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Article

Involvement of Evaporite Layers in the Formation of Iron Oxide-Apatite Ore Deposits: Examples from the Luohe Deposit in China and the El Laco Deposit in Chile

by
Dongwei Guo
1,2,*,
Yanhe Li
1,*,
Chao Duan
1 and
Changfu Fan
1
1
Ministry of Natural Resources Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
2
School of Earth and Space Sciences, Peking University, Beijing 100871, China
*
Authors to whom correspondence should be addressed.
Minerals 2022, 12(8), 1043; https://doi.org/10.3390/min12081043
Submission received: 2 August 2022 / Revised: 15 August 2022 / Accepted: 18 August 2022 / Published: 19 August 2022
(This article belongs to the Special Issue Magmatic-Hydrothermal Fe Deposits and Affiliated Critical Metals)
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Abstract

:
Iron oxide-apatite (IOA) deposits are important sources of iron. The role of evaporite layers in the formation of IOA ore deposits remains controversial. The Luohe deposit in eastern China and the El Laco deposit in Chile are representative IOA deposits. In this study, the S isotope characteristics of these two deposits are revisited. The formation of the Luohe ore deposit is closely related to marine evaporite layers in the Middle Triassic Dongma’anshan Formation. At Luohe, most of the sulfides and sulfates have high δ34SV-CDT values (concentrated from 6‰ to 10‰ and 16‰ to 20‰, respectively). The δ34SV-CDT values of sulfates are similar to those of marine evaporite layers (28–30‰) in the Dongma’anshan Formation. Estimates show that 46–82% of sulfur in the Luohe deposit is derived from marine evaporite layers. Unlike the Luohe deposit, the El Laco ore deposit formed in close relation to terrestrial evaporite layers from the Cretaceous-Tertiary Salta Group. At El Laco, the sulfides and sulfates have lower δ34SV-CDT values of −2.3‰ to −0.9‰ and 6.8‰ to 10.5‰, respectively. The δ34SV-CDT values of sulfates from the El Laco deposit are similar to those of sulfates from terrestrial evaporite layers (9.5‰) in the Salta Group. Estimates reveal that more than 70% of sulfur comes from terrestrial evaporite layers. These results indicate that evaporite layers have been involved in IOA ore-forming systems of both hydrothermal and magmatic deposits. Evaporite layers are proposed to have played key roles in the ore-forming processes of the Luohe and the Laco deposits and in other IOA deposits elsewhere.

1. Introduction

Iron oxide-apatite (IOA) deposits, also known as magnetite-apatite deposits or Kiruna-type deposits, are an important type of porphyrite Fe deposits that occurred sporadically over a considerable time span ranging from the Paleoproterozoic to the Pliocene; they are mainly found at convergent boundaries, such as the Kiruna and Grängesberg districts in Sweden, the Bafq and Zanjan districts in Iran, the Great Bear zone in Canada, the Adirondack and Missouri districts in the United States, the High Andes and iron belt in Chile, and the Ningwu and Luzong districts in eastern China [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. IOA deposits are characterized by massive and disseminated magnetite-apatite ± actinolite ores with Na, Ca, and K alteration and have a close spatial relationship with evaporite layers [2,4,18,23,24,25,26,27,28,29], which dominantly comprise gypsum/anhydrite, halite, and carbonates [2,30,31,32,33]. However, whether the evaporite layers are involved and play an important role in the formation of IOA ore deposits remains controversial. As the most important oxidation barrier, assimilation of evaporate layers played a key role in the formation of IOA deposits [2,4,14,18,23,26,28,29], while other researchers argue that evaporate layers are not needed to form IOA deposits [13].
The Luohe deposit in eastern China and the El Laco deposit in northern Chile are two typical IOA deposits. The Luohe deposit has been described as a hydrothermal IOA deposit comprising massive, disseminated, and vein-type magnetite-apatite-diopside/actinolite ores, with widespread anhydrite-diopside alteration [34,35,36,37,38]. At Luohe, marine evaporite layers appear in the mining area and have a close spatial relationship with ore bodies [4,39]. The El Laco deposit is the youngest and best-preserved IOA deposit in the world and contains typical magmatic Fe ores [40,41,42,43], with terrestrial evaporite layers occurring in the region [44,45,46]. Sulfur isotopes are sensitive tracers of sulfur sources and ore-forming processes [47,48,49,50]. In this study, the published sulfur isotope data of these two deposits are reviewed and revisited. The proportions of sulfur from the evaporate layers in the two deposits are calculated and discussed to reveal the involvement of evaporite layers and provide new insights for understanding the formation of IOA deposits.

2. Deposit Geology

2.1. Luohe Deposit

The Luohe deposit is located in the western part of the Luzong ore district, which is one of the major Fe ore districts in eastern China (Figure 1). The strata of the Luzong ore district can be divided into three parts: (1) pre-Sinian metamorphic basement; (2) Sinian to Early Triassic marine clastic and carbonate rocks; and (3) Middle-Late Triassic to Cretaceous terrigenous clastic and volcanic rocks [36,51,52]. In the Early Cretaceous (135−127 Ma) [53], intense volcanism developed, producing the Longmenyuan, Zhuanqiao, Shuangmiao, and Fushan Formations. The IOA deposits are related to intrusions that occurred in the late depositional interval of the volcanic Zhuanqiao Formation [35] at ca. 130 Ma [39,54,55,56].
At the Luohe deposit, lime dolomite, breccia limestone and anhydrite limestone of the Middle Triassic Dongma’anshan Formation, trachyandensite and volcaniclastic rocks of the Zhuanqiao Formation, and basaltic trachyandesite of the Shuangmiao Formation occur in the mining area [13,34,57,58,59]. There are three types of ores. Anhydrite ores occur in the upper part, while pyrite ores and magnetite ores occur in the middle and lower parts [34]. The Fe ore bodies are distributed in the volcanic rocks of the Zhuanqiao Formation, with bedded and lenticular structures (Figure 2).
There are three vertical alteration/mineralization zones from top to bottom: (1) an upper, light-colored zone consists of silicate, kaolinite, chlorite-epidote, alunite, and anhydrate alteration with pyrite mineralization with a thickness of 400 m; (2) a middle, dark-colored zone consisting of diopside–garnet, chlorite–epidote, scapolite alteration, and Fe ores represents the main mineralization zone and has thicknesses of 300–500 m; and (3) a lower, light-colored zone is characterized by strong alkaline alteration consisting of major albite or K-feldspar alteration with retrograde alteration to sericite, carbonates, zeolites, and pyrite, with thicknesses of 250–300 m [18]. The main ore types include vein, veinlet, massive and disseminated types. Magnetite is one of the most important metallic minerals in the deposit, with a prevalent assemblage of diopside–magnate–apatite, which can be divided into disseminated and veined occurrences (Figure 3A). Pyrite ores are usually formed after the crystallization of magnetite, mainly comprising anhydrite–pyrite, magnetite–pyrite and anhydrite–pyroxene–pyrite assemblages (Figure 3B,C). Anhydrite is distributed widely, occurring mainly in the middle and upper alteration zones, some of which form anhydrite ore bodies (Figure 3D).
In addition, the Xiaobaozhuang skarn iron deposit has been found underneath the Luohe deposit, near the contact zone between trachyandensite of the Zhuanqiao Formation and evaporite layers of the Dongma’anshan Formation; this skarn deposit shows features similar to those of the Luohe deposit (Figure 4).
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Figure 1. Geological map of the Luzong ore district (after [55]).
Figure 1. Geological map of the Luzong ore district (after [55]).
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Figure 2. Section of the Luohe IOA deposit with mineralization and alteration zones (modified from [60]).
Figure 2. Section of the Luohe IOA deposit with mineralization and alteration zones (modified from [60]).
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Figure 3. Photographs of Fe ore textures in the Luohe deposit. (A) Breccia apatite–magnetite–actinolite ore, with coarse-grained purple gypsum and sporadic apatite and actinolite. (B) Disseminated pyrite in magnetite ore with anhydrite. (C) Iron ore with anhydrite and pyrite replacements. (D) Anhydrite ore. Abbreviations: Anh = anhydrite; Mt = magnetite; Py = pyrite; Ap = apatite; Act = actinolite.
Figure 3. Photographs of Fe ore textures in the Luohe deposit. (A) Breccia apatite–magnetite–actinolite ore, with coarse-grained purple gypsum and sporadic apatite and actinolite. (B) Disseminated pyrite in magnetite ore with anhydrite. (C) Iron ore with anhydrite and pyrite replacements. (D) Anhydrite ore. Abbreviations: Anh = anhydrite; Mt = magnetite; Py = pyrite; Ap = apatite; Act = actinolite.
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Figure 4. Geological section of the No. 3 prospecting line in the Luohe-Xiaobaozhuang system (after [37,61]).
Figure 4. Geological section of the No. 3 prospecting line in the Luohe-Xiaobaozhuang system (after [37,61]).
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2.2. El Laco Deposit

The El Laco IOA deposit is located on the Pune Plateau of the Andes in northern Chile on the slope of the Pliocene El Laco stratovolcano (5.3−1.6 Ma [62]), which consists of typical andesite flows (Figure 5) [63,64]. The El Laco deposit is underlain by thick sedimentary sequences, including limestones and sulfate evaporites of the Salta Group with a total thickness of 5000 m [31,44,64] and phosphatic siderite ironstones in the Paleozoic basement [43,65]. The Salta Group can be divided into three main units from base to top: (1) the Middle Cretaceous Pirgua Subgroup [66], which is composed of red beds; (2) the Maastrichtian to early Paleocene Balbuena Subgroup, which is composed of white sandstones (Lecho Formation), gray limestones with some layers of anhydrite and gypsum (Yacoraite Formation) and dark pelites (Olmedo/Tunal Formations); and (3) the Paleogene Santa Barbara Subgroup, which is dominated by red fine-grained sandstone and siltstone and green mudstone. Several sulfate-rich units of the Yacoraite Formation are exposed to the east and west of the complex and likely extend laterally beneath El Laco [44,46]. The IOA deposits are closely related to the Yacoraite Formation, which formed in a carbonate shallow sea to hypersaline lake environment and is mainly composed of limestone and sandstone [31,64].
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Figure 5. Location (A) and regional geologic setting (B) of the El Laco deposit in Chile (modified from [62]).
Figure 5. Location (A) and regional geologic setting (B) of the El Laco deposit in Chile (modified from [62]).
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The magmatic rocks in the El Laco deposit are mainly composed of andesite, ignimbrite, pyroclastic andesite, dacite, and rhyolite and covered by Quaternary fluvial sediments in Quaternary (Figure 5B) [62,67]. The ore bodies crop out around the El Laco volcanic plug. Most mineralization occurs as strata-bound ore bodies interbedded with andesite flows or subvertical veins [63,64], forming the two large Laco Norte and Laco Sur ore bodies, with four other smaller ore bodies (Cristales Grandes, Rodados Negros, San Vicente Alto, and San Vicente Bajo). The ore bodies show prominent volcanic features, including flow banding, pipe vesicles, pahoehoe texture, and columnar jointing (Figure 6A,B) [63,64,68,69].
The alteration can be divided into three vertical zones from bottom to top: (1) an earlier, deep zone of Na-Ca alteration; (2) a dominant K-Ca alteration assemblage with diopside, magnetite, scapolite, pyroxene, garnet, apatite and anhydrate and small amounts of pyrite and chalcopyrite mineralization; and (3) a later, shallow zone of acid-sulfate alteration, characterized by abundant gypsum, montmorillonite, kaolinite, alunite, quartz and hematite, together with deposition of sulfides (Figure 6C) [64]. Vast quantities of anhydrite and gypsum occur as stockwork veins, mounds, and sulfate-rich zones in the shallow zone of acid-sulfate alteration (Figure 6D) [64].
A brief comparison of geological characteristics between the Luohe IOA deposit in eastern China and the El Laco IOA deposit in Chile is shown in Table 1.
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Figure 6. Photographs of geological features in the El Laco deposit. (A) Highly vesicular and oxidized magnetite from San Vicente Alto. (B) Columnar magnetite from Rodados Negros. (C) Large zones of acid-sulfate alteration in Laco Norte. (D) Gypsum-rich mound in the zone of acid-sulfate alteration, consisting of gypsum with minor cristobalite and alunite, at Pasos Blancos (after [64,68]).
Figure 6. Photographs of geological features in the El Laco deposit. (A) Highly vesicular and oxidized magnetite from San Vicente Alto. (B) Columnar magnetite from Rodados Negros. (C) Large zones of acid-sulfate alteration in Laco Norte. (D) Gypsum-rich mound in the zone of acid-sulfate alteration, consisting of gypsum with minor cristobalite and alunite, at Pasos Blancos (after [64,68]).
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3. Discussion

3.1. Sulfur Source

Sulfur isotopes can provide important constraints on the origin of sulfur in ore deposits [47,50,72]. Sulfide (pyrite) and sulfates (gypsum/anhydrite), with unique diopside–gypsum–magnetite (gypsum–pyroxene) and garnet–gypsum–magnetite assemblages, are developed in the Luohe and El Laco IOA deposits [2,4,18,38,64,73]. The relative 34S enrichment in sulfates is much larger than that in sulfides [72]. If the sulfur in deposits is mainly derived from magma, the δ34SV-CDT value of the total sulfur is approximately 0. Based on mass balance calculations, the δ34SV-CDT values of sulfides should be significantly negative in the presence of widespread sulfates and magnetite (or hematite) [74]. On the other hand, if the sulfur comes mainly from evaporite layers, the δ34SV-CDT values of sulfides should be higher in the high-temperature mineralization system.

3.1.1. Luohe Deposit

The sulfur isotope compositions of pyrite and gypsum at Luohe are shown in Table 2 [18,38,61,73,75]. The results yield a range of δ34SV-CDT values from −14.0‰ to 11.1‰ (n = 174, average of 4.2) for pyrite and 13.6‰ to 24.4‰ (n = 126, average of 18.1) for anhydrite. The pyrite and anhydrite δ34SV-CDT values are concentrated at 6‰ to 10‰ and 16‰ to 20‰, respectively (Figure 7). The sulfur isotope compositions of the Luohe deposit and evaporite layers show the following trends. (1) The pyrite δ34SV-CDT values are markedly positive, higher than that of mantle-derived magma and lower than that of anhydrite in the ore body. (2) All anhydrite in the ore body is significantly enriched in 34S relative to pyrite, and the δ34S V-CDT values are similar to those of anhydrite from the evaporite layers (n = 3, average of 29.7‰) in the Dongma’anshan Formation near the ore district [18]. Therefore, sulfur in the Luohe IOA deposit is probably sourced from a mixture of primitive magma and evaporite layers.
The δ34SV-CDT values of pyrite in the Luohe IOA deposits vary widely. The δ34SV-CDT values of pyrite in the magnetite mineralization stage are higher than those of pyrite in the sulfide stage [77]. The δ34SV-CDT values of pyrite in the Luohe deposit gradually decrease from deep to shallow levels, but the δ34SV-CDT values of anhydrite are relatively stable (Figure 8) [34,77]. These results indicate that sulfate plays a dominant role in this sulfur-containing magmatic–hydrothermal ore-forming system and governs the stable δ34SV-CDT values of sulfates, while the changes in sulfur isotopes in pyrite are mainly controlled by the sulfide-sulfate mineral equilibrium temperature [32,61,77]. Based on the calculated isotopic fractionation factors between sulfide and sulfate (Table 2), the sulfide-sulfate equilibrium temperatures are approximately 720–770 °C and 750–810 °C in the Luohe and Xiaobaozhuang deposits, respectively [74]. The mineralization temperature gradually decreases from deep to shallow levels, leading to decreases in the δ34SV-CDT values of sulfides. The evaporite layers with high δ34SV-CDT values are the main sulfur source endmember for the deposit.
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Figure 7. Sulfur isotope compositions of pyrite and anhydrite in the Luohe deposit and gypsum in the Dongma’anshan Formation near the Luohe deposit (data are listed in the Table S1 and obtained from [18,38,61,73,75]).
Figure 7. Sulfur isotope compositions of pyrite and anhydrite in the Luohe deposit and gypsum in the Dongma’anshan Formation near the Luohe deposit (data are listed in the Table S1 and obtained from [18,38,61,73,75]).
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The origin of sulfur can be discussed on the basis of the isotopic composition of total sulfur. In the region of high fO2 where sulfate species are significant, δ34Ssulfate ≈ δ34S∑S [72,74]. Therefore, the sulfur isotope value of sulfate represents the sulfur isotope characteristic of total sulfur in the ore-forming fluid [61,77]. Assuming the δ34SV-CDT value of the original magmatic sulfur to be 0‰ and the δ34SV-CDT value of the evaporite layers to be 29.7‰ [18], according to the δ34SV-CDT values of sulfates in the Luohe deposit (from 13.6‰ to 24.4‰) [61,73,75] and binary mixing calculations (Equation (1)), an estimated 46%–82% of the sulfur in the Luohe deposit comes from the evaporite layers in the Dongma’anshan Formation.
δ34S∑S = δ34Smagma × X1 + δ34Sevaporite layers × (1 − X1)
X1, 1−X1 are proportions of sulfur from the magma and evaporate layers, respectively.
The Xiaobaozhuang deposit is located underneath the Luohe deposit. Note that pyrite and anhydrite from the deeper deposit have higher δ34SV-CDT values than those from the Luohe deposit. The δ34SV-CDT values of pyrite decrease vertically from bottom to top, ranging from 6.9‰ to 14.2‰, and δ34SV-CDT values of anhydrite are relatively stable at approximately 20‰ (Figure 8) [38,73], indicating that the mineralization temperature gradually decreases from deep to shallow levels and that the sulfur in the Xiaobaozhuang deposit also comes mainly from evaporite layers in the Dongma’anshan Formation.

3.1.2. El Laco Deposit

As shown in Table 2 and Figure 9, most of the δ34SV-CDT values of pyrite in the El Laco deposit are near zero, ranging from −2.3‰ to −0.9‰ (n = 4) [64]. However, the average δ34SV-CDT value of anhydrite is 8.45‰, with a narrow range from 6.8‰ to 10.5‰ (n = 8) [64], which is similar to that of sulfates from the evaporite layers in the Salta Group (average of 9.5‰, n = 12) [64,76]. Therefore, a possible source of sulfur for the 34S-enriched anhydrite would be the evaporite layers in the Salta Group.
The El Laco deposit has widespread gypsum and includes some assemblages accompanied by large amounts of anhydrite and minor pyrite or chalcopyrite, indicating that sulfates also play a dominant role in the sulfide-sulfate system and that the sulfur isotope values of sulfates (from 6.8‰ to 10.5‰) represent the sulfur isotope characteristics of total sulfur in the ore-forming fluid [72,74]. According to the sulfate δ34SV-CDT values in the El Laco deposit and binary mixing calculations (Equation (1)), more than 70% of the sulfur is estimated to originate from the evaporite layers of the Salta Group by assuming the δ34SV-CDT value of primitive magmatic sulfur to be 0‰ and the δ34SV-CDT value of sulfates in evaporite layers from the Salta Group to be 9.5‰.
According to the equilibrium equation and the relative sulfur isotope enrichment factor between sulfate and sulfide (Table 2) [74], the calculated sulfate-sulfide equilibrium temperature is almost 820–900 °C. These results indicate that S2− and SO42− could have formed at high temperatures when magmas assimilated and mixed with evaporite layers during magmatic ascent. Meanwhile, the El Laco deposit could likely be a magmatic–hydrothermal system in which the involvement of evaporite layers played a key role.
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Figure 8. Variation in sulfur isotope characteristics with depth in the Luohe deposit in eastern China (after [61]).
Figure 8. Variation in sulfur isotope characteristics with depth in the Luohe deposit in eastern China (after [61]).
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Figure 9. Sulfur isotope compositions of pyrite and anhydrite in the El Laco deposit and gypsum from the Salta Group (data are listed in the Table S1 and obtained from [64,76]).
Figure 9. Sulfur isotope compositions of pyrite and anhydrite in the El Laco deposit and gypsum from the Salta Group (data are listed in the Table S1 and obtained from [64,76]).
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In summary, the sulfur isotope composition of total sulfur in the ore-forming fluid within the magmatic–hydrothermal system depends on the sulfur source, and is strongly controlled by physicochemical conditions, including temperature, fO2 and pH [2,32,47,72,77]. As mentioned above, the calculated proportion of sulfur from the evaporite layers shows that these layers are involved in the ore-forming process at both the Luohe and El Laco IOA deposits. The difference is that the sulfur in the Luohe deposit is mainly sourced from marine evaporite layers, while the sulfur in the El Laco deposit mostly originates from terrestrial evaporite layers. The δ34SV-CDT values of sulfates from terrestrial evaporite layers are lower than those of marine evaporite layers, resulting in lower δ34SV-CDT values of sulfate and sulfide minerals in the El Laco IOA deposits. It is reasonable to conclude that sulfur is derived only from magma when δ34SV-CDT values of total sulfur are near zero in the mineralization system. However, δ34SV-CDT values of sulfides do not represent the sulfur isotope composition of total sulfur in the presence of widespread sulfates and Fe-oxides. Under the conditions of large-scale sulfates and Fe-oxides, if magmatic sulfur is the signal sulfur source in the deposits, δ34S∑S ≈ 0 and δ34SV-CDT values of sulfides will be significantly negative rather than 0. On the other hand, if δ34SV-CDT values of sulfides are near zero, δ34SV-CDT values of total sulfur will be markedly positive. Therefore, the sulfur source in the deposit cannot be determined simply based on the δ34SV-CDT values of sulfides. The physicochemical conditions and the geological and geochemical characteristics, especially whether sulfates and iron oxides exist on a large scale, need to be considered when applying the sulfur isotopes in tracing ore-forming material sources of IOA deposits.

3.2. The Participation of Evaporite Layers in the Process of Mineralization

As mentioned above, the evaporite layers are significantly involved in ore formation at both the Luohe and El Laco IOA deposits. Questioning when involvement occurs is logical.
Li et al. (2015) [4] found fluid inclusions in garnet that formed at the stage before the main mineralization and contain various daughter crystals such as anhydrite and halite; these observations are quite consistent with the widespread albite and scapolite alteration in the early mineralization within the Luohe deposit. The same authors also found that the initial fluids are high-temperature (~780 °C and hypersaline (~90 wt.% NaCl eq) fluids with high oxygen fugacity. Fluid and melt inclusions, especially the earliest and most primitive melt inclusions, are valuable features that record the origin and evolution of ore-forming fluids [78]. The compositions of fluid and melt inclusions imply that fluids evolved from magmas and assimilated substantial amounts of evaporite layers during magmatic ascent and that the involvement of evaporite layers might have started in the initial stages of Fe mineralization.
The involvement of evaporite layers in the mineralization system from the initial ore-formation stage is also supported by the carbonate-sulfate Fe-rich fluid and melt inclusions reported in the El Laco deposit in Chile; the Buena Vista and Iron Springs deposits in the United States; and the Meishan, Nihe and Luohe IOA deposits in eastern China [4,29,79]. Moreover, igneous rocks associated with El Laco IOA systems show direct evidence of the assimilation of evaporite layers in the form of sedimentary xenoliths from the Yacoraite Formation [44].
The addition of evaporite layers (rich in Na+, K+, Ca2+, Cl−, CO32−, SO42− and H2O) into an Fe-bearing magma is considered to be a key factor in IOA ore formation [3,4,14,17,18,26,32]. Evaporite layers are important oxidation barriers that can oxidize Fe2+ in the silicate melt to Fe3+ and prevent Fe from entering the lattice of Fe-rich silicate minerals such as pyroxene and amphibole, forming Fe-poor diopside and tremolite, while Fe oxides enter the melt (as shown in Equations (2) and (3)). This is proven by the large-scale anhydrite–diopside and actinolite/tremolite alteration in the Ningwu and Luzong districts and the extensive development of diopside alteration and anhydrite alteration and iron ore magma in the El Laco deposits in Chile [4,42].
FeMgSi2O6 + CaSO4 → CaMgSi2O6 + Fe3O4 + FeS2
Ca2(Mg, Fe)5[Si4O11](OH)2 + CaSO4 → Ca2Mg5[Si4O11] (OH)2+ Fe3O4 + FeS2
On the other hand, evaporite layers could provide P, NaCl and volatiles, which would promote the occurrence of liquid immiscibility in magma, forming an Fe oxide-P-volatile-rich melt and a Si-rich melt [80,81,82,83]. These enrichment mechanisms of Fe for IOA deposits have been proven by many natural phenomena and laboratory simulation experiments [6,11,42,43,84,85,86]. Moreover, the addition of oxygen agents increases the oxygen fugacity of the magmatic system, leading to the formation of numerous Fe oxides above the solidification temperature of silicate liquids, which has been proven by previous experiments [87,88]. Furthermore, Fe-volatile-rich melt evolves to an iron ore magma and a high-temperature, high-salinity, iron-rich magmatic–hydrothermal fluid. During the magmatic–hydrothermal process, hydrothermal fluid can activate the evaporate layers and transfer materials to the upper part, leading to reprecipitation of gypsum/anhydrite [2,18].
In the hydrothermal stage, with the continuous pulsed addition of SO42−, NaCl and H2O from evaporite layers, Fe2+ is oxidized to promote hydrothermal Fe-oxide precipitation, SO42− itself is reduced to S2−, and S2− combines with Fe2+ to form hydrothermal Fe-sulfide ores (Equation (4)). Evaporite layers can provide large numbers of agents such as Na+ and Cl− and cause Fe2+ transport as complexes (e.g., Na-Fe-Cl), probably forming hydrothermal-type ores in the distal zone from the evaporite layers [32].
Fe2 + + SO42− → Fe3O4 + FeS2
The coexistence of Fe oxides, Fe sulfides and sulfate in different stages of one IOA deposit is often observed in the field. The close genetic relation of these ores is shown in Equations (2)–(4). In IOA deposits, magma-type and hydrothermal-type ore bodies coexist, showing a close vertical relationship in space where magma-type and skarn-like ores occur in the deep contact zone between intrusive rocks and evaporite layers. IOA-type and hydrothermal–metasomatic-type ores are located in the shallow part in intrusive rock-related mineralization [2,32]. This is consistent with the Luohe IOA ores hosted in the upper part in intrusions and Xiaobaozhuang skarn Fe ores in the deeper part of the contact zone between intrusions and evaporites in this study. Therefore, Fe-oxide ores, Fe-sulfide ores and sulfate ores have a close spatial relationship and are potential prospecting indicators for each other in IOA deposits controlled by evaporite layers.

4. Conclusions

Evaporite layers were involved in the ore-forming process at both the Luohe and El Laco IOA deposits in the early mineralization stage. As the most important oxidation barrier, evaporite layers are a critical factor in the formation of an IOA ore deposit. The difference is that the sulfur in the Luohe deposit is derived mainly from marine evaporite layers, while the sulfur in the El Laco deposit mostly originates from terrestrial evaporite layers, resulting in higher δ34SV-CDT values of sulfate/sulfide in the Luohe deposit but lower δ34SV-CDT values in the El Laco deposit. It is unreasonable to conclude that sulfur is sourced only from magma based on the δ34SV-CDT values (~0‰) of sulfides (e.g., El Laco deposit) and to ignore the involvement of evaporite layers. This study highlights that evaporite layers are involved in the ore-forming process of IOA deposits related to both marine and terrestrial evaporite layers. This implies that IOA ores and Fe-sulfide ores have an intrinsic genetic relationship and belong to the same Fe mineralization system.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min12081043/s1, Table S1: Sulfur isotope values of pyrite and sulfate from the Luohe and El Laco IOA deposits, and anhydrite from the Dongma’anshan Formation and Salta Group.

Author Contributions

Data curation, D.G. and Y.L.; Formal analysis, C.F.; Funding acquisition, Y.L. and C.D.; Writing—original draft, D.G.; Writing—review and editing, Y.L. and C.D. All authors have read and agreed to the published version of the manuscript.

Funding

This study was jointly supported by the National Natural Science Foundation of China (No. 41973022, 41627802 and 42172102) and Central Public-interest Scientific Institution Basal Research Fund (No. KK2015).

Acknowledgments

We gratefully thank two anonymous reviewers for their helpful comments and insightful reviews, which have greatly improved the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Table
Table 1. A brief comparison of geological characteristics from Luohe and El Laco IOA deposits.
Table 1. A brief comparison of geological characteristics from Luohe and El Laco IOA deposits.
DepositsLuoheEl Laco
Geological
Characteristics
Host rockTrachyandesite and diorite porphyriteAndesite and rhyolite
Evaporite layersMarine evaporite layers of the Dongma’anshan FormationTerrestrial evaporite layers of the Salta Group
Ore-controlling structureRock fissures and fractures near contact zoneDiatreme
Ore bodyNearly bedded and lenticular shapesStratified upper part and lower part with vertical veins and domes
Ore structuresVeined, net-veined, disseminated, breccia and massive structuresMassive, vesicular, lava flows, skeleton and columnar structures
Essential mineralsMag, (Hem), Py, Ccp, Ab, Kfs, Di, Grt, Ap, Wo, Anh, Chl, Cal, QzMag, (Hem), Di, Scp, Ap, Kfs, Anh, Act
AlterationDeep dark zone with skarn and anhydrite pyroxene alteration and shallow light zone with silicate and kaolinite alterationSeep alkali-calcic alteration and shallow acid-sulfate alteration
Main metallogenic age131.0–129.1 Ma(5.3 ± 1.9)–(1.6 ± 0.5) Ma
(host volcanic rock)
Fluid temperature and salinity in early mineralization>830 °C,
~90% NaCl eq		>900 °C (Magnetite–diopside oxygen isotope temperature);
40%–60% NaCl eq
References[4,58][42,64,70,71]
Table
Table 2. Sulfur isotope compositions of pyrite and gypsum in the Luohe and El Laco IOA deposits and gypsum from evaporite layers near the deposits.
Table 2. Sulfur isotope compositions of pyrite and gypsum in the Luohe and El Laco IOA deposits and gypsum from evaporite layers near the deposits.
Deposits/
Evaporite Layers		δ34SV-CDT (‰) of Pyriteδ34SV-CDT (‰) of GypsumΔ34SSO4-S2-
(‰)		References
Variation RangeAverageVariation RangeAverage
Luohedeposit−14.0–11.14.2 (174)13.6–24.418.1 (126)13.9[61,73,75];
this study
Xiaobaozhuangdeposit6.9–14.210.8 (17)16.4–32.223.3 (17)12.5[38]
El Lacodeposit−2.3–0.9−1.8 (4)6.8–10.58.45 (8)10.25[64,76]
Dongma’anshan Formationevaporite layers 29.4–29.929.7 (3) [18]
Salta Groupevaporite layers 4.9–14.19.5 (12) [76]
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Guo, D.; Li, Y.; Duan, C.; Fan, C. Involvement of Evaporite Layers in the Formation of Iron Oxide-Apatite Ore Deposits: Examples from the Luohe Deposit in China and the El Laco Deposit in Chile. Minerals 2022, 12, 1043. https://doi.org/10.3390/min12081043

AMA Style

Guo D, Li Y, Duan C, Fan C. Involvement of Evaporite Layers in the Formation of Iron Oxide-Apatite Ore Deposits: Examples from the Luohe Deposit in China and the El Laco Deposit in Chile. Minerals. 2022; 12(8):1043. https://doi.org/10.3390/min12081043

Chicago/Turabian Style

Guo, Dongwei, Yanhe Li, Chao Duan, and Changfu Fan. 2022. "Involvement of Evaporite Layers in the Formation of Iron Oxide-Apatite Ore Deposits: Examples from the Luohe Deposit in China and the El Laco Deposit in Chile" Minerals 12, no. 8: 1043. https://doi.org/10.3390/min12081043

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