Fun la serena octubre 2015 dada e Arc scale hydrothermal …biblioteca.sernageomin.cl/opac/DataFiles/14905_v2_pp_… ·  · 2016-10-11new geologic mapping (deposit to transect scale),

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Arc-scale hydrothermal alteration, and the distribution and origin of Cordilleran Fe-oxide(-Cu-Au-P-REE) systems Mark D. Barton* Department of Geosciences and Lowell Institute for Mineral Resources, University of Arizona, Tucson, AZ 85721 U.S.A. *Contact email: [email protected] Abstract. Regional to deposit-scale geologic mapping and related lab studies demonstrate: (1) that Na-Ca-K-dominated hydrothermal alteration modifed large parts of the upper 4-8 km of Mesozoic arc crust in the cordillera of SW North America and the central Andes, and (2) that this regionally extensive alteration is closely associated with diverse styles of Fe-oxide(-Cu-Au-P-REE-Co) mineralization including both Mt(-Act-Ap) (Kiruna) and Mt/Hm(-Cpy) (ʻIOCGʼ) types. In many cases these two types are clearly parts of the same hydrothermal system. Variability in style and composition of the Fe-Cu/Na-Ca-K systems reflects structure, stratigraphy, associated magmatism, and exposure level. In the Chilean Coastal Cordillera and the SW USA published work combined with new geologic mapping (deposit to transect scale), new U-Pb dates, and petrological and geochemical studies shows that: (1) Na-Ca-K-Fe hydrothermal systems are remarkably widespread in space and time, (2) they accompanied many intrusions but are largely independent of igneous compositions, (3) they commonly zone from deep Mt(-Act-Ap) to shallow Mt/Hm(-Cpy), and (4) they had a major component of external brines. Understanding the types, scale, and timing of alteration places economic Fe and Cu mineralization in a firmer context, allowing for better exploration and improved interpretation of this enigmatic family of ore deposits. Keywords: hydrothermal alteration, IOCG, Kiruna, Punta del Cobre, Candelaria, Chilean Iron Belt, scapolite

1 Introduction

The Fe-oxide(-Cu-Au-P-REE) family is a geochemically defined group of hydrothermal systems characteized by abundant low-Ti Fe oxides and the eponymous suite of anomalous minor elements (Barton, 2014). As originally used, this includes Kiruna and “IOCG” type deposits as well as their voluminous associated Na-Ca-K metasomatism (e.g., Hitzman et al., 1992; Barton and Johnson, 1996; Sillitoe, 2003).

Most studies of Fe-oxide(-Cu-Au-P-REE) mineralization have emphasized either deposit-scale geology, specific genetic hypotheses, or the general aspects of the geologic settings. In contrast, this contribution focuses on system to regional scale patterns of Mesozoic Fe-oxide-rich systems in northern Chile and the SW North America. It derives mainly from field, lab, and theoretical investigations conducted over the last several decades by the ecnoomic

geology group at the University of Arizona.

2 Regonal Context

Fe-oxide(-Cu-Au-P-REE) mineralization is widespread along the Mesozoic margins of both North and South America (Fig. 1; Barton et al., 2011; Sillitoe, 2003). In both areas, Na-Ca-K-rich hydrothermal systems overlap in space and time with multiple styles of hydrothermal Fe-oxide-rich mineralization ranging from Kiruna to IOCG to skarn types (Barton, 2009). These areas share many characteristics including prolonged (if punctuated magmatic histories), arid paleoenvironments, and episodes of extensional to strike-slip tectonics.

Figure 1. Approximate extents of Mesozoic and Cenozoic Fe oxide(-Cu-Au-P-REE) (= IOCG sensu lato) occurrences in the cordillera of the Americas. From Barton (2009).

2.1 Mesozoic SW North America

Among the Phanerozoic Fe-oxide-dominated systems in SW North America, Jurassic examples are particularly well developed (Fig. 2A). In their geologic and metallogenic synthesis of the Jurassic magmatic arc, Barton et al. (2011) showed that there Fe oxide systems and related Na-Ca-K alteration formed at many times (195-150 Ma) and in many places along the subaerial portions of the arc. The Fe-oxide systems coincide broadly with the locus of arid paleoenvironments; in contrast, VMS systems formed where magmatism was submarine (Fig. 2B).

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Fe-oxide deposits include Mt- and some Hm-dominated varieties including a number of systems with significant Cu mineralization (e.g., Pumpkin Hollow, NV; Runyon et al., 2015). Tilted crustal sections allow a 3-dimensional view of many of these systems. As discussed below, these sections reveal vertical zoning from deep, barren Na-Ca alteration up through Fe-P mineralization into shallow Cu-bearing Hm/Mt mineralization. In contrast to the Fe-oxide-rich systems, which do not correlate with magmatic compositions, the metal suites and alteration of other Jurassic hydrothermal deposit types (porphyry, skarn, greisen, replacement) correlate closely with magmatic compositions.

Figure 2. Jurassic metallogenic patterns in SW North America (modified from Barton et al., 2011a). A) Distribution of Fe-oxide(-Cu) and VMS deposits. B) Distribution of seawater and more saline surface fluids, arid (evaporitic) environments, and areas of extensive Na-Ca or stratabound K metasomatism. Not shown are the Jurassic W, Cu, Zn, and Ag-Pb-rich systems (skarn, porphyry, etc.) which also occur throughout this area.

2.2 Mesozoic central Andes

Patterns analogous to North America are well known along

the Jurassic to Cretaceous arc of the central Andes. Fe-oxide(±Cu-Au-P-REE) systems are widespread (Sillitoe, 2003; Chen, 2010) and best known along the Chilean Iron Belt where they are roughly coextensive with Early Cretaceous porphyry Cu occurrences (e.g., Maksaev et al., 2006). As in North America, the arc produced diverse, typically intermediate composition rocks in a complex, but generally extensional to strike slip setting (e.g., Parada et al., 2007). The overall paleoenvironment was arid, with widespread evidence for evaporitic materials (e.g., Arcuri and Brimhall, 2002).

2.3 Fe-oxide(-Cu-Au-P-REE) Systems in the Coastal Cordillera 26˚ to 29˚S

Figure 3 shows the generalized distribution of the Mesozoic igneous rocks, erosion levels, general extent of Na-Ca hydrothermal alteration, and representative Fe ± Cu deposits between ~26 and 29˚S. New work has focused on defining the types, ages, and distribution of hydrothermal alteration across the Mesozoic arc and is based on new deposit to district-scale mapping, regional transects, >100 new U-Pb zircon and titanite dates, and related geochemical and petrological studies. The fundamental observation is that hydrothermal alteration, especially Na-Ca types, is remarkably extensive, and concentrated in the upper 5-10 km of the Mesozoic arc crust. As discussed next, there are varied but systematic relationships with several families of mineralization.

Figure 3. Synopsis from ~26˚ to 29˚S in the coastal cordillera of the generalized regional patterns in: (A) exposure levels constrained by barometry and stratigraphy, and (B) main tracts of intense hydrothermal alteration (blue fill). These results are based on the new district and transect studies mentioned in the text. See Fig. 4 for more detailed results along the Rio Copiapó. Also shown on both panels are the better known Fe-oxide(-Cu-Au-P-REE) family deposits (Mt/Hm-Cpy = red; Mt-Ap-Act = blue;). Background patterns include generalized magmatism (Jurassic = blue; Cretaceous = green), and principal strands of the Atacama Fault System (blue lines).

A case study, shown in Figure 4, summarizes regional-scale alteration patterns and mineralization along the Rio

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Copiapó based on recent district and transect field-based projects (e.g., Barton et al., 2005, 2011b; Girardi, 2014; Girardi and Barton, 2013; Kreiner and Barton, 2011). These projects have built on work done by many others and reviewed elsewhere (e.g., Arévalo et al., 2006; Chen, 2010; Marschik and Fontbote, 2001; Sillitoe, 2003).

Figure 4. Synopsis of results of transect and district mapping across the Mesozoic arc mostly ≤ 25 km of the Rio Copiapó showing the broad types and distribution of (A) hydrothermal alteration, and (B) mineralization. See text for discussion.

Alteration patterns are complex and superimposed and require detailed district-scale observations to resolve (e.g., the Punta del Cobre and Ojancos Viejo districts; Barton et al., 2011; Kreiner and Barton, 2011). Based on several other transects, related reconnaissance, and published studies (e.g., Menard, 1995) we have found that similar relationships hold over the entire region examined (26-29˚S). Observations show that: 1) Na-Ca(-K) alteration and Fe-oxide(±Cu) mineralization is present with essentially all the major intrusive suites; 2) these features are best developed at shallow levels and in favorable stratigraphic and structural settings; 3) single systems may extend >10 km laterally and several km vertically; 4) indisputably magmatic-hydrothermal (e.g., porphyry) types are restricted to the vicinity of the more felsic, water-rich (early Hbl-crystallizing) plutons; and 5) the cumulative patterns represent superposition of many systems over >100 m.y. (~195-65 Ma).

3 Vertical Zoning and the Relationships of Mt-Act-Ap to Mt/Hm±Cpy Mineralization

Possible linkages between Kiruna and Olympic Dam types have been debated since originally suggested by Meyer (1988) and Hitzman et al. (1992). Districts in the Andes and SW North America demonstrate that vertical zoning can be present in single systems (Fig. 5). In these relatively young and well exposed systems, varied geologic settings exhibit similar overall alteration and mineralization patterns: vertical zoning goes from deep intense, metal-depleted Na-Ca alteration through a regular upward progression that includes Mt-Ap-Act deposits and terminates in high-level hematite-rich and acid-altered assemblages. Excellent 3-D exposures occur in the extended and tilted Basin and Range province of SW North America (see, for example, Dilles et al., 2000; Johnson and Barton, 2000; Runyon et al., 2015) but similar relationships can be inferrred in Chile (Fig. 5). Such patterns are consistent with predicted reaction pathways for brine-rock interaction (Barton and Johnson, 2000). Likewise, differences are predictable: specific assemblages and accessory element contents should and do vary systematically with setting, for example with igneous rock compositions or, for Cu in particular, with the availability of reduced sulfur. Nevertheless, even though these patterns occur widely, there are many exceptions in style and type in the Cordillera and elsewhere (Barton, 2009, 2014).

Figure 5. Synthesis of vertical zoning in well described Fe-oxide(-Cu-Au-P-REE) systems in northern Chile and the SW USA (summary from Barton et al., 2013, and based on sources cited in that paper). See text for discussion.

4 Final Considerations

Understanding the Fe-oxide(-Cu-Au-P-REE) clan remains a challenge, yet regular patterns emerge when system-scale observations are made. The regular patterns observed in cordilleran settings constrain possible genetic models and may aid exploration. The similarity in vertical zoning

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given different geologic environments (e.g., magmas, host rocks, tectonic setting) requires an origin independent of these features and one that operates in the upper 5-8 km of the terrestrial crust (as evidenced by well defined, metal-depleted bottoms at such levels). These geologic observations when combined with other evidence support a key role in many systems for non-magmatic brines proposed by Barton and Johnson (1996, 2000) and investigated by many since (e.g., Chiardia et al., 2006).

Acknowledgements

University of Arizona studies in the Chilean Iron Belt began with mapping in support of near-mine exploration at Candelaria and evolved to deposit (Candelaria, Santos, Tigressa, Santo Domingo Sur), regional, and topical (geochemical, petrological, mineralogical) projects. These complemented earlier and continuing Fe-oxide-related projects in North America. Many of my students have collaborated on these projects, notably David Johnson, Douglas Kreiner, James Girardi, Giancarlo Daroch, Eric Jensen, Simone Runyon, Frank Mazdab, Maria Zavala, and Ana Collins. Many organizations have supported this work including the U.S. National Science Foundation (EAR08-38157), the U.S.G.S. MRERP (08HQGR0060), Freeport McMoRan (Phelps Dodge), Capstone (Far West), and Minera Carmen Bajo. Among the many company geologists who have helped make these studies possible, the decade-long help of Ralph Stegen, Walter Martin, and Bob Jenkins was particularly important.

References Arcuri, T.L.; Brimhall, G.H. 2002. Animation model of west central

South America from the Early Jurassic to Late Miocene, with some oil and gas implications. Search and Discovery Article no. 10033. University of California, Berkeley. www. searchanddiscovery. net.

Arévalo, C.; Grocott, J.; Martin, W.; Pringle, M.; Taylor, G. 2006. Structural Setting of the Candelaria Fe Oxide Cu-Au Deposit, Chilean Andes (27°30′S). Economic Geology 101(4): 819-841.

Barton, M. D. 2009. IOCG deposits: A Cordilleran perspective: In Proceedings of the 9th Biennial Meeting, Society for Geology Applied to Ore Deposits: 5-7. Townsville.

Barton, M.D. 2014. Iron oxide(-Cu-Au-REE-P-Ag-U-Co) systems.: Treatise on Geochemistry (2nd edition), Volume 13, Ore Deposits (Scott, S.D.; editor). Elsevier: 515-541. Oxford.

Barton, M.D.; Johnson, D.A. 1996. An evaporitic-source model for igneous-related Fe-oxide(-REE-Cu-Au-U) mineralization: Geology: 24(3). 259-262.

Barton, M.D.; Johnson D.A. 2000. Alternative brine sources for Fe-oxide(-Cu-Au) systems: Implications for hydrothermal alteration and metals. In Hydrothermal iron oxide copper-gold & related deposits: A global perspective (Porter, T.M., editor). Australian Mineral Foundation: 43–60, Adelaide.

Barton, M.D.; Dilles, J.H.; Girardi, J.D.; Zürcher, L.; Haxel, G.; Seedorff, E.; Kreiner, D.C. 2011b. Jurassic igneous-related metallogeny of the southwestern North America. In Great Basin Evolution and Metallogeny (Steinger, R.L.; Pennell, B.; editors), Geological Society of Nevada: 373-396. Reno.

Barton, M.D.; Jensen, E.P.; Ducea, M. 2005. Fluid sources for IOCG

and porphyry Cu-style mineralization, Copiapo batholith, Chile: Geologic and Sr isotopic constraints: Geological Society of America Abstracts with Programs 37(7): 316.

Barton, M.D.; Johnson, D.A.; Kreiner, D.C.; Jensen, E.P. 2013. Vertical zoning and continuity in Fe oxide(-Cu-Au-Ag-Co-U-P-REE) (or 'IOCG') systems: Cordilleran insights. In Proceedings of the 12th Biennial Meeting, Society for Geology Applied to Ore Deposits. 1348-1351. Uppsala.

Barton, M.D.; Kreiner, D.C.; Jensen, E.P.; Girardi, J.D. 2011b. Superimposed hydrothermal systems and related IOCG and porphyry mineralization near Copiapó, Chile. In Proceedings of the 11th Biennial Meeting, Society for Geology Applied to Ore Deposits. 521-523. Antofagasta.

Chen, H.Y. 2010. Mesozoic IOCG mineralization in the central Andes: an updated review. In Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective 3 (Porter, T.M., editor). PGC Publishing: 259-272. Adelaide.

Chiaradia, M.; Banks, D.; Cliff, R.; Marschik, R.; De Haller, A. 2006. Origin of fluids in iron oxide–copper–gold deposits: constraints from δ37Cl, 87Sr/86Sri and Cl/Br. Mineralium Deposita 41(6): 565-573.

Dilles, J.H.; Einaudi, M.T.; Proffett, J.M.; Barton, M.D. 2000. Overview of the Yerington porphyry copper district: Magmatic to non-magmatic sources of hydrothermal fluids, their flow paths, alteration affects on rocks, and Cu-Mo-Fe-Au ores: Society of Economic Geologists Guide Book Series 32: 55-66.

Girardi, J.D. 2014. Comparison of Mesozoic magmatic evolution and iron oxide (-copper-gold) ('IOCG') mineralization, Central Andes and western North America. Ph.D. thesis (Unpublished), University of Arizona: 394 p.

Girardi, J.D.; Barton, M.D. 2013, Mesozoic magmatic fluxes, northern Chile. Geological Society of America Abstracts with Programs 45(7): 292.

Hitzman, M.W.; Oreskes, N.; Einaudi, M.T. 1992. Geological characteristics and tectonic setting of Proterozoic iron oxide (Cu-U-Au-REE) deposits. Precambrian Research 58(1): 241-287.

Johnson, D.A., Barton, M.D., 2000. Time-space development of an external brine-dominated, igneous-driven hydrothermal system: Humboldt mafic complex, western Nevada. Society of Economic Geologists Guidebook Series 32: 127-143.

Kreiner, D.C.; Barton, M.D. 2011. District-scale view of the upper levels of iron-oxide(-Cu-Au) (‘IOCG’) vein systems, Copiapó, Chile. In Proceedings of the 11th Biennial Meeting, Society for Geology Applied to Ore Deposits: 497-499. Antofagasta.

Maksaev, V.; Munizaga, F.; Fanning, M.; Palacios, C.; Tapia, J. 2006. SHRIMP U–Pb dating of the Antucoya porphyry copper deposit: new evidence for an Early Cretaceous porphyry-related metallogenic epoch in the Coastal Cordillera of northern Chile. Mineralium Deposita 41(7): 637-644.

Marschik, R. Fontboté, L. 2001. The Candelaria-Punta del Cobre iron oxide Cu-Au (-Zn-Ag) deposits, Chile. Economic Geology 96(8): 1799-1826.

Menard, J.J. 1995. Relationship between altered pyroxene diorite and the magnetite mineralization in the Chilean Iron Belt, with emphasis on the El Algarrobo iron deposits (Atacama region, Chile). Mineralium Deposita 30(3-4): 268-274.

Meyer, C. 1988. Ore deposits as guides to geologic history of the Earth. Annual Review of Earth and Planetary Sciences 16: 147.

Parada, M.A.; Lopez-Escobar, L.; Oliveros, V.; Fuentes, F.; and 10 others 2007. Andean magmatism. In The Geology of Chile (Moreno, T.; Gibbons, W.; editors). The Geological Society. 115-146. London.

Runyon, S.E.; Barton, M.D.; Dilles, J.; Ohlin, H.; Seedorff, E.; Johnson, D.; Carpenter, K. 2015. Iron oxide-rich mineralization and related alteration in the Yerington District, Lyon County, Nevada. In Proceedings of the Geological Society of Nevada Symposium: Geological Society of Nevada. 251-283. Reno.

Sillitoe, R.H. 2003. Iron oxide copper-gold deposits: an Andean view. Mineralium Deposita 38(7): 787-812.