Economic Geology Vol. 89, 1994, pp. 82{)-839 Magmatic Features of Iron Ores of the Kiruna Typein ChileandSweden: Ore Textures andMagnetite Geochemistry JAN OLOV NYSTR/SM Swedish Museum of Natural History, S-10405 Stockholm, Sweden AND FERNANDO HENRiQUEZ Departamento deMinas, Universidad deSantiago, Casilla 10233, Sa•tiago, Chile Abstract Magnetite lavas andfeeder dikes onthe flanks of the volcano El Laco in the Chilean Andes arecharacterized by textures demonstrating rapidcrystal growth fromsupersaturated melts. Columnar magnetite, a conspicuous form of magnetite at El Lacowith occasional dendritic branching, has been found in two otherapatiteiron provinces: the Cretaceous iron belt in Chile, a 600-km-long zone along the Pacific with about 40 deposits, andthe EarlyProtero- zoic Kirunaore field in Sweden. Presence of columnar magnetite in an iron ore is suggested to be diagnostic of a magmatic origin. Platymagnetite, another dendritic formwidespread at Kiruna,also occurs at El Laco.Moreover, many ores of the three provinces contain pyroxene or pseudomorphs afterit withdendritic morphology. The occurrence of similar rapid-growth textures in the investigated apatite ironores demonstrates a similar origin with emplacement of ore magmas at or near the surface. In fact, existence of vesicular ore lavaandpyroclastic ore at Kiirunavaara shows that this deposit is volcanic. A common origin of the oresis supported by similar compositions of their magnetites. Analysis of ca. 50 concentrates from 17 deposits shows that the magnetites are very poor in Cr (<10 ppm)andrelatively rich in V (ca.1,000-2,000 ppm); the Ti content istypically low (ca.100-1,000 ppm, withoccasional values up to 5,000 ppm). Common ranges (inppm) for otherelements are AI -- 200 to 1,500, Mg -- 500 to 2,000, Mn -- 200 to 900, Ni = 100 to 250, Co = 20 to 140, Zn = 50 to 120, andCu = 10 to 50. The magnetites fromE1Laco andKiruna are remarkably similar with the exception of Mg values which areabout fivetimes higher at El Laco (4,000-8,000 ppm). Magnetite in sedimentary ores appears to be significantly lower Introduction APATITE iron ores, also known as ores of the Kiruna type (Geijer,1931), occur in many parts of theworld associated with volcanic rocks or high-levelintru- sions. They are usually composed of magnetite with varying amounts of apatite andactinolite, andrange in sizefrom largebodies containing manyhundred millions of tons of high-grade ore, to small dikes and veinlets. The discussion of their origin has focused on the Kiirunavaara deposit--the type locality--and other iron deposits in the Kiruna area of northern Sweden. There are stillwidelydiverging opinions about the origin of theapatite ironores, in spite of a century of studies of Kiirunavaara and the discovery of better preserved deposits of the Kiruna type elsewhere. Many authors have advocated a magmatic origin, in- volving emplacement of volatile-rich magmas or de- position from residual fluids.Amongthe processes suggested are m..agmatic differentiation (Geijer, 1910; Geijer andOdman,1974; Frietsch, 1978) and liquid immiscibility (Lundbergand Smellie, 1979; Weidner, 1982; Lyons, 1988). Generation of apatite ironores through leaching of cooling plutons by late- magmatic (deuteric) fluids wasproposed by Hilde- brand(1986), M&nard (1986), and Ruiz andPeebles (1988). However, Parftk (1975a,b, 1984, 1985) and severalearlier authors(see references in Frietsch, 1978) haveargued that the Kiruna ores are exhala- tive-sedimentary, i.e., precipitated as chemical sedi- ments in a volcanic-marine environment. The principal evidence for a magmatic origin isthe tabular or dikelike shape of undeformed orebodies, thepresence of"ore breccia" (anetwork ofdikes and anastomosing veinlets of magnetite with a structure suggestive of forceful injection) at the contact with thehost rocks which aremagmatic, typically volcanic rocks deposited under continental conditions, and the primaryore textures (Geijer, 1910, 1931, 1967; GeijerandOdman, 1974;Frietsch, 1978, 1984; Lundberg and Smellie, 1979; Nystr/Sm, 1985; Wright, 1986;Lyons, 1988; Nystr/Sm andHenr•quez, 1989). The sedimentary evidence offered by ParS, k (1975a, b, 1984, 1985) for the Kiruna ores includes structures like banding, graded bedding, and cross- bedding, featuresattributed to erosion (e.g., ore fragments in the hanging wall) and a continuous up- 0361-0128/94/1580/820-20$4.00 820
Transcript
Economic Geology Vol. 89, 1994, pp. 82{)-839
Magmatic Features of Iron Ores of the Kiruna Type in Chile and Sweden: Ore Textures and Magnetite Geochemistry
JAN OLOV NYSTR/SM Swedish Museum of Natural History, S-10405 Stockholm, Sweden
AND FERNANDO HENRiQUEZ
Departamento de Minas, Universidad de Santiago, Casilla 10233, Sa•tiago, Chile
Abstract
Magnetite lavas and feeder dikes on the flanks of the volcano El Laco in the Chilean Andes are characterized by textures demonstrating rapid crystal growth from supersaturated melts. Columnar magnetite, a conspicuous form of magnetite at El Laco with occasional dendritic branching, has been found in two other apatite iron provinces: the Cretaceous iron belt in Chile, a 600-km-long zone along the Pacific with about 40 deposits, and the Early Protero- zoic Kiruna ore field in Sweden. Presence of columnar magnetite in an iron ore is suggested to be diagnostic of a magmatic origin. Platy magnetite, another dendritic form widespread at Kiruna, also occurs at El Laco. Moreover, many ores of the three provinces contain pyroxene or pseudomorphs after it with dendritic morphology. The occurrence of similar rapid-growth textures in the investigated apatite iron ores demonstrates a similar origin with emplacement of ore magmas at or near the surface. In fact, existence of vesicular ore lava and pyroclastic ore at Kiirunavaara shows that this deposit is volcanic.
A common origin of the ores is supported by similar compositions of their magnetites. Analysis of ca. 50 concentrates from 17 deposits shows that the magnetites are very poor in Cr (<10 ppm) and relatively rich in V (ca. 1,000-2,000 ppm); the Ti content is typically low (ca. 100-1,000 ppm, with occasional values up to 5,000 ppm). Common ranges (in ppm) for other elements are AI -- 200 to 1,500, Mg -- 500 to 2,000, Mn -- 200 to 900, Ni = 100 to 250, Co = 20 to 140, Zn = 50 to 120, andCu = 10 to 50. The magnetites from E1Laco and Kiruna are remarkably similar with the exception of Mg values which are about five times higher at El Laco (4,000-8,000 ppm). Magnetite in sedimentary ores appears to be significantly lower
Introduction
APATITE iron ores, also known as ores of the Kiruna type (Geijer, 1931), occur in many parts of the world associated with volcanic rocks or high-level intru- sions. They are usually composed of magnetite with varying amounts of apatite and actinolite, and range in size from large bodies containing many hundred millions of tons of high-grade ore, to small dikes and veinlets. The discussion of their origin has focused on the Kiirunavaara deposit--the type locality--and other iron deposits in the Kiruna area of northern Sweden.
There are still widely diverging opinions about the origin of the apatite iron ores, in spite of a century of studies of Kiirunavaara and the discovery of better preserved deposits of the Kiruna type elsewhere. Many authors have advocated a magmatic origin, in- volving emplacement of volatile-rich magmas or de- position from residual fluids. Among the processes suggested are m..agmatic differentiation (Geijer, 1910; Geijer and Odman, 1974; Frietsch, 1978) and liquid immiscibility (Lundberg and Smellie, 1979; Weidner, 1982; Lyons, 1988). Generation of apatite
iron ores through leaching of cooling plutons by late- magmatic (deuteric) fluids was proposed by Hilde- brand (1986), M&nard (1986), and Ruiz and Peebles (1988). However, Parftk (1975a, b, 1984, 1985) and several earlier authors (see references in Frietsch, 1978) have argued that the Kiruna ores are exhala- tive-sedimentary, i.e., precipitated as chemical sedi- ments in a volcanic-marine environment.
The principal evidence for a magmatic origin is the tabular or dikelike shape of undeformed orebodies, the presence of"ore breccia" (a network of dikes and anastomosing veinlets of magnetite with a structure suggestive of forceful injection) at the contact with the host rocks which are magmatic, typically volcanic rocks deposited under continental conditions, and the primary ore textures (Geijer, 1910, 1931, 1967; Geijer and Odman, 1974; Frietsch, 1978, 1984; Lundberg and Smellie, 1979; Nystr/Sm, 1985; Wright, 1986; Lyons, 1988; Nystr/Sm and Henr•quez, 1989). The sedimentary evidence offered by ParS, k (1975a, b, 1984, 1985) for the Kiruna ores includes structures like banding, graded bedding, and cross- bedding, features attributed to erosion (e.g., ore fragments in the hanging wall) and a continuous up-
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KIRUNA-TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 82 ]
ward gradation from apatite iron ore into quartz- banded ore. The deuteric hypothesis is mainly based on the strong hydrothermal alteration associated with some Chilean and Canadian deposits (Ruiz et al., 1968; Hildebrand, 1986).
Several attempts have been made to use the chemi- cal composition of the Kiruna ores or magnetite sepa- rated from them for genetic purposes. Landergren (1948), Hegemann and Albrecht (1954), Frietsch (1970), Par•tk (1975a, b), and Loberg and Horndahl (1983) compared the geochemistry of the Kiruna ores and other types of iron deposits, but these stud- ies were inconclusive because the magmatic refer- ence material used for the comparisons (magnetite segregations and accessory or minor magnetite in igneous rocks) is too different in character from the magnetite constituting the predominant phase of apatite iron ores. The discovery of lava flows of mas- sive magnetite at the volcano E1 Laco in the Chilean Andes (Park, 1961; Henr•quez and Martin, 1978) demonstrates that apatite iron ores can form from melts, and provides an opportunity to determine if the composition of the Kiruna magnetites is consis- tent with a magmatic origin.
Here, we give the results of a comparative geo- chemical study of magnetite from E1 Laco, other well-preserved deposits of the Kiruna type in Chile, and the Kiruna area and describe ore textures diag- nostic of a magmatic origin which show that the Kii- runavaara deposit is volcanic. It is not our purpose to discuss how the ore magmas formed. The geochemis- try of the associated apatite will be treated by Henr•quez and Nystr/Sm in a forthcoming paper. The emphasis during the sampling of the Kiruna material was on ore with primary textures in order to allow a comparison with analogous textural types at E1 Laco and elsewhere, and to avoid possible influence on the chemistry by recrystallization during metamorphism. Textures can provide much information on ore for- mation: "In recent years so much emphasis has been placed on isotopes, fluids, chemistry, and deposit and process models that the textural features have been ignored" (Barton, 1991).
Investigated Deposits
Our samples come from three different apatite iron ore provinces, each represented by several deposits: the Pliocene E1 Laco ores (Fig. 1), an extensive Cre- taceous province parallel to the Pacific coast of cen- tral Chile, generally referred to as the Chilean iron belt (Fig. 2), and the Early Proterozoic Kiruna ore field (Fig. 3). Samples from a limestone-hosted strati- form deposit in the iron belt (Bandurrias; Fig. 2) are also included for comparison.
The E1 Laco iron deposits (Park, 1961; Ruiz et al., 1965; Haggerty, 1970; Frutos and Oyarzfn, 1975; Henr•quez and Martin, 1978; Wegner, 1982; Garde-
FIG. 1. Iron deposits at El Laco, a volcano in northern Chile belonging to a Pliocene-Recent volcanic arc (see Fig. 2 for loca- tion). Total resources of ore (>60 wt % Fe equiv) is on the order of 500 Mr. The deposits consist of sul)horizontal lava flows (Laco Norte, Laco Sur, and San Vicente Alto), a domelike intrusion (San Vicente Bajo), and dikes. Ore dikes within deposits are shown in white (Rodados Negros includes a boulder field and is much smaller than the map suggests). Modified from Ruiz et al. (1,9t35).
weg and Ramirez, 1985; Frutos et al., 1990) are situ- ated on the flanks of an andesite-rhyodacite volcano at an altitude of 4,700 to 5,300 m. The ores were emplaced at about 2 Ma ago according to fission- track data for apatite in the ore (2.1 _ 0.1 Ma; Mak- saev et al., 1988) and K-Ar whole-rock dating of an andesite from the eastern flank of Pico Laco (2.0 _ 0.3 Ma; Gardeweg and Ramirez, 1985). Within an area of 30 km 2 there are seven deposits (Fig. 1), with total resources on the order of 500 million tons corre-
sponding approximately to one-fourth of the premin- ing tonnage of Kiirunavaara (cf. Figs. 1 and 3). Laco Norte is the largest body. Drilling and surface obser- vations here show the following upward succession: andesite lava, pyroclastic ore (up to 30 m thick), mag- netite lava (up to 50 m), a second pyroclastic unit with fragments of massive ore (about 20 m), and a second andesite flow.
The ores occur as lava flows and related feeder dikes (Laco Norte, Laco Sur, and San Vicente Alto), pyroclastic material, and subvolcanic bodies (San Vi- cente Bajo, a domelike intrusion, and Rodados Ne- gros, Laquito, and Cristales Grandes which are
rio. 2. Iron deposits in the Chilean iron belt (Ruiz et al., 1965) and some associated apatite veins. The country rocks of the Cre- taceous deposits consist of basic to intermediate lavas and grani- toids. The geology is simplified from SERNAGEOMIN (1982). The locations of the El Laco (Pliocene) and Magnetita Pedernales (MP; Tertiary) iron deposits are given in the inset.
dikes). Apatite is locally abundant in the intrusions but is merely an accessory phase in the lavas. The ores contain highly variable amounts of pyroxene in the form of thin elongated prisms, frequently with dendritic branching. It has been erroneously re- ported as amphibole (actinolite) in the literature due to the unusual habit. The pyroxene is partly to com- pletely altered to talc, opal, goethite, and smectite. The octahedral faces of magnetite crystals lining
open spaces are often pitted by molds with a square cross section after pyroxene.
Hematite is a ubiquitous oxidation product after magnetite in extrusive and pyroclastic ore; occasion- ally cavities are lined by well-developed crystals of hematite. The orebodies are surrounded by narrow metasomatic aureoles (up to 1 m thick in lava but con- siderably wider in tuff) where the andesitic host rock
Fla. 3. Early Proterozoic iron deposits in the Kiruna area, northern Sweden (simplified from Par•tk, 1975a). The reserves of high-grade ore (>60 wt % Fe) at Kiirunavaara, the largest individ- ual orebody, exceed 2,000 Mt. The ores dip 50 ø to 75 ø to the east and are hosted 1)v acid and intermediate volcanic rocks of the Por-
KIRUNA-TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 823
is transformed to pyroxene, scapolite, quartz, and garnet. The andesite shows no granoblastic texture close to the ore. The volcanic complex including ores is affected by a locally strong hydrothermal alteration with silicification (cristobalite, tridymite, and amor- phous silica), development of gypsum, jarosite, and alunite, and deposition of fumarolic sulfur in frac- tures (Vivallo et al., 1991).
Frutos and Oyarz6n (1975), quoting Thomas (1970), reported that incompletely digested frag- ments of itabirite occur in the magnetite lavas at E1 Laco. This is totally mistaken; no such thing has ever been observed and the whole idea is based on a mis-
understanding by Thomas. Whether there are sedi- mentary iron strata ("lower Paleozoic ferruginous schists") or not below the volcanic complex as im- plied by Frutos and Oyarz6n (1975) and Frutos et al. (1990) is not known. To the best of our knowledge there is no outcrop of such rocks in the E1 Laco area (cf. Boric et al., 1990). The nearest sedimentary iron deposit (itabirite) is situated in Argentina about 200 km northeast of E1 Laco, across the general north- south strike of the formations.
The Chilean iron belt (Geijer, 1931; Ruiz et al., 1965, 1968; Park, 1972; Espinoza, 1984a, 1990; Oyarz6n and Frutos, 1984; Menard, 1986; Ruiz and Peebles, 1988) is approximately 600 km long and 25 km wide. It consists of seven large (•100 million tons of high-grade ore) and about 40 medium-sized and small deposits of apatite iron ore (Fig. 2). The sizes refer to premining reserves since many of the ores have been exploited; only Romeral, Algarrobo, Los Colorados, and E1 Tofo were in production during 1993. Three of the large deposits (Boquer6n Chafiar, Cerro Negro Norte, and Cristales) are unexploited or only partially mined. The apatite content shows a large variation within and between the different de- posits; genetically related apatite veins occur abun- dantly in the ore-bearing region.
The deposits of the iron belt have been divided into four groups according to their geologic setting (Espinoza, 1984a, 1990). The first group, comprising a few orebodies situated approximately 20 km east of the belt axis, are stratified and sediment hosted (e.g., Bandurrias; Cisternas, 1986; Espinoza, 1986). They lack apatite and were deposited in an Early Cre- taceous shallow-marine basin. The ores of the other
three groups were formed in a magmatic arc along the western margin of this basin (Espinoza, 1990). Some deposits are hosted by granitoids, others by volcanic rocks but most of them--including all the large ones--occur associated with metavolcanic rocks in tectonic contact with granitoids within the north-trending Atacama megafault zone (Espinoza, 1984a, b, 1990; Oyarz6n and Frutos, 1984). The iron belt coincides spatially with this zone, and individual orebodies are elongated parallel to it.
The Cretaceous volcanic rocks and granitoids host- ing the iron deposits are interpreted as comagmatic (Bookstrom, 1977' Pich6n, 1981; Montecinos, 1983; Oyarzfin and Frutos, 1984) or derived from different parent magmas (Gonzalez and Henr[quez, 1991). The volcanic pile is dominated by basaltic to andes- itic lava flows, and most of the granitoids are dioritic members of a large batholith, seemingly younger than the lavas. The ores are coeval with the volcanic
rocks according to Espinoza (1984a) and Oyarzfn and Frutos (1984), but Ruiz et al. (1965, 1968) con- sidered them to be somewhat younger. Available ra- diometric ages of the rocks associated with the ores fall in the range 100 to 128 Ma (Zentilli, 1974; Pi- ch6n, 1981; Montecinos, 1983; the range is based on 11 K-Ar mineral and whole-rock ages for four de- posits).
The lavas away from ore deposits and granitoids are affected by nondeformational regional metamor~ phism, usually at prehnite-pumpellyite facies (Levi et al., 1989). The grade of the alteration reflected by extensive development of amphibole is higher near large orebodies due to contact metamorphism from nearby granitoids or processes related to the forma- tion of the ores, or both (Espinoza, 1984a). The am- phibolization is overprinted by hydrothermal alter- ation of lower temperature (Galatz•tn and Henr•quez, 1979).
We have sampled all the iron deposits indicated by names in Figure 2 except Cerro Negro Norte and Bo- quer6n Chafiar. Romeral (Bookstrom, 1977), repre- sented in this study by several samples, is typical of the large deposits. It is composed of two major bodies (Cuerpo Principal and Romeral Norte), partly delim- ited by zones of faulting and mylonitization, and three small bodies. Five samples (ROM-59 to ROM- 62, and ROM-66) were collected from Cuerpo Prin- cipal and its envelope of ore breccia. This body is lenticular, with a length of 850 m, an average width of 250 m, and a depth of 600 m. One sample (ROM- 58) represents the strongly deformed and martitized Romeral Norte orebody (300 m long, 50-120 m wide, and up to 240 m deep), and two come from veins and patches of ore in meta-andesite and quartz- apatite-calcite-scapolite rock near a younger diorite intrusion (the low-grade Siciliano ore; ROM-63 and ROM-64). Bookstrom (1977) regarded Romeral Norte as a replacement ore hosted by schists in a late Paleozoic basement. In addition, samples from two magnetite-bearing apatite veins in the Romeral area are included for comparison (La Escoba = ESC and Yayita -- YAY in Fig. 2).
Algarrobo (Geijer, 1931; Ruiz et al., 1965; Montecinos, 1983; Espinoza, 1984a; Gonzalez and Henr•quez, 1991) is another large deposit very simi- lar to Romeral. It also consists of two major bodies (Algarrobo C and Penoso); our samples come from
824 N¾$TROM AND HENR[Q UEZ
the former which is 1,100 m long, has an average width of 400 m, and reaches a depth of 400 m. Ojos de Agua (Geijer, 1931, 1967; Dobbs and Henr•quez, 1988), situated in the northern continuation of the Algarrobo district, is composed of six steep dikelike bodies of apatite-rieh ore (the largest is 200 m long, 4 m wide, and 100 m deep). This small well-exposed and unexploited deposit is of special interest because it shows many of the features of the larger but now exploited ores. Two other deposits rich in apatite treated here are E1 Dorado and Carmen. The latter is
a 500-m-long, 25- to 60-m-wide, and 50- to 200-m- deep tabular body hosted by volcanic rocks without known intrusions in the vicinity (Henrlquez et al., 1991). E1 Dorado and two other ores included in the study, E1 Tofo (Geijer, 1931) and Cerro Im•tn (Ruiz et al., 196.5), belong to the Romeral-Algarrobo group.
The Kiruna ore field (Fig. 3) and surrounding areas in northern Sweden constitute the greatest concen- tration of apatite iron ores in the world. The deposits occur as tabular bodies within a thick sequence com- posed largely of intermediate to acid volcanic rocks (the Porphyry Group), which contains ignimbrites and whose lower part. was interpreted as a caldera filling by Geijer and Odman (1974). The deposits at the top of the Porphyry Group (Rektorn, Henry, Nu- kutusvaara, and Haukivaara; Fig. 3), collectively re- ferred to as the Per Geijer ores, differ from Kiiruna- vaara and Luossavaara by higher apatite and hematite contents and the presence of quartz and/or carbonate in significant amounts. Radiometric dating of the host rocks and a crosscutting granophyrie dike indicates that the ores formed at about 1880 to 1890 Ma (We- lin, 1987; Cliff et al., 1990; U-Pb zircon and Sm-Nd whole-rock data).
Veins with gypsum and pyrite in the ore are ex- pressions of hydrothermal activity (Nystr/Sm, 1985). The ores are locally deformed by shearing and are reerystallized, but relict primary textures and struc- tures can nevertheless be seen, especially at Kiiruna- vaara (Nystr/Sm, 1985; Nystr/Sm and Henr•quez, 1989). This is consistent with the nondeformational nature of the greenschist facies regional metamor- phism that characterizes the country rock. Contact metamorphic effects in the latter, if present, are in- conspicuous (reerystallization and eoarsening of grain size). Detailed geologic descriptions of the Ki- runa area can be found in the numerous publications by Geijer, Frietseh, and Par•tk (see references in Frietseh, 1978, 1984).
Primary Textures
The existence of pyroelastic ore (Fig. 4A and B) and ore lavas with flow layering and highly vesicular upper parts at E1 Laco (Figs. 5A and B, and 6A) is
telling evidence of crystallization from volatile-rich melts. Several of the E1 Laeo samples included in this study are from ore of unequivocal volcanic character: a volcanic bomb, lava with ropy surface, and vesicu- lar lava. The bomb (FHL-121) is 9 by 16 em in cross section and has a regular fusifbrm shape with longitu- dinal fluting. Its inner part is a porous aggregate of ca. 0.2-mm-large magnetite oetahedra coated by a film of iron phosphate. The porous ore lavas also are composed of oetahedra which coalesce in massive parts.
The pyroelastic ore at E1Laeo occurs in the vicinity of feeder dikes and comprises volcanic bombs and finer air-fall material, alone or mixed with blocks of ore lava. The structures are indistinguishable from those normally found in near-vent pyroelastic mate- rial. The E1 Laeo material is poorly consolidated and friable, which causes the structures to be easily de- stroyed except where the ore has been sintered by heat from a nearby magnetite flow or dike. The pyro- elastic ores consist of iron oxide with magnetite as the primary constituent, and some oxide grains have films of green iron phosphate. Apatite, usually absent or accessory, can be present in considerable amounts and gives rise to a fine stratification that can be fol- lowed up to several meters (Fig. 4A). Presence of bomb sags demonstrates the fallout nature of the ma- terial.
The stratification can be attributed to sorting dur- ing the eruption. Magnification reveals that the apa- tire layers consist of euhedral apatite prisms, gener- ally lying with their long axis within the stratification plane, and some magnetite oetahedra (Fig. 4B). A varying number of apatite prisms also occur in the magnetite layers (the amount of apatite is constant at each level), but the prisms show a less regular orienta- tion here, consistent with simultaneous deposition of the two minerals. Their euhedral shape and the fabric of the stratified ore (Fig. 4B) identify the material as pyroelastic. Tiny plates composed of magnetite oeta- hedra in parallel intergrowth (el. Fig. 7D) are wide- spread.
Ores with apatite banding are more common in the Kiruna area. In many eases the banding is clearly of tectonic character, but some structures appear to be primary, caused by a process of sedimentation. They are represented in this study by samples with cross- bedding (PG-531; illustrated in Nystr/Sm, 1985, fig. 8) and fine stratification of uniform thickness (Fig. 4C-D; see detailed description of the stratification in Geijer, 1967). The stubby apatite prisms in the apa- tite layers (Fig. 4D) have the same orientation as that described above for the pyroelastic ore at E1 Laco, and tiny magnetite plates are abundant in the magne- tite layers where octahedra also can be discerned. The stratification is locally disturbed by intraforma- tional folding and brecciation at a small scale.
KIRUNA-TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 825
Fic. 4. Stratified (apatite-banded) pyroclastic ore. A. Crystal tuff, Laco Sur (sample ELS-2; black = magnetite, white = apatite and reflecting crystal faces of magnetite), The wavy stratification is only apparent because of surface irregularities. B. SEM photomicrograph of an apatite layer and surrounding ore in the same sample (ELS-2) showing the euhedral habit of the constituent minerals and the low degree of consolidation in the loose crystal aggregate. The small rounded growths on the apatite prisms and magnetite octahedra consist of SiOn. C. Crystal tuff, Kiirunavaara (sample PG-235; black = magnetite, white = apatite, the latter partly removed by weathering). D. Photomicrograph of a thin section of the same sample (PG-235; crossed nicols).
Numerous blocks of ore with vesiclelike holes
were found in a few places during a recent visit to Kiruna (Fig. 6B). The blocks were dumped during the open-pit stage of the mining of Kiirunavaara, and their present position indicates that they came from the hanging-wall side of the southern half of the ore- body. The ore consists solely of magnetite except in one dump where it is strongly oxidized to hematite and hydrous ferric oxides. There is no evidence that the holes represent cavities generated by weathering of sulfides, calcite, or other minerals. The holes are often elongated although highly irregular in detail because the ore seems to be a crystal aggregate. Sev- eral samples show that the aggregates are composed of octahedra, which here and there may be inter-
grown to form small plates. Minor quartz is deposited in many holes.
Some ore textures at El Laco constitute indepen- dent evidence of a magmatic origin. Henr•quez and Martin (1978) described a columnar form of magne- tite that locally grades into spherulitic fiber and/or octahedral crystal (Fig. 8D) and showed that the col- umns and spherulites are rapid growth features caused by sudden supersaturation of degassing oxide melts. The columnar magnetite is common at El Laco where it typically occurs as uniform arrays of parallel 0.5- to 15-cm-long columns oriented perpendicular to nearby open spaces, like flow tops and gas escape tubes in feeder dikes (Fig. 8A and D). Often several columnar arrays are superimposed above each other,
826 NYSTROM AI•D HE•RfQUEZ
FIG. 5. Magnetite lava with contorted flow structure. Note the drawnout shape of the vesicles due to flowage. A. Laco Norte (out- crop). B. Laco Norte (sample ELN-19,).
resulting in a banded structure. Column surfaces dis- play a striation at an angle of 45 ø to 90 ø to the axis of the column; the striae are the edges of very thin plates stacked on each other. Locally the magnetite shows a dendritic morphology, occurring as radiating arrays of branching fibres and small columns.
Columnar magnetite has also been observed at Kiirunavaara (Geijer, 1910, 1967; Nystr6m, 1985)as composite bands of parallel 1- to 12-cm-long columns
(Fig. 8C) which form lenses up to a few meters long in the ore. The columns of each band are uniform in
size and oriented perpendicular to the extension of the lens; the lenses are parallel with the contacts of the orebody. A detailed study of a few crosscuts in the Kiirunavaara mine has shown that less regular small arrays of columnar magnetite are quite wide- spread and can be found in ore types of very different apatite content (Nystr6m and Henriquez, 1989).
A search for columnar magnetite in the Chilean iron belt revealed that it is present locally in six out of eight investigated deposits (Table 1). It was not seen at Bandurrias which is a nonapatitic stratiform ore or at El Tofo where the ore appears to be partly recrys- tallized. However, poorly preserved magnetite col- umns have been observed in a small iron deposit just north of the main El Tofo orebody. The parallel ar- rangement of the columns found at El Laco (Fig. 8A) and Kiirunavaara (Fig. 8C) is rare in the iron belt, where they tend to be oriented at random (Fig. 8E). This nonoriented variety of columnar magnetite also occurs at El Laco.
Columnar magnetite is not the only form of magne- tite that may show dendritic morphology in the Ki- runa ore field. The so-called skeleton ore (Geijer,
FIG. 6. Vesicular magnetite lava. A. Laco Sur (sample ELS-20; photographed with a coating of ammonium chloride in order to eliminate reflexes). B. Kiirunavaara (sample KIR-34).
KIR UNA- TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 827
FIG. 7. P!aty magnetite. A. Skeleton ore, i.e., dendritic magnetite plates embedded in apatite, Kiirun- avaara (sample PG-36:lA). B. Closeup of a magnetite plate in skeleton ore showing parallel growth of octahedra, Kiirunavaara (sample KIR-55). C. Apatite-free analogue of skeleton ore, Laeo Sur (sample ELS-I). D. SEM photomierograph of a tiny magnetite plate composed of intergrown octahedra in pyro- elastic ore, Laeo Sur (crystal tuff, sample ELS-2; cf. Fig. 4A-B).
828 N¾STROM AHD HENRfQUEZ
A B
mm mm i I
D
2 mm
FIG. 8. Columnar magnetite. A. Array of parallel columns, Laco Norte (sample FHL-76). B. Artifi- cially grown columnar magnetite, Degerfors Ironworks. Note the gas bubbles in the lower and upper part of the sample. (From the collection of C. Benedicks l1875-19581, an outstanding Swedish experimental metallurgist. The collection of Benedicks was donated by the Swedish Institute for Metal Research in 1989 to the Swedish Museum of Natural History. No note or report describing the conditions under which the magnetite was grown accompanied the collection.) C. Coarse array of parallel columns, Kiirun- avaara (sample K1R-15). D. Columns terminated by octahedral faces, Laco Sur (sample ELS-19). E. Chaotic array of broad columns, Algarrobo (sample ALG-24).
KIR UNA- TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 89.9
TABLE 1. Minor and Trace Element Compositions of Magnetite from Swedish and Chilean Apatite Iron Ores (in ppm)
El Laco samples come from seven different deposits (Fig. 1); the Chilean iron belt (Fig. 2) is represented by samples from Carmen (CMN), Cerro Imfm (CIM), Bandurrias (BAN; nonapatitic), Ojos de Agua (ODA), Algarrobo (ALG), E1 Tofo (TOF), Romeral (ROM), E1 Dorado (DOR), and two related apatite veins (La Escoba = ESC and Yayita = YAY); the first ten Kiruna samples are from Kiirunavaara, followed by one from Henry and two from Rektorn (Fig. 3); magnetite is largely replaced by hematite in KRE-1, PG-K5, and ROM-58 (Romeral Norte) and significantly replaced in FHL-18 and FHL-82
Abbreviations: ap = apatite, cc = calcite, mt = magnetite, "px" = pseudomorphs after pyroxene or amphibole, q = quartz, scap= seapolite
830 N•'$TROM AND HENR[Q UEZ
1910; Par•k, 1975a, b; Nystr6m, 1985) is a more common dendritic variety. It consists of magnetite plates embedded in apatite. The plates are oriented at random or form subparallel arrays, locally with off- shoots like branches from a stem (Fig. 7A). Nystr6m and Henrlquez (1989) observed that the plates are made up of octahedra showing parallel growth (Fig. 7B). Skeleton ore has not been reported from any Chilean deposit, but apatite-free arrays of platy mag- netite dendrites analogous to those in Figure 7A are found as a rare textural type at E1 Laco (Fig. 7C), and small magnetite plates seem to be quite widespread in the pyroclastic ore (Fig. 7D).
At Kiirunavaara columnar and platy magnetite are intimately associated with each other in apatite segre- gations found occasionally in the skeleton ore (see illustrations in Nystr6m and Henrlquez, 1989). The contact between the about 1- to 10-cm-large segrega- tions and surrounding ore is marked by a millimeter- thick shell of subparallel magnetite columns oriented perpendicular to the segregation. The magnetite out- side (and inside) the shell occurs as platy dendrites embedded in a matrix of apatite prisms. Some of the segregations display a trachytoid texture defined by slender apatite prisms and thin magnetite plates; others have been interpreted as miniature diapiric structures by Nystr/•m and Henrlquez (1989).
Amphibole (actinolite) is the dominant silicate min- eral in apatite iron ores according to the literature. However, some deposits contain pyroxene instead of amphibole, for example, the extrusive and intrusive orebodies at E1 Laco. The pyroxene tends to be con- centrated in layers of subparallel elongated prisms growing toward flow tops, cavities, and dike inte- riors, alternating with layers of columnar magnetite and layers of conical apatite prisms in intrusive bod- ies (a type of comb layering; Fig. 9A). The 0.5- to a few centimeters-long pyroxenes often form fanlike dendrites similar to those illustrated in Figure 9B. Both monoclinic (diopside) and orthorhombic (ferroan enstatite) pyroxenes are found. The pyrox- ene in the aureoles at the ore-host rock contacts oc-
curs as anhedral grains and prisms without dendritic branching.
At Kiirunavaara there are two types of amphibole: one (normal) of a fibrous-prismatic and one of a den- dritic variety. The latter was discovered by Geijer (1910) in a 150- by 10-m-large zone within the ore- body where it characterized a distinctive ore type now removed by mining ("ophitic ore"). The den- dritic amphibole is a pseudomorph after pyroxene, and microprobe analysis of relicts of the original py- roxene (Wo44En44Fsl•2) shows that it was similar in composition to the clinopyroxene in the E1 Laco ore (Wo48En4•Fs •; Nystr6m, 1985). The arrangement of the pseudomorphs defines a flow texture; comb layering like that at E1 Laco has not been described.
No pyroxene has been reported from the iron belt; however, there are dendritic pseudomorphs resem- bling the E1 Laco pyroxene in many of the deposits (Fig. 9B-C). Whether the original mineral was pyrox- ene, amphibole, or both is not known; in the follow- ing we refer only to a pyroxene precursor for simplic- ity. The pyroxene pseudomorphs consist of actinolite and varying amounts of talc, quartz, and calcite. They are common in apatite-rich parts of ores growing on apatite prisms or forming comb layers and they are often associated with colun•nar magnetite.
Analytical Procedures
Fifty-one concentrates of magnetite from 18 iron deposits and two apatite veins (Figs. 1-3) were ob- tained by repeated crushing, washing in destilled water, and treatment with a Franz isodynamic mag- netic separator. A few samples were considerably martitized (Table 1). Small amounts of apatite re- mained in samples from very apatite rich ore types; no attempt was made to remove it by selective leach- ing. After an XRD purity control of the magnetite, 1 g of each sample was dissolved in warm 5N HC1 to which a small quantity of hydroxylamine was added as a reducing agent. Silicates (mostly actinolite and quartz) difficult to separate completely from some magnetite concentrates remained as solid residue (up to 0.04 g in one sample; usually less than 0.01 g). The dissolved magnetites were analyzed for Fe, Ti, V, Cr, A1, Mg, Mn, Ni, Co, Zn, Cu, Si, Na, K, Ca, and P with ICP emission spectrometry at the Department of Ge- ology and Geochemistry, Stockholm University, by Birgitta Bostr/•m. The values for the elements from Ti to Si are given in Table 1. Kiruna samples with num- bers prefixed by PG come from Per Geijer collec- tions, sample PF-HE was donated by Paul Forsell (Kiruna), and sample HjL-1 is marked with the name Hj. Lundbohm; all the other samples were collected by us. The material is deposited at the Swedish Mu- seum of Natural History.
Magnetite Geochemistry
The minor and trace element compositions of the magnetites presented in Table i are uncorrected for the small amounts of apatite in some samples. Com- parison of apatite-free and apatite-rich samples from the same orebody shows that the included apatite does not affect the listed elements. Examples are sam- ples HjL-1, KUJ-3, and PG-530 (apatite-free) and PG-235, PG-531, and PG-K5 (apatite-rich) from Ki- runa.
Inspection of polished thin sections at high magni- fication reveals that a few Kiruna magnetites contain small inclusions of TiO2, probably rutfie, and there are indications that some grains too small to analyze quantitatively consist of i]menite (an Fe-Ti phase ac- cording to SEM-EDS). Geijer (1910) reported rare
KIRUNA-TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 831
FIG. 9. Dendritic pyroxene (including pseudomorphs after pyroxene or amphibole, referred to as "pyroxene"). A. Comb layering defined by alternating bands of columnar magnetite, dendritic pyroxene, and apatite, Rodados Negros (El Laco, sample ELR-3; sketch of irregular surface). B. Curved branching "pyroxene" in iron ore, Carmen (sample CMN-28, thin section). C. Coarse dendritic "pyroxene" embed- ded in columnar magnetite, Romeral (sample ROM-66).
8 3 2 NYSTROM AND HENR[Q UEZ
ilmenite on jointing planes, crystals of titanitc near the footwall, and a few grains of allanite, all in the ore. Very thin, crystallographically oriented (ex- • solved) lamellae occur in many magnetites from E1 Laco. The largest of them (2 #m thick) is ilmenite and a rough proportion between the amount of lamellae 2 and the Ti content of the magnetite also suggests that these lamellae are ilmenite. Minute grains at their borders look like spinel. Henrlquez and Martin 08 (1978) identified small euhedral crystals of rutfie oc- casionally associated with magnetite oetahedra in eav- 0.6 ities. Ilmenite was observed as inclusions in relatively 0.• Ti rich magnetites from the iron belt.
The following conclusions can be drawn from an inspection of Table 1: 02
1. The magnetites form two populations with re- gard to V, Ni, and A1. One group consists of the Ban- durrias samples (the stratiform iron deposit in lime- stone), and the other group comprises all the magne- tites from apatite iron ores and associated apatite veins (Fig. 10).
2. The magnetite in the apatite iron ores and apa- tite veins has low Cr (<10 ppm) and relatively high V values (about 1,000-2,000 ppm; higher in the veins). The Ti content is typically low (about 100-1,000 ppm, with occasional values up to 5,000 ppm). Com- mon values (in ppm) for the other elements fall into the following ranges: A1 = 200 to 1,500, Mg = 500 to 2,000 (4,000-8,000 for E1 Laco), Mn = 200 to 900, Ni = 100to 250, Co = 20 to 140, Zn = 50 to 120, and Cu-- 10to50.
3. The magnetites of different deposits and ore districts have distinct chemical variation patterns, il- lustrated for a few elements in Figures 11 and 12. For example, all the E1 Laco magnetites including pyrox- ene-free samples are Mg rich, whereas the Kiruna magnetites are characterized by low Mg contents; the V/Ni ratios of the latter are uniform and low (Fig. 12). However, some elements (especially Ti; Fig. 13) show a large variation even within the same orebody (e.g., Rodados Negros; Table 1).
4. Magnetites from apatite iron ores of a different character in a district may form compositional sub- groups. For instance, the E1 Laco samples from pyro- clastic ores are low in Ti and A1, whereas the opposite is true for pyroxene-rich samples from ore dikes (Ta- ble 2). The contents of these elements show no corre- lation with the values for Si and Mg (Table 1), demon- strating that they cannot be derived from pyroxene which anyway is insoluble in HC1. Analysis of several Si-bearing magnetites with SEM-EDS indicates that Si is incorporated in the magnetite lattice.
Discussion
The existence of similar primary textures in the in- vestigated apatite iron ores, the overall chemical simi-
0.1
0.08
0.06
0.0/,
Bnndurrins
i I i i i I i i
Ti V Mn A[ Ni Co Mg Zn
FIG. 10. Geochemical patterns for maguetite from apatite iron ores in the Kiruna area (thin line; average of 13 samples), the Chi- lean iron belt (dotted line; average of 21 samples), and a stratiform volcanic-exhalative iron deposit belonging to this belt (Bandurrias; thick line; average of 2 samples), normalized against the average composition of the E1 Laeo magnetite (broken line; 13 samples; Ti = 933, V = 1,360, Mn = 486, AI = 944, Ni = 201, Co = 124, Mg = 5,990, and Zn = 90; all in ppm). The stippled band shows the compositional range for all the Kiruna and iron belt magnetites (except Bandurrias), excluding the highest and lowest values which are plotted as poiuts outside the band. Note that the V and Ni contents of the Baudurrias samples are maximum values.
larity of their magnetites, and other features in com- mon (shape of the orebodies, presence of ore brec- cias, mineralogy, type of host rocks, and alteration) strongly suggest a similar genesis. The magmatic ori- gin established for E1 Laco by HenHquez and Martin (1978) can therefore be extended to the other de- posits. There is definite evidence that the ore melts reached the surface not only at E1 Laco but also at Kiruna and in some parts of the iron belt. We suggest that the occurrence of columnar magnetite in iron ores is diagnostic of a magmatic origin and that the magnetite of apatite iron ores is characterized by very low Cr and relatively high V values. Textures
The ore with vesiclelike hones at Kiirunavaara (Fig. 6B) is almost indistinguishable from some of the ore lavas at E1Laco and should therefore also be volcanic.
Both are composed of tiny octahedra in different de- grees of coalescence, resulting in a continuous grada-
KIRUNA-TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 833
FIG. l l. Vanadium vs. Ni + Co for magnetites from apatite iron ores in the Kiruna area (3 deposits), E1 Laco (7 deposits), and the Chilean iron belt (8 deposits and 2 related apatite veins). Note that the V and Ni contents in the nonapatitic Bandurrias samples are maximum values. Analyses represented by a number within a circle are taken from the literature: 1 -- apatite iron ores, northern Sweden; 2 -- apatite iron ores, Missouri; 3 = stratified iron ores, northern Sweden; 4 -- stratified iron ores, northern Finland; 5 -- nonapatitic iron ores, central Sweden; 6 = Lahn-Dill ores (Co given as <20 ppm); 7 -- hematite ore, E1 Laco (Co unreported); 8 -- magnetite ore, Bafq, Iran (Co unreported; the ore is of apatite iron type, see F6rster and Knittel, 1979); 9 -- apatite iron ores, Kiruna (median values for P-rich and P-poor ores from Kiirunavaara and the Per Geijer ores); 10 -- banded iron-formations, Quadril/ttero Ferrffero, Minas Ge- rais, Brazil. The box in the lower left corner of the diagram represents the normal range in hand specimen samples of ore from iron-formations (Davy, 1983; V down to <6 ppm). Analyses 1 to 6 are averages for magnetite from Frietsch (1970) and references therein, analyses 7 and 8 refer to ore concentrates from Loberg and Horndahl (1983), analysis 9 is average ore from Parftk (1975a, table 35), analysis 10 is the average iron oxide composition from Kessler and Miiller (1988).
tion from highly vesicular to porous to massive ore in the same hand specimen. It is unlikely that the vesicu- lar ore is hypabyssal, because when vesicles have been observed in magnetite dikes in Chile (e.g., at Amancay just north of Algarrobo), their number is low and the ore between them lacks porosity. The local transition from octahedra to plates of magnetite at Kiirunavaara is consistent with rapid growth. This is supported by the occurrence of small arrays of par- allel, poorly preserved columnar magnetite in a few
vesicular samples. In addition, Geijer's (1910) de- scription of Kiirunavaara during the early stages of mining proves that the vesicular ore was situated in the top of the orebody and suggests that the body was eroded before the overlying volcanic rocks were de- posited.
Geijer (1910, p. 90) reported that a considerable area near the hanging wall in the claim Professorn (located in the southern part of Kiirunavaara) was composed of oxidized ore with an abundance ofirreg-
8 3 4 NYSTROM AND HENR[Q UEZ
Mg ppm
7000
6000
5000
4000
3000
2000
1000
øø o
o
Oo
ß Kiruna
0 E[ Laco
[• Romeral
ß A[garrobo ß Ojos de AguQ /• Cerro Im(Jn
V E[ Tofo
X CQrmen
+ E( DorQdo
V Ap(3tite veins
X
ß
+/x •+
2'o 3• •'o 5'o do V/Ni FIG. 12. Magnesium vs. V/Ni for magnetites from apatite iron ores in the Kiruna area, El Laco, and the
Chilean iron belt (including two apatite veins).
ular, often fiat, and thin cavities. It is obvious that this corresponds to our vesieular ore, which in all likeli- hood represents the top of a magnetite flow; the flat- tening of the vesicles can be explained by flowage.
The oxidation suggests exposure at the surface in anal- ogy with the conditions at E1 Laeo. Abundant relicts of magnetite reveal the original composition of the ore, which with (stratigraphic) depth passed into un-
T^BI• 2. Analyses of Magnetite and Iron Ore from E1 Laco and Kiruna in this Study and the Literature (in ppm)
All the samples in this study are magnetite separates; pyroclastic ore = FHL-82, FHL-101, FHL-121; ore lavas = EL-2:6, FHL-18, FHL-67; pyroxene-rich ore dikes = EL-C:24, FHL-106, FHL-113; intrusion = FHL-126; SVB = San Vicente Bajo
The relatively high Cr value for the pyroxene-rich E1 Laco ore dikes is due to one Cr-rich sample (cf. Table 1); Kiruna data from Frietsch (1970) include many other apatite-iron ores from northern Sweden; the values from Par'•k (1975a) are averages for the Kiiruna- vaara and Per Geijer ores
KIRUNA-TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 835
altered magnetite, first vesicular, then nonvesicular. The oxidized flow top must have been a palcosurface, since the ore in the rest of the Kiirunavaara mountain
ridge was unoxidized. The occurrence of vesicular ore along only a part of the hanging wall indicates considerable erosion, which is supported by the exis- tence of common Kiruna ore types as angular and rounded fragments in the immediately overlying vol- canic rocks (see illustrations in Par•tk, 1975a).
It appears that Geijer did not realize the nature of the vesicular ore, because he did not use it as evi- dence for his original opinion that Kiirunavaara erupted as lava (Geijer, 1910). Later on he found ve- sicular ore at Nukutusvaara, one of the Per Geijer ores (Fig. 3), and reported that "A local phase of the main ore body of this field [Nukutusvaara] is vesicu- lar, with... structure... analogous to that of an amygda- loid lava rock" (Geijer, 1919, p. 18). However, at that time he had just changed his interpretation of the Kiruna ores, from then on regarding them as in- trusive (sills), due to the local occurrence of ore brec- cia at the hanging-wall contacts.
Emplacement of the ore melts at the surface in the Kiruna area is supported by a new interpretation of Geijer's (1910, 1967) "stratified ore," i.e., the apa- tite-banded ore with a fine layering of uniform thick- ness illustrated in Figure 4C and D. We consider it to be a crystal tuff, analogous with the partly consoli- dated magnetite-apatite fallout ash at E1 Laco (Fig. 4A and B). In both areas the material is composed of magnetite and apatite of the same dimensions and habits, including platy magnetite, arranged in a simi- lar way. These features, taken together, cannot be generated by later recrystallization or replacement.
According to Geijer (1967), the stratified ore was found near the footwall in some sectors of the Kiirun-
avaara orebody, consistent with an early eruption of gas charged with tiny crystals of magnetite and apa- tite. The intraformational folding and brecciation could have been caused by the subsequently erupt- ing ore lava, whose movements destroyed the stratifi- cation except very locally. This, in combination with later shearing, probably explains why brokenup pieces of apatite-poor ore often are found embedded in apatite-rich, originally pyroclastic (?) deformed ore in the lowermost part of the body. Whether the Kiirunavaara deposit consists of a single or more than one lava flow is under study.
The presence of columnar magnetite in almost all the investigated apatite iron deposits is remarkable considering the rarity of this textural form of magne- tite. It has to the best of our knowledge only been described from one other deposit in the world, Mag- netita Pedernales (Fig. 2; Grez et al., 1991), which is also an apatite iron ore. The reason why the columnar magnetite in the iron belt usually occurs in chaotic rather than parallel arrays is not known. Their coexis-
tence at E1 Laco and occasionally in the iron belt shows that they are varieties of the same general type.
The most conspicuous variety of columnar magne- tite at Kiirunavaara is strikingly similar in arrange- ment and size to the magnetite columns near flow tops and in feeder dikes at E1 Laco (Fig. 8C and A) and to artificially grown columnar magnetite (Fig. 8B). This similarity is a strong argument for rapid cooling of a melt at Kiirunavaara. The parallel arrays of columnar magnetite resemble morphologically the monomineralic layers of "columnar-dendritic" feld- spar described by Petersen (1985) from a contact zone of a lardalitic ring complex in the Oslo region, Norway. According to Petersen the layers formed under conditions of supercooling, an explanation which has also been given by Henr•quez and Martin (1978) for the crystallization of columnar magnetite at E1 Laco.
The magnetite dendrites with branchlike offshoots at Kiruna and E1 Laco (Fig. 7A and C) resemble the "christmas-tree" form of dendritic chromite de-
scribed by Greenbaum (1977) and Leblanc (1980) from the Troodos ophiolite complex, Cyprus. They attributed the dendritic texture to rapid crystalliza- tion of a magma supersaturated with respect to chro- mium. Skeletal crystals of other minerals (galena and sphalerite) morphologically similar to the branching magnetite plates are illustrated by Barton (1991). The sulfides formed at lower temperatures from fluids rather than from a melt, but an essential feature in common with the platy magnetite is supersatura- tion during crystal growth. Existence of dendritic magnetite plates, per se, does not prove crystalliza- tion from a melt, as shown by their occurrence in Pahtohavare, a tuffRe-hosted stratiform Cu-Au de- posit in the Kiruna region.
Nystr6m and Henrlquez (1989) suggested that the apatite segregations with shells of columnar magne- tite found in skeleton ore at Kiirunavaara resulted
from local differentiation in a crystallizing ore melt. The segregations can be regarded as volatile-rich pockets relative to the surrounding ore, and their shells represent interfaces where volatiles diffused into the accumulating pockets. The growth rate might be relatively high at the interface, leading to crystallization of columnar magnetite; slower growth away from it produced platy dendrites. The transfer of volatiles is analogous to a local degassing of the ore melt. Crystallization from a melt is supported by the trachytoid texture; the distribution and deformation pattern of the dendritic pseudomorphs after pyrox- ene in the ophitic ore (Nystr6m, 1985, fig. 7) consti- tute independent evidence of flow in a melt.
Studies of basaltic systems show that dendritic py- roxenes grow at relatively high cooling rates (Lof- gren, 1980), indicating that the dendritic pyroxene
836 NYSTROM AND HENR[OUEZ
in the E1 Laco ore is also a rapid growth feature. Its close association with columnar magnetite here (Fig. 9A) and in the iron belt (Fig. 9C) supports the ge- netic interpretation given above for the columnar magnetite. The dendritic pyroxene at E1 Laco cannot be a metamorphic mineral because the ore is unaf- fected by metamorphism--neither are the relict py- roxene at Kiirunavaara and the inferred pyroxene in the iron belt metamorphic phases. The magnetite surrounding them shows no sign of recrystallization, and the regional metamorphic grade in the two ore provinces (greenschist and prehnite-pumpellyite fa- cies, respectively) is too low for pyroxene to form. Moreover, the Kiruna pyroxene differs chemically from the more Fe rich pyroxenes in metamorphosed iron-formations (cf. Deer et al., 1978).
Magnetite can form in an exhalative-sedimentary environment. It has been reported from the Atlantis II Deep in the Red Sea rift where it is an uncommon mineral associated with hematite and pyroxene (Zier- enberg and Shanks, 1983). This magnetite is a recrys- tallization product after goethite and hematite and does not show dendritic development. The pyroxene, which occurs as aggregates of prismatic crystals or is intergrown with magnetite, ranges in composition from low A1 aegirine-augite to hedenbergite. Apatite has not been described. Thus, the mineralogy here and that in well-preserved apatite iron ores bear no resemblance to each other.
The upward gradation from apatite iron ore into the quartz-banded hematitic ore described by Parilk (1975a, b) from some places in the Per Geijer de- posits is suggestive of an exhalative-sedimentary pro- cess. However, all the examples of quartz-banded ore seen by us in these deposits bear the imprint of defor- mation accompanied by oxidation of magnetite and introduction of silica, and we consider them to be formed by eruption of very volatile-rich melts at the surface. There is even evidence for emplacement by pyroclastic flow at Nukutusvaara, one of the deposits (Fig. 3).
Magnetite geochemistry
Primary magnetite and secondary hematite from E1 Laco plot similarly in Figure 11, indicating that mar- titization at low temperature produced no significant changes in the trace element composition of the iron oxide. The chemistry of the magnetite also seems to be unmodified by recrystallization, as suggested by a comparison of ores with primary textures and de- formed ores from Kiruna. The magnetites have no vis- ible inclusions that can influence their composition, the only exception being Ti phases.
Figure 10 illustrates the overall geochemical re- semblance between the investigated magnetites from apatite iron ores. The similarity is notable if the com- parison is restricted to E1 Laco and Kiruna (Figs. 11
and 13; Table 2). It can also be seen in Table 2 that the data obtained in this study for the Kiruna magne- tites are in good agreement with the analyses of Frietsch (1970) and Parilk (1975a), and that our E1 Laco data are similar to the values reported by Wegner (1982) and Loberg and Horndahl (1983). However, the E1 Laco analyses given by Parilk (1975a) differ considerably with regard to Cr (much too high), Ni, and Co (both high). A possible explana- tion for his strange results is the use of different ana- lytical methods and laboratories for the Kiruna and E1 Laco material. In addition, the E1 Laco analyses pre- sented by Frutos and Oyarzfin (1975) are somewhat low in V and high in Cr.
The widely varying Ti content of the magnetites from both E1Laco and Kiruna (Table 1; Fig. 13) em-
Ti ppm
4000
3OOO
2000
1000
i
ß Kiruno
0 E[ to½o
[] Romero[
ß A[gorrobo ß 0jos de Aguo /q Cerro Im(Jn
V E[ Tofo X Carmen
Jr E[ Dorado
V Apotite veins
o []
L••,) oß [] j • [] [] [] -•- I I I I 10 20 30 /-,.0 50 •'o V/N i
FIc. 13. Titanium vs. V/Ni for magnetites from apatite iron ores in the Kiruna area, El Laco, and the Chilean iron belt (includ- ing two apatite veins).
KIRUNA-TYPE ORES: TEXTURES AND MAGNETITE CHEMISTRY 837
phasizes the danger of using only a few samples in order to characterize a deposit chemically. The scat- tered high values of Ti in magnetites from E1 Laco and the iron belt can be attributed to exsolved ilmen-
itc. The relatively high Ti in some Kiruna samples is more difficult to explain, because the most Ti rich of them (sample PG-K5) lacks visible inclusions. Rutile and titanitc, both insoluble in HC1, cannot account for the Ti. Apatite iron ores with Ti-rich magnetite (0.5-2 % TiO2) containing exsolved ilmenite occur in the Bafq region, Iran (F/firster et al., 1973), which shows that a low Ti content cannot be used as a diag- nostic feature of this ore type.
The similarity in magnetite geochemistry as well as in primary ore textures documented here for E1 Laco and Kiruna constitutes independent evidence for a similar origin. Parftk's (1975a) conclusion that the Kiruna and E1 Laco ores are too different chemically to allow a genetic comparison cannot be supported by the data now available. Instead of posing a prob- lem, the geochemistry of the ores favors a close rela- tionship.
The Romeral samples vary more in composition than those from E1 Laco and Kiruna (Table 1), but the variation is small taking into account that they com- prise such contrasting types as massive pure magne- tite ore, ore breccias, and patches of ore in quartz- apatite-calcite-scapolite rock. The strongly deformed and recrystallized hematitic ore from Romeral Norte is very similar chemically to the columnar magnetite in the main orebody (Table 1), arguing against Book- strom's (1977) conclusion that the two orebodies are of different origins. Dating by Munizaga et al. (1985) of the foliated "Paleozoic" host rock of Romeral Norte yielded a normal iron belt age (110 _+ 3 Ma), supporting the opinion of the geologists at Romeral that the difference between the orebodies is merely due to deformation (Hugo Aguirre, pers. commun., 1983).
We have been unable to find chemical data in the literature for magnetite in ores which can be estab- lished beyond doubt to be exhalative-sedimentary in origin. Frietsch (1970) analyzed magnetite and he- matite from a large number of mostly Swedish iron ores, many of them considered to be metamorphosed chemical sediments related to basic volcanic rocks (the stratified and nonapatitic ores in Fig. 11). The Bandurrias ore, analyzed by us, is interpreted as a contact metamorphosed exha]ative-sedimentary de- posit by Cisternas (1986).
The most important chemical difference between the nonapatitic ores classified as sedimentary and the apatite iron ores is the low V contents in the former (Fig. 11). A low V content also seems to characterize iron-formations. Kessler and Milllet (1988) reported the following mean values for about 170 iron oxide concentrates from the banded iron-formation in
Minas Gerais, Brazil: Ti -- 114, V -- 138, Cr = 77, A1 = 1,068, Mg = 40, Mn -- 170, Ni -- 167, Co = 120, and Cu = 40 (all in ppm). The ranges of Davy (1983) plotted in Figure 11 refer to ores. Another apparent difference is the higher Cr content in the sedimen- tary ores. Frietsch (1970) obtained an average of 90 ppm for the stratified iron ores in northern Sweden which agrees well with the iron-formation value and is approximately 10 times higher than the apatite iron ore data (Table 1).
In volcanic rocks the iron oxide is usually titano- magnetite which is much richer in Cr, A1, and Mn than the magnetite of our apatite iron ores (common rock values are 0.01-0.2 wt % Cr2Oa, 1-3% AlcOa, and 0.5-1% MnO; Haggerty, 1976; Frost and Linds- ley, 1991). The iron oxide in layered mafic intrusions also differs by high Ti, Cr, and V contents, and ex- solved bodies of ilmenite are typical of the iron oxide in plutonic rocks. The differences in composition demonstrate that genetic conclusions for iron ores based on chemical comparisons of ore and rock mag- netite are meaningless.
Conclusions
This study of apatite iron ores from E1 Laco, the Chilean iron belt, and the Kiruna ore field shows that well-preserved deposits have several features in common besides a tabular shape, association with sub- aerial volcanic rocks, presence of ore breccia, and other features supporting a magmatic origin. The magnetites of the ores are similar to each other in composition (very poor in Cr and relatively rich in V) and morphology. Columnar magnetite, which is a rare rapid-growth form according to the literature, is found in almost all the investigated deposits. It may be accompanied by platy magnetite, locally with den- dritic branching, and dendritic pyroxene or pseudo- morphs after it.
The volcanic nature of the magnetite lava flows and pyroclastic ore on the flanks of the E1 Laco volcano has been established beyond doubt. Ores with similar texture and structure also exist at Kiirunavaara, e.g., highly vesicular ore and stratified magnetite-apatite tuff, which show that this deposit is volcanic and formed by eruption of lava and air-fall material. Min- erals indicating hydrothermal activity overprint the ore in both provinces, and in the iron belt, consistent with a volcanic setting.
Apatite iron ores with volcanic features are far from unique, suggesting that emplacement at or close to the surface is characteristic of this ore type. They have been reported from E1 Laeo, Cerro de Mercado in Mexico (extrusive and pyroelastic ore; Lyons, 1988), the Bafq region in Iran (flows and pyroelastic layers of magnetite; F6rster and Knittel, 1979), and Magnetita Pedernales in Chile (fragments of ore lava with flattened vesicles indicating flow; Grez et al.,
8 3 8 N•STflOM •ND HENflfOUEZ
1991). To this list now can be added lava flows and pyroelastic sheets of magnetite recently discovered in the iron belt (V. Travisany, F. Hendquez, and J. O. Nystr6m, unpub. data) and at Kiirunavaara, the type locality of the apatite iron ores.
Acknowledgments
We thank U. H•lenius, B. Levi, and B. Lindqvist (Stockholm), and two Economic Geology reviewers for constructive criticism of the manuscript. Uno Sam- uelson (Stockholm) did the photographic work and Solveig Jevall and Inger Arnstr6m (Stockholm) drew the figures. We are grateful for economic support from the Swedish Agency for Research Co-operation with Developing Countries (SAREC, grant 86/199), the Swedish Board for Industrial and Technical Devel-
opment (NUTEK, grant 92-00224P), the Fondo Na- cional de Desarrollo Cienfifico y Tecno16gico (FON- DECYT, grant 89-0759), and the Departamento de Investigaciones Cienfificas y Tecno16gicas-Universi- dad de Santiago de Chile (DICYT, grant 05-92- lSHB).
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