Evolution, Magmatic Source and Metallogenesis of A-Type Granites in the Fanchang Volcanic Basin, Middle and Lower Yangtze Metallogenic Belt: A Review

Abstract

The Fanchang volcanic basin (FVB) is located in the Middle and Lower Yangtze Metallogenic Belt (MLYMB) between the ore districts of Ningwu and Tongling. The existing ore deposits in the FVB are relatively small in scale and related to late Mesozoic A-type granites. In this paper, the crystallization age, major and trace element composition, and Sr-Nd and Hf isotope compositions of the A-type granites are summarized from the literature; in addition, the magnetite composition, H and O isotopes of fluid inclusions, and sulfur isotope composition of metal sulfides in some typical ore deposits in the FVB are also summarized to give insights into the petrogenesis and mineralization of the A-type granites intruding into the FVB. The results show that: (1) Orthopyroxene, plagioclase, K-feldspar, and biotite are the main fractionating minerals controlling the evolution of the magmas of A-type granites in the FVB and other areas in the MLYMB. (2) The whole-rock Sr-Nd and zircon Hf isotopic characteristics show that the source of A-type granite magma is complex and includes the enriched mantle, lower crust, and upper crust, probably with stronger participation of Archaean–Paleoproterozoic crustal materials in the FVB granites than in other regions of the MLYMB. (3) The ores in the FVB are dominated by skarn and hydrothermal deposits. H and O isotopes of fluid inclusions indicate that ore-forming fluids have been derived from mixtures of magmatic hydrothermal fluid, meteoric waters, and deep brine related to gypsum layers. S isotopes of metal sulfides indicate that the sulfur may be a mixture of magmatically derived sulfur and sulfur originating from the Triassic gypsum-bearing layers. The deposit and ore characteristics of the main deposits in the FVB are also illustrated, and the evaluation of metal resources indicates that the skarn and hydrothermal iron–zinc ores in the FVB also have potential as sources of Cd, Ga, and Se. In addition, in terms of the oxygen fugacity, rock type, and geochemical characteristics of magmatic rocks, the metallogenic characteristics and potential of the A-type granites in the FVB are evaluated. It is considered that in addition to the dominant constituents of iron and zinc and the minor constituents listed above, the FVB could have the potential for providing copper, gold, molybdenum, uranium, and other metals as well.

1. Introduction

A-type granites, an important category of granites [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17], are closely related to some key metals or strategic minerals, including iron, zinc, cobalt, tin, cadmium, niobium, gallium, cobalt, REE, uranium, etc. [18,19,20,21,22,23,24,25,26,27,28,29,30].

In the 1960s, with the advent of the theory of plate tectonics, the genesis of granite was mostly explained in its framework, and in the 1970s and 1980s, research on granite classification reached its peak [31,32,33]. Up to now, there have been about 20 genetic classification schemes for granites, among which the MISA classification based on the character of presumed source rocks is one of the more commonly used [34,35,36,37]. The classification of I-type (igneous source) and S-type (sedimentary source) is based on the study of the granites in the Berridale–Kosciuszko area of the Lachlan Fold Belt, Australia [31]. The model provides a good explanation for the appearance of these two types of granites and indicates that the unique compositional features of these granites were inherited from their source rocks [38].

Loiselle and Wones [33] first defined A-type granites as alkaline, anhydrous, and anorogenic granites, named after the initial letter “A” of the three words. The widespread use of the concept of A-type granite started with the determination of the granite complex in southeastern Australia, which is mainly characterized by high contents of Nb, Ga, Y, and REE and low contents of Al, Mg, and Ca [1]. Then, Whalen et al. [3] constructed discriminant diagrams for A-type granites as opposed to orogenic granites, based on the Ga/Al ratio, Zr, Nb, Ce, Y contents, and so forth.

At present, the concept of A-type granite contains a wide range of rock types, such as alkaline granites, quartz syenites, and charnockites [5,8,39]. The distinguishing geochemical indicators include high contents of Na₂O + K₂O, Y, Nb, REE, and Ga, as well as high FeO_T/MgO, K₂O/Na₂O, and Ga/Al ratios, and low contents of V, Cr, Ni, Sr, Ba, and large negative Eu anomalies, plus flat HREE partitioning characteristics [1,3,15,37,40]. So far, there has been no unified understanding of the genesis of A-type granites, and the main genetic models include magmatic differentiation or low-degree partial melting of mantle-derived tholeiitic magma, residual melt generated by the differentiation of mantle-derived alkaline magma, and partial melting of F-rich residue after partial melting to form I-type granite, as well as interaction between mantle-derived alkaline magma and crustal materials [1,3,5,7,8,9,10,14,16,17,19,22,26,40,41,42,43,44,45,46,47,48]. Regardless of the origin of A-type granites, however, it is generally believed that their formation temperatures are relatively high [13,37,42,43,45].

So far, there have been many classification schemes for A-type granites [8,9,10,49,50]. Eby [8] proposed discriminant diagrams corresponding to the tectonic environment, dividing A-type granites into two types: A1 and A2, based on their Nb/Y ratios. The former suggests a non-orogenic intraplate setting related to a continental rift environment or mantle hot spot, while the latter is attributed to a post-collision or post-orogenic environment. In addition, Hong et al. [49] divided A-type granites into two categories, i.e., non-orogenic (AA type) and post-orogenic (PA type), and Liu et al. [50] further discussed this classification. King et al. [10] also proposed the concept of aluminous A-type granite, and many Chinese scholars then carried out some further research work, including the division between metaluminous, aluminous, and peraluminous granites [51,52,53]. Nowadays, geologists have extended the concept even to extrusive rocks; still, the extensional system is the main tectonic environment for their formation, while the magmatic sources may be diverse [2,3,11,46].

Systematic studies on the relationship between the geochemical properties of granite (such as oxygen fugacity, etc.) and metallogenic types began in the early 1980s [54,55,56]. At present, scholars have obtained some understanding of the relationship between the differentiation degree of magmatic rocks, the redox state of the magma, and the dominant metallic mineral deposits of granite (such as Sn, W, Mo, Cu, and Au) [57,58,59,60]. For example, porphyry Cu and Mo deposits are mainly associated with I-type granites with high oxygen fugacity, tungsten mineralization can occur in any granite type, and tin mineralization is generally associated with heavily fractionated, reduced felsic granites [21,31,37,58,61,62].

Ores closely related to A-type granites include zinc, tin, molybdenum, bismuth, niobium, tungsten, thallium, REE, fluorine, and uranium [1,3,6,24,63,64,65,66], with different characteristics in different ore-forming belts. For example, in the Middle and Lower Yangtze Metallogenic Belt (MLYMB), iron, zinc, gold, molybdenum, and uranium are generally dominant [23,67,68,69,70,71,72], while in South China, tin, tungsten, and REE mineralization are the main types [24,48,73,74].

There are two A-type granite belts that are parallelly and symmetrically distributed on both sides of the Yangtze River in the Anhui Province (Figure 1 and Figure 2), within the MLYMB, in eastern China. The granite belt on the north bank stretches from the Dalongshan granite massif in the Anqing area to Chengshan and Huangmeijian in Zongyang County, in a NE direction, for about 75 km. On the south bank, the A-type granite belt extends from the granite massifs of Huayuangong and Maotan in the Guichi area to Banshiling and Fushan in the Fanchang volcanic basin (FVB), for about 100 km and in a NE direction as well [75,76].

The FVB is located between the Tongling and Ningwu ore fields (Figure 1B). The formation and evolution of the FVB is controlled by Mesozoic plate collision, intracontinental orogeny, and transtension processes. It is a volcanic basin superimposed on a Middle Triassic–Early and Middle Jurassic sedimentary basin [77], which is intruded by A-type granites. Compared with other areas in the MLYMB, the A-type granite suite in the FVB has unique ore characteristics, mainly because of its enrichment in iron and zinc [78,79,80,81].

The crystallization ages, petrogenesis, and evolution of the A-type granites in the FVB have been defined earlier based on the major and trace element geochemistry, zircon U-Pb dating, and whole-rock Sr-Nd isotope and zircon Hf isotope investigations [82,83,84,85]. However, there are two main issues that remain unresolved: (1) The contribution of mantle-derived materials to the A-type granite source has generally been recognized, but the end-member characteristics of the crustal components are still unclear. Lou et al. [82] believed that a mantle-derived alkaline basaltic magma interacted with a siliceous magma from the lower crust, and the resulting evolved magma in turn interacted with Meso-Neoproterozoic shallow metamorphic rocks and formed a shallow magma chamber, from which A-type granites evolved through separation and crystallization processes. Yan et al. [85,86] considered that A-type granites in this area were formed by a mixture of mantle-derived and crust-derived magma in different proportions, with crust-derived magma being the product of middle-upper crust melting. There has been no conclusion as to whether Archean–Paleoproteroic crustal materials have been involved. (2) The comparison in terms of magmatic evolution, petrogenesis, and ore-forming characteristics of A-type granites in the FVB with other A-type rocks in the MLYMB is insufficient; therefore, the similarities and differences between them are not clear, particularly when it comes to their geochemical characteristics [84,86].

Figure 1. (A) Tectonic sketch map showing A-type granite belts in eastern China (modified from [76]); (B) sketch map showing the A-type granites of the MLYMB (modified from [72,87]); (C) main ore deposits related to the A-type granites of the FVB.

Figure 1. (A) Tectonic sketch map showing A-type granite belts in eastern China (modified from [76]); (B) sketch map showing the A-type granites of the MLYMB (modified from [72,87]); (C) main ore deposits related to the A-type granites of the FVB.

Figure 2. Discrimination diagrams of A-type granites in the MLYMB (figure (a) after [3] and (b) after [8]). Data sources: A-type granites in the FVB [46,82,85,88,89]; A-type granites in the Chizhou area (the south bank of the Yangtze River) [46,90,91,92,93,94,95,96]; A-type granites in the Anqing–Zongyang area (the north bank of the Yangtze River) [97,98,99,100,101,102,103].

Figure 2. Discrimination diagrams of A-type granites in the MLYMB (figure (a) after [3] and (b) after [8]). Data sources: A-type granites in the FVB [46,82,85,88,89]; A-type granites in the Chizhou area (the south bank of the Yangtze River) [46,90,91,92,93,94,95,96]; A-type granites in the Anqing–Zongyang area (the north bank of the Yangtze River) [97,98,99,100,101,102,103].

Research on mineralization related to A-type granite in the MLYMB, such as ore types, ore-controlling characteristics, ore deposit genesis, etc., has not been systematically summarized before. This is especially so for the FVB, in which the dominant metals related to A-type granites are only iron and zinc, and deposits are relatively small [79,80,81,104,105,106]. Previous summaries on ore-forming types, deposit characteristics, and ore characteristics in this area are not thorough enough [78], and the discussion on the geochemistry of the deposits is still insufficient [80], in particular with respect to the characteristics of ore-forming fluid and the source of ore-forming materials. In addition, new breakthroughs have been carried out in this area, which makes it urgent to give insights into the metallogenic potential of the A-type granite suite and expand the list of prospective metals in this area.

In view of this, on the basis of the data on A-type granites in the MLYMB systematically collected and analyzed during the last few years, such as crystallization ages [46,83,84,85,89,92,93,94,96,98,100,101,107,108,109], major and trace elements [46,82,84,85,88,89,92,93,94,96,98,100,101,107,108], Sr-Nd [85,86,89,91,93,94,100,101,108,110,111], and Hf-O isotopes [85,88,92,93,94,96,100,112] (

Table 1. Summary of A-type granites in MLYMB.

No.	Area	Rock Massif	Location (Approximate Center)		Outcrop Area (km2)	Lithology	Minerals		Sr-Nd		Hf-O		References
								Ages (Ma)	(87Sr/86Sr)	εNd (t)	εHf (t)	δ18O‰
1	Fanchang	Banshiling	30°59′59″ N	118°11′06″ E	16.29	Biotite quartz monzonite	Kfs (45%) + Pl (35%) + Qtz (10%) + Bt (6%)	125.3 ± 1.4	-	-	-	-	[46]
								124.9 ± 1. 7	0.7072	−6.8	−2.7~−6.3	6.7~7.4	[85,86]
								125.3 ± 2.9	-	-	-	-	[83]
								125.4 ± 1.6	0.70827	−11.2	-	-	[89]
2	Fanchang	Fushan	31°09′05″ N	118°03′08″ E	15.25	Syenogranite	Kfs (55%) + Qtz (30%) + Pl (5%) + Bt (5%)	124.9 ± 2.0	-	-	−5.8~−10.0	-	[88]
								126.8	-	-	−7.52	-	[84]
								126.4 ± 1.7	0.7076	−7.7	−1.6~7.9	7.1~9.1	[85,86]
3	Fanchang	Binjiang	31°09′05″ N	118°03′08″ E	12	Granitic porphyry	Kfs (60%) + Pl (20%) + Qz (15%) +Bt (5%)	124.3 ± 2.5	-	-	-	-	[83]
								124.6 ± 4.7 (coarse-grained granite)	0.7078	−3.4	0~−6.6	8.0~10.3	[85,86]
								123.0 ± 1.8 (granite porphyry)
4	Fanchang	Xiangxingdi	31°02′00″ N	118°15′30″ E	6	Granitic porphyry	Qtz(25%) + Pl(70%) + Hb(2%)	124.3 ± 1.2	-	-	-	-	[89]
5	Fanchang	Suishan	31°05′00″ N	118°12′00″ E	-	Granite	Qtz(22%) +Pl(20%) + Kfs(53%) + Bt(4%)	124.3 ± 1.2	0.70755	−10.5	-	-	[89]
6	Fanchang	Zhuhouling	31°05′00″ N	118°16′00″ E	4.85	Granitic porphyry	Kfs(70~80%) + Qtz(<5%) + Pl(10~15%)	127.6 ± 1.8	0.70827	−11.2	-	-	[89]
7	Fanchang	Xiaoyang- chong	31°05′30″ N	118°07′00″ E	0.13	Quartz diorite and granodiorite	Kfs(15%) + Qtz(18~20%) + Pl(50~60%) + Bt(5~10%)	126~128	-	-	-	-	[113]
8	Chizhou	Huayuan- gong	117°36′00″ N	30°37′00″ E	220	Quartz syenite	Kfs(70~80%) + Qtz(<5%) + Pl(10%)	126.2 ± 1.2	0.7081	−6.7	−7.4	-	[93]
						Syenogranite	Kfs(64~67%) + Qtz(25~33%) + Pl(2.5~3.0%) + Bt(1.0%)	125.3 ± 1.2 [47]	--	-	−7.3, −7.89 [42]	-	[42,46]
						Quartz monzonite	Kfs(30~35%) + Qtz(5~10%) + Pl(45~55%) + Bt(3~6%) + Hb(2~4%)	127 ± 1	0.709776	−7.42	-	-	[108]
						Quartz syenite	Kfs(65~75%) + Qtz(5~10%) + Pl(7~9%) + Bt(1~3%) + Hb(1~4%)	127	0.713653	−7.67	-	-	[108]
						Syenogranite	Kfs(55~65%) + Qtz(5~10%) + Pl(2~5%) + Bt(1~2%)	127	0.740000	−7.97	-	-	[108]
						Syenogranite		122.6 ± 1.3	-	-	−4.7	-	[92]
						Syenogranite		122.6 ± 1.3	-	-	−6.7~−2.1	-	[94]
9	Chizhou	Bashan	117°38′00″ N	30°35′00″ E	40	Syenogranite	Kfs(66%) + Qtz(25%) + Pl(2%) + Ab(5%)	121.6 ± 2.8	0.7082~0.7091	−7.2~−7.5	-	-	[111]
10	Chizhou	Guilinzheng	117°40′00″ N	30°25′00″ E	-	Granitic porphyry	Kfs(40~60%) + Qtz(35~45%) + Pl(5~10%) + Bt(<5%)	127.0 ± 0.5 [94]; 127.6 ± 1.5 [114]	-	-	−2.9~5.9	-	[94,114]
11	Chizhou	Yangshan	117°50′00″N	30°30′00″ E	30	Syenitic porphyry	Kfs(40~45%) + Qtz(5~10%) + Pl(40~50%)	127.0 ± 0.6	0.7107~0.7140	−7.02~−5.78	−5.5~−3.7	-	[94]
						Syenogranitic porphyry	Kfs(30~35%) + Qtz(55~65%)	126.0 ± 1.0	0.7094~0.7065	−6.03~−5.47	−6.4~−4.4	-	[94]
						Syenogranite	-	127.6 ± 0.6	-	-	−7.5~−2.3	-	[94]
12	Chizhou	Maotan	117°47′00″ N	30°42′00″ E	25	Syenite	Kfs(55~65%) + Qtz(30%) + Pl(10%) + Bt(2~5%)	127.7 ± 1.8	0.70076	−7.03	-	-	[91]
						Syenogranite	Kfs(76~79%) + Qtz(7~22%) + Pl(1~4%) + Bt(0.5~3%)	125.4 ± 2.2	-	-	-	-	[46]
13	Chizhou	Xiangshui- jian	118°14′00″N	31°02′00″ E	20	Syenogranite	Kfs(64~67%) + Qtz(22~28%) + Pl(2~3%) + Bt(5~8%)	125.4 ± 1.4	-	-	-	-	[46]
14	Anqing-Guichi	Dalongshan	117°04′00″ N	30°36′00″ E	90	Quartz syenite	Kfs(60~70%) + Qtz(10~15%) + Pl(10~15%) + Bt(<5%)	125.8 ± 1.6, 126.4 ± 3.5 [112]; 123.8.4 ± 2.1 [84]	0.706444 [115]	−6.8~−7.7 [115]	−4~+1.1,−7.8~−3.6 [112]; −3.41 [84]	-	[84,112,115]
15	Anqing-Guichi	Huashan	117°09′00″ N	30°42′00″ E	21	Syenogranite	Kfs(70%) + Qtz(20%) + Pl(10%) + Bt(<5%)	126.2 ± 0.8; 124.4 ± 2.2	-	-	−3.51	-	[84]
16	Anqing-Guichi	Zongyang	117°14′00″ N	30°43′00″ E	10	Syenogranite	Kfs(70%) + Qtz(20%) + Pl(12%) + Bt(<1%)	124.8 ± 2.2 [98]; 125.4 ± 1.5 [84]	-	-	−3.57 [84]	-	[84,98]
17	Anqing-Guichi	Chengshan	117°14′00″ N	30°46′00″ E	19	Syenogranite	Kfs(70%) + Qtz(20%) + Pl(8%) + Aegirine(a small amount)	126.5 ± 2.1 [98] 125.0 ± 1.7 [84]	0.7076 [115]; 0.70695~0.70742 [101]	−5.0, −6.3~−4.2 [101]	−4.72 [84]	-	[84,98,101,115]
18	Anqing-Guichi	Hejiaao	117°13′00″N	30°43′00″ E	5	Syenogranite	Kfs(75%) + Qtz(20%) + Pl(8%) + Ae(a small amount)	128 ± 1	0.70795~0.70931	−6.4~−5.8	-	-	[101]
19	Anqing-Guichi	Meilin	117°12′30″ N	30°43′00″ E	7	Syenogranite	Kfs(75%) + Qtz(20%) + Pl(5%) + Ae(a small amount)	128 ± 2	0.7364~0.7659	−5.2, −6.0, −5.4	-	-	[101]
20	Anqing-Guichi	Huangmeijian	117°34′00″ N	30°55′30″ E	120	Quartz syenite	Kfs(86%) + Qtz(12%) + Pl(<2%)	127.6 ± 2.1; 127.2 ± 2.1 [112]	0.7078 [115]; 0.7089 [110]	−7.7 [115]; −2.5 [110]	−3.3~+2.1 [112]; −3.8~−0.1 [112]; −3.38 [84]	-	[84,110,112,115,116]
21	Anqing-Guichi	Changgang	117°12′00″ N	31°20′00″ E	0.5	Syenogranitic porphyry	Kfs(45~55%) + Qtz(15~20%) + Pl(20~30%) + Bt(a small amount)	120 ± 2	0.7082	−14.9	−18.3	5.99	[100]

and Supplement Tables S1–S4), this paper gives insights into the evolution characteristics of the A-type granite suite in the FVB and other parts of the MLYMB, as well as the end-member characteristics of the magma source area. Based on summaries of the characteristics of typical ore deposits related to A-type granites in the MLYMB [77,113], combined with the compositions of magnetite, H and O isotopes of fluid inclusions in ore and gangue minerals, as well as S isotopes of metal sulfides in typical deposits in the FVB (Supplement Tables S5–S7) [80,81], the ore-controlling factors, ore-forming fluids, and the sources of materials in the ores of the FVB are discussed. Based on the oxygen fugacity, rock types, and geochemical characteristics of magmatic rocks, this paper interprets the metallogenic potential of A-type granites in this area and expands their potential ore types. In addition, this paper preliminarily evaluates the comprehensive utilization potential of associated key metals in the main ores in the FVB, and finally, a petrogenic and metallogenic model for the A-type granite suite in the FVB is proposed.

2. Geological Background of the Fanchang Volcanic Basin

According to the geological characteristics, mineral assemblages, and metallogenic ages of the deposits in the MLYMB, two metallogenic stages are distinguished: one related to Hercynian submarine eruptive volcano-sedimentary processes, and the other related to Yanshanian (Late Jurassic and Early Cretaceous) intermediate-acid intrusive rocks [117,118]. The metallogenic stage related to the Yanshanian intermediate-acid intrusive rocks is the most important and is connected to the thinning of the continental lithosphere in eastern China [67,71,72,117,119,120]. The Yanshanian metallogenic stages can be subdivided into three types (Figure 1B): (1) the skarn-porphyry Cu and Au mineralization associated with high-K calc-alkaline, adakitic rocks (147~137 Ma), mainly developed in Jiurui, Anqing-Guichi, Tongling (fault uplift area) [121,122,123,124,125], and the southeastern Hubei province in a transitional zone of fault depression and fault uplift [126,127,128,129]; (2) the subvolcanic or porphyrite-type Fe mineralization related to Na-rich calc-alkaline subvolcanic diorite (135–127 Ma), mainly developed in volcanic basins such as Ningwu and Luzong [107,130,131,132]; and (3) Fe, Au, Mo, and U mineralization related to alkaline or A-type granites (130–120 Ma), developed both in fault uplift areas and volcanic basins, mainly the FVB, Luzong volcanic basin, and the uplift area of Chizhou [67,69,72,118].

The FVB is a compound basin covered by Mesozoic volcanic rocks, which are superposed on an earlier fault-bounded basin [105,133], found between the fault uplift area (Tongling) and the volcanic basin (Ningwu) (Figure 1B). Late Mesozoic magmatic activity in the FVB was intense, and the volcanic rocks are mainly distributed in the southern part, whereas A-type granites are mainly found in the central and northern parts (Figure 1C).

Volcanic rocks are mainly distributed around Tadpole Mountain, Maren Mountain, and other craters, with a total thickness of between 220 and 2250 m. There are three eruptive cycles from bottom to top, namely the Kedoushan Formation, Chisha Formation, and Zhongfencun Formation, characterized by successive bimodal eruptions of basalt and rhyolite [113], dated between 129 and 131 Ma [134]. The crystallization time of the intrusive rocks is slightly later than that of the extrusive rocks, and they are dominated by A-type granites [75,89,135]. There are similar abundance patterns of trace elements (especially incompatible elements), Sr-Nd isotopes, and zircon Hf-O isotopes between the intrusive rocks and volcanic rocks in the FVB [85,90,134].

Previous studies have shown that the mantle endmember of the Yanshanian magmatic rocks in the MLYMB appears to be enriched [71,72,136,137,138,139,140,141]. On the other hand, crustal materials were also obviously involved during the petrogenetic process [122,142,143,144,145,146,147]. Our previous studies have also shown that the patterns of trace elements in the intrusive rocks in the FVB are similar to those in typical crustal rocks, indicating the participation of crustal materials during the formation of the A-type granites [83,89].

3. Evolution of the A-Type Granites in the FVB

The A-type granites in the FVB mainly belong to the high-K calc-alkaline and shoshonite suites (Figure 3a). They are mainly metaluminous and peraluminous (Figure 3c). Granites and quartz mozonites are the main rock types of A-type granites in the FVB (Figure 3b and Figure 4d,e), followed by a few syenite and granodiorite rocks. The A-type granites in the FVB and MYLMB are enriched in light rare-earth elements with flat, heavy rare-earth element curves (Figure 3d). They are enriched in Rb, Th, U, Ce, Pb, Nd, Sm, and Gd, but depleted in Ba, Nb, Ta, Sr, P, and Ti, reflecting a trace element abundance pattern roughly consistent with the crust, with more marked peaks and troughs (Figure 3e). The crystallization age of Mesozoic intrusive rocks in the FVB is mainly between 128 and 123 Ma, which essentially overlaps with the crystallization ages of A-type granites in the rest of the MLYMB (Figure 3f).

The Harker diagram and trace element covariant diagram are used to investigate the magmatic evolution process. As can be seen from Figure 5, the intrusive rocks in the FVB are similar to other A-type granites in the MLYMB, and fractional crystallization plays a dominant role in the magmatic evolution process. According to the Harker diagram (Figure 6), SiO₂ content is negatively correlated with Fe₂O3_T, MgO, P₂O₅, and TiO₂ content, indicating the separation and crystallization of Fe-bearing mafic minerals (pyroxene, amphibole, and biotite), apatite, and titanite. The negative correlation with Al₂O₃, CaO, and Eu/Eu* indicates the crystallization of plagioclase.

The Zr-Zr/Y covariant diagram (Figure 7) can further identify the controlling role of orthopyroxene during the fractional crystallization or partial melting process. It can crystallize as separate cumulate phases during the crystallization process or remain as residual phases during the partial melting process [152].

Figure 7. Zr/Y vs. Zr diagrams of A-type granite rocks in the FVB and other areas in the MLYMB (after [152]). Gt—garnet; Hbl—hornblende; Cpx—clinopyroxene; Opx—orthopyroxene; Ol—olivine; Pl—plagioclase.

Figure 7. Zr/Y vs. Zr diagrams of A-type granite rocks in the FVB and other areas in the MLYMB (after [152]). Gt—garnet; Hbl—hornblende; Cpx—clinopyroxene; Opx—orthopyroxene; Ol—olivine; Pl—plagioclase.

The logarithmic diagrams of Ba-Sr and Rb-Sr (Figure 8) can reflect the fractional crystallization of K-feldspar, plagioclase, and other rock-forming minerals during the evolution of granitoid magmas [153]. The A-type granites in the FVB and in other areas of the MLYMB show trends suggesting fractionation of K-feldspar, plagioclase, and biotite.

Figure 8. (a) Ba vs. Sr diagram and (b) Rb vs. Sr diagram of intrusive rocks in the FVB and A-type granites in other parts of the MLYMB (after [153]). Pl_An50: plagioclase (An = 50); plagioclase (An = 15); Kfs: K-feldspar; Bt: biotite; Ms: muscovite; Grt: garnet; Amp: amphibole.

Figure 8. (a) Ba vs. Sr diagram and (b) Rb vs. Sr diagram of intrusive rocks in the FVB and A-type granites in other parts of the MLYMB (after [153]). Pl_An50: plagioclase (An = 50); plagioclase (An = 15); Kfs: K-feldspar; Bt: biotite; Ms: muscovite; Grt: garnet; Amp: amphibole.

In summary, we consider that the magmatic evolution of the A-type granites in the FVB and other areas in the MLYMB are mainly controlled by fractional crystallization, with orthopyroxene, plagioclase, K-feldspar, and biotite as the main crystallization phases.

4. Magmatic Source Characteristics of A-Type Granites in the FVB

According to ε_Nd(t)–(^S7Sr/⁸⁶Sr)_i (Figure 9), the variation trend of Sr-Nd isotopes in the source area of the A-type granites in the FVB indicates that the source area is a mixture of three possible sources: the lithospheric mantle, late Archean Paleoproterozoic lower crust, and Neoproterozoic upper crust. The mantle endmember can be considered to consist of a somewhat enriched mantle, similar in composition to the basalts of the Kedoushan Formation (129.5 ± 3.3 Ma) [90,134,138]. The lower crustal endmember suggests late Archean or Paleoproterozoic crustal materials in the MLYMB, perhaps represented by the Kongling Group [154,155,156,157]. In addition, the Sr-Nd isotopes also show an obvious trend towards the Neoproterozoic crust in the Yangtze block [85,86]. However, this trend is mainly seen for A-type granites in other parts of the MLYMB, while the A-type granites in the FVB are dominated by the older lower crustal component in addition to the enriched mantle.

Figure 9. Diagram of initial ε_Nd(t) vs. initial (^S7Sr/⁸⁶Sr)_i of A-type granites in the MLYMB. Data sources (see Supplement Table S3): Ocean basalt and primitive mantle [158]; metasomatically enriched mantle [138]; global subducted sediments [159]; Neoproterozoic basement [160]; Lower Yangtze crust [161]; A-type granites of the FVB [86,89]; A-type granites of the MLYMB [91,93,94,96,100,101,108,110].

Figure 9. Diagram of initial ε_Nd(t) vs. initial (^S7Sr/⁸⁶Sr)_i of A-type granites in the MLYMB. Data sources (see Supplement Table S3): Ocean basalt and primitive mantle [158]; metasomatically enriched mantle [138]; global subducted sediments [159]; Neoproterozoic basement [160]; Lower Yangtze crust [161]; A-type granites of the FVB [86,89]; A-type granites of the MLYMB [91,93,94,96,100,101,108,110].

The initial ε_Hf(t) in zircon from A-type granites in the FVB dominantly falls between –4 and 10 (Figure 10), indicative of a Neoproterozoic crustal source. Therefore, some scholars believed that the Archean ancient material did not contribute to the magmatic source [85,134]. This is, however, at odds with the Nd isotope data from A-type granites in the FVB above, which indicated an Archean or Paleoproterozoic crustal source for these granites (Figure 9). In addition, the inherited zircons in the late Mesozoic magmatic rocks in the FVB and other parts of the MYLMB have multi-stage age ranges, also reflecting the involvement of Archaean–Paleoproterozoic and Neoproterozoic crustal materials [154,155,156,162].

Thus, when it comes to the apparent discrepancy between the indications of crustal components (Archean–Paleoproterozoic versus Neoproterozoic) from zircon Hf and whole rock Nd-Sr isotope data in the FVB, this may be simply an effect of too little Hf isotope data from the FVB granites, failing to capture the whole range of Hf isotope compositions, in combination with the fact that the Hf and Nd-Sr data have not been obtained from the same samples and therefore not directly comparable.

Compared to the FVB, the initial zircon ε_Hf(t) of A-type granites in other areas of the MLYMB ranges from –20 to +6 (Figure 10 and Supplement Table S4). The low range of –20––10 can be considered the result of the involvement of the enriched mantle or Archaean–Paleoproterozoic crust in the magmatic source [154], while the high range of 0–6 could be due to juvenile crust as a source [163], with the initial ε_Hf(t) range of −10–0 corresponding to Neoproterozoic crustal materials. Combing with ε_Nd(t)–(^S7Sr/⁸⁶Sr)_i (Figure 9), it can be concluded that the magmatic source area of A-type granites in other parts of the MLYMB included enriched mantle and Neoproterozoic crustal materials; perhaps Archean–Paleoproterozoic crust and juvenile crust are also involved.

To sum up, we believe that there are several sources of A-type granites in the FVB, with the enriched mantle, Archean–Paleoproterozoic lower crust, and Neoproterozoic upper crust involved. Compared with other areas in the MLYMB, the participation of ancient lower crust materials in the source area of A-type granites in the FVB may be higher.

5. Ore-Controlling Characteristics and Typical Deposits

Skarn-type and hydrothermal-type iron and zinc deposits are the main ore types closely related to the A-type granites in the FVB, and there is more of a gradual shift between them. They are mainly hosted in interlayer faults or other fractures in carbonate rocks as well as in contact zones with granites. The difference between them lies in whether there are obvious skarn minerals [113].

Gravity and magnetic anomaly features show that there are granite massifs below these mining areas; combined with the outcrops, these anomalies are likely to be A-type granites [77,113]. In addition, taking the Taochong and Suishan deposits as examples, granite can be seen in the deep part of the drillings at the edge of these mining areas (Figure 3d–f, Figure 11b and Figure 13). In terms of mineralization time, the Re-Os model age of pyrite in the Xiaoyangchong skarn stage indicates that its metallogenetic age is about 125.7 Ma [89], which is clearly located in the same age region as the A-type granites in the FVB.

The skarn-type ore bodies are always restricted to the contact zone between magmatic rocks and carbonate rocks or occur within faults in the granite massif. The hydrothermal deposits are mainly hosted within fracture zones in granite massifs or carbonate strata. The characteristic gangue minerals include calcite, diopside, and quartz, and the ore minerals are mainly specularite, hematite, magnetite, and sphalerite (Figure 3) [77,81,113,164].

The deposit and ore characteristics of the main typical deposits in the FVB are as follows.

5.1. Taochong Iron Ore Deposit

(1)

Geological characteristics

The main structure of the mining area is a fan-shaped anticline with an axis trending northeast. The deposit is located in the northwest limb of the anticline (Figure 1). The ore body is controlled by the faults between the upper and lower limbs of a recumbent fold. There are diorite porphyrite, microcrystalline diorite, and syenite porphyry veins interspersed in the deposit, and the alteration mainly consists of skarn alteration, epidotization, and carbonation. The ore body occurs near the contact zone between skarn and limestone, and it is sickle-like or lamellar in profile (Figure 11) [80,113].

(2)

Ore mineral characteristics

The natural types of ore are as follows: (1) iron ore in the outer skarn zone and (2) iron ore in the inner skarn belt. The content of pseudohematite and specularite is higher in the outer skarn zone than in the inner skarn belt, while the content of magnetite is relatively lower. The gangue minerals in the outer skarn zone are mainly calcite and quartz, while in the inner skarn belt, they are mainly garnet, followed by pyroxene, calcite, and quartz [77].

Figure 11. (a) Three-dimensional metallogenic model map (modified from [165]); (b) profile of the Taochong iron mine in the FVB (modified from [164]).

Figure 11. (a) Three-dimensional metallogenic model map (modified from [165]); (b) profile of the Taochong iron mine in the FVB (modified from [164]).

5.2. Xiaoyangchong Zinc-Iron Ore

(1)

Geological characteristics

The deposit is located in the southeastern limb of an anticline, and early Permian limestone and pre-mineralization diorite porphyrite are the main host rocks (Figure 12). The diorite porphyrite is shaped like an irregular tube that branches toward the surface but is connected at depth. The surrounding limestone is altered to marble or skarn [77,113].

(2)

Ore mineral characteristics

The natural types of ore are as follows: (1) massive zinc-iron ore; (2) disseminated magnetite; (3) iron-bearing breccia; and (4) massive limonite. The first has a dense and massive structure, while the latter three have disseminated, vein-like and brecciated, and massive structures, respectively.

Ore minerals are mainly sphalerite, magnetite, and hematite, with a small amount of specularite, limonite, pyrite, chalcopyrite, and gold. Gangue minerals include garnet, tremolite, epidote, wollastonite, quartz, and opal.

5.3. Suishan Zinc Ore Deposit

(1)

Geological characteristics

The deposit is hosted in the southeast limb of the Fanchang compound syncline, with the limestone of the upper member of the Lower Triassic Nanlinghu Formation being exposed in the area (Figure 1). The ore bodies are mainly lens-like (Figure 13) [113].

Skarnization and marmarization are closely related to zinc mineralization, with dolomitization, chloritization, and silicification processes also being involved. Skarn formation is the main alteration phenomenon in the area, and pyroxene and garnet skarns often have strong zinc mineralization, forming rich ore.

(2)

Ore mineral characteristics

The natural types of ore are as follows: (1) massive sphalerite; (2) zinc-bearing skarn; and (3) zinc-bearing marble. The ore has massive, disseminated, banded, and brecciated structures.

Ore mineral composition: mainly sphalerite, pyrite, and cobaltite, followed by chalcopyrite, galena, specularite, and chalcocite, and with secondary minerals including limonite, siderite, and lead oxide. Gangue minerals are mainly pyroxene, garnet, and actinolite, followed by calcite, quartz, tourmaline, and epidote.

6. Metallogenesis

6.1. Genesis of Mineral Deposits

The ore minerals in the zinc–iron deposits in the FVB are mainly of hydrothermal origin [113] (Figure 14). Taking the Xiaoyangchong zinc–iron deposit as an example, the electron probe analysis shows that the magnetite in the zinc–iron deposit principally formed during the post-magmatic hydrothermal stage [89], and the main genetic types are the contact metasomatic-type and the skarn-type.

The studies on fluid inclusions and S isotopes of metal sulfides show that during the evolution process of skarn fluid to quartz–sulfide-bearing hydrothermal fluid, in addition to the mixing of magmatic water and meteoric water [80], there is also the presence of brine (hot brine formed by deep circulation of meteoric water or sealed hot brine), probably in association with the Triassic gypsum–salt formation (such as Triassic gypsum solution breccia, gravelly dolomite, etc.) (Figure 15) [81].

The range of sulfur isotopic compositions of metal sulfides in skarn and hydrothermal deposits related to A-type granites in the FVB is relatively narrow (δ³⁴S = 5.8‰–19.2‰) (Figure 16); only metal sulfides are found in the ore rocks, no sulfate minerals, so the δ³⁴S of these metal sulfides should approximately represent the sulfur isotope composition of the hydrothermal system [171]. Possible sulfur sources of these deposits related to A-type granites include magmatic rocks (0.09‰–7.87‰, with an average value of 3.50‰), sedimentary clastic rocks (−16.7‰–−31.1‰, with an average value of −24.0‰), Triassic gypsum–salt strata (25.3‰–34.4‰, with an average value of 30‰) (Figure 16) [117,169]. Therefore, we consider that the sulfur in the skarn and hydrothermal deposits comes from two sources: A-type granite magma and gypsum–salt strata.

6.2. Metallogenic Potential

Pijajno [21] considered that metal precipitation depends on the oxygen fugacity of intrusive rocks in the order Sn–W–Mo–Cu-Mo–Cu-Au, with increasing oxygen fugacity, as well as on the iron content of intrusive rocks in the order Mo–Sn–W–Cu-Mo–Cu-Au, with increasing iron content. Blevin [20] suggested that four factors could determine the metallogenic potential of granitoids and related rocks: (1) the oxidation state; (2) compositional character (including alkalinity, petrogenic type, SiO₂ content, K₂O content, etc.); (3) degree of compositional evolution (e.g., Rb/Sr and K/Rb); and (4) the presence of fractionation. Zhao et al. [62] simplified the criteria to determine the mineralization potential of granitoids and believed that the types of granitoids (diorite, monzodiorite, monzonite, alkaline granite, etc.) reflected the comprehensive characteristics of the above four parameters [20], except for the oxidation state. Therefore, the combination of oxygen fugacity and rock type can ideally characterize the mineralization potential of granitoids [62].

As shown in Figure 17, Au and Cu (Au, Fe) correspond to the oxidized alkaline ore-forming system related to diorite and monzodiorite. Cu, Mo, W, Pb, and Ag correspond to the relatively reduced and peraluminous ore-forming system related to granodiorite (monzonite). Cu, Au, Mo, Fe, and Zn are related to the oxidized and peraluminous ore-forming system related to tonalite and monzonite. Sn, W, Be, Mo, F, and Zn correspond to a strongly reduced and peraluminous greisen metallogenic system related to granite or alkaline granite. Zn, Mo, F, W, and Ag tend to be formed in oxidized and peralkaline systems [62]. This is very consistent with the known metallogenic regularity of granite [172,173].

Our previous studies have shown that for the A-type granites in the FVB, both the magmatic rocks and post-mineralization hydrothermal fluids are relatively oxidized [81,89]; the rock types are mainly K-feldspar granite, granite, and monzonite; and they essentially belong to the metaluminous intrusive types [81,89,174]. It can be seen from Figure 17 that besides Fe and Zn, the A-type granites in the FVB have prospecting potential for Cu, Au, Ag, Mo, etc. [172,173].

In other areas of the MLYMB, the mineralization related to A-type granites consists of Fe, U, Mo, and Au, etc. (

Table 2. Summary of deposits related to A-type granites in the MLYMB.

No.	Location	Related Intrusive Rock Type	Deposit	Ore Type	Amount	Grade	Type of Ore	Alteration	Ore Minerals	Metallogenic Age			Metallogenic Type	References
										Mineral	Method	Age
1	Fanchang	Banshiling, biotite quartz monzonite	Zishanling	Cu	No data	0.35%	Copper-bearing limonite ore and copper-bearing marble ore	Marbleization, skarnization, silicification, and chloritization	Chalcopyrite, bornite, and limonite	-	-	-	Hydrothermal-type	[113]
2	Fanchang	Binjiang, granitic porphyry	Taochong	Fe, Zn	34.71 Mt	44.29%	Skarn-type iron ore	Skarnization, breccification, marblelization, and silicification	Magnetite, hematite, and specularite	-	-	-	Layered skarn-type	[164]
3	Fanchang	Suishan, granite	Suishan	Zn	7331 t	10%	Massive zinc ore, zinc-bearing skarn ore, zinc-bearing marble ore	Skarnization, dolomitization, carbonation, chloritization, silicification, etc.	Mainly sphalerite, pyrite, and cobaltite	-	-	-	Skarn-type	[77]
4	Fanchang	Suishan, granite	Songyuan	S(Fe)	No data	28.35%	Pyrite ore	Garnet skarnization, carbonation, and silicification	Pyrite and specularite	-	-	-	Skarn-type	[77]
5	Fanchang	Xiaoyangchong, granodiorite	Xiaoyangchong	Zn (Fe)	Zn: 91,962 t; Fe: 2898 t	Zn: 6.7%; Fe: 37.97%	Massive zinc-iron ore, disseminated magnetite ore	Marbleization and skarnization	Sphalerite, magnetite, and hematite	Pyrite	Re-Os	125.7 Ma	Skarn-type	[77]
6	Chizhou	Huayuangong, Syenogranite	Liwan	Cu	40,000 t	0.62%	Copper-bearing pyrite, copper-bearing sulfur skarn, lead-zinc skarn	Marbleization and skarnization	Chalcopyrite, bornite, sphalerite, pyrite, and molybdenite	-	-	-	Skarn-type	[92]
7	Chizhou	Guilinzheng, granitic porphyry	Guilinzheng	Mo (W)	0.15 Mt	0.13%	Disseminated ore and banded ore	Silicification, sericitization, skarnization, and serpentinization	Molybdenite, sphalerite, molybdenum-rich scheelite, magnetite, and galena	-	-	-	Skarn-type	[114,179]
8	Anqing-Zongyang	Dalongshan, quartz syenite	Dalongshan	U	Small deposit	0.81%	Sandstone type ore and quartz syenite type ore	Hydromica, albitization, hematite, carbonation, silicification, pyritization, and chloritization	Pitchblende, microcrystalline quartz, hematite, and pyrite	Pitchblende	U-Pb isochron method	130.0 Ma and 111.7 Ma	Hydrothermal-type	[178]
9	Anqing-Zongyang	Huangmeijian, quartz syenite	Dingjiashan	U	No data	0.1~0.2%	Sandstone type ore and quartz syenite type ore	Silicification, carbonation, chloritization, discoloration, pyritization, brass mineralization, and kaolinization	Pitchblende and uranium	Pitchblende	U-Pb isochron method	108.7 Ma	Hydrothermal-type	[176]
10	Anqing-Zongyang	Huangmeijian, quartz syenite	Xucun	U	No data	0.28%	Felsic sandstone type and quartz syenite type	Silicitization, pyritization, carbonatization, greenization, and hydromicatization	Pitchblende and uranium	Single mineral zircon in pitchblende	U-Pb	108 ± 1.5 Ma and 71.3 ± 1.0 Ma	Hydrothermal-type	[116]
11	Anqing-Zongyang	Huangmeijian, quartz syenite	Makou	Fe	0.08 Mt	No data	Reticulated and massive magnetite ore	Potassic mineralization	Magnetite, apatite, pyrite, and sphalerite	Phlogopite	40Ar-39Ar	127.3 ± 0.8 Ma	Hydrothermal-type	[175]

) [77,92,114,164,175,176,177,178,179]. The A-type granites in the FVB are mostly identical to those in other parts of the MLYMB in terms of magmatic evolution characteristics (Figure 5, Figure 6, Figure 7 and Figure 8), characteristics of major and trace elements [70,86,89], crystallization ages (Figure 3f), and magmatic source (Figure 9 and Figure 10). The dominant metals in the ores related to the A-type granites in the FVB are mainly Zn and Fe, which seem to be different from other parts of the MLYMB (Table 2). However, from the perspective of metallogenic types and mineral compositions (Table 2), skarn and hydrothermal deposits are dominant, and sphalerite and magnetite are common in all these deposits. This indicates that the A-type granites in the FVB also potentially contain U, Mo, Au, and other metals.

6.3. Metallogenetic Mechanism

Most scholars consider that many A-type granites contain an abnormally high F content [1,3], and F is positively correlated with Fe, Zn, Nb, Zr, and Ga. These ore-forming elements can form stable complexes with fluorine [14]. In an oxidizing environment, Fe in a magmatic hydrothermal fluid mainly migrates in the form of a Fe³⁺–F⁻ complex, and after entering a relatively reducing environment, the Fe³⁺ in the ore-forming fluid changes to Fe²⁺, and the Fe³⁺–F⁻ complex decompounds, resulting in the precipitation of a large amount of iron to form magnetite or pyrrhotite [180,181]. At high temperatures, zinc also forms a complex with F (such as ZnF₂) [182,183]. The above findings may explain the metallogenic mechanism of zinc and iron deposits formed by A-type granite magmatism in the FVB.

6.4. Preliminary Analysis of the Metallogenetic Potential of Key Metals

The chemical analysis results of iron and zinc ores from typical deposits in the FVB, both of skarn-type and hydrothermal-type ores, show their additional utilization potential as sources of Cd, Ga, and Se (

Table 3. Key metals associated with important deposits in the FVB [77].

Name of Mine	Exploration Stage	Ore Type	Associated Key Metal and Grade
Xiaoyangchong zinc-iron mine	Mining	Massive sphalerite and magnetite ores	Cd: 100–900 ppm
Suishan zinc mine	Mineral prospecting	Sphalerite ore and pyrite ore	Cd: 1111 ppm Se: 25–60 ppm
Shunfengshan iron mine	Detailed mineral prospecting	Magnetite ore	Ga: 21 ppm
Fuchengdun copper mine	Mineral prospecting	Chalcopyrite ore	Cd: 100 ppm

) [77].

The metallic element cadmium has a very low abundance in the crust (0.2 ppm, [153]). Cadmium is mainly produced from lead–zinc mines, which account for 90% of the total cadmium resources. It has been generally believed that cadmium is mainly enriched in sphalerite by replacing Zn [184,185].

In this new round of comprehensive evaluations of metal resources in the FVB, it was found that the utilization potential of rare, scattered elements in zinc ores was great (Table 3), especially for cadmium. Taking the Xiaoyangchong zinc–iron deposit as an example, the Cd content of the massive sphalerite ore in the Xiaoyangchong mine reached an economic level for industrial production, with the content of Cd in massive magnetite ore also being relatively high. The content of Cd in sphalerite ore in the Suishan zinc deposit was also high, at 1111 ppm and 379 ppm, respectively, in two samples of sphalerite ore [77].

Ga can be enriched in metal sulfides instead of Fe, Zn, and Al, such as chalcopyrite, sphalerite, and bornite [186,187,188]. In the magnetite ore of the Shunfengshan iron mine, the content of Ga reaches 21 ppm. In addition, the anionic sulfur in sphalerite can be replaced by selenium to form selenium-rich sphalerite ore. The content of Se in sphalerite ore and pyrite ore in the Suishan zinc mine is high (25–60 ppm), reaching the economic levels of selenium.

7. Regional Petrogenetic and Metallogenetic Model

The crystallization temperatures of intrusive rocks in the FVB show a roughly rising trend (802 °C–931 °C) (Figure 18) [89], indicating that the high temperature conditions required for the formation of A-type granites were present during the extensional process [1,2,3,5]. This extension may have been caused by the roll-back movement of the Paleo-Pacific plate, resulting in upwelling of the asthenosphere [189,190,191,192]. As mentioned above, the A-type granites in the FVB are similar to coeval intrusions in the MLYMB when it comes to the magma source, with a mixture of enriched mantle and crust [71,72,84,86,136,137,141,143] components. It can be suggested that asthenospheric upwelling led to the partial melting of the enriched subcontinental mantle [5,8]. Then, the enriched magma migrated upward and underwent assimilation and contamination from the lower crust to form intermediate magma [1,3,7]. During the continued upward migration of intermediate magma, assimilation and contamination with the upper crustal materials occurred [16,17], and plagioclase and other minerals crystallized as cumulate phases. In addition, fluorine-containing minerals in the crust entered the magma system [14], in which F⁻ could replace OH⁻, forming skarn and hydrothermal ore deposits, as fluorine formed complexes with metals enriched in the A-type magma, which were concentrated into ore-forming hydrothermal solutions (Figure 18) [9,30].

The A-type granite suite in the FVB mainly intruded into the dominating Triassic carbonate strata, forming skarn-type deposits in the contact zone between the granites and the carbonate strata or yielding hydrothermal-type mineralization along fault systems after the magmatic period. The ore-forming magmatic–hydrothermal fluid of the skarn deposits in the FVB was affected by mixing with gypsum salt brine and meteoric waters [89]. The gypsum salt layer is rich in CaSO₄, MgSO₄, and Cl⁻, which is conducive to the extraction, migration, and transportation of iron, zinc, and other metal elements (Figure 19) [193,194].

8. Conclusions

The A-type granites in the FVB are mainly similar to those in the other parts of the MLYMB in terms of magmatic evolution characteristics, with fractionation caused by the crystallization and separation of orthopyroxene, plagioclase, K-feldspar, and biotite. The magmatic source of A-type granites in the FVB is mixed and includes the enriched mantle, lower crust, and upper crust materials as components, perhaps with relatively stronger participation of Archaean–Paleoproterozoic crustal materials than that of other regions in the MLYMB. The main ore types of A-type granites in the FVB are skarn-type and hydrothermal-type ores, with the ore-forming fluid being a mixture of magmatic hydrothermal fluid, meteoric waters, and deep brine related to the gypsum–salt layer. The sulfur of metal sulfides may be a mixture of magmatically derived sulfur and sulfur originating from the Triassic gypsum-bearing layers. The iron and zinc ores in the FVB may be of interest also for the recovery of Cd, Ga, Se, and other minor elements contained within the ores. Furthermore, it is concluded that the FVB potentially harbors Cu, Au, Ag, Mo, etc., in addition to the already known metals in the area.