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MININGGEOLOGY,34(1),2131,1984

Tectonic and Petrological Frame of the Cretaceous Iron Deposits

of North

Jorge OYARZUN*and Jose FRUTOS**

Abstract: The Cretaceous (-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 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 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- 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 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 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 , 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 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 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 (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 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 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 vein in Boqueron Chanar, and 102 m.y. for a muscovite from Cerro Iman. PICHON released 108 m.y., 110 m.y. and 111 m.y. for a diorite, a post mineral dike and an from Los Colorados iron deposit. With reference to this information it is important to note that the youngest age (102 m.y.) may be a consequence of post-mineral plutonic activity. In exchange, the dating at Los Colorados seems reliable, and portraits the chronological proximity of the volcanic, plutonic, and mineralization stages. BOOKSTROM(op. cit.) reached to a Fig. 3 Idealized profiles of iron deposits. similar conclusion for El Algarrobo deposit and 1: El Romeral diorite; 2: fault ; 3: La Liga considered the diorite and the mineralized intrusive porphyry andesite; 4: Punta de Piedra andesitic rock as comagmatic and Neocomian granodiorite (post mineral intrusive); 5: intramineral in age, on the basis of petrological and dikes; 6: Iron ore (40-49%); 7: ; 8: stratigraphical criteria. shale; 9: limestone; 10: carbonate breccia; 11: The major iron deposits (El Algarrobo, El garnet skarn; 12: diorite intrusive; 13: andesitic Romeral) present segmentation in addition to intrusive; 14: and Quaternary ; strong faulting and fracturing. Thus, 2 or 3 15: rhyo-dacitic intrusive; 16: ignimbrites; 17: Lower Palaeozoic basement; 18: iron ore (>50%). major bodies are in line after the strike of the Modified after BOOKSTROM,1977 (El Romeral); main longitudinal fault (which is also CISTERNAS, 1982 (Bandurrias) and FRUTOS and coincident with the elongation of the bodies OYARZUN,1975 (El Laco). and with the direction of the iron belt). Two reasons make it doubtful to assign the between an older quartz-monzonite (west) mineralized rocks of these deposits as typical and a younger granodiorite (east) (Fig. 3). The roof pendants. First, it is not clear whether the original shapes of the mineralized bodies, ' andesitic' bodies are always extrusive (RUIZ, resemble to inverted elipsoidal cones. The op. cit.) or may also have an intrusive height of such cone at El Romeral, before character, as concluded by BOOKSTROM(op. erosion, may be estimated as close to 1 km cit.) for El Romeral. Second, the structural (OYARZUNand FRUTOS, op.Cit.). relationships between the 'andesitic' miner- Small to medium deposits show different alized rocks and the flanking granitoids are characteristics, and are emplaced in plutonic, different from those of a roof pendant. At El volcanic and sedimentary rocks. Those of the Romeral (BOOKSTROM,op. cit.), the miner vein type have generally low magnitude, and alizing diorite, and the comagmatic intrusive, are present in granitoids as well as in the mineralized andesitic rock, are both emplaced volcano-sedimentary pile. Some of them 26 J. OYARZUN and J. FRUTOS MINING GEOLOGY: contain chalcopyrite and represent a transition The Ti content in the from the to copper deposits with actinolite and minor iron belt as well as from El Laco (FRUTOSand magnetite (RUIZ, op. cit.). Others, may be OYARZUN, 1975) is very low compared to properly considered as apatite deposits, for those in magnetites from volcanic rocks or their higher content of this mineral. from magmatic segregation iron deposits in A few of the deposits emplaced in the mafic or ultramafic rocks, as Boreumdo in volcano-sedimentary pile present a stratiform Korea (So, 1978). Ti content in the Chilean character. They have a minor economic iron ores is also remarkably lower than those importance (pre-mining ore reserves under found in nelsonites (PHILPOTTS, 1967) or 10m.t.), but are interesting in genetical terms. jacupirangites (BERGSTOL,1972), which are The best known of them is Bandurrias probably generated by process of magmatic (27•‹51' S/70•‹35'W), which has been con inmiscibility. sidered by some authors as sedimentary in Deposits Ti in Mgt References(%) origin (LORAM, 1977 in ST CLAIR 1965; ESPINOZA, 1979; ROUTHIER, 1980; PICHON, El Laco 0.01-0.1 FRUTOSand 1981). At Bandurrias (Fig. 3) the magnetite OYARZUN(1975); ore has grades over 50%Fe, 0.06%P, 0.08%S PICHON(1981) and 0.1%Ti (ST CLAIR, op. cit.), and is Cretaceous 0.01-0.1 BOOKSTROM(1977); emplaced in Lower Cretaceous limestones, ores OYARZUN(1980) belonging to a volcano-sedimentary monoclinal Kiruna 0.02-0.08 PARAK(1975) formation. Granodiorite stocks crop out close Vaara to the ore body, and the mineralized strata are Boreumdo 1.7 So (1978) pierced by a diorite porphyry. The limestone flows 2-7 DUNCANand is partially metamorphosed to andradite skarn. TAYLOR(1968) Although CISTERNAS (1982) pointed out that (Chile, FRUTOSand the mineralization is restricted to this meta N. Zealand) OYARZUN(1975) morphic rock facies, replacement seems Phosphorous and sulphur content exceeding unlikely considering the clear cut contact 0.5% and 0.3% respectively are rarely found between the limestone and the overlying in major Chilean iron deposits (Espinoza, stratiform magnetite bodies. An exhalative 1971). Phosphorous, that is contained in Cl origin related to the submarine volcanic or F apatite tend to increase with the Fe activity in the Neocomian basin seems much grade, while concentrations are probable for the genesis of this deposit. distributed independently with regard to those of Fe or P. Mineralogy and Geochemistry High grade mineralization in major deposits The typical paragenesis of the Cretaceous is concentrated in veins and lenses filling iron deposits emplaced in igneous rocks faults or subvertical fractures from which has includes magnetite, actinolite and apatite. All partially or totally replaced the host rock. of them are early minerals, and their (RUIZ, op. cit.; PARK, 1972). Close to the intergrowth is frequent, especially between massive mineralization polydirectional zones magnetite and actinolite. Other early minerals of veinlet filling are found, where the ore has are and sphenon. The sulfurized partially replaced the host rock. Mineralization stage, represented by and minor cuts and is intruded by intramineral alkali rich arsenopyrite and chalcopyrite is late, and fill dikes, formed from a differentiated dioritic fractures cutting both the rocks and the iron magma (BOOKSTROM,op. Cit.). Severe alteration mineralization. Fe2O3occurs mainly as martite, makes difficult to identify accurately the and also as primary (RUIZ, op. cit.; original structure and composition of the RUIZ et al., 1968; BOOKSTROM, op. cit.;PICHON, replaced rocks. This alteration, together with op. Cit.; OYARZUNand FRUTOS,op. Cit.). dynamic metamorphism, is responsible for the 34 (1), 1984 Cretaceous Iron Deposits of North Chile 27

generation of actinolite and biotite schists in authors to explain the origin of major iron the flank of major faults. Although these deposits. Also, the chronological and structural schists were thought to be associated to relationships between the 'andesitic' miner a Paleozoic regional metamorphism by alized rocks and the granitoids, have been BOOKSTROM(op. cit.), Chilean iron mines object of different interpretations. Thus, geologists (e.g. L. VERGARA, P. CARRASCO, GEIJER (op. cit.) concluded that the andesitic etc., pers. comm.) consider them as a product rocks are younger, and intruded the dioritic of fault shearing metamorphism of Cretaceous granitoids. BRUGGEN(op. cit.) had the opposite andesitic rocks. K-Ar dating of El Romeral view, and associated the mineralization to the schists (R. BROUSSE,pers. Comm., 1981) dioritic intrusives bodies. Ruiz (op. cit.) related indicates a Lower Cretaceous age. the granitoids to a Cenomanian intrusive Chlorite, epidote and quartz are also abun episode, and the andesitic rocks to the Lower dant alteration minerals, and are associated to Cretaceous volcanism, and BOOKSTROM(op. iron mineralization as a peripheric zoning. cit.) considered both types of rocks at El A mineral assemblage similar to the potassic Romeral as contemporary and comagmatic. zone of porphyry copper deposits is also In the following paragraphs, a general discus present in some deposits, (e.g. El Romeral) sion of the genetical problem will be intended, including biotite, muscovite and tourmaline. from the view point of the present evidences Extreme grades of host rock silicification are and the current ideas concerning the evolution spatially linked to intense faulting and frac of the Cretaceous basin. turing (RUIZ, op. cit.; RUIZ et al., op. cit.; A first point in the discussion is the conven BOOKSTROM, op.cit.; GALATZAN,1978). ience of relating the origin of the major magnetite ore deposits to the remobilization of The Genesis of the Cretaceous pre-existent iron ores. In first place, there are Iron Ores not clear evidences of the presence of pre Several hypothesis for the origin of the - sedimentary-metamorphic ores in major iron deposits of the Cretaceous iron belt the iron belt, nor in the region (iron bearing in Chile have been proposed. They can be schists described by BOOKSTROM(op. cit.) at El summarized according to the following scheme: Romeral are probably dynamic Cretaceous I Source: 1. Differentiation of an andesitic schists made up out of a mineralized andesitic magma (GEIJER, 1931); 2. Differentiation of a protolith). Regarding the scarce and rather granodioritic magma by fractional crystalliza small stratiform deposits, as Bandurrias, they tion (BRUGGEN,1934); 3. Fe-enrichment in the appear as a different but contemporaneous volatile phase during the crystallization of a iron mineralization, instead of as a probable granodioritic (RUIZ, op. cit.) or dioritic source for the huge mineralizations of El (BOOKSTROM,op. cit.) magma. 4. Aplitization Algarrobo or El Romeral. of granodioritic intrusives by deuteric If remobilization is not a satisfactory alter alteration (RUIZ, et al., op. cit.). 5. Re- native, then the source for the major iron mobilization of iron ores emplaced in Pre deposits and in general for the Cretaceous iron cambrian metamorphic rocks (PARK,op. cit.), mineralization, should be looked for at the or in Neocomian volcano-sedimentary rocks magmatic activity of the Cretaceous basin. (ESPINOZA, op. cit.; ROUTHIER, op. Cit.). II On the other hand, geological studies by Transport: a) Ore magma (GEIJER, op. cit.; BOOKSTROMand the available radiometric BRUGGEN, op. cit.); b) Pneumatolytic dating, indicate a simultaneity and possibly, a - hydrothermal fluids (RUIZ, op. cit.; RUIZ, et comagmatic relationship between andesitic al., op. cit.; BOOKSTROM, op. cit.) (or basaltic) magmatism and granitoid From this review of ideas, it is evident that emplacement. In consequence, the iron most, if not all the the possibilities have mineralization appears as related to volcano already been considered by the different plutonic complexes, in the style of the model 28 J. OYARZUN and J. FRUTOS MINING GEOLOGY:

by SILLITOE(1973) for the porphyry copper Mineralization: According to LIOU et al. deposits, rather than to roof pendant structures (1974), the actinolite-magnetite paragenesis is as proposed by RUIZ (op. cit.). stable between 475•‹ and 550•‹C, a rather Although it is not the aim of this paper to moderate range of temperature when compared

present a new hypothesis for the origin of the to those of the early stages of mineralization Cretaceous iron deposits of North Chile, some of porphyry copper deposits (e.g. 550 to 750•‹C general basis for a genetical model will be at El Salvador, GUSTAFSON and HUNT, 1975). discussed in the following paragraphs. On the other hand, the abundance of actinolite,

Magmatism and structural conditions : and the presence of widespread alteration A first insight into the genesis of these iron zones around the deposits, suggest that min deposits comes from their restricted chro eralization was linked to a H2O rich phase. nological and spatial distribution. Therefore, Considering the dry character of the magmas a particular coincidence between magmatism related to mineralization (the rocks contain and structure in the Lower Cretaceous basin, little or no hornblende at all), participation of favorable for the production of iron mineraliza deep ground water in the mineralization tion may be inferred. Lower Cretaceous process is probable. This participation has magmatism is characterized by a dominance been proved for porphyry copper systems of mantle derived, andesitic to basaltic dry (GUSTAFSON and HUNT, 1975), formed under magmatism (ABERGet al., op. cit.). As these close thermodynamic conditions, compared magmas are rich in FeO (up to 10%), and a to the open ones of the iron deposits (favorable large part of the iron is not linked to the for the contamination of magma by meteoric silicate phases (where low Fe is water). A possible clue to the mechanism of dominant, CHAVEZ,1974), they have a high iron mineralization associated to hydrous potential for iron mineralization (OYARZUN phases is in the description by RUIZ (op. cit., and FRUTOS,op. cit.). p. 237) of some rock samples from the Boqueron The association of the Cretaceous iron belt Chanar deposit. In these samples, large to a megafracture zone, and the lineament of amounts of magnetite-crystals are scattered in larger deposits along main fault paths , suggest the groundmass and in the labradorite and that structural conditions were a major control pyroxene phenocrystals. In some cases, for the genesis of the belt. However, distensive, pyroxene is replaced by actinolite, and normal faulting (ZEIL, 1979), also determined magnetite-actinolite veinlets are formed, in the emplacement of magmatism and as a dicating the activity of a H2O rich phase that consequence, its control upon the genesis of helped to segregate and transport the mag the iron deposits may be only indirect , although netite. Another reason to sustain the impor distensive faulting is also linked to iron tance of an hydrous phase for the magnetite mineralization at the ore deposit scale. The segregation from magma is the low Ti content emplacement of the iron mineralizations of magnetite from the Cretaceous iron deposits ocurred in fault margined furrows , developed which is not compatible with a segregation by a mechanism of the cauldron subsidence process by simple liquid inmiscibility or type (as described by MYERS , 1975 for the fractional crystallization under moderate to emplacement of magmatic complexes in the high pressure (OSBORN et al., 1979). Coastal Batholith of Peru). This hypothesis is Regarding to depth of emplacement of the based both on the structural and petrological mineralizing-system, the only available figures relationships of the mineralized complexes are those of BOOKSTROM(op. cit.) for El and on the presence of fluidization structures Romeral: 7km, at 2 k.b. pressure, and PICHON well exposed west of El Romeral (J . NYSTROM, (op. cit.) for Los Colorados: 1.5-3 km, at personal comm). Distensive conditions also 0.5-1.0 k.b. BOOKSTROMfigures, considered controlled the intrusion of intramineral dikes excessive by the authors, are based on geologi at El Romeral (BOOKSTROM,op. Cit.). cal interpretation, while those of PICHONwere 34 (1), 1984 Cretaceous Iron Deposits of North Chile 29

calculated after mineralogical equilibria and in the absence of actinolite. In exchange, analysis. magnetite is accompanied by variable but Genetical comparisons with other types rather important apatite concentrations. of deposits: A comparison of the environ Considering the absence of actinolite, the ment of generation of major Cretaceous iron intervention of a H2O-rich phase may not be ore deposits with that of the classical porphyry stated for El Laco (although H2O may have copper system (e.g. El Salvador, GUSTAFSON participated in the oxidation of iron). On the and HUNT, op. Cit.) may help to illustrate this other hand, the low Ti content of the magnetite point. In general, the following preliminary (FRUTOS and OYARZUN, op. cit.), implies conclusion may be disclosed: a) Porphyry that, if this mineral was segregated by liquid copper mineralization was deposited under inmiscilibity (PHILPOTTS, op. cit.), the moderate compressive conditions, beneath segregation should have occurred at a very strato volcanoes of andesitic to dacitic composi shallow level (OSBORN,written comm. 1980). tion (SILLITOE, 1980). In exchange, iron Acknowledgements: The authors are mineralizations of the type found at El greatly indebted to Profs. R. BROUSSE(Univ. of Romeral, El Algarrobo, etc. were formed in Paris), P. ROUTHIER (C.N.R.S., France), B. distensive, open systems, from magmatic LEVI and J. NYSTROM(Univ. of Stockholm) complexes of mafic andesitic-dioritic com and Dr. J. MENARD(Univ. of Paris) for critical position, emplaced by a cauldron subsidence discussions of the hypothesis presented in this mechanism. b) Higher oxydating conditions, paper. Geologists from C.A.P. (Chilean Steel coupled with open conditions, favored the Co.) provided access to the mines and to general scape of magmatic sulphur (as SO2) from the informations. iron mineralizing systems. In consequence Dr E. F. OSBORN, (Carnegie Institution, only a small amount of this element remained Washington) communicated unpublished in the system, and was deposited as pyrite and results and opinions concerning the genesis of minor arsenopyrite and chalcopyrite. low Ti magmatic deposits. Dr. R. OYARZUN A Cenomanian belt of porphyry copper (Univ. of Conception) helped to improve the deposits (Andacollo, Domeyko; LLAUMETT, form of this manuscript. 1975) lies at the eastern flank of the iron belt References (Fig. 1), and it is related to granodioritic (hornblende rich) magmatism generated ABERG, G., AGUIRRE, L., LEVI, B. and NYSTROM,J. during the compressive tectonic episode (1983): Spreading-subsidence and generation of between 118 and 85 m.y. (FRUTOS,op. Cit.). ensialic marginal basins. An example from the Early Cretaceous of Central Chile. Jour. Geol. Finally, it is interesting to consider a Soc. London, Special Issue (in print). comparison of the Cretaceous iron deposits BERGSTOL,S. (1972): The jacupirangite at Kodal, with the late Pliocene magnetite mineralization Vestfold, Norway. A potential magnetite, ilmenite of El Laco (1.9 m.y, BELLON in PICHON, 1981). and apatite ore. Mineral. Deposita, 7, 233246.

This deposit, that belongs to the recent volcanic BOOKSTROM, A. A. (1977): The magnetite deposits of belt, is in the Chilean Altiplano, where El Romeral, Chile. Econ. Geol. 64, 1101•`1130. BRUGGEN,J. (1934): Grundzuge der Geologie and pyroxene andesites are dominant, and its emplacement is in coincidence with the Lagerstattenkunde Chile. Tubingen. Math. Nat. intersection of two major volcanic lineaments. K1. d. Heidelberger Akad. d. Wiss. 362 p. CHAVEZ, L. (1974): Metamorfismo de contacto y The deposit consists of five major magnetite alteration regional de rocas volcanicas andesiticas bodies (total reserves over 500 m.t. ore 50% en Santa Gracia. In Publ. No. 41, Depto. de Fe), emplaced on the borders of an eliptical Geologia. Univ. de Chile, , 139•`196.

(7•~3 km) cauldron structure that has a CISTERNAS, M. E. (1982): Relaciones texturales entre dacitic intrusive needle at its center (Fig. 3). los minerales de la mena y la roca en el yacimiento

Compared to the Cretaceous deposits, El de hierro Bandurrias, III Region, Chile. Revista

Laco differs both in its level of emplacement Geologica de Chile, 15, 27•`40. 30 J. OYARZUN and J. FRUTOS MINING GEOLOGY:

DUNCAN, A. R. and TAYLOR, S. R. (1968): Trace III, 25•`39.

element analysis of magnetites from andesitic and OSBORN, E. F., WATSON, E. B. and RAWSON, S. A.

dacitic from Bay of Plenty, New Zealand. (1979): Composition of magnetite in subalkaline Contr. Mineral. Petrol., 20, 30•`33. volcanic rocks. In Annual Report of the

ESPINOZA,S. (1971): Origen y distribucion de fosforo, Director Geophys. Laboratory, Carnegie Inst.

azufre y silice en el yacimiento de El Algarrobo, (Washington), 475•`481. prov. de Atacama, Chile. Thesis Univ. de Chile, PARAK, T. (1975): Kiruna iron ore deposits are not Santiago, 150 p. "Intrusive -magmatic ores of the Kiruna Type".

ESPINOZA, S. (1979): Una hipotesis sobre la metalo Econ. Geol., 70, 1242•`1258.

genesis de la franja ferrifera chilena. Actas PARK, C. F. (1972): The iron ore deposits of the Pacific 2•‹Congreso Geologico Chileno (Arica), T2, p. basin. Econ. Geol., 76, 339•`349.

Cl•`C21. PHILPOTTS, A. R. (1967): Origin of certain iron

FRUTOS, J. (1981): Andean tectonics as a consequence oxide and apatite rocks. Econ. Geol.,

of sea floor spreading. Tectonophysics, 72, T21 62, 303•`315. •` T32. PICHON, R. (1981): Contribution a l'etude de la FRUTOS, J. and OYARZUN, J. (1975): Tectonic and ceinture du fer du Chili. Les gisements de

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チ リ北部 白堊紀鉄鉱床 の造 構造的岩石学 的背景

J.オ ジ ャ ス ン ・J.フ ル ト ス

要 旨:チ リ地 域 の 白堊 紀 磁 鉄 鉱(燐 灰 石― ア ク チ ノ閃 石) は,特 殊 な造 構 造 的 条 件 と火成 作 用 条 件 の 重 複 に よ っ て 鉱 床 は,南 緯25°～31°の 間 に発 達 す る南 北 方 向 の細 長 生 起 した も の と考 え られ る.地 下 水 の 深部 循 環 に よ る苦 い地 帯 に集 っ て い る.本 論 文 で は,鉄 鉱 化 作 用 の主 要形 成 鉄 質 ドライ マグ マ の水 和 作 用 が,お そ ら く,こ れ らの 鉱 期 と,遅 くて一 定 した プ レー トの 収 束(FRUTOS,1981)に 床 の 形 成 に主 要 な役 割 を果 した の で あ ろ う.卓 越 苦 鉄 質 伴 う,地 殻 伸 張― 苦 鉄 質 火 成 作 用 の 時 期 とを 関 連 づ け 相 と して鉄 含 量 の 低 い 単 斜 輝 石 を伴 う苦 鉄 質 ドラ イマ グ た. マ は,大 部 分 の鉄 が 珪酸 塩 相 とし て保 持 され る 角 閃石 に これ ら磁 鉄 鉱 鉱 床 の成 因 に 関 す る い くつ か の仮 説 につ 富 む ウェ ッ トマ グ マ に 比 較 して,一 般 に よ り高 い鉄 鉱 化 い て考 察 した.結 論 と して,こ れ らの 白堊 紀 鉄 鉱 化 作 用 作 用 の ポ テ ンシ ァル を有 して い る もの とい え よ う.