation is produced when there is preferential adsorption and incorporation of one or more elements from the fluid on a specific growth surface of a crystal (Hollister, 1970). Commonly, sector zonation is considered to have formed under conditions of rapid growth such that homogeneous equilibrium

could not be maintained along the whole of the crystal. An example could be precipitation of a sulphide grain immediately adjacent to one side of a tourmaline needle. The mineral tourmaline is characterized as a bipolar mineral with the opposite poles along the c-axis having different charges. Some workers have related distinct compositional differences between zones that developed at the (+) and (-) c-poles (Henry and Dutrow, 1992, 1996). Henry et al, 1999 have observed sector zones in tourmalines from the cap rock of a salt dome where the (-) c-poles are strongly enriched in Fe3+, Ti, Ca and O while the (+) c-poles are enriched in Al, Mg and OH. Such zonation may also occur in our tourmalines, however, relating zoning to specific crystal faces is virtually impossible due to incorporation or precipitation of tourmaline in a silicate mineral matrix. DISCUSSION One of the salient features of this crystal chemical study of tourmaline is the similarity of the tourmaline in all three of the giant and complex porphyry copper deposits. All of the tourmalines analyzed from the various settings are basically Mg-rich members of the schorl-dravite solid solution series. Some variability in which some tourmalines are either Al-rich or Al-poor has also been observed between deposits and even with deposits. Tourmaline is an accessory mineral associated with potassic and phyllic alteration zones and hydrothermal breccias in numerous porphyry Cu deposits worldwide. The chemical and isotopic signature of these tourmalines can provide insight on the magmatic-hydrothermal ambient and mineralizing environment. The relatively low Al and correspondingly high Fe contents attest that ferric iron is an important constituent in certain tourmalines and provides an indication of the relatively oxidizing conditions of the fluids. In general, oxidizing conditions could be produced by either mixing magmatic fluids with more oxidizing fluids or boiling of the magmatic fluids. In their study, Drummond and Ohmoto 1985 demonstrated that boiling caused a loss of H into the vapor phase and a resulting oxidation of Fe in the aqueous phase. Fluid inclusion observations in tourmaline and coexisting quartz (this project and Skewes 1992) demonstrates that oxidation is not due to mixing with an oxidizing fluid but rather the boiling of an evolving magmatic-hydrothermal fluid. Furthermore, recent research by Hernandez et al, 2002 on apatites in the felsic intrusive host rocks within these deposits, indicate that the magmas were rather oxidizing to begin with. The chemical variation and fine-scale oscillatory and patchy zonation are consistent with rapid growth of the tourmaline under fluctuating conditions of growth both on a macroscopic and microscopic scale. This is to be expected and is characteristic of an open system behavior. Compared to counterparts in the north of Chile, characteristic features distinguishing the deposits of the El Teniente Belt are the abundance of tourmaline and breccia units. The origin of Boron and the mechanism of formation of the breccias however remains enigmatic. The presence of tourmaline in these deposits may be viewed as an indication of material provenance. Boron isotope studies have shown on numerous occasions that B in volcanic arcs is typically derived from subducted sediments (see Palmer 1991 for review). Thus the presence of

tourmaline is often interpreted to indicate the incorporation in the hydrothermal system of subducted slab material. In areas were metaseds form a substantial component of the supracrustal sequence, a contribution from the surrounding rocks cannot be univocally dismissed. The anomalously large concentration of boron in the El Teniente belt may be related to the particular tectonic setting of the region. Skewes 1992, noted that flattening of the subduction angle may have increased rates of sediment subduction and subduction erosion along the continental margin resulting in an increase in the proportion of continental components incorporated into the source. A boron isotope study on tourmaline and their varied host rocks is currently underway to further elucidate this point (King et al in prep.). The mechanism for the formation of the breccia pipes remains ambiguous and needs further work. Skewes 1992 hypothesized that erosional unroofing of the Miocene magmatic system, in combination with cooling of the magmatic system as magma supply decreased, may have triggered the formation of the breccias by pressure-release exsolution of dissolved volatiles. In a different study on breccias, London and Manning 1999 questioned how volatile exsolution stemming from adiabatic decompression alone provides a mechanism for the sudden appearance of massive tourmaline from the quenching of a felsic melt. London et al, 1990 further demonstrated from experiments involving the hydrothermal synthesis of tourmaline, that no dependance of rates of nucleation of growth of tourmaline on P at a fixed T. Tourmaline would not become suddenly stable in large quantities due to decompression. Furthermore, were does the large quantities of Fe and Mg needed to precipitate tourmaline comes from? In an evolving granitic system –unlike B- Fe and Mg do not behave incompatibly –i.e. they cannot be concentrated. It is clear that further work is needed to understand the origin and mechanism of emplacement of the breccia bodies. Selected References Henry, D.J. and Dutrow, B.L., 1992. Tourmaline in a low grade clastic metasedimentary rocks: an example of the petrogenetic potential of tourmaline. Contrib. Mineral. Petrol. Vol. 112, p. 203-218. Henry, D.J., Dutrow, B.L.. 1996. Metamorphic tourmaline and its petrologic applications. In Grew, E.S., Anovitz, L.M. (Eds.), Boron: Mineralogy, Petrology and Geochemistry. Min. Soc. Amer., Rev. Mineral. Vol. 33. p. 503-557. Henry, D.J., Kirkland, B.L, and Kirkland, D. W. 1999. Sector-zoned tourmaline from the cap rock of a salt dome. Eur. J. Mineral. Vol. 11. p. 263-280. King, R.W., 1990. Tourmaline from Mesothermal Gold Deposits of the Superior Province, Canada: Textural, Chemical and Isotopic Relationships. Ph.D. thesis. Univ. of Saskatchewan, Saskatoon, Saskatchewan. Skewes, M.A., 1992. Miocene and Pliocene Copper-Rich Breccia from the Andes of Central Chile (32 – 34 S). Ph.D. thesis. Univ. of Colorado, Boulder, Colorado. Skewes, M.A. and Stern, C.R., 1995. Genesis of the Giant Late Miocence Copper Deposits if Central Chile in the Context of Andean Magmatic and Tectonic Evolution. Int. Geol. Rev. Vol. 37. p. 893-909. London, D., and Manning, D.A.C., 1995. Chemical variations and significance of tourmaline from southwest England. Econ. Geol. Vol, 90. p. 495-519. Mattson, S.M., and Rossman, G.R., 1987. Fe2+ - Fe3+ interaction in tourmaline. Phys. Chem. Minerals. Vol. 14. p. 163-171. Hollister, L.S., 1970. Origin, mechanism and consequences of compositional sector-zoning in staurolite. Am. Mineral. Vol. 55. p. 742-766. Hernández, L.B., Rabbia, O.M., King, R.W. and López Escobar L. (2002) Sulfur-rich apatite from intrusive rocks associated with the supergiant the El Teniente porphyry Cu-Mo deposit, Chile. 18th IMA Meeting, Edinburg, Scotland, 2002, Abstract vol. p. 268. Palmer, M.R., 1991. Boron isotope systematics of hydrothermal fluids and tourmaline; a synthesis. Isotope Geosci. Vol. 14. p. 111-121. Drummond, S.E., and Ohmoto, H., 1985. Chemical evolution and mineral deposition in boiling hydrothermal systems. Econ. Geol. Vol. 80. p. 126-147.