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Keynote presentation The roots and tops of - mineralization: evolving magmatic-hydrothermal systems

Fernando Tornos IGEO, Consejo Superior de Investigaciones Científicas, Spain

John M. Hanchar Memorial University of Newfoundland, St-John's, Canada

Francisco Velasco Universidad del País Vasco UPV/EHU, Spain

Rodrigo Muñizaga Compañía Minera del Pacífico,

Gilles Levresse Universidad Autónoma de México, México

Abstract. Magnetite-apatite deposits form a well-defined Cordillera of the , Missouri, the Urals, China, and type of mineralization characterized by the presence of a . Furthermore, some of them are potential sources of core of massive magnetite ± apatite with an extensive Co and REE. Despite grouped within the - aureole of alkali-calcic alteration hosting a low grade -Gold (IOCG) style of mineralization (Williams et mineralization. Geology and geochemistry suggest that al. 2005), MtAp deposits form an independent group that these deposits are the product of the crystallization of oxidized water-bearing iron-rich melts that separated from a is easily discernible from the dominantly replacive sensu parental silicate . The temporal and vertical stricto IOCG deposits. evolution of these systems is controlled by the timing and The existence of melt inclusions and crystallized depth of separation of large amounts of aqueous fluids, a (nelsonite) with a composition similar to MtAp process that leads to the formation of complex magmatic- deposits, the high temperature of formation, the geologic hydrothermal systems. These magnetite-apatite systems relationships and the common presence of structures grade upwards into bodies of massive apatite or an similar to that of crystallized igneous rocks suggest that extrusive magnetite-apatite mineralization. A key feature of these rocks are the product of the crystallization of iron- this style of mineralization is the formation of large rich melts (Frietsch 1978; Naslund et al. 2002; Tornos et pipes and diatremes that are related to melt degassing at shallow depths. al. 2016).

1 Magnetite-apatite deposits 2 The roots of the system: Immiscible iron- rich melts Magnetite-apatite (MtAp) rocks form a well-defined group of ore deposits characterized by the presence of massive There is widespread evidence that immiscible P-bearing magnetite with variable amounts of F-apatite, iron-rich melts are present in different magmatic systems /actinolite, and ; this type of massive within the Earth’s crust (e.g., Clark and Kontak 2004; orebody has sharp non-replacive contacts with the host Jakobsen et al. 2005). Recent work (Kamenetsky et al. rocks, which are usually affected by a large aureole of 2013) shows that there is a solvus surface between Fe-rich alkali-calcic hydrothermal alteration that is zoned around melts enriched in Ca, Mg, Ti and P, and silica-rich melts the massive mineralization. They typically have a enriched in Al, Na and K. Water and volatiles are strongly characteristic mineral assemblage including K fractionated into the Fe-rich end member (Lester et al. feldspar/albite, diopside/actinolite and scapolite with a 2013). The ultimate cause of melt immiscibility can be low grade-mineralization in , stockworks or crustal contamination by P-, Fe- or silica-rich rocks disseminated ore. All these deposits are poor in sulphides (Philpotts 1967). In the Andes, the Sr-Nd isotope and but have variable amounts of anhydrite. In geochemistry of MtAp ores tracks the composition of the detail, the massive magnetite mineralization form vertical underlying basement and in several sites it is likely that the ellipsoidal pipes or lenses controlled by major transcrustal primitive andesitic magmatism interacted with P-rich faults; only when located at or near the surface form evaporite-bearing sedimentary rocks. stratiform deposits. The shape of the solvus is critical for the formation of Despite not being as economically significant as the MtAp mineralization; only systems crystallizing at banded iron formations (BIF), they can form large decreasing temperatures or with high degrees of crustal deposits such as Kiruna () and as well as several contamination will form immiscible iron-rich and other medium-sized ones clustered in the Coastal

SY03 – IOCG-IOA ore systems and their magmatic-hydrothermal continuum: A family reunion? 831 melts. Otherwise, the immiscible melts will have Magnetite occurring as microphenocrysts or in melt intermediate compositions; - and - inclusions in the parental rocks, as well as sometimes in magnetite. This seems to be a critical feature of these the alkali-calcic hydrothermal assemblage, are enriched in systems, since only iron-rich melts having low Si contents Ti (≈>2 wt%), commonly containing exsolution lamelae of – and hence low viscosities but high density – can Ti-rich oxides (Velasco et al. 2016). These major effectively separate from the much more viscous silicate differences cannot be due solely to changes in the melt that should be left behind. Otherwise, it is likely that temperature-fO2 but are interpreted as controlled by the KD the two immiscible components do not separate. Evidence between the iron-rich melt and the exsolving volatile (P- or of the existence of a silica-rich melt that should accompany F-rich) hydrothermal fluids or by mineral equilibria the magnetite-apatite mineralization are rare. However, involving Ti-bearing magnetite, titanite or . The last some have been found near several MtAp deposits like Los two minerals are widespread in the massive MtAp Colorados, La Perla, El Laco in Chile and Rektorn mineralization and the host rock hydrothermal alteration. (Kiruna, Sweden). In the Coastal Cordillera of the Andes, the MtAp deposits are located along the major Atacama Fault System, which behaves as a strike-slip structure during the ore forming event in the Late Jurassic-Early Cretaceous (Sillitoe 2003). As in many other magmatic-hydrothermal systems, the formation of small zones of vertical extension such as pull-apart structures or fault splays should promote the upwards intrusion of melts and fluids to shallow depths. Density-controlled buoyancy seems to play a minor role in this part of the system. Locally, near these deposits there are dykes of microdiorite with up to cm-sized drops of magnetite that are here interpreted as crystallized remnants of the iron- rich melt trapped prior to coalescence. They show no appreciable chemical zoning but complex intergrowths with titanite (Fig. 1). The water-rich nature of these melts is evidenced by the presence of an aureole of hydrothermal Figure 1. Drops of magnetite showing no chemical zonation and alteration. enclosed in an aureole of hydrothermal (alkali-calcic) alteration w the host microdiorite. Magnetite shows abundant exsolution lamelae of ülvospinel and is intergrown with titanite (Los 3 Water separation and melt crystallization Colorados Mine, Chile).

The solidus temperature of an iron-rich melt strongly If iron-rich melts behave similar to silicate melts, the depends on the phosphorous, fluorine, and water contents, ascent and later crystallization of the magnetite rock which significantly lower the temperature of should promote the exsolution of large amounts of aqueous crystallization from ca. 1600°C to less than 1000°C. The fluids enriched in incompatible elements (Na, K), volatiles crystallization of an almost monomineralic magnetite rock and abundant Fe. As in other magmatic-hydrothermal is due to the high oxygen fugacity and low aSiO2 of the systems, when water exsolution takes place at intermediate melt, something that inhibits the formation of most iron- to shallow depths (i.e., <4 km) and above the two-phase bearing silicates; when present, they often form an surface, it will separate in two immiscible phases, a external zone of coarse-grained actinolite accompanied by dominant low density vapour carrying most of the volatiles interstitial magnetite. After the crystallization of and a residual relatively immobile brine enriched in magnetite, only apatite and small amounts of elements that form chloride complexes (Driesner and actinolite/diopside can crystallize. Heinrich 2007); the relative proportion, and the salinity, of One of the most striking features of these deposits is the immiscible fluids depends on the depth but in very the highly variable Ti content of the magnetite. While the shallow environments this process can form small amounts classical nelsonite has Ti-rich magnetite, magnetite of the of an (hydro)-saline melt as has been described in El Laco MtAp mineralization is usually regarded as impoverished by Broman et al (1999) accompanied by large amounts in Ti (≈<5000 µg/g) (Dare et al. 2015; Knipping et al. (>99 wt%) of a low density gas. Reaction of the high 2015; Velasco et al. 2016; Broughm et al. 2017; density chloride-rich brine with the host rock would Kołodziejczyk et al. 2017). Despite that is generally true, produce the alkali-calcic alteration observed around the the Ti content of magnetite crystallized from immiscible massive magnetite-apatite zone and associated low grade melts can be very low as has been described in several mineralization while low-density gas would tend to flow localities. In MtAp deposits it varies significantly between upward. A mixture of gas with the melt would ascend in a the different generations of magnetite crystallization. form similar to that described by Woods and Cardoso

832 Mineral Resources to Discover - 14th SGA Biennial Meeting 2017, Volume 3 (1997) and promote the formation of flows of vesiculated apatite in irregularly distributed pods or finely magnetite and explosive volcanism as observed at El Laco disseminated, or absent. The upper zones are characterized or Cerro del Mercado (); this mechanism is by the presence of up to meter-sized crystals of fluor- predicted to be more important and efficient for the upflow apatite in bands or veins – usually in the selvage of the of the iron-rich melts than tectonic stress in shallow bodies - showing unidirectional growth structures. These systems. Eventual reaction of this gas with surficial waters textures as well as the presence of melt and high and low would produce large zones of steam heated alteration such density fluid inclusions recording complex processes of as is observed at El Laco (Tornos et al 2016). silicate and water immiscibility (Velasco and Tornos 2009). The extent of the hydrothermal alteration zone This suggests that these rocks formed during the separation accompanying the MtAp mineralization is significantly of aqueous fluids from a melt in genesis similar to that of large, up to 5-10 times the size of the massive ore. In the pegmatite in felsic magmatic-hydrothermal systems. Coastal Cordillera of the Andes, the alkali-calcic alteration shows a marked but gradual zonation with an internal zone in which actinolite, , K feldspar and variable 3 The tops of the system proportions of scapolite coexisting with stockwork- and breccia-like magnetite mineralization; locally, there are The crystallization at depth of the magnetite ± apatite rock zones enriched in grandite and epidote that likely reflect leave a volatile-rich residual melt that crystallizes in the the existence of more oxidizing fluids. The proportion of massive apatite ± actinolite rocks that sometimes cap sub- calc-silicates and the K/Na ratio of the feldspars decrease outcropping mineralization (Fig. 2). Here, coarse grained outwards. In the outermost part of the aureole of apatite coexists with actinolite and minor amounts of hydrothermal alteration, the magnetite occurs disseminated ilmenite and and forms homogeneous bodies in the altered groundmass. This alteration is similar to that along fractures or forms the groundmass of large found in IOCG deposits but the major difference is the lack magmatic-hydrothermal breccia pipes. The presence of of sulphides other than rare . The outmost these massive apatite-rich bodies suggest that here the hydrothermal zone consists of propylitic alteration. system never reached the surface. Sulphur is abundant in these systems but in the However, the high fluid pressure generated during the oxidized form of anhydrite. Anhydrite is found in the melt upflow and crystallization ofMtAp rocks allows many inclusions within the parental andesite-diorite but also magnetite-apatite systems to reach the surface, producing makes up a significant part of the mineral assemblage in local extrusive magnetite flows, such as those several of the deposits. This suggests that the lack of described in El Laco and Cerro del Mercado, or breccia- sulphides in these rocks is due to the oxidized nature of the diatreme complexes. Along with the well-described iron-rich melts which inhibits the formation of sulphides. example of Olympic Dam there are abundant examples of magnetite-apatite deposits forming part of diatreme breccia complexes including Abovian (Armenia), Gushan (China), Bafq (Iran), Artillero and Peña Colorada (Mexico), Tunguska basin (Russia), El Laco and perhaps the Per Geiger deposit (Kiruna). Since these breccias are located in the top part of MtAp mineralizing systems it is very likely that many of them have been eroded away. At El Laco and Artillero the tops of the systems are well preserved as possible maars or crater lakes. At Artillo, the subaqueous pyroclastic deposits include welded fragments of the immiscible silicate melt (Levresse et al, this volume). In some of these extrusive deposits the amount of apatite is quite low, something probably related to degassing of the melt in the atmosphere.

4 Superimposed alterations

Downward crystallization and cooling of the magnetite- apatite rocks lead to the collapse of the magmatic- hydrothermal system and the hydrothermal alteration of Figure 2. Breccia with oriented fragments of andesite with the massive MtAp ores. This widespread phenomenon has alkali-calcic alteration supported by apatite (Maria Ignacia Mine, sometimes produced a pervasive disturbance of the earliest Chile). rocks, masking the original magmatic features. In some Magnetite-apatite deposits show a marked vertical localities, the MtAp rocks are crosscut by stockwork-like continuum; the deep part of the systems is dominated by veins of hydrothermal magnetite and the early fluor- massive magnetite poor in vesicles and with the fluor-

SY03 – IOCG-IOA ore systems and their magmatic-hydrothermal continuum: A family reunion? 833 apatite is replaced by Cl- and CO3-rich apatite. Later, calc- magma mixing in the Antauta Subvolcanic Center, Peru: silicates and feldspars are locally replaced by chlorite, Implications for the origin of nelsonite and iron oxide-dominated carbonates, quartz and clays, and magnetite by hematite. hydrothermal deposits. Econ Geol 99:377-395. Dare SAS, Barnes S-J, Beaudoin G (2015) Did the massive magnetite In the deposits studied there are no evidences of “lava flows” of El Laco (Chile) form by magmatic or transition from MtAp mineralization to coeval IOCG hydrothermal processes? New constraints from magnetite mineralization. Some of the MtAp rock, as well as other composition by LA-ICP-MS. Miner Deposita 50:607-617 magnetite-rich rocks, act as a trap for later superimposed Driesner T, Heinrich CA (2007) The system H2O-NaCl. Part I. genetically unrelated hematite-rich Cu-Au (IOCG) Correlation formulae for phase relations in temperature-pressure- composition space from 0 to 1000ºC, 0 to 5000 bar and 0 to 1 mineralization, as can be observed in some of the major XNaCl. Geochim Cosmochim Acta 71:4880-4901. hematite-rich Cu-Au deposits of the Coastal Cordillera of Frietsch R (1978) On the magmatic origin of iron ores of the Kiruna Chile such as Mantoverde, Sierra Norte or Cerro Negro type. Econ Geol 73:478-485. Norte. Jakobsen JK, Veksler IV, Tegner C, Brooks CK (2005) Immiscible iron- and silica-rich melts in basalt petrogenesis documented in the Skaergaard intrusion. Geology 33:885-888. Kamenetsky VS, Charlier B, Zhitova L, Sharygin V, Davidson P, Feig 5 Conclusions S (2013) –scale liquid immiscibility in the Siberian Traps represented by melt pools in native iron. Geology The geology and geochemistry of several MtAp deposits 41:1091-1094. reveal some common features that suggest that these Knipping JL, Bilenker LD, Simon AC, Reich M, Barra F, Deditius deposits represent complex and unusual magmatic- AP, Wolle M, Heinrich CA, Holtz F, Munizaga R, 2015, Trace elements in magnetite from massive iron oxide-apatite deposits hydrothermal systems related to the crystallization of indicate a combined formation by igneous and magmatic- immiscible iron-rich melts that separated from parental hydrothermal processes: Geochim Cosmochim Acta, v. 171, p. intermediate to mafic silicate melts. Only systems showing 15-38. a significant degree of melt immiscibility can form large Kołodziejczyk A, Hanchar JM, Tornos F, Velasco F, 2017, Trace volumes of iron-rich melts that locally flow upwards along element composition of iron oxide ores from the El Laco major faults and produce large volumes of aqueous fluids magnetite-apatite deposit, northern Chile (this volume). Lester GW, Clark AH, Kyser TK, Naslund HR (2013) Experiments that are responsible of the formation of pegmatite-like on liquid immiscibility in silicate melts with H2O, P, S, F and Cl: rocks and large maar-diatreme complexes and lava flows implications for natural magmas. Contrib Mineral Petrol when extruding in subaerial environments. Reaction of 166:329-349. these fluids with the host rock also produces large volumes Levresse G, Tornos F, Velasco, F, Corona Esquivel R (2017) New of a characteristic alkali-calcic hydrothermal alteration. insights on the formation of magnetite-apatite deposits: the Artillero magnetite prospect (Mexico) (this volume). These systems are perhaps broadly equivalent to Naslund HR, Henriquez F, O. NJ, Vivallo W, Dobbs FM (2002) magmatic Cu-Ni deposits in mafic-ultramafic rocks. In Magmatic iron ores and associated mineralisation; examples both cases, the ultimate cause of melt separation has been from the Chilean High Andes and Coastal Cordillera In: Porter interpreted to be crustal contamination. However, in MtAp TM (ed) Hydrothermal Iron Oxide Copper-Gold & Related systems contamination by oxidized crustal rocks produces Deposits: A Global Perspective, vol 2. PGC Publishing, Adelaide, highly oxidized systems that favour the formation of pp 207-226. Philpotts AR (1967) Origin of certain iron- oxide and apatite sulphide-poor magnetite and apatite rocks instead. rocks. Econ Geol 62:303-315. Sillitoe, R. H., 2003, Iron oxide-copper-gold deposits: an Andean Acknowledgements view: Miner Deposita, v. 38, p. 787-812. Tornos F, Velasco F, Hanchar J (2016) Iron-rich melts, magmatic This study has been funded by the Spanish SEIDI project magnetite and superheated magmatic-hydrothermal systems: The El Laco deposit, Chile. Geology 44:427-430. 2014-55949R. We acknowledge colleagues working on Velasco F, Tornos F (2009) Pegmatite-like magnetite-apatite deposits this intriguing group of ore deposits including K. Ehlig, M. of northern Chile: a place in the evolution of immiscible iron Lagos, H. Lledó, A. Pozolov, M. Rojo N. White, and T. oxide melts? In: Williams P et al (eds) Smart Science for Zhou for sharing their knowledge about this intriguing Exploration and Mining. Economic Geology Research Unit. group of deposits. We also thank CAP Minería, LKAB, James Cook University, Townsville, pp 665-667. Grupo Acerero del Norte and MINOSA for granting access Velasco F, Tornos F, Hanchar JM (2016) Immiscible iron- and silica- rich melts and magnetite geochemistry at the El Laco volcano to the mine sites and sharing information. (northern Chile): Evidence for a magmatic origin for the magnetite deposits. Ore Geol Rev 79:346-366. References Williams P, Barton MD, Johnson DA, Fontboté L, Haller Ad, Mark G, Oliver NHS, Marschik R (2005) Iron Oxide Copper-Gold deposits: Geology, space-time distribution, and possible modes of Broman C, Nyström JO, Henriquez F, Elfman M (1999) Fluid origin In: Hedenquist JW, Thompson JFH, Goldfarb RJ, Richards inclusions in magnetite-apatite ore from a cooling magmatic JP (eds) Economic Geology – 100th anniversary volume. Society system at El Laco, Chile. GFF 121:253-267. of Economic Geologists, Littleton, pp 371-406. Broughm S, Hanchar JM, Tornos F, Attersley S, Westhues A (2017) Woods AW, Cardoso SSS (1997) Triggering basaltic volcanic Mineral chemistry of magnetite from magnetite-apatite ores and eruptions by bubble-melt separation. Nature 385:518-520. their host rocks in Sweden and Chile. Miner Deposita. http://doi: 10.1007/s00126-017-0718-8. Clark AH, Kontak DJ (2004) Fe-Ti-P oxide melts generated through

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