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Rationale: A novel laser ablation‐isotope ratio mass spectrometry (LA‐IRMS) method for in situ analysis of sulfur isotopes in sulfides has been developed. Instead of the in situ reaction applied by the traditional laser microprobe, the analyte gas preparation in this method is separated temporally and spatially from the LA, resulting in improved precision and accuracy. Methods: Our LA‐IRMS system combines an ultraviolet LA system, an elemental analyzer (EA), a custom‐built cryogenic concentration system, a continuous‐flow interface, and an IRMS. The sulfide aerosol particles generated from LA were transferred by a helium carrier gas from the ablation cell into the reaction tube and were then converted into SO2. Subsequently, SO2 was enriched in two cold traps and was finally introduced into the ion source of the IRMS through the continuous‐flow interface. Results: We measured three synthetic and four natural sulfide reference materials to test the performance of this method. Precisions of ±0.25‰–±0.48‰ and ±0.32‰–±0.64‰ (1SD, n = 5) for δ34S values of synthetic and natural sulfide standards can be obtained for spot sizes ranging from 64 to 80 μm. Measured values and their recommended values showed a good linear relationship (R2 within 0.998 and 0.9995) with the slope of approaching unity (within 1.0509 and 1.1313). Conclusions: Data from the measurement of reference materials showed that the precision and accuracy of our method were satisfactory. This method is a powerful tool for in situ sulfur isotope measurement of sulfides and can be further applied to in situ carbon and oxygen isotope analyses. [ABSTRACT FROM AUTHOR]

The traditional method for sulfur isotope measurement using EA‐IRMS commonly requires sulfur content greater than 2 μmol. Such a large sample size limits its application to low‐S materials, the size mainly being due to ineffective utilization of sample gas, almost 99.7% of which is discarded with carrier gas through the split port of the continuous‐flow interface. A modified EA‐IRMS system with a gas chromatographic (GC) column and a custom‐built cryogenic concentration device is used in this study. We measured six reference materials to test the performance of this method. The results were consistent with those obtained through traditional EA‐IRMS. Precisions ranging from ±0.24‰ to ±0.76‰ (1σ) can be obtained with samples equivalent to ∼80 nmol sulfur, which were similar to results obtained from an alternative method using an absorption column. Our improved method is a powerful tool for sulfur isotope measurement in ultrasmall sulfide and sulfate samples, which can be further applied to carbon, nitrogen and oxygen isotope analyses of samples at about 100 nmol level. [ABSTRACT FROM AUTHOR]

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

A large number of Paleoproterozoic borate deposits are hosted by the lower units of a volcanic-sedimentary sequence in Liaoning Province, northeastern China, and are a major source of boron in China. The ore-bearing wall rocks in the deposits are serpentinized ultrabasic rocks and carbonates, with layered leptynites, leptites, amphibolites, and migmatites adjacent to the ore. Both the borate ores and country rocks contain tourmaline, although the country rocks have much lower abundances of the mineral. Based on in situ boron isotope measurements using laser ablation–multi-collector–inductively coupled plasma–mass spectrometry (LA–MC–ICP–MS), boron isotope data show that: (1) δ 11 B values of borate ores range from + 6.8‰ to + 13.9‰ (mean + 10.8‰); (2) tourmalines from the borate ores have δ 11 B values from + 9.5‰ to + 12.7‰; and (3) the wall rocks within the borate ores yield slightly lower δ 11 B values ranging from + 5.7‰ to + 7.6‰, and those outside the deposits from − 9.9‰ to − 5.9‰. Positive δ 11 B values in borates as well as in tourmalines inside the mining area indicate that boron in these Paleoproterozoic borate deposits was derived from marine evaporites. δ 34 S V-CDT (where V-CDT is Vienna Canyon Diablo Troilite) values of borate ores, serpentinized marbles, and anhydrites range from + 16.1‰ to + 24.7‰, whereas δ 13 C V-PDB (where V-PDB is Vienna Pee Dee Belemnite) values of marbles range from + 3.2‰ to + 5.9‰. These isotopic characteristics are interpreted to reflect formation in a marine evaporative environment. LA–MC–ICP–MS zircon weighted 207 Pb/ 206 Pb ages of leptite and serpentinized olivine basalt from the hanging wall of the borate deposits are 2139 ± 13 Ma and 2130 ± 19 Ma, respectively. Therefore, the (~ 2.2 Ga) borate deposits may have originated from marine evaporative boron-bearing sediments, which were interbedded within bimodal volcanic rocks during the early stages of development of the Liaoji rift. [ABSTRACT FROM AUTHOR]

The origin of the Zhaoanzhuang serpentine-magnetite deposit in the southern North China Craton (NCC) is highly disputed, with some investigators having proposed an ultramafic origin, whereas others favor a chemical sedimentary origin. These discrepancies are largely due to the difficulty in determining the protolithic characteristics of the highly metamorphosed rocks. Sulfur, magnesium, and boron isotope geochemistry combined with detailed petrography was carried out in this study to constrain the original composition of the Zhaoanzhuang iron orebodies. Anhydrite is present as coarse crystals intergrown with magnetite, indicating that the anhydrite formed simultaneously with the magnetite during metamorphism rather than as a product of later hydrothermal alteration. The anhydrite has a narrow range of positive δ34S values from +19.8 to +22.5‰ with a mean value of +21.1‰. These values are significantly higher than that of typical magmatic sulfur (δ34S = 0 ± 5‰) and deviate away from primary igneous anhydrite towards mantle-sulfur isotopic values, but they are similar to those of marine evaporitic anhydrite and gypsum (~+21‰). The sulfur isotopic compositions of several samples show obvious signs of mass-independent sulfur fractionation (Δ33S = −0.47‰ to +0.90‰), suggesting that they were influenced by an external sulfur source through a photochemical reaction at low oxygen concentrations, which is consistent with the Neoarchean-Paleoproterozoic atmosphere. Coarse-grained tourmaline from the tourmaline-rich interlayers of the orebodies occurs closely with Mg-rich minerals such as phlogopite, talc, and diopside, indicating that it has a metamorphic origin. The δ11B values of the tourmaline range from −0.2‰ to +3.6‰ with a mean value of +2.0‰, which is much positive relative to that of magmatic tourmaline but is consistent with that of carbonate-derived tourmaline. The magnesium isotopic analyses of the serpentine–magnetite ores and the magnesium-rich wall rocks revealed a wide range of very negative δ26Mg values from −1.20‰ to −0.34‰ with an average value of −0.80‰. The value is higher than that of ultramafic rocks (δ26Mg = −0.25‰) and exhibits minor Mg isotopic fractionation. However, these values are consistent with those of marine carbonate rocks, which have lower δ26Mg values and larger Mg isotopic variations (δ26Mg = −0.45‰ to −4.5‰). Collectively, the S–Mg–B isotopic characteristics of the Zhaoanzhuang iron orebodies clearly indicate a chemical sedimentary origin. The protoliths of these orebodies most likely reflect a series of Fe–Si–Mg-rich marine carbonate rocks with a considerable evaporite component, indicating a carbonate-rich superior-type banded iron formation precipitated in an evaporitic shallow marine sedimentary environment. [ABSTRACT FROM AUTHOR]

Evaporites have played important role in the formation of diverse metallic ore deposits, especially in the case of magmatic–hydrothermal deposits. However, the relationship between evaporites and skarn Fe deposit remains poorly constrained. In this contribution, we present new sulfur isotope data of pyrite, as well as the composition of halogen-rich minerals (scapolite and amphibole) in the Jinshandian skarn Fe deposit. The data are used to evaluate the evidence for incursion of evaporites in the skarn Fe deposit. The δ 34 S values for pyrite from the early and late retrograde stage range from +17.4‰ to +18.7‰ ( n = 4) and +16.4‰ to +19.4‰ ( n = 13), respectively. Both these values are markedly heavier than the common δ 34 S values of sulfides from magmatic–hydrothermal fluid, indicating that sulfur in the Jinshandian ore-forming system was mostly derived from evaporites. Compared to the amphibole from endoskarn, the scapolite and amphibole from exoskarn show high Cl content up to 4.04% and 3.01%, respectively, suggesting that the hydrothermal fluid with high NaCl content was probably derived from evaporites. The amphiboles from endoskarn are more enriched in F which is probably of magmatic in origin. The data presented in our study suggest that the hydrothermal system of the Jinshandian skarn Fe deposit probably experienced significant incursion of evaporites before or during the late prograde stage. [ABSTRACT FROM AUTHOR]

Copper and iron skarn deposits are economically important types of skarn deposits throughout the world, especially in China, but the differences between Cu and Fe skarn deposits are poorly constrained. The Edong ore district in southeastern Hubei Province, Middle–Lower Yangtze River metallogenic belt, China, contains numerous Fe and Cu–Fe skarn deposits. In this contribution, variations in skarn mineralogy, mineralization-related intrusions and sulfur isotope values between these Cu–Fe and Fe skarn deposits are discussed. The garnets and pyroxenes of the Cu–Fe and Fe skarn deposits in the Edong ore district share similar compositions, i.e., dominantly andradite (Ad 29–100 Gr 0–68 ) and diopside (Di 54–100 Hd 0–38 ), respectively. This feature indicates that the mineral compositions of skarn silicate mineral assemblages were not the critical controlling factors for variations between the Cu–Fe and Fe skarn deposits. Intrusions associated with skarn Fe deposits in the Edong ore district differ from those Cu–Fe skarn deposits in petrology, geochemistry and Sr–Nd isotope. Intrusions associated with Fe deposits have large variations in their (La/Yb) N ratios (3.84–24.6) and Eu anomalies (δEu = 0.32–1.65), and have relatively low Sr/Y ratios (4.2–44.0) and high Yb contents (1.20–11.8 ppm), as well as radiogenic Sr–Nd isotopes (εNd(t) = − 12.5 to − 9.2) and ( 87 Sr/ 86 Sr) i = 0.7067 to 0.7086. In contrast, intrusions associated with Cu–Fe deposits are characterized by relatively high Sr/Y (35.0–81.3) and (La/Yb) N (15.0–31.6) ratios, low Yb contents (1.00–1.62 ppm) without obvious Eu anomalies (δEu = 0.67–0.97), as well as ( 87 Sr/ 86 Sr) i = 0.7055 to 0.7068 and εNd(t) = − 7.9 to − 3.4. Geochemical evidence indicates a greater contribution from the crust in intrusions associated with Fe skarn deposits than in intrusions associated with Cu–Fe skarn deposits. In the Edong ore district, the sulfides and sulfates in the Cu–Fe skarn deposits have sulfur isotope signatures that differ from those of Fe skarn deposits. The Cu–Fe skarn deposits have a narrow range of δ 34 S values from − 6.2‰ to + 8.7‰ in sulfides, and + 13.2‰ to + 15.2‰ in anhydrite, while the Fe skarn deposits have a wide range of δ 34 S values from + 10.3‰ to + 20.0‰ in pyrite and + 18.9‰ to + 30.8‰ in anhydrite. Sulfur isotope data for anhydrite and sedimentary country rocks suggest that the formation of skarns in the Edong district involved the interaction between magmatic fluids and variable amounts of evaporites in host rocks. [ABSTRACT FROM AUTHOR]