The Ore-Forming Process of Washan Porphyrite Iron Deposits in the Ningwu District Associated with Iron Oxide Apatite (IOA) Deposits and Iron Oxide Copper Gold (IOCG) Deposits

Abstract

The Washan iron deposits in Ningwu district contain different magma-related genetic natures, including magmatic, magmatic–hydrothermal and hydrothermal types, and their ore-forming processes remain a subject of debate. To elucidate the ore-forming processes of iron ores from Washan, we present textural, major element analytical, and thermal data of magnetites from various ore bodies in Washan, probing the crystallization conditions and subsequent formation sequence of magnetites. SEM analysis with back-scattered electron (BSE) imaging reveals diverse magnetite textures, including mineral inclusions, exsolution lamellae, and recrystallization features, reflecting the transitional environment from magmatic to hydrothermal. Based on Ti, V, and Cr compositions of magnetite from different ore bodies, two distinct evolution trends of genetic processes are identified, including evolution paths from porphyry-type to IOA- and IOCG-type magnetite. High-resolution WDS mapping highlights the intensifying alterations during this process. Calculated magnetite crystallization temperatures among different types of magnetite range from 597 °C to 378 °C, suggesting a cooling trend from porphyry-type magnetite (~558 °C) to IOA-type magnetite (~515–439 °C) and IOCG-type magnetite (~378 °C). These results underscore the significant role of magma-derived hydrosaline liquids and vapors in the formation of iron ores from Washan, where variations in the salinity of ore-forming fluids lead to different evolutionary paths for subsequent generations of magnetite. The metallogenic model of the Washan iron deposit suggests that highly saline, iron-rich fluids connect the varying geneses of magnetite, transitioning from deeply formed porphyry-type magnetite to IOA- or IOCG-type magnetite generated in the subaerial zone.

1. Introduction

Porphyrite iron deposits, located in the Middle-Lower Yangtze River Metallogenic Belt (MLYRMB) of China, are primarily associated with various types of iron ores with transitional geneses, from iron oxide apatite (IOA) to iron oxide copper–gold (IOCG) and skarn deposits [1,2,3,4,5,6,7,8,9]. Their evolution process among diverse geneses is still under discussion. The magmatic IOA deposits, commonly referring to the magnetite lava flows or their analogs in Kiruna-type IOA deposits, are characterized by magnetite-rich ores of magmatic origin, and by vesicle-like structures, tabular magnetite–apatite dykes, massive magnetite lava flows, or flow bands within ore bodies [10,11,12,13,14,15]. Analogues of these deposits can be found in the Ningwu region within the MLYRMB of China [16,17,18,19]. Ore magma iron deposits (OMIDs), a special type of magmatic deposit, using the geological terms from the “Encyclopedia of China”, is nominated as these deposits are formed directly by the precipitation of iron from Fe-rich melts, and are often characterized by stratabound lenses of ore bodies, magnetite flows preserving vesicles and gas-escape tubes, as well as flow bands outlined by concentrations of magnetite, which can be observed in Meishan, Washan, and Gushan iron deposits [1]. However, iron ores of OMID exhibit various origins, from magmatic [18,20,21] to magmatic–hydrothermal [16,19,22,23,24] and hydrothermal geneses. The Washan iron deposit in the middle section of the Ningwu region is classified as an analog of Kiruna-type IOA deposit [1,25]. The iron ores from Washan share some similarities related to both magmatic and hydrothermal processes. The magmatic affinity of magnetite compositions, flow-banding textured iron ores, and sharp contact between massive ores and dioritic porphyrite within vein-type ore bodies show the features of magmatic genetics, but the deposit also exhibits distinct features underwent during intense fluid–rock interactions related to the hydrothermal mineralization process [26,27,28,29]. The details of mineralization types in Washan are not fully understood and are likely varying as the ore-forming process progressed. Therefore, further understanding the specific mechanisms of mineralization and their evolution would be crucial to elucidate the unique ore-forming process of the Washan deposit.

The origin of the magmatic iron ores from the Kiruna-type IOA deposits is still controversial, and can be summarized into three representative views: (1) a purely magmatic origin that invokes the floatation and emplacement of magnetite lava flows separated from the andesitic magma [11,12,21,30], (2) a combined magmatic and hydrothermal origin involving the crystallization of the massive magnetite from iron-rich melts and subsequent hydrothermal alteration through an exsolution of hydrosaline and vapor [30,31,32,33], and (3) a hydrothermal metasomatic origin attributed to the high-temperature hydrothermal replacement processes [34,35]. Despite this progress on studying the origins of magmatic iron ores from Kiruna-type IOA deposits, there remain several interrelated but distinct hypotheses regarding their formation. Some researchers propose a magmatic process responsible for the iron-rich formation, where igneous magnetite crystallizes from an Fe–P-rich melt that separates from an intermediate silicate melt [30,36,37]. This separation is facilitated by the addition of elements such as phosphorus, chlorine, fluorine, and carbon, which leads to the expansion of the immiscibility gap and the lowering of the solidus temperature. Other researchers favor the magmatic–hydrothermal origin and propose the combination of igneous and magmatic–hydrothermal processes, suggesting a floating iron-rich suspensions/liquids model [38,39,40]. This model combines the segregation of magmatic magnetite crystallized from magma and precipitation-regrowth of hydrothermal magnetite from medium temperature iron–oxide-rich fluids. A different hypothesis suggests a hydrothermal origin involving the injection of high-temperature iron-rich liquids, which scavenged iron by high concentrations of Na, K, and Cl, and are emplaced into the subaerial zone through fissures [10,41]. Despite the above models revealing possible ore-forming mechanisms, the distinctive features of iron ores from porphyrite iron deposit led to a limited understanding for these analogs.

The iron ores from the Washan deposit exhibit both magmatic and hydrothermal genetic characteristics, with flow-banding structured massive ore bodies within veins in contact zones and disseminated ores within deep sub-volcanic intrusions probably being of magmatic origin while the brecciated ores near the surface and miarolitic cavities within iron ores were formed by hydrothermal fluids [1]. The composition of magnetite from massive ores exhibits a high concentration of Fe, Ti, V, P, and REEs that indicates a magmatic process involving liquid immiscibility [42], while ores from later stages show increasing geochemical signs of fluid–rock interactions (e.g., Mg, Al, Mn, Ni, and Ga) [43]. These contrasting geneses introduce a question about the key factors in the formation of the OMID in Washan, with magnetite compositions that are neither like those formed during the separation of iron-rich melts nor the pure metasomatism from hydrothermal fluids. Therefore, revealing the ore-forming conditions of magnetite throughout the whole ore-forming process is crucial for understanding the genetic type of mineralization in Washan. The Washan deposit exhibits a complex, multistage iron mineralization history, with the occurrence of disseminated-, massive-, and breccia-type ores in different structural levels, which provides the opportunity to probe the iron-rich mechanism of ore magma-type iron-rich process.

In this study, we provide in situ analyses of magnetites in representative iron ores from the Wanshan iron deposit, presenting it’s the textural features, major composition, and thermal data of magnetites. By analyzing the conditions associated with the textural features of magnetite and their genetic types, combining that with ore deposit geology, we reveal the ore-forming environment among different types of magnetite. We utilize the cooling trend of magnetite, calculated using a T_Mg-mag thermometer [44], to reveal the crystallization sequence of different generations of magnetite. The purpose of this study is to investigate the genetic type of ore magma-type mineralization by reconstructing the evolution process of mineralization of the Washan deposit. This is helpful to fingerprint the iron-rich mechanism of ore magma-type mineralization in Washan, and further verify the genetic links between ore magma-filled-type ores and other potential end members in the metallogenetic process of porphyrite iron deposits.

2. Geological Setting

2.1. Regional Geology

Porphyrite iron deposits are predominantly found in the Cretaceous volcanic basins that lie between Nanjing and Wuhu in the MLYRMB, which is known as the Ningwu iron district. The Ningwu volcanic basin evolved from a pre-existing sediment basin filled with Jurassic terrigenous rocks (Xiangshan and Xihengshan Formation) and Triassic marine sediments (Qinglong Formation and Huangmaqing Formation), which are overlaid on the stable marine or continental (metamorphic) basement that has developed since pre-Sinian times. Among these marine sediments, the evaporite layers mainly developed in the Triassic Huangmaqing Group (T_3h). Starting from the late Jurassic, the Yanshanian tectonic movement led to the transformation of the tectonic framework between the North China block and the Yangtze block, accompanied by widely developed intraplate volcanic activities and associated mineralization in the MLYRMB (Figure 1A). As a result, andesitic magmas were generated and passed through these sediments, forming a series of volcanic rocks, including the Longwangshan, Dawangshan, Gushan, and Niangniangshan formations, and associated sub-volcanic intrusions in the Ningwu volcanic basin during the early Cretaceous period (Figure 1B). These volcanic rocks are rich in alkaline and are controlled by regional fault systems, emplaced within the intersections of the NE-, NNE-, or NW-striking faults (Figure 1B).

2.2. Iron Deposit Geology

The iron mineralization in Washan is mainly characterized by the emplacement of gas-charged, iron oxide-rich liquids within syn-magmatic fractures at the top of pyroxene diorite porphyrite intrusions and fractures extending to andesitic wall rocks (Figure 2) [1]. The ore-controlling structures of Washan exhibit a series of types, developing on the basis of the syn-magmatic structures. The ore bodies are typically presented in a vertical sequence and exhibit various shapes, with the coarse vein-shaped ore body developed from the deep annular fractures in the lower part, the chamber-shaped ore body formed via the shrinkage–collapse effect at the top of sub-volcanic intrusions caused by rock mass cooling in the middle part, and the collapsed brecciated ore body formed by crypto-explosions in the upper part (Figure 2). In the coarse vein-shaped ore body, the iron ores are characterized by distinct combined magmatic–hydrothermal styles, including a sharp contact between massive ores and dioritic porphyrite, and mineral assemblages of diopside–albite–apatite–magnetite, which show similarities with ore magma-type ores. Within these ores, vesicle-like miarolitic structures commonly present and are filled by quartz and calcite. In the chambered and brecciated ore bodies, the iron ore exhibits a classic mineral assemblage of Kiruna-type IOA deposits, evidenced by the diopside–apatite–magnetite mineral assemblage within pegmatitic ores, as well as the ore magma-style iron ores characterized by iron ore-cemented brecciated fragments. The margin parts of mineralization zones are influenced by pyritization and hematitization, such as extensively developed pyrite veinlets and martite overgrown on magnetite. The Washan deposit also shows simultaneous alkaline metasomatism both in ore bodies and wall rocks. The melanocratic alteration zone in the middle level is characterized by sodic–calcic alteration, which plays a crucial role in the processes of iron mineralization. The leucocratic alteration zone in the upper and lower level is distinguished by potassic–siliconized alteration, which notably modifies the ore bodies and wall rocks.

The ore bodies are typically dominated by a single type of mineralization, which transitions into other types in the vertical direction. The paragenetic sequence of mineralization is identifiable through four distinct ore structure types (Figure 3) within the deposit. (1) The first type is the disseminated structured ores, characterized by fine-grained magnetites dispersed within pyroxene diorite porphyry [45]. (2) The second type is the brecciaed iron ore, formed by the infilling of magnetite as a cementing agent among brecciated wall rock fragments. (3) The third type is the vein-type ores, which occur as veins within fractures, comprising a mineral assemblage of magnetite, actinolite, apatite, and quartz. (4) The fourth type, exhibiting the ore magma-filled style, is the pegmatitic structured ore, which is similar to the third type in mineral assemblage but with coarser grains. The first type of ore structure emerged during the initial phase of mineralization, with the second type initiated at the second stage. The remaining types primarily materialized during the second and third stages of mineralization. The ore-forming mineral is mainly magnetite, which forms masses, patches, breccia cement, or stockworks of veinlets within iron ores. The gangue minerals are garnet, diopside, apatite, calcite, dolomite, quartz, and anhydrite.

3. Sample Selection and Analytical Methods

3.1. Sample Selection and Preparation for Analysis

To ensure the samples were representative, we selected and prepared iron ores from four types of ore bodies in the Washan iron deposit, specifically the coarse iron ores within deep veins (AS03B), apatite-bearing massive iron ores from a chambered ore body (AS02B), the pyrite-bearing brecciated iron ores from the brecciated zones (AS02A), and fine-grained massive iron ores (AS05) from the marginal zone.

3.2. EPMA Analysis with Associated SEM Photographing and WDS Mapping

To investigate the magmatic–hydrothermal evolution of magnetite in ore magma-type ores from the Washan iron deposit, a multi-method analytical approach was employed, combining Electron Probe Microanalysis (EMPA), Scanning Electron Microscopy (SEM), and Wavelength Dispersive Spectroscopy (WDS) mapping.

The electron microprobe analyses (EMPA) of magnetite were performed using a JEOL JXA 8230 electron microprobe at the School of Resources and Environmental Engineering, Hefei University of Technology. The major elements analysis was conducted under the conditions of an accelerating voltage of 15 kV, a beam current of 20 nA, and a beam size of 3 μm. Calibration standards employed were Jianxiong Zhou’s standard sample group (GB/T 17359-1998: oxide-36, silicate-36, sulfide-12, ultra-light element-9, rare earth pentaphosphate-15) and the American SPI International Sample Group (02753-AB 53 Minerals standard), including Mg (periclase, 106 ppm, 15 s), Al (Kyan, 87 ppm, 10 s), Ca (Wo, 111 ppm, 15 s), Ti (Rut, 143 ppm, 15 s), V (V-P glass, 231 ppm, 30 s), Co (CoO, 206 ppm, 30 s), Ni (Oli, 207 ppm, 30 s), Na (Jad, 132 ppm, 10 s), Si (Qtz, 78 ppm, 15 s), Cr (Cr-Sp, 206 ppm, 10 s), Fe (Hem, 169 ppm, 15 s), Zn (ZnWO₄, 256 ppm, 15 s), and Mn (Mn-Fe pyrope, 201 ppm, 10 s). The detection limits for all elements were less than 0.1 wt%. The relative error was controlled as follows: (1) errors were within 5% or 3% for elements exceeding 20% by weight, (2) errors were less than or equal to 8% for elements ranging from 3% to 20% by weight, (3) errors were between 15% and 20% for elements ranging from 1% to 3% by weight, and (4) between 20% and 30% for elements ranging from 0.5% to 1% by weight.

The EMPA analysis was associated with back-scattered electron (BSE) imaging for the identification of magnetite textures using a scanning electron microscope (SEM) equipped with JEOL JXA 8230 EMPA at the School of Resources and Environmental Engineering, Hefei University of Technology. To provide high-resolution images to observe the oxygen exsolution features of the magnetite samples, the SEM detector was operated under fine scan mode with an accelerating voltage of 15 kV and a beam current of 10 nA. After the BSE imaging, the WDS mapping analysis was conducted on a specific type of magnetite (AS05), which exhibited transitional origins. This analysis was performed using a WDS instrument attached with EMPA. The WDS was operated with an accelerating voltage of 15 kV, 10 ms/pixel, and a focused circle beam size of 1 μm for seven major element analyses (Fe, Mg, Cr, Mn, Ni, Ti, and V).

3.3. Temperature Estimations

The T_Mg-mag thermometer, which utilizes the Mg and Fe concentrations in magnetite [46], was employed to calculate the crystallization temperatures of magnetites. The calculation was performed by using the results of the EPMA analysis. The temperatures of magnetite were calculated using the empirical calibration: (T_Mg-mag = −8344 (±320)/[lnX_Mg − 4.1 (±0.28)] − 273), where X_Mg refers to Mg/(Mg + Fe_total). The uncertainty was within ±50 °C.

4. Results

4.1. Magnetite Textures

In the Washan iron deposit, the general characteristics of magnetite include primary textures (mineral inclusions), different styled oxy-exsolutions of ilmenite, and re-equilibrium textures. These magnetite samples from various types of iron ore have been classified into distinct groups based on their textural and compositional characteristics. To identify magnetite types within each iron ore, the magnetite types are labeled by appending the iron ore name with texture types, which are sequenced by their generation from early to late (Type A, B, C, D, E, and F) (Figure 4).

In the deep veins of Washan, Type A magnetite is the most abundant, characterized by ilmenite exsolution lamellae with abundant mineral inclusions that are randomly distributed (Figure 4A). Type A magnetite also shows inclusion-free magnetite rims, which defines Type B magnetite (Figure 4B). Type E magnetite is distinguished by its pure ilmenite exsolution lamellae with minor mineral inclusions (Figure 4E). In the chambered ore body from the middle level, three representative magnetite textures are identified: Type A with ilmenite exsolution lamellae and abundant mineral inclusions, Type C with ilmenite exsolution lamellae forming trellis textures, and Type D with intensified trellis-textured ilmenite exsolution lamellae and lacking mineral inclusions. In the marginal zone at the shallow level, Type F magnetite is characterized by dense sets of ilmenite exsolution lamellae, which are associated with low-Ti content.

4.2. Major Elements of Magnetite

The EPMA analysis results for different types of ores are reported in

Table 1. The major elements and calculated crystallization temperatures of magnetites in different types of iron ores from the Washan iron deposit, Ningwu district, China.

Sample	Magnetite Grains	Iron Ore Types	Magnetite Types	SiO2	TiO2	Cr2O3	Al2O3	ZnO	NiO	Fe2O3	FeO	MnO	V2O3	CoO	Na2O	MgO	Total	XMg	T-min	T-max	T-every
AS02A	AS02A-MAG-1	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type E	0.03	0.14	0.01	0.18	0.03	0.04	68.25	30.91	0.08	0.22	0.08	0.01	0.03	100.00	0.00	431	452	441
AS02A	AS02A-MAG-2	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type E	0.06	0.42	0.03	0.69	0.0050	0.01	67.18	31.22	0.04	0.19	0.05	0.05	0.06	100.00	0.00	479	499	489
AS02A	AS02A-MAG-3	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type E	0.03	0.59	0.02	0.67	0.01	0.02	66.70	31.46	0.06	0.29	0.10	0.02	0.02	100.00	0.00	412	433	423
AS02A	AS02A-MAG-4	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type B	0.02	0.62	0.03	0.69	0.14	BDL	66.65	31.41	0.02	0.25	0.12	BDL	0.05	100.00	0.00	475	495	485
AS02A	AS02A-MAG-5	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type B	0.04	0.38	0.08	0.66	0.13	0.01	67.04	31.24	0.02	0.24	0.13	BDL	0.03	100.00	0.00	440	461	451
AS02A	AS02A-MAG-6	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type B	0.03	0.48	0.05	0.51	0.03	BDL	67.09	31.36	0.01	0.28	0.13	0.02	0.01	100.00	0.00	351	373	362
AS02A	AS02A-MAG-7	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type B	0.04	0.37	0.05	0.13	BDL	0.07	67.74	31.16	0.04	0.23	0.12	BDL	0.04	100.00	0.00	458	478	468
AS02A	AS02A-MAG-8	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type A	0.03	0.42	0.04	0.47	0.06	BDL	67.23	31.34	0.02	0.26	0.11	BDL	0.02	100.00	0.00	417	438	428
AS02A	AS02A-MAG-9	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type E	0.03	0.58	0.05	0.24	0.01	0.04	67.19	31.47	BDL	0.24	0.13	BDL	0.01	100.00	0.00	378	400	389
AS02A	AS02A-MAG-11	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type B	0.03	0.25	0.06	0.28	0.10	BDL	67.77	31.11	BDL	0.27	0.11	BDL	0.03	100.00	0.00	435	456	445
AS02A	AS02A-MAG-10	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type B	0.03	0.29	0.03	0.51	0.05	0.01	67.56	31.14	0.08	0.17	0.09	BDL	0.04	100.00	0.00	458	478	468
AS02A	AS02A-MAG-12	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type B	0.23	0.0016	0.04	0.07	0.04	BDL	68.07	31.06	0.06	0.25	0.14	BDL	0.04	100.00	0.00	454	475	464
AS02A	AS02A-MAG-13	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type B	0.02	0.13	0.07	0.03	BDL	0.0049	68.33	31.07	0.03	0.22	0.09	BDL	BDL	100.00	0.00	BDL	BDL	BDL
AS02A	AS02A-MAG-14	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type C	0.08	0.56	0.05	0.26	0.16	0.07	67.12	31.28	BDL	0.24	0.13	0.0044	0.04	100.00	0.00	463	483	473
AS02A	AS02A-MAG-15	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type E	0.07	0.11	0.05	0.02	0.09	0.0048	68.49	30.70	0.03	0.23	0.10	0.06	0.04	100.00	0.00	448	468	458
AS02A	AS02A-MAG-16	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type E	0.07	0.35	0.03	0.36	0.06	BDL	71.78	25.72	0.02	0.19	0.09	1.30	0.03	100.00	0.00	433	454	444
AS02A	AS02A-MAG-17	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type E	0.03	0.19	0.05	0.12	0.09	0.02	68.23	31.00	BDL	0.18	0.07	0.02	0.0009	100.00	0.00	269	291	280
AS02A	AS02A-MAG-18	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type B	0.09	0.97	0.06	0.51	0.17	BDL	66.22	31.55	0.03	0.23	0.07	0.06	0.04	100.00	0.00	457	477	467
AS02A	AS02A-MAG-19	The pyrite-bearing brecciated iron ores from the brecciated zones	AS02A-Type A	0.08	0.47	0.08	0.15	0.01	BDL	67.54	31.20	0.03	0.25	0.12	0.04	0.04	100.00	0.00	452	473	462
AS02B	AS02B-MAG-1	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type C	0.07	0.74	0.07	0.53	BDL	0.01	66.51	31.58	0.13	0.21	0.04	BDL	0.10	100.00	0.00	520	538	529
AS02B	AS02B-MAG-2	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type A	0.09	1.31	0.05	0.58	0.05	0.08	65.35	32.08	0.09	0.18	0.07	BDL	0.07	100.00	0.00	492	511	501
AS02B	AS02B-MAG-3	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type D	0.06	0.41	0.11	0.43	BDL	BDL	67.28	31.34	0.02	0.18	0.08	BDL	0.08	100.00	0.00	507	526	517
AS02B	AS02B-MAG-4	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type C	0.13	0.53	0.05	0.53	0.08	BDL	66.99	31.06	0.09	0.18	0.17	0.04	0.15	100.00	0.00	554	572	563
AS02B	AS02B-MAG-5	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type A	0.07	0.42	0.05	0.30	BDL	BDL	67.41	31.42	0.01	0.20	0.09	BDL	0.02	100.00	0.00	417	438	427
AS02B	AS02B-MAG-6	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type D	0.04	0.58	0.04	0.71	0.17	BDL	66.76	31.19	0.09	0.18	0.11	BDL	0.13	100.00	0.00	546	564	555
AS02B	AS02B-MAG-7	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type A	0.05	1.49	0.10	0.51	0.11	0.04	65.14	32.08	0.07	0.23	0.12	0.02	0.04	100.00	0.00	455	476	466
AS02B	AS02B-MAG-8	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type A	0.05	2.24	0.12	0.94	0.02	BDL	63.70	31.49	0.16	0.17	0.13	0.01	0.98	100.00	0.02	755	763	759
AS02B	AS02B-MAG-9	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type D	0.25	0.96	0.22	0.72	0.05	BDL	65.38	31.89	0.02	0.19	0.10	BDL	0.21	100.00	0.00	586	602	594
AS02B	AS02B-MAG-10	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type A	0.07	1.24	0.14	0.56	0.14	BDL	65.42	32.08	0.03	0.21	0.06	BDL	0.05	100.00	0.00	471	492	482
AS02B	AS02B-MAG-11	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type A	0.18	1.27	0.06	0.63	0.08	BDL	65.20	32.23	0.02	0.18	0.05	0.01	0.11	100.00	0.00	529	548	539
AS02B	AS02B-MAG-12	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type B	0.06	0.98	0.05	0.49	0.0037	0.04	66.20	31.95	0.02	0.10	0.10	BDL	BDL	100.00	0.00	BDL	BDL	BDL
AS02B	AS02B-MAG-13	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type A	0.03	1.03	0.06	0.72	0.12	0.01	65.93	31.79	BDL	0.18	0.07	0.03	0.04	100.00	0.00	464	484	474
AS02B	AS02B-MAG-14	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type B	0.04	0.54	0.06	0.35	0.05	BDL	67.26	31.38	0.05	0.16	0.09	0.02	0.01	100.00	0.00	350	372	361
AS02B	AS02B-MAG-15	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type C	0.06	0.62	0.06	0.55	0.04	BDL	66.81	31.31	0.08	0.23	0.11	0.01	0.13	100.00	0.00	543	561	552
AS02B	AS02B-MAG-16	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type D	0.10	0.57	0.07	0.54	0.05	0.03	66.82	31.47	0.03	0.15	0.10	BDL	0.07	100.00	0.00	494	513	503
AS02B	AS02B-MAG-17	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type E	0.08	0.30	0.05	0.19	0.04	BDL	67.76	31.23	BDL	0.21	0.11	0.0022	0.04	100.00	0.00	452	473	462
AS02B	AS02B-MAG-18	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type B	0.05	1.08	0.03	0.46	0.0012	BDL	66.04	31.79	0.17	0.21	0.10	0.01	0.07	100.00	0.00	495	514	505
AS02B	AS02B-MAG-19	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type E	0.10	0.28	0.11	0.46	0.06	0.02	67.49	31.04	0.10	0.14	0.10	0.02	0.07	100.00	0.00	497	516	506
AS02B	AS02B-MAG-20	The apatite-bearing massive iron ores from the chambered ore body	AS02B-Type E	0.08	0.33	0.03	0.72	0.03	BDL	67.18	31.20	0.04	0.19	0.13	0.02	0.06	100.00	0.00	487	506	496
AS03B	AS03B-MAG-1	The coarse iron ores within deep veins	AS03B-Type E	0.05	0.23	0.06	0.27	0.02	BDL	68.02	30.96	0.01	0.15	0.13	0.04	0.05	100.00	0.00	467	487	477
AS03B	AS03B-MAG-2	The coarse iron ores within deep veins	AS03B-Type E	0.03	0.33	0.08	0.33	0.05	BDL	67.62	31.19	0.08	0.18	0.13	BDL	BDL	100.00	0.00	BDL	BDL	BDL
AS03B	AS03B-MAG-4	The coarse iron ores within deep veins	AS03B-Type E	0.08	0.07	0.06	0.06	0.06	BDL	68.39	31.01	0.02	0.14	0.10	BDL	0.01	100.00	0.00	393	415	404
AS03B	AS03B-MAG-5	The coarse iron ores within deep veins	AS03B-Type E	0.04	0.35	0.08	0.50	BDL	BDL	67.37	31.07	0.09	0.25	0.11	BDL	0.15	100.00	0.00	553	571	562
AS03B	AS03B-MAG-6	The coarse iron ores within deep veins	AS03B-Type A	0.03	1.03	0.06	0.66	BDL	0.04	66.05	31.55	0.05	0.21	0.10	0.03	0.19	100.00	0.00	575	592	583
AS03B	AS03B-MAG-7	The coarse iron ores within deep veins	AS03B-Type E	0.08	0.47	0.07	0.46	0.07	BDL	67.12	31.27	0.09	0.20	0.11	0.01	0.05	100.00	0.00	475	495	485
AS03B	AS03B-MAG-8	The coarse iron ores within deep veins	AS03B-Type A	0.06	0.96	0.03	0.60	0.12	BDL	66.06	31.65	0.09	0.27	0.05	0.02	0.09	100.00	0.00	517	536	527
AS03B	AS03B-MAG-8RM	The coarse iron ores within deep veins	AS03B-Type B	0.01	0.66	0.06	0.56	0.03	BDL	66.86	31.18	0.06	0.21	0.10	BDL	0.27	100.00	0.01	608	624	616
AS03B	AS03B-MAG-9	The coarse iron ores within deep veins	AS03B-Type A	0.07	1.39	0.04	0.76	0.0037	BDL	65.07	32.14	0.12	0.18	0.10	BDL	0.13	100.00	0.00	541	559	550
AS03B	AS03B-MAG-10	The coarse iron ores within deep veins	AS03B-Type E	0.08	0.73	0.04	0.49	0.04	0.02	66.57	31.19	0.17	0.29	0.15	BDL	0.23	100.00	0.00	592	608	600
AS03B	AS03B-MAG-10RM	The coarse iron ores within deep veins	AS03B-Type A	0.06	1.19	0.03	0.72	0.11	0.03	65.69	31.56	0.09	0.20	0.09	0.05	0.18	100.00	0.00	572	589	580
AS03B	AS03B-MAG-11	The coarse iron ores within deep veins	AS03B-Type A	0.04	2.11	0.03	0.75	0.01	0.02	63.90	32.07	0.28	0.23	0.10	BDL	0.46	100.00	0.01	665	678	671
AS03B	AS03B-MAG-12	The coarse iron ores within deep veins	AS03B-Type A	0.03	1.65	0.05	0.66	0.12	BDL	64.95	31.67	0.30	0.12	0.08	BDL	0.37	100.00	0.01	641	655	648
AS03B	AS03B-MAG-15	The coarse iron ores within deep veins	AS03B-Type A	0.02	1.30	0.03	0.65	0.01	BDL	65.57	31.47	0.22	0.24	0.12	BDL	0.37	100.00	0.01	640	654	647
AS03B	AS03B-MAG-16	The coarse iron ores within deep veins	AS03B-Type D	0.03	0.29	0.0038	0.72	0.02	BDL	67.35	31.02	0.06	0.23	0.14	0.01	0.13	100.00	0.00	541	559	550
AS03B	AS03B-MAG-17	The coarse iron ores within deep veins	AS03B-Type C	0.04	0.80	0.04	0.58	0.07	BDL	66.47	31.59	0.05	0.17	0.12	BDL	0.07	100.00	0.00	493	512	503
AS03B	AS03B-MAG-19	The coarse iron ores within deep veins	AS03B-Type A	0.06	1.02	0.08	0.57	BDL	0.03	66.02	31.53	0.11	0.22	0.17	0.02	0.16	100.00	0.00	562	579	570
AS03B	AS03B-MAG-20(18rm)	The coarse iron ores within deep veins	AS03B-Type B	0.16	0.65	0.11	0.19	0.05	BDL	66.92	31.54	0.08	0.12	0.12	BDL	0.06	100.00	0.00	489	509	499
AS03B	AS03B-MAG-21(18rm)	The coarse iron ores within deep veins	AS03B-Type B	0.39	0.50	0.06	0.27	0.03	BDL	66.69	31.60	0.01	0.18	0.09	0.02	0.17	100.00	0.00	564	582	573
AS03B	AS03B-MAG-22(18rm)	The coarse iron ores within deep veins	AS03B-Type B	0.47	0.42	0.06	0.30	BDL	BDL	66.57	31.73	0.04	0.20	0.08	0.01	0.14	100.00	0.00	551	569	560
AS05	AS05-MAG-1	The fine-grained massive iron ores	AS05-Type F	1.86	0.24	0.17	0.10	0.05	0.01	63.89	33.30	0.26	0.10	BDL	0.02	BDL	100.00	0.00	BDL	BDL	BDL
AS05	AS05-MAG-2	The fine-grained massive iron ores	AS05-Type F	3.32	0.21	0.12	0.07	BDL	BDL	60.74	35.31	0.11	0.07	BDL	0.04	0.0027	100.00	0.00	313	335	324
AS05	AS05-MAG-3	The fine-grained massive iron ores	AS05-Type F	2.22	0.10	0.10	0.01	BDL	0.03	63.56	33.76	0.10	0.07	0.01	0.04	BDL	100.00	0.00	BDL	BDL	BDL
AS05	AS05-MAG-4	The fine-grained massive iron ores	AS05-Type F	0.32	0.20	0.09	BDL	0.04	BDL	67.77	31.40	0.03	0.08	0.01	0.04	BDL	100.00	0.00	BDL	BDL	BDL
AS05	AS05-MAG-5	The fine-grained massive iron ores	AS05-Type F	2.24	0.25	0.10	0.11	BDL	BDL	62.94	33.46	0.74	0.12	0.02	BDL	0.01	100.00	0.00	374	396	385
AS05	AS05-MAG-6	The fine-grained massive iron ores	AS05-Type F	1.97	0.25	0.06	0.25	0.06	0.04	63.53	33.63	0.08	0.10	BDL	0.02	BDL	100.00	0.00	BDL	BDL	BDL
AS05	AS05-MAG-7	The fine-grained massive iron ores	AS05-Type F	2.52	0.21	0.07	0.20	BDL	BDL	62.41	34.26	0.18	0.10	BDL	0.03	0.03	100.00	0.00	434	454	444
AS05	AS05-MAG-8	The fine-grained massive iron ores	AS05-Type F	4.55	0.26	0.09	0.14	0.03	BDL	57.63	37.12	0.09	0.08	BDL	0.01	BDL	100.00	0.00	BDL	BDL	BDL
AS05	AS05-MAG-9	The fine-grained massive iron ores	AS05-Type F	0.19	0.10	0.04	0.01	BDL	BDL	68.34	31.13	0.05	0.07	BDL	0.04	0.01	100.00	0.00	386	408	397
AS05	AS05-MAG-10	The fine-grained massive iron ores	AS05-Type F	2.31	0.27	0.04	0.16	BDL	BDL	62.79	33.68	0.56	0.14	0.01	0.02	0.01	100.00	0.00	387	409	398
AS05	AS05-MAG-11	The fine-grained massive iron ores	AS05-Type F	3.13	0.25	0.09	0.17	BDL	BDL	60.92	35.11	0.21	0.08	BDL	0.02	0.0009	100.00	0.00	269	292	280
AS05	AS05-MAG-12	The fine-grained massive iron ores	AS05-Type F	6.98	0.20	0.08	0.23	BDL	BDL	52.18	38.76	1.40	0.07	BDL	0.06	0.04	100.00	0.00	454	475	465
AS05	AS05-MAG-13	The fine-grained massive iron ores	AS05-Type F	1.14	0.19	0.24	0.01	0.05	BDL	65.57	32.59	0.05	0.14	BDL	0.01	BDL	100.00	0.00	BDL	BDL	BDL
AS05	AS05-MAG-14	The fine-grained massive iron ores	AS05-Type F	5.47	0.27	0.06	0.21	0.03	BDL	55.49	37.96	0.29	0.11	0.05	0.05	0.01	100.00	0.00	389	411	400
AS05	AS05-MAG-15	The fine-grained massive iron ores	AS05-Type F	1.65	0.33	0.12	0.24	BDL	BDL	64.09	33.38	0.07	0.08	0.01	0.02	0.01	100.00	0.00	344	366	355
AS05	AS05-MAG-16	The fine-grained massive iron ores	AS05-Type F	4.17	0.30	0.09	0.18	BDL	0.04	58.43	35.43	1.23	0.08	0.02	0.02	0.0027	100.00	0.00	314	336	325
AS05	AS05-MAG-17	The fine-grained massive iron ores	AS05-Type F	6.14	0.26	0.07	0.43	0.05	0.03	53.73	37.71	1.44	0.07	0.01	0.04	0.02	100.00	0.00	423	444	434
AS05	AS05-MAG-18	The fine-grained massive iron ores	AS05-Type F	3.07	0.23	0.14	0.20	0.05	BDL	61.06	35.07	0.04	0.10	BDL	0.04	BDL	100.00	0.00	BDL	BDL	BDL
AS05	AS05-MAG-19	The fine-grained massive iron ores	AS05-Type F	1.20	0.23	0.20	0.15	BDL	0.02	65.37	32.63	0.03	0.11	0.01	0.04	0.01	100.00	0.00	358	380	369
AS05	AS05-MAG-20	The fine-grained massive iron ores	AS05-Type F	0.18	0.16	0.10	0.07	0.07	0.01	68.09	31.07	0.10	0.10	0.0024	0.04	0.0036	100.00	0.00	324	346	335

BDL: the element is below detection limit. The detection limits are based on Jianxiong Zhou’s standard sample group (GB/T 17359-1998: oxide-36, silicate-36, sulfide-12, ultra-light element-9, rare earth pentaphosphate-15) and the American SPI International Sample Group (02753-AB 53 Minerals standard), including Mg (periclase, 106 ppm, 15 s), Al (Kyan, 87 ppm, 10 s), Ca (Wo, 111 ppm, 15 s), Ti (Rut, 143 ppm, 15 s), V (V-P glass, 231 ppm, 30 s), Co (CoO, 206 ppm, 30 s), Ni (Oli, 207 ppm, 30 s), Na (Jad, 132 ppm, 10 s), Si (Qtz, 78 ppm, 15 s), Cr (Cr-Sp, 206 ppm, 10 s), Fe (Hem, 169 ppm, 15 s), Zn (ZnWO₄, 256 ppm, 15 s), and Mn (Mn-Fe pyrope, 201 ppm, 10 s).

. The major element compositions of magnetite from various deposits exhibit significant variations, reflecting various contents associated with iron ore types (Figure 5). In coarse iron ores (AS03B), the magnetite is characterized by high-Ti content, with TiO₂ concentrations ranging from 0.07 to 2.11 wt%. The contents of Zn, Ni, and Cr are relatively low, typically less than 0.1 wt%, while the average Al₂O₃ concentrations can reach up to 0.5 wt%. The contents of MnO (0.1%) and CoO (0.11%) are relatively high compared with iron ore from shallow levels (AS02). In massive iron ores (AS02B), the magnetites display increasing Ti (TiO₂: 0.84%) and Cr contents. The Cr content average is about 0.08 wt%, with notable enrichments in Si (SiO₂: 0.08%), Al (Al₂O₃: 0.55%), and V (V₂O₃: 0.18%). The contents of these elements increase with depth, indicating a deeply originated high-V-Ti source. The Na, Ni, and Co concentrations generally are similar with those of coarse iron ores (AS03B). In brecciated iron ores (AS02A), magnetite compositions exhibit a considerable decrease in Ti content. The TiO₂ content is relatively low, averaging 0.38 wt%, with a notable decrease in major elements such as Al (Al₂O₃: 0.34%), Cr (Cr₂O₃: 0.05%), and Mn (MnO: 0.03%). The Ni (NiO: 0.02%), V (V₂O₃: 0.23%), and Na (Na₂O: 0.08%) contents of magnetite are larger compared with massive ores. In fine-grained massive iron ores (AS05), the depletion in Ti contents (TiO₂: 0.23%) is significant compared with other types of iron ores, with a considerable enrichment in Si content (SiO₂: 2.73%).

The Wavelength Dispersive Spectroscopy (WDS) mapping of the A05 magnetite sample reveals detailed compositional changes between different major elements within individual magnetite grains (Figure 6). The mapping of Ni and Mg in the A05 magnetite grain show sharp, stable edges, suggesting that these elements remained unchanged post-crystallization. The V and Cr contents exhibited a continuous decrease toward the edges of the magnetite, indicating element activation triggered by magmatic–hydrothermal processes. The distribution of Mn and Ti within the core of the A05 magnetite followed the direction of the dissolved ilmenite lamellae while exhibiting a clear alteration style at the edge, with Mn significantly depleted and Ti enriched.

4.3. Calculated Crystallization Temperatures of Magnetite

The calculated crystallization temperatures of magnetite from the Washan iron deposits are listed in Table 1. The coarse iron ores within deep veins (AS03B) exhibited the highest temperatures. The results of different magnetite types spanned from approximately 597 °C to 503 °C, with average values of ~558 °C. The apatite-bearing massive iron ores from the chambered ore body (AS02B) exhibit temperatures from ~521 °C to 488 °C, with an average temperature of 515 °C. The pyrite-bearing brecciated iron ores from the brecciated zones (AS02A) recorded temperatures ranging from ~473 °C to 418 °C, with averages for different magnetite generations of about ~439 °C. The fine-grained massive iron ores (AS05) from marginal zone yield the lowest temperatures, at ~378 °C.

5. Discussion

5.1. A Transitional Mineralizing Process Based on Textural Features of Magnetites from Different Generations

In the Ningwu basin, the porphyrite iron deposits exhibit diverse mineralization types, and their genetic types are still under discussion. In Washan, various types of iron ores are found within ore bodies from different structural levels. The occurrence of the ore bodies, combined with the compositional and textural features of magnetites within them, are closely associated with the depth.

In the deep levels, the coarse iron ores (AS03B) occur within veins penetrating sub-volcanic rocks, which is different from the apatite-bearing massive iron ores (AS02B). The ilmenite exsolution lamellae in Type A magnetite (Figure 4A) contains abundant magnetite-hosted mineral inclusions. The ilmenite exsolution lamellae within magnetite grains are commonly found in magnetites from Fe-Ti-V oxide deposits [46,47], and are primarily influenced by the high-temperature conditions (550° to 1000 °C) during the exsolution process of the original magnetite-ulvöspinel solid solution [48,49]. Therefore, the textural features in Type A magnetites from coarse iron ores suggest that they were crystallized from a high-medium-temperature magmatic environment. The transition from magmatic to hydrothermal genesis of these magnetites is clearly demonstrated by the inclusion-free rim (Type B) observed in Type A magnetites (Figure 4B) and the presence of low-Ti magnetites (Ti: 0.3%–0.36%) exhibiting the growth of ilmenite exsolution lamellae (Type E) (Figure 4E). These observations suggest interactions between magnetites and hydrothermal fluids with subsequent re-equilibration processes, which are typically found in IOA deposits [40] and porphyry deposits [50]. The Ti contents of magnetites (TiO₂: 0.81%) from coarse iron ores are same as those of andesitic rocks (TiO₂: 0.81%) [1]. Combining with their highest concentrations of MnO and lowest concentrations of Na₂O among all samples, these magnetites exhibit an early-stage magmatic–hydrothermal environment with a growing exsolution of saline from magma. In the Ti vs. V diagram (Figure 7A), these transitional geneses can be clearly traced by two evolving paths among different types of iron ores. The first path initiates from igneous origins (AS03B) and evolves into magmatic–hydrothermal origins (AS02B, AS02A), while the other path migrates into the hydrothermal field (AS05), which is commonly seen in IOCG deposits [51].

In the middle level, the apatite-rich massive iron ores (AS02B) occurred in the fractured zones related to the annealing process of sub-volcanic rocks. They formed dyke-controlled ore bodies with flow-like styles, and their formation is proposed to result from the floatation of high-temperature iron-rich fluids or melts [1,42,43]. Similar geneses have also been revealed in massive iron ores, which have been nominated as magma-type iron ores [1], from the Baixiangshan and Gushan iron deposits in the Ningwu district. Their lava flow-like ore bodies, vesicle-structured iron ores, as well as a significant enrichment in P and HFSE in iron ores, are interpreted as characteristics of magmatic genesis related to the immiscibility of iron-rich melts [20,54]. Apart from Type A magnetites, the trellis-textured ilmenite exsolution lamellae developed on magnetites with abundant inclusions (Type C, Figure 4C) from apatite-rich massive iron ores. Although the trellis-texture is a common feature in magmatic magnetites from igneous rocks and Fe-Ti-V deposits related to mafic intrusions [55,56]; experiments have also proved that they can be generated by the exsolution of ilmenite from high-Ti magnetites in a magmatic system with the increasing participation of fluids during the cooling process [48]. Therefore, this type of texture exhibits transitional geneses during the evolving mineralization process. In these iron ores, the trellis textures become intensified on inclusion-free Type D magnetites (Figure 4D), indicating a decrease in ore-forming temperature and pressure [50]. In the subaerial zones, the pyrite-bearing brecciated iron ores (AS02A), with the classic mineral assemblage of apatite–(diopside)–actinolite–magnetite that can be compared with IOA deposits, primarily overlay on the massive iron ores (AS02B). These iron ores are characterized by magnetites of low-Ti (TiO₂: 0.39%) inclusion-free rims (Type B, Figure 4B) and linear exsolution lamellae of ilmenite on low-Ti magnetites (TiO₂: 0.34%) (Type E, Figure 4E), which indicate the fluid-assisted re-equilibration effect overprinted on magnetite grains during the cooling process [57,58]. Due to the high mobility of Cr⁶⁺ in fluids, its high K_D value in augite, magnetite in Kiruna-type IOA deposits often show high V and low Cr contents in the V vs. Cr diagram [53], which can be used to effectively differentiate IOA from other types of magmatic–hydrothermal iron deposits. The magnetites show two contrasting genetic evolution paths in this diagram. The first path initiates from porphyry fields with the highest contents of V and Cr in magnetites from massive iron ores (AS02B), and transitions into the field of IOCG deposits, where the V and Cr contents of magnetites vary dramatically. In contrast, the second path migrates into the field of Kiruna-type IOA deposits (Figure 7B). The considerable spreading plots in the IOCG zone (samples from AS05) might be attributed to the significant intensifying fluid activities in their mineralization process.

In the shallow level, fine-grained massive iron ores (AS05) from the marginal zone of the Washan deposit is distinguished by its low Ti (TiO₂: 0.23%). The dense sets of ilmenite exsolution lamellae on magnetites (Type F magnetite) suggest considerable cooling conditions that may lead to increasing proportions of interstitial ilmenite in magnetites [48]. The lowest Ti contents in magnetites, by high-Ti and low-Mn contents, along with the corroded rim of the Type F magnetite (Figure 6), indicate a cooling process with intensifying alterations. Ni and Cr can serve as indicators of a magmatic environment due to their high compatibility with magmatic magnetites, and they become mobile when fluid activities increase [59,60]. Although the Type F magnetites show hydrothermal-dominated genesis, their highest Cr contents (Cr₂O₃: 0.1%) among all samples, combined with their comparable values in Cr₂O₃ with those of late-generation magnetites (Type D) from massive iron ores, exhibit magmatic origins inherited from a multistage transitional mineralization.

The textural and compositional features of magnetites from different types of iron ores in Washan suggest that they are genetically related and formed from a transitional metallogenic system, where they were produced through two potential evolving paths from magmatic to hydrothermal origins. Their genetic evolution trends, according to the two paths, are closely related to the structural levels of mineralization and can be identified by the Ti contents in magnetites.

5.2. Evolution of Magnetites from Different Generations

5.2.1. The Thermal Evolution of Magnetites

The temperature data obtained by using the magnetite T_Mg-mag thermometer indicate a cooling process from a deep magmatic environment to shallow magmatic–hydrothermal conditions. The temperatures of different groups of magnetites correlate to their geneses, the mineralization style of iron ores, and the spatial position of iron ore bodies.

The crystallization temperatures of different types of magnetites from deep to shallow show two possible cooling paths. The first path is illustrated by decreasing temperature spans from magnetites in vein-hosted massive iron ores (~558 °C, Figure 8 AS03B), to apatite-rich massive iron ores (~515 °C, Figure 8 AS02B) and pyrite-bearing brecciated iron ores (~439 °C, Figure 8 AS02A). This cooling path is combined with the transition of magnetite genesis from a porphyry field to a Kiruna-type IOA field (Figure 6B), which is similar to those of the magnetites from IOA deposits developed on the top of the sub-volcanic intrusions in Andean [61]. The textural features of magnetites from iron ores with the highest temperatures (AS03B), characterized by ilmenite exsolution lamellae within magnetite grains (Type A magnetite), exhibit magmatic–hydrothermal conditions. This transitional condition then became hydrothermal-predominate under medium to low temperatures, as evidenced by the developed inclusion-free rim (Type B magnetite) and gradual growth of ilmenite exsolution lamellae (Type E magnetite) on magnetites from early generations during the cooling process. Although the thermal evolution of magnetites from different types of iron ores generally demonstrates a positive correlation with their decreasing Ti contents, magnetites from apatite-rich massive iron ores (AS02B) exhibit a reverse relationship. The magnetites from vein-hosted massive iron ores show a moderate Ti content (TiO₂: 0.81%) similar to dioritic porphyrite (Ti: 0.81%) [1] in high-medium temperatures (~671–404 °C), while the apatite-rich massive iron ores show TiO₂ concentrations reaching 0.83% with fluctuating spans of temperatures (~759–361 °C). The other path is characterized by the decreasing temperatures from apatite-rich massive iron ores (AS02B) (~515 °C) to fine-grained massive iron ores (AS05) (~439 °C). Magnetites from fine-grained massive iron ores yield the lowest crystallization temperatures (~489–280 °C). The magnetites from apatite-rich massive iron ores and fine-grained massive iron ores overlapped in temperatures from 361 to 489 °C, exhibiting a genetic transition from the porphyry field into the IOCG field (Figure 7B). Their low-Ti content, the dense sets of ilmenite exsolution lamellae, and altered rims of low Ti and Mn contents, reveal intensifying hydrothermal conditions.

Within each type of iron ore that occurred in a specific structural level, the crystallization temperatures vary between different types of magnetites. The early-formed magmatic magnetite (Type A, B, and C) from vein-hosted massive iron ores (AS03B) exhibits gradually decreasing temperatures from ~597 °C to 503 °C, while magnetites from late generations (Type D) show higher temperatures that increase to ~550 °C. The fluctuations in cooling temperatures among different generations of magnetites might suggest the multistage upwelling of ore-forming fluids from deep levels. The heating in the late generations of magnetites is also present in apatite-rich massive iron ores (AS02B); however, Type C magnetite yield temperatures (548 °C) were higher than Type A magnetites (521 °C), indicating a variation in the mineralizing effect in contrast with the hydrothermal intensifying geneses during the cooling process. Cr can serve as indicator of a magmatic environment due to its high compatibility with magmatic magnetites [61,62]. Combing through the increasing Cr contents (0.04%–0.11%) from Type B to Type D magnetites, the reverse thermal trends in late generations of magnetites probably suggest a pulsing process of ore-forming fluids carrying pristine magnetites from deep levels. The fluctuations in temperature can be also seen in magnetites from the pyrite-bearing brecciated iron ores (AS02A), which support the possible pulsing process of ore-forming fluids.

5.2.2. The Reconstruction of the Ore-Forming Process Based on the Thermal Features of Magnetites

Pristine magnetite is susceptible to modification by hydrothermal metasomatism, which leads to the mingled genetic features spreading from the Fe-Ti-V field to the IOCG field on the Ti + V vs. Mg + Al diagram for most of the IOA deposits [7,62,63,64]. When fluids are involved in the formation of magnetite, they can cause the oxy-exsolution of ilmenite on magnetite grains and the formation of hydrothermal magnetites individually [60] or on the rims of early-generation magnetites under evolved ore-forming conditions [53]. However, these modifications to the composition of magnetites can be distinguished by their different generations. Therefore, by reconstructing the evolution sequences of various types of magnetites, their actual geneses can be revealed, and their evolving path might provide new insights into the evolution of associated mineralizing mechanisms. The evolution paths of magnetites from different types of iron ores in the Washan deposit have been restructured by their crystallization temperatures (Figure 9). The two evolution paths can be identified by their distinct genetic trends that initiated from the porphyry field. However, the iron ores from the deep level (AS03B and AS02B) demonstrated similar evolving paths, crossing the Fe-Ti-V field and the porphyry field. Their associated Ti contents are similar to those from igneous environments (Ti: 0.81%) [1], and their magmatic–hydrothermal textual features, characterized by developed ilmenite exsolution lamellae, imply a potentially different mineralizing effect compared with the above genetic types (IOA and IOCG-Skarn) at the end of evolution paths. Compared with two different types of iron ores from Washan [43], which are the disseminated ores and ores from magnetite–actinolite veins, the iron ores collected in this study exhibit different compositional characteristics. These two types of iron ores from works by Duan et al. (2019) show magmatic origins with genetic features like Fe-Ti-V deposits [43]. The four types of iron ores in this study, without the above two types of iron ores, exhibit magmatic–hydrothermal origins with genetic features spreading from Fe-Ti-V deposits to porphyry- and Kiruna-type IOA deposits. In Kiruna-type IOA deposits, individual magnetite grains across all samples may exhibit significant variability in the concentrations of major and trace elements such as Ti, Al, and Mn. These variations reflect a variety of genetic features associated with different deposit types, including Fe-Ti-V deposits, Kiruna-type IOA deposits, IOCG deposits, and skarn deposits [38,40,53]. In the study of the El Laco iron deposit, it was observed that the major element compositions of magnetite (especially Ti, V, Al, and Mn concentrations) are closely related to its formation depth, with distinct surface-to-depth variations [65]. This relationship reflects the transformation of the mineralization process, resulting in the evolutionary history of magnetites from magmatic to magmatic–hydrothermal conditions. Furthermore, studies of the Meishan porphyrite iron deposits have shown that magnetite samples collected from different depths (between 717m and 1286m in the same drill core) exhibit varying compositions and genetic features associated with Fe-Ti-V and porphyry deposits [66]. Therefore, the differences in the compositional features of magnetites among or within various types of iron ores from the Washan porphyrite iron deposit might be attributed to the ore-forming conditions at specific sampling depths and the different evolutionary histories of magnetite. This might lead to compositional variations in magnetites under evolving conditions from magmatic to magmatic–hydrothermal. Given that there are limited in-situ EMPA studies on magnetites from Washan, further investigation is required to gain a more comprehensive understanding of these aspects in future studies. In studies on major element compositions of magnetite from Kiruna-type IOA deposits, the contents of NiO and MnO have been observed to be below 0.01 wt% or below the detection limit, as noted in references [30,40,53]. The magnetites in these deposits exhibit compositional characteristics of magmatic–hydrothermal origin, with genetic features associated with porphyry, Kiruna-type IOA, and IOCG deposits, which are consistent with those observed in the magnetites analyzed in this study. Additionally, the low contents of NiO and MnO observed in the Meishan porphyrite iron deposit [66] further support the findings on the concentrations of these elements in the magnetites presented in this study.

The evolution paths of different generations of magnetites from coarse iron ores (AS03B: From Type A, to B, D, E, and C) and apatite-bearing massive iron ores (AS02B: From Type A, to C, D, E, and B) generally are accordant with the cooling process but exhibit differences in the formation sequence of re-equilibration textures. The Type B magnetites characterized by inclusion-free rims from AS03B show an earlier presence than those from AS02B. As discussed above, the presence of the exsolution of ilmenite textures on magnetites points to a cooling condition [48], while purified rims indicate the effect from hydrothermal fluids due to alteration [58]. Combined with the subsequently developed exsolution lamellae textures (Type C, D, and E), the different sequences of magnetites in these paths implies a dynamic environment potentially linked to the episodic upwelling of ore-forming fluids. The earlier-occurring Type B magnetites in the deep level (AS03B) under higher temperatures probably suggest an early stage of a magmatic–hydrothermal environment producing abundant iron-rich fluids, which might play an important role in the iron-rich mechanism. A recent study on fluid inclusions within apatite from coarse-grained iron ores in Washan and Taocun deposits indicates that the exsolution of hydrosaline liquid and vapor from sub-volcanic intrusions (diorite porphyrite) played a crucial role in their formation [41]. By facilitating the transportation and accumulation of metals within magmatic–hydrothermal systems, the upwelling process of hydrosaline fluids might efficiently have led to the formation of the Washan porphyrite iron deposits. This evidence suggests that the mineralization that occurred in the subaerial zone of Washan exhibits genetic affinities with IOCG deposits.

Porphyrite iron deposits are characterized by the common occurrence of magma-type iron ores, which exhibit indicative ore geology features, including mostly layered massive ore bodies with stable ductility, the directional presence of vesicular structures in iron ores, magnetite lava flows, minerals with high melting points (apatite and diopside), developed in ore bodies showing a sharp contact relationship with the wall rocks [19,21,67,68,69]. These features suggest their emplacement involves the floating and filling of iron-rich melts. Based on the theory of iron-rich immiscibility, studies have proposed the magmatic genesis of this type of iron ore from the Washan iron deposit, which result from the floatation of immiscible iron-rich melts to the top of the andesitic intrusion [42,69]. However, the coupled compositional and textural features of magnetites from different types of iron ore in this study reveal a magmatic–hydrothermal genesis of the ore magma-type iron ores in the evolving process of mineralization in Washan. In magmatic–hydrothermal systems, studies have shown that chlorides can form complexes with metal elements in hydrothermal fluids, playing a crucial role in the transport and enrichment of Mn [70,71]. The increasing Mn contents from shallow brecciated ores (02A) (MnO: 0.03%) to deep massive ores (AS03B) (MnO: 0.10%) indicate the ascent of chlorides-enriched fluids from the magmatic environment. The highest MnO contents in marginal ores might both be attributed to the episodic upwelling of ore-forming fluids and considerable decrease in temperatures in hydraulic fractures near wall rocks. The relatively higher Na₂O contents (0.03%) of the ores from marginal zones, as well as those of brecciated ores (AS02A: 0.09%) (Figure 5) imply the circulation of these deep-originated Na₂Cl-rich fluids lead to significant sodic metasomatism in the shallow and marginal ore bodies. In the iron ores of many Kiruna-type IOA deposits, the late-stage high-salinity fluids may have caused the loss of chromium after the precipitation of large amounts of magnetite [12,72,73]. Studies have shown that Cr (chromium) is highly mobile in high-temperature saline fluids [74,75], which prevents its enrichment in the early stages of massive iron ore formation and facilitates its precipitation in laterally formed distal ore bodies. Experiments have suggested that, compared to the effect of temperature, the salinity of the fluid is the primary factor controlling the migration of Cr [74], which may promote its ability to reflect the fluid’s salinity characteristics. Therefore, the depletion of Cr in the massive magnetite ore in Washan is likely the result of evolving high-salinity conditions, similar to those observed in Kiruna-type IOA deposits. This is not only consistent with the lower C₂O₃ concentration (0.05%) in magnetites from deeply formed iron ores (AS03B) and the highest C₂O₃ concentration (0.09%) in magnetites from the marginal mineralization zone of Washan (AS05), but also with the lowest Na₂O contents (0.01%) in sample AS03B and the elevated Na₂O contents (0.03%) in sample AS05. Research has demonstrated that high-temperature and high-salinity brine liquids can be directly separated from chlorine-rich, low-water-content dioritic magmas, subsequently undergoing immiscibility in shallow environments [76], thereby leading to changes in the salinity of ore-forming fluids. Furthermore, a study on hydrosaline liquid inclusion in magnetites and garnets from porphyrite iron deposits has revealed the iron-rich mechanism of Na₂Cl-rich fluids, which functioned through the immiscibility of hydrosaline fluids within dioritic magmas [76]. The abundant chlorides can scavenge the iron from magma through metal–chlorine complexes and crystallize magnetites, forming massive magnetite ores with a magma-type ore style in the fractures penetrating the upper wall rocks. Therefore, the variation in salinity of ore-forming fluids might play a crucial role in the formation of magnetites and lead to different evolving paths of later generations of magnetite (Figure 9). The fluids with increasing salinity generated magmatic–hydrothermal magnetites with IOA-like genesis, while fluids with less salinity under considerable cooling conditions produced hydrothermal magnetites identified by their low-Ti content and IOCG-Skarn-like geneses. Based on the findings above, we distinguish this unique deep-occurred mineralization (AS03B, AS02B) in Washan as ore magma-filled-type, in contrast with the IOCG deposits commonly formed by metasomatic processes and the IOA deposits dominated by magmatic geneses. In summary, the metallogenetic model of porphyrite iron deposits in the Ningwu basin are suggested in the following sketch map (Figure 10):

In the scenario illustrated in this model, the contribution of phosphorus contents from evaporites, which widely developed in this area [8,77], may lower the liquidus temperature of the pre-mineralization parental magma, and facilitates the early crystallization of mafic minerals. This process helps in the enrichment of Ti contents in the residual magma and the crystallization of high-Ti magnetites (Ti₂O: 1.33%) characterized by abundant saline inclusions (Type A, AS03B) and Fe-Ti-V genesis (Figure 9) under magmatic–hydrothermal temperatures (~597 °C). As the continually exsolved saline from the andesitic magmas reached a favorable amount, immiscible Fe-P melts may be formed and separated from host hydrous silicic magma [78]. The high-salinity fluids, carrying substantial amounts of iron chlorides, feed into the fractures at the top of sub-volcanic intrusions and generated Type C and D magnetite (in AS03B and AS02B) under ~550–503 °C. This process resulted in the formation of ore magma-type ore bodies in the deep level of Washan. These magnetites are distinguished by their trellis-textured ilmenite exsolution lamellae, medium-Ti content (0.63%–0.55%), and porphyry and partial Fe-Ti-V genesis (Figure 9). As the salinity considerably increased, the ability to scavenge iron of these fluids was promoted. Type B and E magnetites (AS02A) may participate in the outer space around the ore magma-type ore bodies under ~473–451 °C due to decompression and cooling. They exhibited magmatic–hydrothermal geneses through declining Ti content (Ti₂O: 0.39%–0.34%) and IOA genetic features. If the fluids were transported by hydraulic fractures and reached the marginal mineralizing zones, they may have reacted with the surrounding rocks and participate Type F magnetites (AS05) under considerable cooling temperatures (~378 °C). The associated hydrothermal magnetites are depleted in Ti (Ti₂O: 0.23%) and show IOCG-Skarn genetic features. The formation of different generations of magnetite in a magmatic–hydrothermal metallogenic system in Washan not only connects their contrasting late-stage geneses, but also the unique ore magma-filled-type mineralization initiating the whole ore-forming process.

6. Conclusions

One of the unique characteristics of porphyrite iron deposits in China is its widely developed high-titanium iron ores (>1%), which are typically considered to be of igneous origin and are proposed to be magma-type iron ores. In this study, similar iron ores from Washan show distinctive magmatic–hydrothermal genesis. By performing a textural and compositional study on the various generations of magnetites, these transitional-type iron ores are identified as products from ore magma-filled-type mineralization. The evolution paths from ore magma-filled-type ores to other potential genetic types (end members) of iron ores during the ore-forming process have been ascertained. The results of this study shed light on the ore-forming processes of genetically diverse porphyrite iron deposits and the role of different generations of magnetites played in their formation. The initial findings derived from this study are summarized as follows:

• The ore magma-filled-type mineralization, relating to iron-rich high-salinity fluids, is a viable mechanism in the formation of high-Ti magnetites with porphyry genesis within the vein-style ore bodies filling in deep fractures in Washan.
• As temperatures drop, the mineralization evolution processes may split into two paths. The high-salinity fluids, originating from ore magma-filled-type ore bodies, generated medium-Ti magmatic–hydrothermal magnetites of IOA genesis. Decreased-salinity fluids can pass through cracks and react with rocks, causing the precipitation of low-Ti hydrothermal magnetites.
• The crystallization of magnetite from different generations connects the deep ore magma-filled-type mineralization and subaerial iron ores with IOA- and IOCG-like geneses in Washan.