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Post Paleozoic Development at the North and Central Chilean Territory

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Post Paleozoic Development at the North and Central Chilean Territory MININGGEOLOGY,34(1),2131,1984 Tectonic and Petrological Frame of the Cretaceous Iron Deposits of North Chile Jorge OYARZUN*and Jose FRUTOS** Abstract: The Cretaceous magnetite (apatite-actinolite) ore deposits of the Chilean territory are grouped in a N-S narrow band, extended between latitudes 25•‹ and 31•‹S. This paper relates the principal stage of iron mineralization to a period of crustal extension and mafic magmatism (125-110 m.y.), associated to a low and uniform convergence speed of the lithospheric plates (FRUTOS, 1981). Several hypothesis proposed for the genesis of these deposits are discussed. As a general conclusion, the Cretaceous iron mineralization is considered to be a result of the coincidence of particular tectonic and magmatic conditions. Hydration of dry mafic magmas by deep ground water probably had a major role in the genesis of the deposits. As a general principle, it is postulated that dry mafic magmas, with low Fe clinopyroxene as dominant ferromagnesian phase, have a higher iron mineralization potential than the hydrous hornblende-rich ones, where a larger part of iron is retained by the silicate phase. deposits are distributed in Jurassic, Cretaceous Introduction and Tertiary rocks, throughout a large extent The Cretaceous iron orebodies in Northern of the Chilean territory, the post Paleozoic iron Chile occur within a narrow belt between lats ores are confined to a narrow Cretaceous belt, 25•‹S and 31•‹S, although minor and scattered and to a single spot in the Plio-Pleistocene deposits exist beyond these boundaries (Fig. 1). volcanic chain (El Laco, located in the rim of The belt includes about 50 deposits with a volcanic caldera in the Andean Altiplano). a characteristic magnetite-actinolite-apatite The Cretaceous Basin paragenetic assemblage, and are located in Cretaceous volcanic, plutonic, and sedimentary Post Paleozoic development at the North rocks (RUIZ,1965). Besides, some 20 significant and Central Chilean territory is characterized but minor magnetite-poor vein deposits, with by the formation of a 200km wide north apatite dominant as economic mineral, are also trending ensialic synclinorium, comprising within the belt, most of them between lats predominantly volcanic rocks of early Jurassic 28•‹30' and 30•‹10' S. Four of the iron deposits to Paleogene age (ABERGet al., 1983). Both have pre-mining reserves well over 100 m.t. flanks of the synclinorium overlay a Paleozoic, (60% Fe): Cerro Negro-Cristales, Boqueron pre-Andean basement. The cumulative thick Chanar, El Algarrobo-Penoso, and El Romeral, ness of the units in the flanks of the synclinorium and several of them are in the 20-100 m.t. is up to 20km in the western "eugeosynclinal" range. All of the major deposits are found flank, and up to 10km in the eastern, between lats 27•‹S and 30•‹S, that define the "miogeosynclinal" one . Non-deforming meta principal mineralized segment of the belt. morphism is also characteristic, as well as the Unlike the copper mineralization, whose abundance of contact aureoles in volcano Received on September 15, 1983, accepted on December sedimentary rocks as a consequence of granitoid 5, 1983. intrusions of calc-alkaline type (LEVI and * Depto . de Minas, Universidad de La Serena, Chile. AGUIRRE, 1981). The intrusion of granitoids ** Depto . de Geociencias, Universidad de Concepcion, occurred along north trending belts (Fig. 1), Chile. Keywords: Magnetite deposits, Dry mafic magma, progressively younger toward the east, which Groundwater mixing, Ensialic spreading, Northern suggests the action of a plate subduction Chile, El Laco. mechanism. However, the symmetries in the 21 22 J. OYARZIJN and J. FRUTOS MINING GEOLOGY: Fig. 1 The Cretaceous iron belt of North Chile. 1: Iron ore deposits; 2: Cretaceous porphyry copper deposits and prospects; 3: Palaeogene porphyry copper belt; 4: Neogene porphyry copper belt; 5: Lower Cretaceous magmatic arc; 6: Neocomian marine basin; 7: Plio-Quaternary volcanic belt; 8: Magsat anomalies contours (nanotesla units), after HASTINGS(1982, unpubl.). 34(l), 1984 Cretaceous Iron Deposits of North Chile 23 Fig. 2 Paleogeographic setting of the Cretaceous iron-apatite belt and the tectonic and magmatic evolution of the Andes in North and Central Chile (OYARZUNand FRUTOS,1982). 1: Lower Cretaceous (strong distensive conditions); 2: Upper Cretaceous (moderate distensive conditions), 3: Lower Tertiary and 4: Upper Tertiary (Compressive conditions). Ml: iron-apatite belt in North Chile; M2: Cretaceous porphyry copper and vein-type copper and gold deposits; M3 and M4: Tertiary porphyry copper and gold deposits. V1: Mafic calc-alkaline volcanism; V2: intermediate volcanism; V3 and V4: intermediate to acidic volcanism. geological structure of the synclinorium and through a spreading-subsidence process com partly in the chemistry of the volcanic series, bined with plate subduction, alternating coupled with the strong subsidence of the cyclically during the evolution of the basin. basin and the extensional regime of volcanism, Their model is in agreement with, and comple can be better explained in terms of a 'Islandic ment the hypothesis of FRUTOS(1981), who type' rift mechanism. LEVIand AGUIRRE(1981) related the episodes of compressive conditions interpreted this contradictory evidences in the Andean belt (dominance of plate 24 J. OYARZIUN and J. FRUTOS MINING GEOLOGY: subduction over subsidence) to the acceleration 84 m.y. interval, took place (LARSON and of the velocity of plate convergence. PITMAN, op. cit.) and favored folding, intrusion The ensialic furrows generated during of granitoids and uplift. However, although periods of spreading-subsidence, when extrusive activity was dominant during the extrusive activity was dominant, are considered distensive episode, and intrusive activity during by ABERGet al. (op. cit.) as aborted marginal the compressive one (110-85m.y.) both of basins. The basins were aborted in the sense them present extrusive and intrusive activity. that "rifting, spreading and subsidence took Thus, the volcanism in the 125-110m.y. place in an ensialic environment, without interval was also accompanied by granitoid generation of oceanic crust, although mantle emplacement. ZENTILLI (1974) concluded that derived material is present in the form of K-Ar dating for intrusive rocks between lats voluminous flows of flood basalts". Those 26•‹and 29•‹S suggest that emplacement of basalts are often highly porphyritic, with the Cretaceous batholith began during unzoned phenocrysts of labradorite (up to Neocomian times (128m.y. in the western 3 cm), clinopyroxene, magnetite, and altered part of the basin and 120-110 m.y. in the olivine, in a ground-mass of the same minerals, eastern part), and finished during the Upper with minor amounts of K-feldspar and quartz Cretaceous (about 90m.y.). It is interesting intergrows, and may be classified as K-rich calc to note that the granitoids related to the main alkaline basalts according to the classification iron belt were emplaced during the distensive of IRVINEand BARAGAR(1971) in ABERGet al. stage, or during the transitional period prior to (op. cit.). Their aspect and mineralogical the compressive one. They are comagmatic to composition closely resemble those of the the volcanic rocks in which most of the iron labradorite-porphyritic basalts of the low-speed mineralization is emplaced, and have a dioritic oceanic rifts. However, clinopyroxene is to quartz-dioritic composition (BOOKSTROM, strongly dominant in the Chilean rocks under 1977; OYARZUn and FRUTOS, 1982). In consideration. As these rocks have also a high exchange, the younger Cretaceous granitoids iron content (FeO 9-10%, CHAVEZ,1974), and are dominantly granodioritic, and are asso clinopyroxene is low Fe-diopsidic augite, a ciated to copper mineralization. large part of this element is present as magnetite or ulvospinel (normative magnetite is over The Cretaceous Iron Belt 6%). The main part of the iron belt, between lats. The emplacement of the iron mineralization 27•‹and 30•‹S, has a NNE trend, parallel to is coincident with the culmination of a the present coast and is coincident with a spreading-subsidence episode, that occurred large longitudinal fracture zone. The major between 125 and 110 m.y. (Fig. 2) and is iron deposits lie closely to this fracture zone related to a low convergence speed between (which also controlled magmatic emplace- the oceanic and continental plates (5 cm/y.) ment), and were formed by fracture filling and (LARSONand PITMAN,1972; FRUTOS,Op. Cit.). replacement in highly altered 'andesitic' rocks. The volcanism attained a peak in both, volume These rocks are affected by dynamic metamor and, FeO content, during the Hauterivian, phism and display different facies of when flood basalts dominated in the initial hydrothermal alteration, although the iron and intermediate stages, and andesitic flow mineralization is clearly linked to actinolitiza breccias in the terminal ones (ABERGet al., tion. In the major iron deposits of the belt the op. cit.). As a consequence, a thick volcanic original composition, texture and original pile, was built, that attained up to 5-8km in structures of the mineralized rocks are largely the western part of the basin. The distensive oblitered. From a structural and morphological stage and volcanism continued until the point of view, they form 'wedge-like' bodies, Albian, when a strong acceleration of the elongated parallel to the axis of the belt. The convergence speed to 18cm/y, for the 110- bodies are flanked along fault planes by 34(l), 1984 Cretaceous Iron Deposits of North Chile 25 granitoids that may present different composition and ages, although the iron mineralization is related to the dioritic ones, and those of a granodioritic type represent younger, post-mineralization intrusive episodes (BOOKSTROM,OP. Cit.; OYARZUNand FRUTOS, OP. cit.). The only published absolute ages for the Cretaceous deposits are those presented by ZENTILLI(op. cit.) and PICHON(1981), all of them obtained by the K-Ar method. ZENTILLI reported 128m.y. for a biotite vein in Boqueron Chanar, and 102 m.y. for a muscovite from Cerro Iman.
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