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Mineralogy and Geochemistry of Tourmaline in Contrasting Hydrothermal Systems: Copiapó Area, Northern Chile

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Mineralogy and Geochemistry of Tourmaline in Contrasting Hydrothermal Systems: Copiapó Area, Northern Chile MINERALOGY AND GEOCHEMISTRY OF TOURMALINE IN CONTRASTING HYDROTHERMAL SYSTEMS: COPIAPÓ AREA, NORTHERN CHILE by Ana C. Collins A Prepublication Manuscript Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 2010 STATEMENT BY THE AUTHOR This thesis has been submitted in partial fulfillment of requirements for the Master of Science degree at the University of Arizona and is deposited in the Antevs Reading Room to be made available to borrowers, as are copies of regular theses and dissertations. Brief quotations from this manuscript are allowable without special permission, provided that accurate acknowledgment of the source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the Department of Geosciences when the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained by the author. __Ana Collins________________________________ ________________ (author’s signature) (date) APPROVAL BY RESEARCH COMMITTEE As members of the Research Committee, we recommend that this thesis be accepted as fulfilling the research requirement for the degree of Master of Science. _Dr. Mark D. Barton__________________________ ________________ Major Advisor (type name) (signature) (date) _Dr. Eric Seedorff____________________________ ________________ (type name) (signature) (date) _Dr. Robert Downs___________________________ ________________ (type name) (signature) (date) 2 Abstract Tourmaline group minerals can be useful for petrogenetic studies due to their refractory nature, chemical and isotopic variability, and widespread occurrence in many geologic settings. Near Copiapó, Chile, tourmaline occurs in a wide range of igneous- related hydrothermal systems with widely varying types of mineral assemblages and chemical compositions, making this area an exceptional locality for studying controls on tourmaline chemistry. Copiapó tourmalines cover the majority of known tourmaline compositions excluding those associated with Li-rich pegmatites. Tourmaline formed in multiple, complex stages and is commonly intermediate schorl-dravite with a general progression in later tourmaline generations towards more Fe-rich and Al-deficient compositions with a dominant substitution of Fe3+ for Al. This compositional trend, along with the presence of several tourmaline generations, is consistent with time-varying, relatively oxidizing, saline, acidic, boron-bearing fluids and reflects a greater host rock influence with progressive hydrothermal alteration. Tourmalines from other saline environments, both mineralized and otherwise, show similar compositional trends, reflecting analogous tourmaline-forming fluid compositions. The correlation between iron enrichment and highly saline fluids may reflect progressively more effective leaching and transport of iron from the host rock with time. Boron isotope analyses of tourmaline indicate a mixed fluid source, reflective of both magmatic and evaporitic sources, and is consistent with previous fluid-related studies of mineralizing fluids associated with iron oxide-copper-gold (IOCG) mineralization in the Candelaria-Punta del Cobre district. The study of tourmaline in these settings has the potential to constrain the origin(s) of this puzzling style of mineralization and can yield insights on the diversity of conditions under which tourmaline forms. Introduction Tourmaline occurs in a variety of geological environments and is a common accessory mineral in granitic pegmatites, low- to high-grade metamorphic rocks, and clastic sedimentary rocks. However, hydrothermal environments comprise some of the most common and diverse occurrences. Tourmaline’s complex composition reflects changes in its chemical and physical environment which, combined with its refractory 3 nature and wide range of stability, make it well-suited to explore the conditions under which it formed (Henry and Guidotti, 1985). Consequently, tourmaline has been the subject of many studies and is useful for investigating differences between contrasting hydrothermal systems. Tourmaline is a complex borosilicate mineral group that has a general structural 1+ 2+ 2+ formula of XY3Z6[T6O18](BO3)3V3W, where X = Na, Ca, K, and □, Y = Li , Mg , Fe , Mn2+, Al3+, and Ti4+, Z = Al3+, Mg2+, Fe3+, V3+, and Cr3+, T = Si, Al, and B, V = OH, O, and W = OH, O, and F (Dyar et al., 1998; Hawthorne and Henry, 1999). Usually tourmaline is considered in terms of its end members, of which there are fourteen IMA- recognized species (Table 1; Hawthorne and Henry, 1999). Solid solution in tourmaline is ubiquitous as simple or coupled substitutions. Table 2 summarizes common exchange vectors in tourmaline. Al, Fe, Na, Ca, and Mg comprise some of the most important substituent elements. Li can be important in tourmalines from rare-metal granites and pegmatites; however, it is minor or absent in most other types of settings (e.g., Henry and Guidotti, 1985). Documenting and understanding variations in these constituents is central to interpreting the significance of tourmaline in hydrothermal systems. Tourmaline is commonly associated with and co-precipitated during the formation of numerous types of mineral deposits, including copper, silver-gold, tin(-tungsten), massive sulfide, and uranium deposits, and occurs as breccia cement and clasts, veins, alteration envelopes and assemblages, and other metasomatic bodies (Slack et al., 1984; Pirajno and Smithies, 1992; Slack, 1996; Xavier et al., 2008). Tourmaline is commonly the principal host of boron in these deposits, and its durability allows it to preserve a detailed record of its formation even when dispersed during weathering and erosion. Tourmaline chemistry reflects the diverse compositions of both host rock and hydrothermal fluids, as well as differences in temperature and pressure of formation. This compositional record provides insight into mineralizing conditions, fluid flow, and possible sources of constituents in hydrothermal systems (e.g., in magmatic-hydrothermal systems: Pirajno and Smithies, 1992; Mlynarczyk and Williams-Jones, 2006; Dini et al., 2008; those sourced from external fluids: Palmer and Slack, 1989; Peng and Palmer, 2002; Xavier et al., 2008). Major-element trends have been used as guides for exploration (i.e., Clarke et al., 1989). For instance, Fe/(Fe+Mg) ratios in tourmaline vary systematically in Sn and Sn-W 4 hydrothermal deposits, with ratios decreasing with increasing distance from the magmatic source of mineralizing fluids and increasing interaction with the host rock (Pirajno and Smithies, 1992). Boron isotopes in tourmaline have been used to fingerprint the source of mineralizing fluids and can provide new insight as to the metallogenesis of various hydrothermal deposits (Palmer and Slack, 1996; Xavier et al., 2008). The abundance of tourmaline can also serve as a prospecting guide for undiscovered borate bodies and stratabound mineral deposits (Peng and Palmer, 2002; Slack, 1982). Care must be taken when using tourmaline as an exploration tool, as the compositions can be strongly influenced by the composition of the host rock, and the final composition may reflect an amalgamation of multiple sources and chemical interactions. Relatively little work has been done on the mineralogy and stability of tourmaline in many high-temperature hydrothermal systems, and tourmaline petrology and geochemistry have only recently been considered in iron oxide-copper-gold (IOCG) deposits (i.e., Xavier et al., 2008). IOCG systems are characterized by voluminous magnetite and/or hematite, variable amounts of Cu- and Fe-sulfides, gold, and REE, and low Ti contents compared to most igneous rocks (Hitzman et al., 1992; Barton and Johnson, 1996; Williams et al., 2005). In contrast to porphyry-type systems, magmatic compositions play only a secondary role on IOCG alteration mineralogy and elemental abundances (Barton and Johnson, 1996). IOCG deposits are generated by hypersaline, variably CO2-bearing, Cl-rich, and S-poor fluids, are formed at shallow to mid-crustal levels, and are closely associated with variably intense and voluminous sodic(-calcic) and potassic alteration. The origins of the ore-forming fluids are unsettled: they might be exsolved from magmas (Marschik and Fontboté, 2001; Sillitoe, 2003; Pollard, 2006) or be derived from external, evaporitic brines (Barton and Johnson, 1996; Xavier et al., 2008). Thus, understanding tourmaline in these settings has the potential to elucidate and constrain the origin(s) of this puzzling style of mineralization and, in well-chosen cases, can yield an independent set of constraints on the diversity of conditions under which tourmaline forms. This study systematically looks at igneous-related tourmaline occurrences in the Copiapó region of northern Chile, a part of the Chilean Iron Belt and one of the world's classic areas for IOCG mineralization (Marschik and Fontboté, 2001; Sillitoe, 2003). This 5 area is also the locus for widespread, though economically unimportant, porphyry copper mineralization (Maksaev et al., 2007). The first part of this project evaluates the petrographic, chemical, and isotopic characteristics of tourmaline in the diverse hydrothermal environments present near Copiapó, of which the IOCG systems in the Candelaria-Punta del Cobre district are pre-eminent examples. We then use
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