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11 th SGA Biennial Meeting Let’s Talk Ore Deposits 26-29th September 2011 ,

High-level alteration in Iron-Oxide(-Cu-Au) (IOCG) vein systems, examples near Copiapó, Chile

Douglas C. Kreiner, Mark D. Barton Institute for Mineral Resources, Department of Geosciences, University of Arizona

Abstract. IOCG deposits of the Chilean Iron development of AAA is now represented by pyrophyllite Belt, including the "simple" vein systems hosted by the and/or andalusite assemblages, is devoid of sulfide Copiapó batholith, allow comparison of contrasting levels minerals and may have a few percent . Hypogene within IOCG systems. Exposed at the highest levels of has not been observed. These features are the systems studied near Copiapó are zones of hydrolytic analogous to the more complicated, larger and alteration characterized by either calcite- specularite±chalcopyrite veins with chlorite- economically important IOCG deposits in the region. alteration, or specularite dominated vein(let)s with This paper summarizes recent studies of alteration sericitic and/or advanced argillic alteration (AAA). and zoning about IOCG veins near Copiapó, with a Multiple zones of AAA in the Copiapó area are hosted in focus on sericitic and advanced argillic alteration volcaniclastic rocks and form strataboumd, structurally developed in the shallower levels. controlled, and -pipe bodies. Mineralization is texturally complex typically with superimposed 2 Geologic framework assemblage (e.g., contact metamorphic andalusite on hydrothermal pyrophyllite). In some cases, AAA clearly overlies Cu-Fe mineralization (Jesus Maria, Ojancos The is part of one of the world’s Viejo) whereas in others the relationship is uncertain premier IOCG belts; it lies along the Mesozoic arc of (Vinita Azul, Ojancos Nuevo). AAA associated with IOCG northern Chile and southern Peru. Deposits are hosted in systems in Chile and elsewhere is sulfide-poor, the Coastal Batholith and related supracrustal rocks evidenced by an abundance of hypogene hematite and (mostly volcanic) (Sillitoe 2003). Multiple styles of little or no pyrite. These systems contrast markedly with IOCG mineralization occur varying from strataboumd high total sulfide AAA in epithermal and (manto-type) to discordant breccia bodies, both hosted systems. Acid sources are problematic In low�S primarily in supracrustal rocks. These texturally varied systems, we suggest that AAA can result from the precipitation of metals (principally iron) from Cl bodies typically exhibit complex superposition of complexes at depth in systems with very high W/R. events. In contrast, plutonic rocks generally host somewhat simpler, smaller, but locally economic veins Keywords. IOCG, advanced argillic, hydrolytic, that exhibit systematic zoning from shallow or distal epithermal advanced argillic alteration to deeper, proximal Na(-Ca) styles of alteration.

1 Introduction 2.1 Copiapó Batholith

Use of zoning patterns in iron-oxide(-Cu-Au) (‘IOCG’) The Copiapó Batholith is an Early Cretaceous (95-127 deposits can be a key to exploration and an improved Ma) composite batholith of broadly dioritic composition understanding of their origin, however the tops of few (Marschik and Söllner 2006). Plutons range in systems are well documented. Some models for IOCGs composition from a two- diorite to predict moderate to intense acid alteration should form monzodiorites and tonalites. The batholith was at shallow levels (Barton and Johnson, 2000), and it is emplaced into Jurassic and Cretaceous volcanic and known that various styles of hydrolytic alteration marine sedimentary rocks with local evaporite-bearing including hematite-bearing sericitic and advanced layers (Marschik and Fontboté 2001). Each pluton in the argillic alteration (AAA) occur in a number of IOCG Copaipó batholith has an associated and distinct systems (Barton et al. 2010; Kreiner and Barton 2010). hydrothermal system associated with its emplacement Hydrolytic alteration in IOCG systems contains (Barton et al. this volume). These plutons have hypogene (as specularite) and lacks abundant alteration ranging from proximal and deep high-T Na(- sulfide minerals that can constitute 10-20% of AAA in Ca), to moderate and low-T Na with local potassic other ore forming environments. alteration; high-levels are characterized by sericitic and The Ojancos Viejo and adjacent districts, near advanced argillic assemblages. Excellent exposure of Copiapó in northern Chile have field patterns and pluton-supracrustal environments in the batholith allows petrologic and fluid inclusion evidence that indicate examination of IOCG veins, including their upper preservation of high levels in these Early Cretaceous levels. IOCG systems. Hydrolytic alteration is distal and overlying many small, yet locally high-grade 2.2 District scale mineralization bearing and hematite dominated vein deposits (Fig. 1). AAA is present in the overlying The largest IOCG occurrences in the vicinity of Copiapó volcanisedimentary sequence and has not been observed include the volcanic-hosted strataboumd to discordant in the plutonic units. Intense acid leaching and breccia bodies of La Candelaria and similar deposits of

497 11 th SGA Biennial Meeting Let’s Talk Ore Deposits 26-29th September 2011 Antofagasta, Chile

the Punta del Cobre district (Marschik and Fontboté the 127 Ma mineralization. Overprinting and 2001). There are, however, several well defined IOCG overgrowing the AAA are tourmaline-andalusite- - vein districts located in the Copiapó batholith and calcite±hematite (Fig. 2a). Superimposed andalusite and surrounding areas (Segerstrom et al. 1963); these perhaps some of the pyrophyllite on early dumortierite- provide geologically simpler systems suited for pyrophyllite may represent a metamorphic overprint comparative study. New geologic mapping, structural related to the voluminous 118-115 Ma plutons that are reconstruction, and petrologic studies indicate the vein related to the Ojancos Viejo and Nuevo districts. systems continue upward into penecontemporaenous volcanic rocks and have vapor-rich, CO2-poor inclusions which imply low pressures and indicates the veins are exposed from near the contemporaneous paleosurface to paleodepths of 2-3,000 m. Investigating the vein systems at multiple points along strike has allowed for the development of a paragenesis that indicates the veins are exposed over multiple structural levels. Veins zone from proximal and commonly deep magnetite--dominated, copper- poor assemblages with Na-Mt alteration to mushketovite-chalcopyrite-pyrite-quartz assemblages with Hm-Na-K(-H+) alteration to distal and shallower calcite-hematite with hematite-chlorite alteration (Fig. 1). Overlying and/or distal to mineralization are zones of Cu-poor, sericitic and advanced argillic assemblages (Fig. 1). Zones of AAA are restricted to the northern end of the Ojancos Nuevo vein district, the southern end of the Ojancos Viejo vein district, the Vinita Azul district and the northern end of the Jesus María district (Fig. 1).

3 Distribution and characterization of shallow alteration

Shallow level alteration associated with the vein-style mineralization in the batholith is characterized by sericite-hematite(-quartz-pyrite) and/or chlorite-hematite alteration. Zones of AAA are commonly devoid of veins or mineralization. Mineralization in shallow levels is manifested by calcite-specularite±chalcopyrite veins (up to several meters in width) with chlorite- hematite(±carbonate) dominated envelopes. Zones of calcite, sericite, hematite and AAA are overlying the zones of the main magnetite-mushketovite±hematite- chalcopyrite mineralization. Calcite-specularite±chalcopyrite veins typically have Figure 1. Location map and vein zoning. Zones of Mt- local alteration envelopes of intense chlorite- dominated veins with Na-Mt alteration in core are enveloped hematite(±carbonate±albite) (Fig. 1) and are present at by Hm-dominated veins with Na-K-Hm(H+) alteration. Zones the distal ends of mineralized structures (e.g., San of AAA and Ser-Hm found coincident with supracrustal rocks. Francisco vein). Calcite-dominated assemblages are one Base map compiled from Arevalo (1999) and Tilling (1963) of the latest paragenetic stages of vein development and and modified by Barton et al. (this volume). thus may also be present at deeper levels of mineralization. In the Vinita Azul area, AAA is strongly localized on Sericitic alteration is more widespread than AAA, NNE oriented structures. Zones of AAA are hosted in a and well developed along pyrite and specularite package of interbedded volcanic and clastic rocks with dominated structures. Sericitic alteration can be interbedded sill-like bodies of fine-grained diorite. texturally destructive and pervasive, with minor Assemblages are dominated by quartz-pyrophyllite- hematite and quartz, and often contains little or no pyrite lazulite±hematite (Fig. 2b), locally with andalusite near and may zone inwards to AAA. a cross-cutting pluton, and flanked by zones of sericite- The Jesus Maria area contains zones of strataboumd hematite±pyrite alteration. AAA alteration is cut and AAA hosted in volcaniclastic rocks. The AAA overprinted by potassic (biotitic) and sericitic alteration structurally overlies deeper magnetite--actinolite that likely formed with a 98 Ma tonalite. Deeper styles and intermediate Cu-bearing mineralization (Fig. 1); of mineralization are not exposed in this area. these are cut by intramineral dikes (127 Ma; U-Pb In the northern part of La Brea pluton (Fig. 1), in the zircon). AAA is characterized by early dumortierite- Ojancos Nuevo district, a zone of AAA has a breccia- pyrophyllite±hematite (Fig. 2a) assemblages related to pipe geometry. Host rocks are porphyritic .

498 11 th SGA Biennial Meeting Let’s Talk Ore Deposits 26-29th September 2011 Antofagasta, Chile

Alteration assemblages are characterized by andalusite- region, and Carolina Slate Belt all have examples of hematite-quartz±pyrophyllite that zone out into sericite- IOCG-related AAA (Barton et al. 2010; Kreiner and hematite. The intense acid alteration overlies a Barton 2010). prominent magnetic anomaly, is truncated by the San The understanding of the mineral assemblages Gregorio fault, does not affect the San Gregorio pluton. characteristic of the upper levels of IOCG systems have The southern end of the Ojancos Viejo district has implications on effective mineral exploration. Zones of volcaniclastic-hosted, restricted zones of intense acid AAA and calcite-dominated iron-oxide veins may assemblages overlying the magnetite and hematite represent, respectively, the fluid-buffered and rock- dominated, Cu-bearing vein systems (Fig. 1). buffered flow paths generated by the precipitation of Volumetrically sericite-quartz±hematite is the most metals (particularly iron from Fe-Cl complexes) at abundant but is locally cut by pyrophyllite±quartz veins. depth. These zones may be overlying or distal to copper- bearing mineralization.

Acknowledgements

We thank Ralph Stegen and Freeport McMoran for support, and Joaquin Errazuriz and Ioan Filip of Minera Carmen Bajo for logistical support and access to their vein deposits. Additional funding has been provided by National Science Foundation grant EAR08-38157, Science Foundation Arizona, Hugh E. McKinstry fund.

References

Arevalo C (1999) The coastal cordillera/precordillera boundary in Figure 2. AAA Examples. a) photomicrograph dumortierite the Tierra Amarilla area (27º20’-27º40’S / 70º05’-70º20’W), (Dum)-hematite-quartz(Qz) overgrown by tourmaline(Trm)- northern Chile, and the structural setting of the Candelaria Cu- andalusite(And) with late calcite(Cc) from Jesus María; b) Qz- Au ore deposit. PhD dissertation, Kingston University, 204 p lazulite-pyrophyllite-hematite from Vinita Azul, scale 1 cm. Barton MD, Johnson DA (2000) Alternative brine sources for Fe- oxide(-Cu-Au) systems: Implications for hydrothermal 4 Discussion alteration and metals. Hydrothermal Iron Oxide Copper-Gold & related Deposits: A Global Perspective. Adelaide, Australian The upper levels of IOCG systems contain distinct low- Mineral Foundation pp 43-60 temperature styles of alteration and mineralization. Barton MD et al (2010) Jurassic igneous-related metallogeny of the southwestern North America. Geol Ore Deposits Great Portions of the vein systems are dominated by calcite- Basin, Geol Soc Nevada, Reno, Nevada pp 75-98 rich assemblages and these commonly occur distal to the Kreiner DC, Barton MD (2010) Diversity of intense acid main zones of mineralization. Overlying and distal to (advanced argillic) alteration. GSA Abstracts with Programs mineralization are intensely acid-altered zones, 42(5) p. 207 characterized by sericite-hematite and local AAA. Marschik R, Fontboté L (2001) The Candelaria-Punta del Cobre AAA associated with IOCG systems is marked by the iron-oxide Cu-Au(-Zn-Ag) deposits, Chile. Econ Geol 96:1799-1826 scarcity of pyrite and alunite and by the presence of Marschik R, Sollner F (2006) Early Cretaceous U–Pb zircon ages hypogene iron oxides. This contrasts with high-S for the Copiapó plutonic complex and implications for the systems, such as epithermal and porphyry environments IOCG mineralization at Candelaria, Atacama Region, Chile. where pyrite constitutes 5-20% of the altered zones and Miner Deposita 41:735-801 alunite may be abundant. Low-sulfidation (LS) Ray GE, Dick LA (2002) The Productora prospect in north-central Chile: An example of intrusion-related, Candelaria-type Fe- epithermal systems also contain zones of AAA but only Cu-Au hydrothermal system. Hydrothermal Iron Oxide locally in steam-heated zones. All of these systems share Copper-Gold & related Deposits: A Global Perspective. similar geometries with strong structural controls. Adelaide, Australian Mineral Foundation pp 43-60 AAA associated with IOCG systems is not unique to Segerstrom K, Thomas H, Tilling RI (1963) Cuadrangulo Pintadas. the Copiapó region. Mantoverde, has zones of intense Instituto de Investigaciones Geologicas, , Chile sericitic alteration and El Laco and Productora have Sillitoe RH, Burrows DR (2002) New field evidence bearing on the origin of the El Laco magnetite deposit, northern Chile. local AAA (Vila et al. 1996; Ray and Dick 2002; Sillitoe Econ Geol 97:1101-1109 and Burrows 2002). Sillitoe and Burrows (2002) suggest Sillitoe RH (2003) Iron oxide copper-gold deposits: an Andean the zones of AAA at El Laco are formed in a steam- view. Miner Deposita 38:787-812 heated environment, similar to the LS epithermal Vila T, Lindsay N, Zamora R (1996) Geology of the Manto Verde environment. However, formation in a steam heated copper deposit, northern Chile: A specularite-rich, zone requires the presence of H S as a product of hydrothermal-tectonic breccia related to the Atacama Fault. 2 Soc Econ Geol Spec Publ 5:159-175 boiling in a sulfide-bearing fluid. This intrinsically Williams PJ, Barton MD, and others (2005) Iron oxide copper-gold drives the system to high-sulfidation states, which is not deposits; geology, space-time distribution, and possible modes recorded in the AAA mineral assemblages associated of origin. In: Hedenquist JW, Thompson JFH, Goldfarb R, with IOCG. AAA related to IOCG mineralization is not Richards J (eds) Economic Geology 100th Anniversary unique to the deposits in northern Chile. Several Volume (1905–2005). Society of Economic Geologists, systems in southwest North America, the VA piedmont Littleton, Colorado, USA, pp 371-406

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