Sulfur and lead isotope geochemical characteristics of Pb-Zn deposits in the Khau Loc zone, northeastern Vietnam, and their significance

In northeastern Vietnam, the Khau Loc zone is considered to have high potential for lead-zinc mineralization. The lead isotope data for 18 galena samples and 18 ones of δS isotope data (including galena and pyrite samples) were collected from lead-zinc ore deposits in some areas in the Khau Loc zone, including Phia Dam, Khuoi Man, Ban Lin, Lung Dam, and Ta Pan. These were employed to investigate the sulfur and lead isotope geochemical characteristics of Pb-Zn deposits and their significance in this study. The samples were analyzed using the LA-ICP-MS to show that the Pb isotopic ratios of Pb/Pb, Pb/Pb, and Pb/Pb in the galena samples range from 17.8908 to 18.6012, 15.5794 to 16.1025, 38.4420 to 39.2118, with the average values of 18.296, 15.749, and 38.812, respectively. The pyrite and galena samples had the δS isotope, ranging from 9.0 to 15.106. The sulfur isotope systematics implies that most of the lead-zinc ore formations originated from marine sedimentary evaporation deposits and magmatic intrusion-volcanic eruption sources rich in silica. The distribution of lead isotopic ratios had a well-defined cluster for each deposit, indicating the formation of lead-zinc deposits and lower crust and orogen trends. In addition, these findings of lead isotopic ratios and δS isotopes proved that the Khau Loc zone is an activated structure with continuous growth in continental crust thickness during the early Proterozoic and Cenozoic periods. Furthermore, the study results also presented the evolution of material sources involved in the formation of lead-zinc ores in the Khau Loc zone.


INTRODUCTION
In northeastern Vietnam, the Khau Loc zone is one with high potential for lead-zinc mineralization and has attracted considerable attention from geologists (Tri et al. 2011, Hung et al. 2020). In addition, lead-zinc mineralization plays a critical role in supplying the industry with useful metals because zinc, lead, and copper can be strongly contained together in polymetallic mineralization in the deposits to be found in Phia Dam, Khuoi Man, Ban Lim, and Lung Dam (Quoc 2000, Tinh 2000, Graedel et al. 2015. These areas have been surveyed in geological mapping and mineral prospecting at 1:500,000-1:50,000 scale since 1965 (Dovzhikov 1965, Binh 2005, 2010, Bac 2011. The majority of them are located in lower and middle Devonian carbonate deposits with various parts of terrigenous and volcanic elements. They have been categorized by many Vietnamese researchers as being of the stratiform type (Duong 1990, Binh 2005, 2010, Bac 2011. In terms of the sediment-hosted massive sulfide deposit forms, Binh et al. (2005Binh et al. ( , 2010 suggested that it would be prudent to clarify the genesis of ore deposits. As for mineral deposits of multiple genetic types, their ore-forming products have major variations; however, most of these studies have been carried out based on the geological characteristics, their geochemistry as well as mineralization age. However, to date, not much attention has been paid to ore-forming material sources. In this paper, the lead and sulfur stable isotopes are used to reveal the source of sulfur and lead in the lead-zinc ore and to discuss the deposit's genesis. The lead and sulfur isotopes of lead-zinc mineralization have been studied to understand the genesis of ore deposits and ore-forming material sources; therefore, geochemical isotope diagrams can be used successfully by geochemists to interpret ore-forming material sources and their genesis (Colemal 1977, Claypool et al. 1980, Chambers 1982, Sakai et al. 1982, 1984, Kerridge et al. 1983, Chaussidon et al. 1989. Various discrimination plots are presented to discriminate the different material sources sequentially. The results of δ 34 S isotope values on the genesis of ore deposits were shown in diagrams of Kerridge et al. (1988) and Ohmoto & Rye (1979). In contrast, ore-forming material sources were illustrated in the diagrams of Sakai et al. (1982Sakai et al. ( , 1984 and . The analysis results of lead isotopes, including 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb expressed characteristics of material sources involved in ore formation that was presented by means of the schematic diagrams of Zartman & Doe (1981), Davies (1984), Newsom et al. (1986), Allegre et al. (1988), and Rudnick & Goldstein (1990). The ore-forming environmental settings and material sources were displayed in the diagrams of Zartman & Haines (1988).

Geological setting
The geological structure of the Khau Loc zone is one part of the Viet Bac block that mainly developed from the formation of plateaus during the early-middle Paleozoic (Quoc et al. 2000, Tinh et al. 2000, Xuyen et al. 2000; Fig. 1). Furthermore, there was also a small volume of Carboniferous-Permian terrigenous, terri genouscarbonate sediments distributed in the northwestern part of the Khau Loc zone. In northeast Vietnam, the Khau Loc zone is separated by the Quan Ba-Ba Be deep fault in the West and divided from the Song Hien zone by the Song Nang-Ba Be deep fault in the East. This structure is separated from the Pho Ngu zone by the Ba Be strike-slip fault in the South. Moreover, the deep fault system in the center of the Khau Loc zone is characterized by open-circuit fault evidence, forming the late Permian-early Triassic granitoid massifs (Tinh et al. 2000, Xuyen et al. 2000, Binh 2005, 2010. Strike-slip fault systems such as the Song Gam and Du Gia-Minh Son faults divide the Khau Loc zone into three different blocks, showing different mineralizations in each block (Quoc et al. 2000, Tinh et al. 2000, Xuyen et al. 2000Fig. 2).
The lithology of the study area is mainly dominated by quartz-sericite-chlorite schist, terrigenous bearing quartz sediments that were determined as early Proterozoic. Cambrian and Ordovician formations composed mainly of carbonate and marlstone. These formations are covered by Silurian rhyolite formation. The Devonian formation is distributed in small areas, and exposed along narrow basins filled with limestones and dolostones. The lead-zinc deposits are mainly restricted in Neoproterozoic and lower and middle Devonian formation (Quoc et al. 2000, Tinh et al. 2000, Xuyen et al. 2000; Fig. 2).  (modified from Quoc et al. 2000, Tinh et al. 2000, Xuyen et al. 2000

Deposit geology
Most of the Pb-Zn deposits were found in the middle of the Khau Loc zone that is mainly restricted in the strata-bound brecciated dolostone (i.e., Du Tien, Lung Dam, Ta Pan, Ban Lin, Phieng Phat, and Yen Tho deposits), with minor terrigenous bearing quartz sediments (i.e., Ban Bo, Khuoi Man, and Phia Dam deposits) and rhyolite formation (i.e., Na Son deposit) (Duong 1990, Binh et al. 2005, 2010, Bac 2011Fig. 2). In the Khau Loc zone, the Pb-Zn deposits mainly contain sphalerite, galena, barite, pyrite, and chalcopyrite, with minor pyrrhotite and andorite. The minerals of the gangue include primarily calcite and dolomite, supplemented by quartz, calcite, and minor fluorite. The modification of the host rock involves silicate dolomitization, pyritization, and baratization. Several stratiform, platy, nest-like, and lenticular types were found in the primary ore bodies. The ore textures are medium to coarsegrained, euhedral, and mainly brecciated ore structures, with massive minor vein and stockwork structures. The modification of the wall rock involves silicate dolomitization, pyritization, and baratization.

Mineralogy of lead-zinc ores
According to Binh et al. (2010), the mineralogy of lead-zinc ores in the study area is galena, sphalerite, barite, pyrite, chalcopyrite, minor pyrrhotite, andorite, and a few supergene minerals, including limonite and malachite. Sphalerite, smithsonite, hydrozincite, sheridanite, and zinc-bearing dolomite, and calcite are among the zinc-containing minerals discovered. Galena and minor quantities of anglesite and andorite are all lead-containing minerals. Limonite (which also includes lead and zinc), native silver, pyrite, minor quantities of chalcopyrite and enargite, quartz, chlorite, mica, feldspar, and essonite are among the other minerals found. Hydrothermal breccias also occur in this region. They appear as banded, broken veins or vein-like features.
As previously stated, lead-zinc ores are geographically associated with lower Devonian carbonate rocks and are sometimes restricted in Neoproterozoic formation; rock-forming minerals include calcite, dolomite, clay minerals group, quartz, barite, and organic material. In interaction with opaque minerals, rock-forming minerals often exhibit dissolution and embayment textures. The presence of sulfide minerals appears to be linked to the amount of quartz, barite, hydrothermal calcite, and dolomite in rocks and/or mineralization zones.

MATERIALS AND METHODOLOGY
We collected thirty lead-zinc ores samples from various deposits and ore occurrences; among them, 18 galena samples are used for testing leadzinc isotopic analysis, and 18 galena and pyrite samples are used for testing sulfur isotopic analysis (Figs. 1B, 2). Of these, this study analyzed two galena samples from the Na Son deposit, three galena and pyrite samples from the Du Tien deposit, five galena and pyrite samples from the Ta Pan deposit, three galena samples of the Lung Dam deposit, one galena sample of the Po Pieng deposit, two galena and pyrite samples from the Phieng Phat deposit, one galena sample from the Ban Bo deposit, two galena samples from the Yen Tho Pass and one pyrite sample of the Yen Tho deposit, five galena and pyrite samples from the Ban Lin deposit, one galena sample from the Khuoi Man deposit, and four galena and pyrite samples from the Phia Dam deposits.

Sulfur isotope analysis
The sulfide minerals were isolated from typical ore samples obtained from the examined ore veins, and ore lens and sulfur isotope investigations were performed. Crushing, grinding, and sifting (from −250 to +125 m) were used to release unaltered pyrite and galena mineral particles, then hand-picked under a reflected light microscope. Finally, the mineral separates were powdered. The δ 34 S ratios were analyzed at the University of Science and Technology of China using a EA1500 elemental analyzer connected to VG/Fisons/Micromass 'Isochrom-EA' system that was operated in the continuous He flow mode. The amount of the sample tested is determined by the sulfur concentration of the sulfide material. For sulfides having 50 wt.% S (pyrite, marcasite), an amount as little as 0.5 mg is employed; for sulfides containing 12-13 wt.% S, such as galena, the quantity tested is more than 2 mg. Before being thrown into a furnace at 1030°C, the samples are individually crimped in a tin capsule. The sample is flash combusted at 1800°C under helium atmosphere and with the simultaneous injection of O 2 into the system. The gases are subsequently oxidized, and the extra oxygen is absorbed in copper wires. The resultant gases are then separated in a chromatographic column, their peaks quantified, and the SO 2 gas is isolated to be analyzed directly in the mass spectrometer, all carried by He.
For the sulfide minerals, an accuracy of 0.1 for δ 34 S has been achieved. The findings are related to Canyon Diablo Triolite. One standard is utilized to monitor the run at the start of the day and frequently during the run. Its work also includes checking to see if the surplus O 2 has been completely absorbed. In addition, six standards were utilized to calibrate the corrections to the raw values acquired for the samples. They span more than 50 degrees, from −32 to +20‰. For every five samples, one standard is analyzed to confirm or eventually adjust the calibration, and for every ten analyses, one blank is performed to "flush" the system.

Lead isotope analysis
The isotopic samples of Pb-Pb and U-Pb were analyzed with the LA-ICP-MS method at MC-A-ICP-MS Laboratory in the Institute of Geology and Geophysics, Chinese Academy of Sciences.
To determine the isotope ratio of U-Pb, there was approximately 50 mg of rock and 50 mg of galena with the Pb-Pb isotope to be used. The samples were destroyed in a mixture of HF-HClO 4 in a Teflon beaker and placed in a sealed steel beaker at 180°C for six days to ensure complete digestion. Pb was separated and purified by the conventional cation-exchange technique with diluted HBr that was then measured on a Finnigan MAT-262 mass spectrometer at the University of Science

Ore mineralogy of the study samples
Calcite, dolomite, clay minerals, quartz, barite, and organic material are the rock-forming minerals in carbonate (Fig. 3A, B). The abundances of quartz, barite, hydrothermal calcite, and dolomite in rocks and/or mineralization zone seem to be related to the occurrence of sulfide minerals. Galena, sphalerite, barite, pyrite, chalcopyrite, minor pyrrhotite, andorite, and a few supergene minerals, including limonite and malachite, were included found in the analyzed samples ( Fig. 3C-I).
Sphalerite and galena are typically found as anhedral crystals with grain diameters ranging from 0.3 to 2.5 mm in samples. They replace the previous rock-forming minerals and are intimately related to pyrite (Fig. 3D-I). It is typical to record the texture of sphalerite and galena filling in microcrack systems, which form veinlets or branches. Pyrite is most investigated samples are euhedral to subhedral, with grain sizes ranging from 0.1 to 2.5 mm. The pyrite rim is frequently eroded and replaced by sphalerite, suggesting that pyrite is the ore's oldest crystalline phase.

Sulfur and lead isotope compositions and their constraints on the ore genesis of the Pb-Zn deposits
18 galena samples were studied, and the results are provided in Table 1. The δ 34 S values range from +9.00 to +15.106‰ (mean = 11.557 ±0.006, 1δ). The δ 34 S values can be used to discuss the genesis of the lead-zinc deposits into the three following groups, including hydrothermal-sedimentary, hydrothermal-volcanic, and hydrothermal types (Tab. 1).
The results of the isotopic ratio measurements are illustrated in Table 2 Hung K.T., Tung T.D., Binh D.Q., Sang P.N., Cuc N.T., Linh N.T.H., Tin Q.D.   Lead-zinc-barite and lead-zinc ores of hydrothermal-sedimentary type were identified as the most popular mineralization in the Khau Loc zone with their main concentration in the central and southern parts of the structure, characterized by the lead-zinc seams which are distributed in quartz-sericite schists and limestone-dolostone of the Neoproterozoic and Devonian strata, and a small volume of early Carboniferous formation overlapping these formations (Fig. 2). The ore-forming age was established based on the mineralization model ages of Pb/Pb and U/Pb in the eruptive formations containing these ores, and ore-forming origin is determined depending on δ 34 S isotope values as shown in Tables 1 and 2.
The schematic of the isotope values of δ 34 S indicating ore-forming material sources is shown in Figure 4. From this diagram of ore-forming origins, it can be concluded that most of the lead-zinc ore formations started from marine sedimentary evaporation deposits and magmatic intrusion-volcanic eruption sources rich in silica.
The main types of ore-forming origin are plotted in Figure 5. It is indicated that lead-zinc ore formations were generally originated from shallow sources, basically related to sediment-hosted massive sulfide deposits such as Red-bed, and Mississippi-valley types. These results are similar to the previous research of Hoa et al. (2008), Ishihara et al. (2010), andBac (2011).
Formations of hydrothermal-volcanic origin in the Khau Loc zone are not very concentrated; instead, they are popular in the Na Son formation and a small part of eruptive rhyolite formations in the Ban Lin area (Figs. 2-6). The main characteristic of these formations is the presence of lead-zinc-silver ore. An analysis of the results of sulfur and lead isotopic ratios of lead-zinc ores collected from the Na Son formation and U/Pb from the Ban Lin rhyolite formation revealed that lead-zinc ores from the two places have similar ages (Tab. 2). The analysis results of δ 34 S isotopes from the Na Son deposit, as shown in Figure 4, proved that they have such material origins as volcanic sources and intrusive ones. Their distribution characteristics meant that there is a relationship between them and the eruptive source of the Na Son formation. Coleman 1977, Claypool et al. 1980, Chambers 1982, Sakai et al. 1982, Kerridge et al. 1983, Chausidon et al. 1989  In place of frequent denotation, lead-zinc mineralization of a hydrothermal origin is mainly distributed over existing paths during previous mineralization ages, adjacent to ore fields and deposits. Their scale is small, as seen in orebody II Na Son, Po Pieng, and Yen Tho (Fig. 6). The analysis results of δ 34 S isotope values showed that these ore formations are still shallow hydrothermal sources and related to intrusive and volcanic formations. They were formed in the late Mesozoic era (Jurassic to Cretaceous) (Tab. 2).

Fig. 4. Natural sulfur isotope reservoirs, showing lead-zinc forming material sources in the Khau Loc zone (data from
Based on the lead isotope ratios in ore deposits and occurrences in the Khau Loc zone, as shown in Table 2, schematic diagrams of lead-zinc ore formation and development characteristics throughout different mineralization ages were synthesized and plotted. The results of isotope ratios of 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb of the ore deposits and occurrences in the Khau Loc zone are plotted in the diagrams of Zartman & Haines (1988) ( Fig. 7) on the mineral deposit setting and location of the material supply sources through the metallogenic ages from Proterozoic-Cambrian to Paleozoic, and Mesozoic eras are mentioned as followed.
The Proterozoic-Cambrian metallogeny, developing with many lead-zinc deposits and lead-zinc occurrences as lead-zinc ore fields in the Du Tien, Lung Dam, Ta Pan, Ban Bo, Ban Lin-Lung Thom, and Phieng Phat deposits, are presented in Figure 7A, B. The Paleozoic metallogeny with leadzinc deposits of the Na Son, Ban Lin types during the eruption stage, namely Yen Tho Pass, Khuoi Man, Phia Dam, are illustrated in Figure 7C, D. Besides, the late Mesozoic metallogeny with lowpart ore deposits of Na Son, Ta Pan, Po Pieng, and Yen Tho are plotted in Figure 7E, F.
The plumbotectonics model can be used to constrain the source of Pb (Zartman & Doe 1981). As shown in the 208 Pb/ 204 Pb vs. 206 Pb/ 204 Pb and  Ohmoto & Rye 1979, Eldridge et al. 1988, Kerridge et al. 1988, and Naylor et al. 1989 of the imperfect mixing of normal lead in different source regions. Therefore, it is pointed out from the above Pb isotopic composition characteristics that the lead sources are not only the lower crust but also some other source regions, for example, the basement of the Caledonian and Yanshanian orogenic zones (Velasco et al. 1996, Golonka et al. 2006, Hoa et al. 2008, Hung 2010, Milot et al. 2021). From these diagrams, it can be seen that there has been an evolution of the material supply sources gradually shifting to the deep sources of the lower crust and forming in the orogenic settings. Furthermore, Allegre's et al. (1988) schematic also presented the characteristics of material sources forming lead-zinc ores in the Khau Loc zone as shown in Figure 8 with the  (Zartman and Haines 1988). The ticks on each curve are at 100 Ma intervals of the ore deposits and occurrences in the Khau Loc zone during the Proterozoic-Cambrian (A, B), Paleozoic (C, D), and late Mesozoic (E, F) metallogeny A C E B D F mineralization ages starting from Proterozoic-Cambrian to Paleozoic and late Mesozoic eras with the above-discussed ore deposits and occurrences.
Based on the sulfur and lead isotope analysis of the Pb-Zn deposits and their occurrences in the Khau Loc zone, the source of ore-forming materials mainly originated from the lower crust and orogenic field. According to Table 2, it is inferred that ore-forming material, which was derived predominantly from basement rock and partially from Devonian carbonaceous rocks, is the source of the abundance of Pb-Zn mineralization of strata in the northeast margin of the Khau Loc region. Otherwise, rich in organic matter may be produced by carbonaceous rocks. The decomposition of organic matter is helpful for sulfate reduction (Velasco et al. 1996, Xue et al. 2007, Sangster et al. 2011, Duan et al. 2016, Gill et al. 2019, Ünal-Çakır 2020.
The ore-forming processes of Pb-Zn deposits and events can be summarized by means of an overview. Pb and Zn were collected by underground geothermal brine in the basement rock and carbonaceous rocks, with the Devonian dolomite dissolved. As ore-forming fluid migrates to the organic matter-rich dolomite breccia belt, ore forming matters precipitate Pb and Zn due to reduction, with the formation of hydrothermal dolomite.