Geology, geochemistry and genesis of the world-class Shizhushan wollastonite deposit, Mengshan area, South China

The recently discovered Shizhushan wollastonite deposit in the Mengshan district contains about 54 Mt of wollastonite, making it the world ’ s largest wollastonite deposit. Zircon U – Pb geochronological results indicate that the Mengshan composite pluton evolved during a prolonged magmatic history (ca. 230 – 218 Ma). The pluton and the surrounding carbonate units of the Maokou Formation have distinct initial 87 Sr/ 86 Sr ratios, with values of 0.7107 – 0.7162 and 0.7070 – 0.7075, respectively. Pure wollastonite crystals in the Maokou Formation are characterized by uniform 87 Sr/ 86 Sr values of 0.7070 that are indistinguishable from the carbonates, suggesting that the wollastonite was formed during contact metamorphism. Limestones of the Maokou Formation are characterized by variable δ 13 C V-PDB ( (cid:0) 5.2 to 4.0 ‰ ) and δ 18 O SMOW (10.3 to 22.6 ‰ ) values. Carbonates with the lowest δ 13 C V-PDB and δ 18 O SMOW values (and elevated 87 Sr/ 86 Sr ratios) occur in the vicinity of a regional structure, suggesting that magmatic – hydrothermal fluids and CO 2 were channeled along this fault. Based on new geological, geochronological and geochemical data, we propose that the world-class wollas-tonite deposit formed as a result of several factors related to a relatively stable depositional environment during the Late Paleozoic as well as structural deformation and magmatic events during the Early Mesozoic. These features include: 1) juxtaposition of abundant sediment-hosted chert (SiO 2 ) and limestone (CaCO 3 ) units, 2) a prolonged heat source provided by multiple magmatic events, 3) CO 2 degassing along the regional structure promoted wollastonite-forming reactions, and 4) the proximity of the Si-and Ca-rich sedimentary layers sub-parallel to the intrusion contact. Our study documents the important roles of Early Mesozoic deformation and magmatism in the genesis of non-metallic mineral deposits in south China.


Introduction
Wollastonite [Ca 3 (SiO 3 ) 3 ] is an important non-metallic commodity with common applications in the ceramics industry, metallurgy, construction industry, plastic production and coating processes due to its high whiteness, and excellent insulation and heat resistance properties (Asar et al., 2010;He et al., 2020;Miu et al., 2021).There are two key geological processes required for the formation of economic wollastonite deposits.The most important process is represented by wollastonite skarns, which can result from contact metamorphism of siliceous limestone or from chemical alteration of limestone by siliceous, deuteric hydrothermal fluids (e.g., Grammatikopoulos and Clark, 2006;Heinrich, 1993;Higgins et al., 2001;Joesten, 1974;Joesten and Fisher, 1988;Lackey and Valley, 2004;Milke and Heinrich, 2002).The second mechanism is represented by regional high-grade metamorphism, which forms as a result of deep burial processes (e.g., Sengupta et al., 2009;Zelt, 1980).A significant example of a metamorphic wollastonite deposit is the Garies deposit in the Bushmanland Subprovince of South Africa, where the host rock underwent granulite-facies metamorphism, with peak temperatures and pressures of 800-860℃ and 7 kbar, respectively (Seto et al., 2006;Zelt, 1980).
Skarn deposits commonly develop at the contact zone between igneous intrusions and carbonaceous wallrocks, and can be associated with economic grades of W, Sn, Mo, Cu, Zn, Pb, and Fe (e.g., Einaudi et al., 1981;Meinert et al., 2005;Shu et al., 2020;Shu and Chiaradia, 2021;Xiong et al., 2020;Zhao et al., 2022).The genesis of wollastonite in these settings is typically related to the reaction between carbonates and deuteric fluids exsolved from the pluton (Chang and Goldfarb, 2019;Meinert et al., 2005).Many large wollastonite deposits that have been documented formed in this way, such as the St. Lawrence, Platinova, and Olden wollastonite skarns in Ontario, Canada, which contain 9 Mt, 4 Mt, and 2.8 Mt of wollastonite resources, respectively (e.g., Grammatikopoulos and Clark, 2006;Grammatikopoulos et al., 2005;Higgins et al., 2001;Lackey and Valley, 2004).It is important to note, however, that wollastonite formation in the absence of significant hydrothermal activity has also been reported, such as the Bufa del Diente deposit in Mexico and the Christmas Mountain deposit in USA (e.g., Joesten and Fisher, 1988;Milke and Heinrich, 2002).In these scenarios, wollastonite was formed by the reaction of chert and calcite during thermal contact metamorphism (e.g., Joesten and Fisher, 1988;Milke and Heinrich, 2002).
The Shizhushan wollastonite deposit in the Mengshan district, Jiangxi Province, China, represents a recently discovered world-class wollastonite deposit, with a total wollastonite resource of 54 Mt (Hu et al., 2018), ranking it as the largest wollastonite deposit in the world (see details in appendix Table S1).The discovery of the Shizhushan deposit has been reported in several Chinese publications (e.g., Chen and Chen, 2018;Wang et al., 2019), but little work has been done to describe the mineralization mechanisms.In a recent study, Yang et al. (2021) described the petrology of the Mengshan composite pluton, but only briefly mentioned the wollastonite mineralization.The geology of the Shizhushan deposit and the key factors that led to the development of world-class wollastonite mineralization, therefore, remain poorly constrained.
In this contribution, we document, in detail, the geological, petrographic, and mineralogical features of the Shizhushan wollastonite deposit in the Mengshan district.We constrain the age of mineralization by dating the genetically related composite granitic pluton using zircon U-Pb geochronology.Additionally, we systematically characterize the Sr-C-O isotope compositions of representative rock samples collected from drill hole ZK501, which intersects the complete lithological sequence of host sediments, the intrusion, and high-grade wollastonite mineralization.The geological and geochemical characteristics are integrated to develop a genetic model to (1) elucidate the metamorphic vs. hydrothermal origin of this giant wollastonite deposit, (2) characterize the key factors that contributed to the wollastonite mineralization, and (3) highlight the importance of Early Mesozoic deformation and magmatism in forming non-metallic deposits in South China.

Regional geology
The Mengshan area is situated at 114  Plan view of the Shizhushan mining area illustrating the geology of the area, location of regional faults, and outcrops of wollastonite orebodies.The white dots represent the position of drillhole collars used to delineate the deposit (modified after Wang et al., 2019).Three cross-sections in different directions (A-A', B-B', C-C') through the Shizhushan deposit.b.The E-W cross-section (A-A') illustrating the main wollastonite orebodies.c.Crosssection B-B' illustrating the location of drill hole ZK501, which intersects the complete sequence of sedimentary rocks, wollastonite ore, and granite; this drill hole was systematically sampled and analyzed in this study.d.Cross-section C-C' illustrating the morphology of and relationship between the orebodies of the Shizhushan deposit, the Maokou Formation, and the Mengshan granite pluton.
the city of Pingxiang in Jiangxi Province, southern China.It is located in the Pingle Depression within the eastern segment of the Qin-Hang Metallogenic Belt (Mao et al., 2013;You et al., 2009;Zhang et al., 2018) (Fig. 1a).The Qin-Hang Metallogenic Belt was a convergent margin between the Yangtze and Cathaysia blocks during the early Paleozoic, and underwent folding and deformation during the Triassic and Cretaceous (Shu et al., 2008).Several large mineral deposits were produced during these folding and deformation events, including skarn deposits (Chang and Goldfarb, 2019), quartz vein-type deposits (Peng et al., 2006), and porphyry deposits (Mao et al., 2011).
The Pingle Depression, which is bounded by the Pingxiang-Guangfeng and Yifeng-Jingdezhen faults in the north of Jiangxi Province, is a large-scale nappe system that developed on the margin of the Yangtze Block during the late Paleozoic to Middle Triassic (Fig. 1b) (Song et al., 2003).In general, the locations of the Mengshan granitic pluton and related deposits in the Pingle Depression are controlled by NE-trending structures.The main fold structures in the depression are the Mengshan duplex anticline and a series of secondary anticlines, with axes trending in the NE and NNE direction.The lower Permian and Carboniferous strata exposed in the core and flanks of the anticline dip at angles of 40-50 • .Tectonic processes produced folding, thrusting, and stretching structures, resulting in the exposure of late Paleozoic and Mesozoic rock units in Mengshan and surrounding areas.
Late Paleozoic to Mesozoic strata are exposed in the Mengshan and surrounding areas, and are unconformably underlain by Neoproterozoic basement rock.Regional strata comprise 1) Carboniferous dolostone and limestone interbedded with dolostone and carbonaceous mudstone, 2) Permian micritic limestone, bioclastic limestone, and micritic dolostone, and 3) Triassic fine-grained, calcareous sandstone interbedded with oolitic limestone.The Carboniferous Huanglong Formation, and middle Permian Qixia and chert-rich Maokou formations are the carbonate country rocks that are in direct contact with the Mengshan granitic pluton (Fig. 1c).The 185-361-m thick Huanglong Formation is predominantly composed of dolostone.The middle Permian Qixia Formation is composed of a lower sequence of asphaltene limestone, bioclastic micrite, calcareous siltstone, and black carbonaceous mudstone, and an upper sequence of chert-bearing, bedded limestone (You et al., 2006).Outcrop of the chert-rich Maokou Formation, which mainly occurs on the east and west sides of the Mengshan composite pluton, is subdivided into lower and upper sequences; the lower sequence contains bioclastic limestone, marble, and siliceous rocks interspersed with grey-black, thinly bedded limestone, whereas the upper sequence contains dark-light gray limestone, thickly bedded, gray, chert-bearing marble, limestone, and micrite.The abundance of chert can be up to 20-30 vol.% in some intervals of the Maokou Formation.
The intrusive sequence of the Mengshan composite intrusion was constrained by LA-ICP-MS U-Pb geochronology of zircon, which yielded ages that cluster at 236 ± 3 Ma, 220 ± 3 Ma, and 217 ± 1 Ma for the first, second, and third phases, respectively (Zhong et al., 2011); significant variability in the ages within each sample is, however, observed (first phase: 242-222 Ma, second phase: 228-201 Ma, and third phase: 230-210 Ma).The significant overlap in the second and third intrusive phases resulted from the fact that the authors reported the 206 Pb/ 238 U age of each sample as a weighted mean age.A more recent study reported LA-ICP-MS 206 Pb/ 238 U ages that cluster at 226.6 ± 0.5 Ma for the biotite granite porphyry (i.e., the first intrusive phase), although the 206 Pb/ 238 U ages of these zircons varies from 223 to 230 Ma (Yang et al., 2021).
The country rocks adjacent to the composite intrusion were variably altered.The width of the contact aureole ranges from several hundred meters to 2,000 m.In the outer metamorphic zone, the limestone has generally been metamorphosed to marble as a result of thermal contact metamorphism.Calc-silicate skarn dominated by andradite, wollastonite, and minor diopside occurs within the pluton where it contacts the limestone wallrocks (Liao, 2012;Zhong et al., 2011).In the contact aureole, small skarn-type W-Cu-Pb-Zn and Sn-Cu-Pb-Ag polymetallic deposits formed in the Heyi (Yang et al., 2021) and Taizibi mining areas (Liao, 2012;Xiao and Yu, 2019;Yang et al., 2021), respectively.None of these polymetallic deposits, however, have been demonstrated to host economic concentrations of metals.For example, the total Sn and Cu resources of the Mengshan area are on the order of 5,400 and 9,400 tons, respectively (Yang et al., 2021).By contrast, deposits of non-metallic minerals, such as the 54 Mt Shizhushan wollastonite deposit, are typically large and of economic grade.

Wollastonite orebodies
Four separate wollastonite deposits occur in the Mengshan mining area, those being the Caofangmiao, Yueguangshan-Heyi, Yapokeng, and Shizhushan wollastonite deposits, all of which share similar geological features.The orebodies are cut by several thrust faults, namely the NWW-SEE-trending Taizibi Fault (Fig. 1c), and the E-W-trending F2, F31, and F32 faults (Fig. 2a).The main body of the Shizhushan wollastonite deposit is concealed in the mining area, although a few small wollastonite orebodies do outcrop.Compilation of drill core logs indicate that the orebodies occur predominantly as parallel layers and beds.As shown in an E-W-trending cross-section (A-A'), the Shizhushan deposit comprises twelve wollastonite orebodies (Fig. 2b; also see Table 1).The two major orebodies within the Maokou Formation, denoted as the No. II and No. VIII orebodies, appear to be stratified and lenticular in shape, strike in the east-west direction, and dip to the south, with dip angles of ~ 20-40 • .These orebodies stretch for more than 4,800 m and 1,000 m along strike, and more than 1,300 m and 600 m along trend, and have average thicknesses of about 14.1 m and 14.7 m, respectively (Hu et al., 2018;Wang et al., 2019;You et al., 2019).Cross-section B-B' (Fig. 2c) demonstrates that the main orebodies at Shizhushan are greater than 20 m away from the granitic pluton, and the boundary of the pluton parallels the strata of the Maokou Formation.Cross section C-C' illustrates the distribution of the deeper and larger orebodies at Shizhushan (Fig. 2d).Two orebodies are associated with two main faults, F31 and F32.Fault F31 crosscuts and offsets the wollastonite orebodies, indicative of post-mineralization fault formation.The occurrence of Sn-Cu skarn mineralization within fault F32, however, implies that the development of this fault occurred prior to or during skarn mineralization.
Overall, the wollastonite orebodies of the Shizhushan deposit occur in the layered, chert-rich limestone of the Maokou Formation (Fig. 3a-b), and are bedded and lenticular in shape, with thicknesses varying from several decimeters to tens of meters (Fig. 3c-e).Economic orebodies are horizontally distributed at distances of ~ 30-300 m from the Mengshan granitic pluton.

Fig. 3.
Representative photographs of wollastonite ore and calc-silicate nodules from outcrops and drill core in the Mengshan area.a. Fresh, unaltered outcrop of intercalated chert and limestone layers of the Maokou Formation, which is located about 8 km southwest of the Mengshan granitic pluton.b.Outcrop of chert-bearing, metamorphosed limestone of the Maokou Formation; the gray, irregularly shaped wollastonite ores occur in layers.c.Thick, irregular bands of wollastonite, which represent a more complete reaction between layered chert nodules and marble.d.Laminated wollastonite ore occurring subparallel to the bedding of the Maokou Formation.Individual wollastonite layers have a thickness ranging from a few centimeters to a few decimeters, and has sharp boundaries with the marble.e. Highgrade wollastonite ores located at depths of 508-515 m in drill core ZK501.f.Nodular structures indicative of the incomplete reaction between chert nodules and marble; note the chert core, wollastonite shell, and marble groundmass.g.Weathered wollastonite and marble proximal to the Mengshan pluton.Wo = wollastonite.

Petrography and mineralogy
The host rocks of the wollastonite orebodies are marble and limestone.Three types of wollastonite ores are distinguished in hand specimennodular, laminated, and massive, with laminated and nodular varieties being the dominant types.Nodular ores are 5-30 cm in size and exhibit a concentric structure, with a core comprising chert, a mantle comprising wollastonite, and an outer crust comprising marble (Fig. 3f,  g).The wollastonite mantle commonly exhibits two grain sizes of wollastonite, with coarse-grained wollastonite occurring in the outer portions of the mantle adjacent to calcite and fine-grained wollastonite occurring in the inner portions of the mantle adjacent to chert (Fig. 4a-b).Notably, the calcite around the outer boundary of wollastonite mantle is sometimes coarse-grained and light blue in color (Fig. 4c); this association has also been reported by Gerdes and Valley (1994).Laminated ores generally comprise pure wollastonite, apart from the occurrence of minor dissemination of diopside; they range in thickness from several centimeters to several decimeters (Fig. 3c-d).
Massive ores are composed of wollastonite, garnet (andradite), and calcite, but are devoid of chert (Fig. 4d); they generally occur within meters of the contact with the granitic pluton and do not form economic orebodies.
Based on micro-textural observations, wollastonite is divided into different textural types.Type I wollastonite occurs as coarse-to finegrained, euhedral-subhedral crystals, which were replaced by diopside (Fig. 5a).Type II micron-scale wollastonite occurs as veinlets in a marble matrix (Fig. 5b).Type III wollastonite appears as radial clusters of fine-grained, subhedral-fibrous crystals (Fig. 5c).Lastly, type IV wollastonite, which is the most common type within the orebodies at Shizhushan, occurs as coarse-grained crystals or bicrystals adjacent to recrystallized calcite, with diopside microcrystals scattered along the contact (Fig. 5d).Vein-type wollastonite (type II) only occurs at depths of 520 m in drill hole ZK501.Radial wollastonite microcrystals (type III) commonly occur in chert-dominated rocks where calcite is rare.

Samples
Samples utilized for geochemical analyses in this study were collected systematically along drill core ZK501.At 668.36-m long, it is the longest drill core located south of the Mengshan composite intrusion and intersects the entire Maokou Formation, the top of the granitic intrusion, and the major wollastonite orebodies (Fig. 2c).This drill core is, therefore, representative of the deposit and can be used to assess the mechanisms responsible for the formation of wollastonite mineralization.A total of 88 samples were collected at intervals of 5-10 m along the length of the drill core for Sr-C-O isotope analyses.Representative samples of carbonate rock and skarn were also selected for C-O isotope analyses, including the limestone wallrocks, limestone near the fault, wollastonite-associated marble (i.e., the calcite near the margin of wollastonite), and skarn (n total = 5).Two samples of the Maokou Formation (MS44, MS45A) were collected from an outcrop 8 km away from the composite intrusion (Fig. 3a) to avoid the effects of contact metamorphism induced by intrusion of the pluton.Lastly, four pure wollastonite crystals were selected for Sr isotope analyses.
Four samples of the granitic pluton were selected for zircon U-Pb geochronology.Two granite samples were collected at depths of 655 m and 668 m along the length of drill core ZK501, and are considered to be genetically related to the wollastonite mineralization.The other two granite samples (MS30, MS38) were collected from the marginal area of the eastern portion of the granitic intrusion, and are considered likely to be related to wollastonite mineralization (Yang et al., 2021).All of these Details of the methodology utilized for zircon geochronology, geochemical analyses (carbon, oxygen, and strontium isotopes, Sr/Ca ratios), and electron microprobe analyses of wollastonite, pyroxene, and garnet are provided in the Supplementary Material.

Sr isotopes
Most of the carbonates of the Maokou Formation have 87 Sr/ 86 Sr values within the range of 0.707005-0.707097(Table 2 and Fig. 7), which are similar to the 87 Sr/ 86 Sr composition of Capitanian seawater, which is the time when the Maokou Formation deposited (Qu et al., 2021).Significant variations in the 87 Sr/ 86 Sr i and Sr/Ca ratios do, however, occur in samples from 250 to 380 m depth in the drill core, where breccia developed as a result of movement of the F2 fault.The peak 87 Sr/ 86 Sr value (0.717122) of this interval is similar to the 87 Sr/ 86 Sr i of the granitic intrusion (0.7107-0.7162).Among the four wollastonite samples that were analyzed for Sr isotopes, three have carbonate-like 87 Sr/ 86 Sr ratios of 0.707056-0.707063and one is characterized by a slightly higher ratio of 0.707434; the latter value is still notably lower than the 87 Sr/ 86 Sr i composition of the granitic intrusion.

C-O isotopes
The drill core samples exhibit remarkable variability in C and O isotopes (Table 2, Fig. 7).The δ 13 C of limestone in the drill core ranges from − 5.2‰ to 4.0‰.Forty out of the 65 drill core samples have δ 13 C values between 2.3‰ and 3.7‰, which are similar to, or slightly lower than the composition of coeval Permian marine carbonates (3.1‰ to 4.6‰) (Laya et al., 2013).The carbonate samples exhibit a large variation in δ 18 O values between 10.3‰ and 22.6‰.Overall, there is a broad positive correlation between δ 18 O and δ 13 C values along the length of the drill core.Low δ 18 O and δ 13 C values characterize the skarn samples, limestone near the fault zone, and samples associated with wollastonite orebodies.For comparison, two unaltered limestone samples (MS44, MS45A) collected 8 km from the intrusive body are characterized by higher δ 13 C and δ 18 O values that average 3.7‰ and 23.0‰, respectively.

History of magmatism at Shizhushan and the heat source for the wollastonite formation
Zircon U-Pb geochronological results demonstrate a prolonged magmatic history (ca.230-218 Ma) for the Mengshan composite intrusion (Fig. 6).Our results are consistent with the U-Pb geochronological studies of Zhong et al. (2011) and Yang et al. (2021) on the same pluton, as well as with geological evidence indicating that the Mengshan granite represents a composite pluton formed by multiple magmatic pulses.Fig. 6e illustrates all of the age data from this study, and the studies of Zhong et al. (2011) and Yang et al. (2021) as histogram and probability diagrams.Three 206 Pb/ 238 U age peaks are evident, with the youngest peak at 218 ± 1 Ma (n = 60, MSWD = 2.4, 1σ), the middle peak at 224 ± 1 Ma (n = 27, MSWD = 0.19, 1σ), and the oldest age peak at 230 ± 1 Ma (n = 73, MSWD = 0.66, 1σ).
The distribution of zircon ages cannot be an analytical artifact.Zircon is a highly resistant mineral, with high closure temperatures for the U-Pb isotope system (Cherniak and Watson, 2003).In this study, all of the zircon grains exhibit clear oscillatory zoning that is typical of magmatic zircon (Fig. 6f), and the majority of the zircon grains yield concordant 206 Pb/ 238 U and 207 Pb/ 235 U ages (Fig. 6a-d).The ~ 12 Ma (ca.230-218 Ma) interval between age peaks is well above the typical analytical uncertainty of the LA-ICP-MS U-Pb dating method (2-3 Ma).
Additionally, episodic magmatism recorded by zircon grains from composite plutons is not uncommon in the literature.For example, Miller et al. (2007) reported U-Pb concordant ages dispersed over an interval of 5-8 Ma in the Mt.Stuart and Tuolumne batholiths, which were interpreted to be the result of recycling of zircon antecrysts during pulsed magma injection that originated from the same magma chamber.Likewise, protracted emplacement of plutonic systems lasting 10 Ma and 12 Ma has also been documented by Coleman et al. (2004) and Davis et al. (2012), respectively.
Based on the zircon U-Pb data, we suggest that the Mengshan composite intrusion experienced episodic, protracted magmatism extending over 12 Ma.It is important to note that the bimodal distribution of zircon ages exists within individual granitic samples from drill core ZK501, namely 655 m and 668 m (Fig. 6a, b); these two samples are directly below the wollastonite orebodies, which indicates that the wollastonite mineralization at Shizhushan could have a connection to these magmatic episodes.The occurrence of multiple magmatic episodes is important because they can provide abundant and sustained heat to promote the formation of larger volumes of wollastonite compared to a single magmatic event.

Source of Ca and Si for the formation of wollastonite
Formation of wollastonite deposits requires the mobilization and concentration of both Ca and Si given their 1:1 stoichiometry in wollastonite.At Shizhushan, all wollastonite orebodies occur in the Maokou Formation, which is dominated by carbonate rocks.Indeed in the formation of wollastonite deposits the key limiting factor is typically the supply of silica (Abu El-Enen et al., 2004;e.g., Gerdes and Valley, 1994;Grammatikopoulos and Clark, 2006;Higgins et al., 2001).At Shizhushan, silica could have been derived from the banded and nodular chert in the Maokou Formation or, alternatively, supplied from hydrothermal fluids that exsolved from the Mengshan granitic pluton.Geological evidence does not support a hydrothermal origin of Si at Shizhushan because, if this was the case, the wollastonite mineralization would be expected to be concentrated along the lithological contact between the granitic pluton and the limestone wallrocks.However, only minor wollastonite mineralization occurs directly along the pluton-limestone contact.In fact, the main wollastonite orebodies typically occur at distances of greater than 20 m away from the granitic pluton.
The wollastonite mineralization at Shizhushan is stratabound and the orebodies occur subparallel to the sedimentary layering of the Maokou Formation.Additionally, the ubiquitous nodular or banded ores exhibit a concentric structure, with chert in the central portion, calcite in the outer portion, and wollastonite in between (Fig. 3c, d; Fig. 4a, c); this structure is likely due to the incomplete decarbonation between the inherited chert nodules and limestone rather than fluid infiltration.These sedimentary and concentric structures, which are typical features of the entire Mengshan mining area, support the hypothesis that the silica was derived from the Maokou Formation itself, rather than from fluids that infiltrated the Maokou Formation, and that wollastonite formed in a diffusion-driven growth process (e.g., Joesten and Fisher, 1988;Milke and Heinrich, 2002).This is further supported by microtextural evidence.The primary nucleus of wollastonite associated with calcite occurs in siliceous rock (Fig. 5e), and the chert residues are scattered at the boundary between chert and wollastonite (Fig. 5f).These features are an indication that thermal metamorphism affected the calcite-bearing siliceous rock, where wollastonite was formed through diffusion of Ca-Si ions (Milke and Heinrich, 2002).Additionally, the variation in calc-silicates (diopside and wollastonite) in wollastonite ore (Fig. 4a-c; Fig. 5j) indicates that the ores were initially subjected to prolonged high temperatures followed by relatively lower-temperature heating, the latter of which facilitated the formation of small volumes of finer-grained wollastonite.
A significant increase in 87 Sr /86 Sr i (from 0.7070 to 0.7171) and 1000 × Sr/Ca ratios (from 1.7 to 18.2) occurs at depths of 250-360 m, with the higher 87 Sr /86 Sr i values (0.7111-0.7171) being similar to that of the Mengshan composite pluton.In this interval, the drill core intersected the regional fault and minerals indicative of hydrothermal alteration, such as sericite, chlorite, fluorite, and vesuvianite, are present (Fig. 4e,  f).This combination of mineralogical, structural, Sr isotope, and Sr/Ca evidence suggests that hydrothermal fluids exsolved from the magma and caused the carbonate to have elevated 87 Sr /86 Sr i and Sr/Ca ratios (e. g., Peiffer et al., 2022).It should be noted that, given the order of magnitude greater 1000 × Sr/Ca ratios (up to 18.2) of limestone at depths of 250-360 m relative to limestone at other depths (1000 × Sr/ Ca = 0.5-1.7), the hydrothermal fluids must have contained significant amounts of Sr.The corresponding shift in 87 Sr/ 86 Sr i and Sr/Ca ratios is, therefore, a strong indication of hydrothermal activity in which dissolved Si could have been introduced into the system.
The 87 Sr/ 86 Sr i values of three wollastonite crystals (0.707056, 0.707063, 0.707061) in the Maokou Formation are indistinguishable from those of the carbonate wallrocks and Permian seawater, and are much lower than the 87 Sr /86 Sr i values of the Mengshan granite (Fig. 7).This is supportive of a diffusion-driven reaction during wollastonite formation, where the Sr was derived from the carbonate wallrocks accompanying the diffusion of Ca (Faure and Powell, 1973).One exception is the wollastonite crystal collected at a depth of 518 m, which has a 87 Sr/ 86 Sr i value (0.7074) that is slightly different from the other three samples.Localized variation in 1000 × Sr/Ca ratios have also been observed at depths of 506-539 m, with a peak value (1000 × Sr/Ca = 8.9) at 520 m (Fig. 7), which indicates that small amount of wollastonite may have been slightly affected by the infiltrating fluids.
Overall, the geological evidence, namely the predominance of nodular and banded chert in the Maokou Formation, the concentric structure of wollastonite ores, and the occurrence of wollastonite orebodies, as well as the carbonate wallrock-like 87 Sr/ 86 Sr i values of wollastonite crystals, it can be concluded that wollastonite at Shizhushan formed primarily as a result of contact metamorphism.It should be noted that signs of hydrothermal activity, such as localized sulfide mineralization and the appearance of skarn minerals (i.e., andradite, fluorite), are observed, but only within 20 m of the contact aureole.
It should be noted that distinct negative shifts in both δ 13 C and δ 18 O values occur at depths of 250-360 m in drill core ZK501, where carbonate breccia and the regional fault were intersected and no wollastonite formed.The low δ 13 C signatures of carbonate samples in this fault zone could be the result of overprinting by magmatic CO 2 exsolved from the pluton since magmatic CO 2 has low δ 13 C values of − 5‰ to − 8‰ (Troll et al., 2012;Whitley et al., 2019).This interpretation is consistent with the low δ 13 C (-5.2‰ to 0.1‰) and δ 18 O values (11.2‰ to 12.8‰) observed in the five skarn samples that formed via infiltration of magmatic fluids, and the Sr isotopes and Sr/Ca ratios were supportive of hydrothermal activity at this interval as well.
The fault zone could have acted as a channel for degassing of the CO 2 produced by reaction of SiO 2 and CaCO 3 in the Maokou Formation under the heat provided by the composite intrusion.The equation for wollastonite formation (SiO 2 + CaCO 3 = CaSiO 3 + CO 2 ) indicates that removal of CO 2 from the system is crucial to the formation of wollastonite as it represents the key factor required to control this reaction.Indeed, experiments conducted by Harker and Tuttle (1956) demonstrated that the temperature of formation of wollastonite is controlled by the partial pressure of CO 2 (pCO 2 ).Generally, the temperatures required for wollastonite formation can be lower at lower pCO 2 .Development of these fault systems to efficiently remove CO 2 is, therefore, an important contributing factor to the formation of large wollastonite orebodies.It should be pointed out, however, that the presence of fractures, breccia zones, and faults may not always be favorable to the formation of wollastonite given that these structures can also permit the circulation of groundwater and thus promoting heat loss.

Geometric relationships between sedimentary bedding and intrusions
Formation of wollastonite requires the juxtaposition of Si-and Cabearing materials, and reaction of these source materials at sufficiently high temperatures (e.g., Grammatikopoulos and Clark, 2006;Jacobs and Kerrick, 1981).The world-class wollastonite mineralization in the Mengshan district requires large volumes of source rocks and the input of a magmatic heat source producing the critical temperature.The zone of "critical temperature" refers to the lowest temperature required  9d), which leads to the formation of relatively minor wollastonite deposits.c.Parallel relationship between the pluton boundary and the Maokou Formation corresponding to the Shizhushan mining area; this geometric relationship is, in part, responsible for the formation of the large volumes of wollastonite in the Mengshan deposit.d.Schematic map illustrating the distribution of wollastonite deposits in Mengshan.Note that the line for zone of "critical temperature" refers to the lowest temperature in which wollastonite can form, and depends on the volume of the granitic pluton, the distance from the pluton, and the thermodynamic properties of the ore-bearing rocks.
to initiate the wollastonite-forming reaction.In Fig. 9, the zone of "critical temperature" is loosely defined as the temperature within which wollastonite was produced because the temperature was high enough to initiate the reaction.The contact angle between the intrusion and wallrock strongly affects the volume of wollastonite mineralization that was within the zone of "critical temperature".For example, if the bedding of the Maokou Formation was perpendicular to the lithological contact, the volume of wollastonite ore would be limited by the thickness of the zone of "critical temperature".If, however, the bedding of the Maokou Formation was parallel to the lithological contact and within the zone of "critical temperature", then the volume of wollastonite ore would be greatly expanded, which is the case at Shizhushan.
The proximity and parallelism of the sedimentary bedding (i.e., Siand Ca-rich layers) to the intrusion-wallrock interface, therefore, plays a pivotal role in controlling the endowment of wollastonite deposits.Indeed, systematic drilling projects throughout the Shizhushan mining area (Fig. 1) demonstrated that the intrusion-wallrock interface is parallel to the chert-rich layers and chert-derived wollastonite deposits.The fact that the large orebodies in this deposit can extend for kilometers along the trend and strike of the bedding is, therefore, likely due to the parallelism of the Maokou Formation and the intrusion-wallrock interface, which resulted in the enclosure of large slices of Maokou Formation strata within the zone of "critical temperature" around the composite intrusion.In contrast, the smaller wollastonite deposits that occur on the northeast side of the composite intrusion (e.g., the Yueguangshan and Heyi deposits) formed in a setting where the bedding of the Maokou Formation is oblique to the intrusion-wallrock interface (Fig. 9b).It is worth noting that the purity of the carbonate rocks of the Maokou Formation (e.g., a lack of Al) was also a favorable characteristic for the formation of large volumes of wollastonite.This purity limited the complexity of reactions that could have occurred during skarn formation, which would have ultimately decreased the amount of wollastonite that formed (Chang and Goldfarb, 2019).

Genetic model for the Shizhushan wollastonite deposit
A genetic model is proposed here for the Shizhushan wollastonite deposit (Fig. 9).During the late Paleozoic, the presence of a shallow marine environment in South China permitted the accumulation of dolomite, chert-bearing limestone, and siltstone (e.g., Shu et al., 2008).Subsequent tectonic activity resulted in regional-scale folding and thrusting of these sedimentary rocks (e.g., Shu et al., 2015).After this tectonic activity, these sedimentary rocks were intruded by at least three temporally distinct pulses of crustally derived magma along the Mengshan anticline (Yang et al., 2021;Zhong et al., 2011).These pulses occurred between 230 and 218 Ma and provided abundant heat to the wallrocks, promoting the metamorphic conversion of the chert-rich layers to the wollastonite ores via the reaction SiO 2 (s) + CaCO 3 (s) = CaSiO 3 (s) + CO 2 (g).Regional faults served as the channels for removing degassed magmatic volatiles and CO 2 that were produced during wollastonite formation.Subsequent crustal uplift, weathering, and erosion exposed parts of the orebodies.Taken together, the key factors responsible for the formation of the variably sized (1000-4800 m long in strike, 600-1300 m in trend, 2-51 m in thickness) wollastonite deposits in the Shizhushan mining area are i) the variable geometry of the chert-rich Maokou Formation and its lithological contact with the Mengshan granitic pluton, and ii) the availability of chert-bearing limestone intercalations.

Conclusions
The recently discovered Shizhushan wollastonite deposit in the Mengshan district, Jiangxi Province, China represents the largest wollastonite deposit in the world.Geological and geochemical data reveal that the Shizhushan wollastonite deposit was formed as the product of contact metamorphism of the Maokou Formation associated with the episodic emplacement of the Mengshan composite pluton between 230 Ma and 218 Ma.World-class wollastonite mineralization at Shizhushan is the product of several key process:1) the juxtaposition of SiO 2 -and CaCO 3 -bearing rock units; 2) the composite granite acting as a prolonged heat source; and 3) effective CO 2 degassing mechanism, and 4) the proximity of the Si-and Ca-rich sedimentary layers subparallel to the intrusion contact.Our data document the importance of the Early Mesozoic deformation and magmatic events in the genesis of nonmetallic deposits in Southern China.Our study has important implications for the exploration of wollastonite deposits worldwide.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 1 .
Fig. 1.Geological setting of the Mengshan wollastonite deposit.a. Tectonic background of the Mengshan deposit.b.Topography of the Pingle Depression, where the Mengshan deposit is located.c.Geological map of Mengshan (modified after Wang et al., 2019).

Fig. 2 .
Fig.2.Geology of the Shizhushan mining area at Mengshan.a. Plan view of the Shizhushan mining area illustrating the geology of the area, location of regional faults, and outcrops of wollastonite orebodies.The white dots represent the position of drillhole collars used to delineate the deposit (modified afterWang et al., 2019).Three cross-sections in different directions (A-A', B-B', C-C') through the Shizhushan deposit.b.The E-W cross-section (A-A') illustrating the main wollastonite orebodies.c.Crosssection B-B' illustrating the location of drill hole ZK501, which intersects the complete sequence of sedimentary rocks, wollastonite ore, and granite; this drill hole was systematically sampled and analyzed in this study.d.Cross-section C-C' illustrating the morphology of and relationship between the orebodies of the Shizhushan deposit, the Maokou Formation, and the Mengshan granite pluton.

Fig. 4 .
Fig. 4. Photographs of hand samples illustrating the features of the wollastonite ores in the Mengshan deposit.a. Nodules with fine-and coarse-grained wollastonite, and residual chert in a groundmass of dark grey marble.b.Pure, fine-and coarse-gained wollastonite in the inner and outer portions of a nodule.c.Concentric shell of the calcsilicate nodules cut by drill hole ZK501 showing mineralogical zonation.Note the association of light blue and coarse crystalline calcite with wollastonite.d.Massive wollastonite ore that occurs within 20 m of the Mengshan granitic pluton; the wollastonite, garnet, and calcite contacts are ragged, and garnet is embedded in the fibrous wollastonite, indicative of fluid infiltration.e. Sample of vesuvianite skarn from a depth of 282 m hosting a thin red garnet vein.f.Alteration of a carbonate wallrock sample (depth = 376 m) by purple and yellow fluorite, indicative of fluid infiltration.Cal = calcite, Wo = wollastonite, Ves = vesuvianite, Grt = garnet, Fl = Fluorite.

Fig. 5 .
Fig. 5. Photomicrographs illustrating micro-textural features of wollastonite and coexisting minerals.a. Diopside was partially replaced by fine-grained wollastonite.b.Micron-scale wollastonite veinlet that crosscuts recrystallized marble.c.Radial wollastonite microcrystals that have replaced chert and are perpendicular to the interface.d.Various morphologies and sizes of diopside crystals that formed along the interface between coarse-grained wollastonite and recrystallized calcite.e. Wollastonite nucleus within siliceous rock comprising an assemblage of wollastonite and calcite.f.Reaction boundary between calcite and chert.The chert residues scatters at the boundary and where micro-cracks occur.g.Diopside that formed along the boundary between calcite and wollastonite.h and i. Massive wollastonite skarn comprising garnet, wollastonite, fluorite, calcite, and diopside.j.Photomicrograph was taken in one long sample, illustrating the typical variation in calcsilicate mineral assemblage in wollastonite oresfrom left to right, fine-grained diopside + wollastonite, coarse-grained wollastonite, and diopside + chert + calcite.Cal = calcite, Qtz = quartz, Wo = wollastonite, Di = diopside, Fl = fluorite, Grt = garnet.

Fig. 6 .
Fig. 6. (a-d) Concordia diagrams of zircon U-Pb geochronological data.a. Granite sample from a depth of 655 m in drill core ZK501 yields a two-stage age spectrum clustering at 230 ± 2 Ma and 219 ± 2 Ma. b.Bimodal age distribution of a granite sample from a depth of 668 m in drill core ZK501; the ages are indistinguishable from those of the sample from 655 m, within uncertainty.c.Sample MS30 yields a weighted mean 206 Pb/ 238 U age of 230 ± 1 Ma, corresponding to the older age of samples from 655 m and 668 m. d.Sample MS38 yields a weighted mean 206 Pb/ 238 U age of 217 ± 1 Ma, corresponding to the younger age of samples from 655 m and 668 m. e. Histogram and probability diagram summarizing the three-peak distribution of 206 Pb/ 238 U ages.The blue and yellow columns represent data from Zhong et al. (2011) and(Yang et al., 2021), respectively.f.Representative cathodoluminescence images of zircon grains for samples MS30 and MS38, as well as those obtained at depths of 655 m and 668 m in drill core ZK501, showing the locations of U-Pb analyses.

Fig. 7 .
Fig. 7. Stratigraphic column for drill core ZK501 illustrating variations in C-O-Sr isotopes and Sr/Ca ratio.

Fig. 8 .
Fig. 8. Binary diagram illustrating the carbon and oxygen isotopic variation of carbonate samples.The initial isotopic values of δ 13 C (δ 13 C VPDB = 3.7‰) and δ 18 O (δ 18 O SMOW = 23.0‰)utilized in the model are based on the composition of two samples collected distal to the Mengshan pluton, which are thought to be unaltered.The light blue area shows the temperature of wollastonite formation associated with degree of decarbonation.Wo = wollastonite.

Fig. 9 .
Fig. 9. Schematic model illustrating the formation of Mengshan wollastonite deposit.The model also depicts the geometric arrangement between the boundary of the Mengshan composite intrusion and the occurrence of the sediment units of the Maokou Formation.a.Late Paleozoic sedimentary sequences unconformably overlaying the Proterozoic gneiss basement rock; three distinct episodes of granitic magma intruded during the Indosinian.b.The pluton boundary cuts the chert-rich strata northeast of the pluton (i.e., the Yueguangshan and Heyi areas, elliptical dashed line in Fig.9d), which leads to the formation of relatively minor wollastonite deposits.c.Parallel relationship between the pluton boundary and the Maokou Formation corresponding to the Shizhushan mining area; this geometric relationship is, in part, responsible for the formation of the large volumes of wollastonite in the Mengshan deposit.d.Schematic map illustrating the distribution of wollastonite deposits in Mengshan.Note that the line for zone of "critical temperature" refers to the lowest temperature in which wollastonite can form, and depends on the volume of the granitic pluton, the distance from the pluton, and the thermodynamic properties of the ore-bearing rocks.

Table 1
Summary of the wollastonite orebodies in the Mengshan deposit.

Table 2
Carbon, oxygen, strontium isotope data and Sr/Ca ratios for samples from drill core ZK501.

Table 2
(continued ) Skarn samples MS-282, MS-376, and MS-597 refer to samples from depths of 282 m, 376 m, 597 m of the drill core ZK501, respectively.Samples SK-1 and SK-2 are the massive wollastonite ores that occur within 20 m of the pluton contact.Wo refers to wollastonite. *