1. Introduction
Mn–Ag deposits are among the most important types of silver deposits in China. It is difficult to separate Ag from manganese–silver ores due to a lack of research into the occurrence state (form) of silver and a lack of metallogenic models [
1,
2,
3,
4,
5,
6]. Research on mineralogy and metallogenesis is helpful to illuminate the existing state of silver and establish the actual metallogenetic model of these Mn–Ag deposits. Furthermore, there are four main types of silver occurrence state [
1,
7], namely, independent silver minerals, the isomorphic state, ionic adsorption, and the amorphous state.
Mn and Ag are the dominant metals produced in the Sanbao deposit. The Sanbao Mn–Ag deposit, located in the Laojunshan ore district in the southeast of the Yunnan Province of China, is one of the important Mn and Ag metal resources in southwestern China. In addition to Mn, this deposit is characterized by Ag enrichment with a mean value of 221 ppm and special Ag-bearing minerals. The metallogenetic model of the Sanbao Mn–Ag deposit remains in dispute and research about the existing state of silver is almost non-existent. Jia [
8] considered that the Sanbao Mn–Ag deposit has obvious stratabound characteristics and features of volcanic exhalation–sedimentation as well as of distal skarns. Part of the ore body is formed in interlayer fracture zones. The contact zone between marble and schist should be conducive to ore fluid precipitation. Yang et al. [
9] indicated that the Middle Cambrian Tianpeng Formation, with high background values of ore-forming elements, provides the repository for all ores and protores. The Sanbao Mn–Ag deposit is strictly controlled by the Tianpeng Formation and the main ore features a banded structure. In the course of its creation and evolution, the Mn–Ag ore formed through activation, migration, and concentration of Mn and Ag caused by the multiple-epoch metamorphism of the source bed. Although the geology and geochemistry of the Sanbao Mn–Ag deposit are available [
8,
10], the origin of this deposit is still poorly understood.
The study of these Ag occurrences could play an important role in presenting a genetic model for the source and formation of ore-forming materials. The hosting mineralogy and its enrichment are poorly constrained in the Sanbao deposit. From a detailed study of Ag-bearing minerals and chronology, we propose a new genetic model for this deposit. In this study, the results of electron probe microanalysis (EPMA) confirmed that Ag exists mainly in manganite and romanèchite in the state of isomorphism. It is important for a mining company to select a proper mineral processing procedure to recover silver. Additionally, based on the isotopic geochronology and the spatial distribution of industrial orebodies, the genesis of the Sanbao manganese–silver deposit was studied, and a mineralization model established to provide information for future prospecting in the broad region.
2. Geological Background
The South China Craton consists of the Yangtze Block to the northwest and the Cathaysia Block to the southeast (
Figure 1a) [
11,
12]. The South China Craton was welded to the North China Craton to the north and the Indochina Block to the south during the Triassic [
13,
14,
15]. The Yangtze and Cathaysia blocks were amalgamated along the Jiangnan Fold Belt at around 830 Ma (Neoproterozoic time) in South China [
11,
15]. Large-scale mineralization in these two blocks occurred from the Proterozoic to the Cenozoic, making the region one of the most important polymetallic metallogenic provinces in the world. The basement of the Yangtze Block consists mainly of late Archean metamorphic rocks in the north and late Paleo to Neoproterozoic weak metamorphosed rocks in the west and east, all of which were intruded by Neoproterozoic igneous rocks [
16,
17]. The sedimentary succession of the Yangtze Block is mainly composed of Cambrian to Triassic marine sedimentary rocks and Jurassic–Cretaceous and Cenozoic continental sedimentary rocks [
18]. The Cathaysia Block is characterized by widespread, 1.9–1.8-Ga, Neoproterozoic to Early Paleozoic metamorphic rocks [
19]. In particular, there are widespread igneous intrusions with ages ranging from 480 to 400 Ma, 230 to 200 Ma, and 100 to 80 Ma [
19]. Widespread polymetallic mineralization is spatially and temporally associated with the above tectonic-thermal events and igneous activities in South China.
The Laojunshan ore district is located in the southeast of Yunnan Province (southwest China) and occupies the junction of the Yangtze Block to the north and the Cathaysia Block in the east. Several Yanshanian igneous intrusions were emplaced roughly along the juncture of the Yangtze Block and the Cathaysia Block, including the Gejiu, Bozhushan, and Laojunshan intrusions [
20,
21]. A series of super-large ore deposits (e.g., the Gejiu Sn deposit, the Bainiuchang Ag-polymetallic deposit, and the Dulong Sn–Zn–In polymetallic deposits [
22]) are closely related to these igneous activities, forming one of the most important polymetallic tungsten–tin mineralized belts in China. The Laojunshan granite itself is surrounded by a number of large ore deposits, such as the Sanbao Mn–Ag deposit, the Xinzhai Sn deposit, the Nanyangtian W deposit, and the Dulong Sn–Zn–In polymetallic deposits (
Figure 1b). These deposits constitute a large polymetallic Sn–W–Pb–Zn–Cu ore district [
23], also known as the Laojunshan ore district.
Two types of granites occur in the Laojunshan ore district: one is Laojunshan granite formed in the Yanshanian and another is Nanwenhe granitic gneiss formed in the Caledonian. The Nanwenhe granitic gneiss is located mainly in the eastern part of the Laojunshan granite body. The Nanwenhe granitic gneiss is close to a deformed and metamorphosed dome, the Laojunshan-Song Chay Dome [
18]. The Laojunshan Metamorphic Core Complex and Song Chay metamorphic dome extend from the belt between the NW–SE trending Wenshan–Malip fault zone and NW–SE striking Red River shear zone. The dome is the largest in the southwestern part of the South China Block and extends into Vietnam [
8]. The so-called “Nanwenhe granitic gneiss” is composed of light gray intermediate to fine-grained granite with a porphyritic texture and gneissic fabric. The Nanwenhe granitic gneiss was metamorphosed or deformed into gneissic granite and granite gneiss in the Triassic. Porphyritic texture and gneissic fabric are found in the Nanwenhe granitic gneiss. Furthermore, these rocks can be further subdivided petrographically into the Tuantian and Laochengpo units [
20]. Both have similar mineral assemblages, including quartz, feldspar, and mica, with minor sulfides and zircon.
The Sanbao Mn–Ag deposit occurs at the north margin of the Laojunshan ore district (
Figure 1b). Drilling identified potentially ore grade materials, including 13.2% Mn and 221 g/t Ag in this deposit. This deposit has an estimated 200 tons of Ag reserves and 15,000 tons of Mn reserves. It is considered to be a medium-sized reserve and is potentially expected to become a large-scale deposit. The main faults trend NW and NE and dip at a relatively high angle to the east in the Sanbao deposit (
Figure 2a and
Figure 3a). Mn–Ag orebody distribution was strictly controlled by the main faults (
Figure 2a). Outcrops rocks include Middle Cambrian metamorphic rock series, carbonate rocks, and granite associated with the Yanshanian and Caledonian magmatism in the Laojunshan ore district [
8,
10,
20,
24]. The Middle Cambrian Tianpeng Formation is the dominant host for mineralization. It can be sub-divided into five units: unit 1 is chiefly composed of quartz schist and skarn; unit 2 consists of dolomitic marble and minor skarn; unit 3 consists of quartz schist with subordinate mica schist; unit 4 is composed of mica schist, limestone, and dolomitic limestone; and unit 5 consists of mica schist and quartz schist (
Table 1). The Mn–Ag orebodies are mainly hosted in unit 4 and unit 5. The Tianpeng Formation is overlain by the Longha Formation.
The Sanbao Mn–Ag deposit is comprised of primarily stratabound, lenticular, capsular, and irregular orebodies (
Figure 2b). The Sanbao deposit primarily consists of six mineralized belts with 29 orebodies. The no. 1 ore body is the most important in the deposit. It consists of 12 small orebodies and is primarily hosted in the limestone of the Tianpeng Formation unit 4.
The wall rock alteration includes silicification (
Figure 3b), lead–zinc mineralization, pyritization (
Figure 3c), and skarnification (
Figure 3d). Silicification is developed in most Tianpeng units that are composed of quartz schist. Lead–zinc mineralization and pyritization occur in the wall rock; for example, galena, sphalerite, and pyrite are formed within the mica schist. Skarnification occurs mainly in the wall rock, termed skarn, which is not far from the Mn–Ag ore body. The skarn is mainly composed of assemblages containing diopside and tremolite in the Sanbao area.
3. Samples and Analytical Methods
There are three types of ores in the Sanbao deposit: primary, partial oxidized, and strongly oxidized ores. Samples for this study were collected from the three types of ores (
Figure 3,
Figure 4 and
Figure 5). The primary ore, made up largely of rhodochrosite (
Figure 3e), has little Ag and is characterized by a dense texture (
Figure 4a). Additionally, the primary ore contains calcite veins and sulfide minerals, such as pyrite and chalcopyrite (
Figure 3f,
Figure 4b,c). The partial oxidized ore with porous texture is mainly composed of limonite, pyrolusite, and minor amounts of manganite and romanèchite (
Figure 4d,e). The strong oxidized ore mainly consists of manganite, romanèchite, pyrolusite, and a small amount of limonite. The strong oxidized ore showed an unconsolidated and powdery texture (
Figure 4f). We carried out electron probe microanalysis (EPMA) and inductively coupled plasma–mass spectrometry (ICP-MS) for trace elements analysis, and laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) cassiterite U–Pb dating was also conducted.
3.1. EPMA
Mineralogical observations for Mn and Ag were performed using a Shimadzu EPMA-1600 electron microprobe equipped with an energy-dispersive spectrometer and back-scatter electron (BSE) imaging capability at the State Key Laboratory of Ore Geochemistry of the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS, Guiyang, China). SPI (Structure Probe Incorporation) standards were used, and the minimum detection limits of Mn, Ag, Fe, Ba, K, Zn, Ca, Si, Cr, Al, Ti, Na, and Sr were 0.1%. The selected analytical spectral lines and deducted background values were achieved using instrument programs, and some fault spectral peaks were calibrated artificially. In addition, a JEM-2000FX II TEM with an Oxford Link ISIS energy dispersive X-ray spectrometer (EDS) was also used to observe the texture and size of the Ag-bearing minerals at the IGCAS. All data are given in terms of weight percent (wt.%).
3.2. Trace Elements Analysis
Trace elements were analyzed using a Perkin-Elmer Sciex ELAN 6000 ICP-MS at the IGCAS. The powdered samples (50 mg) were dissolved in high-pressure Teflon bombs using a HF + HNO
3 mixture for 48 h at 190 °C [
25]. Rh was used as an internal standard to monitor signal drift during counting. The GBPG-1 (Garnet-Biotite Plagiogneiss) international standard was used for analytical quality control. Analyses of the OU-6 (Penrhyn Slate) and GBPG-1 international standards agreed with the recommended values and the analytical precision was generally better than 5% for all elements [
26].
3.3. LA-MC-ICP-MS Cassiterite U–Pb Dating
LA-MC-ICP-MS cassiterite U–Pb analyses were carried out at the Tianjin Institute of Geology and Mineral Resources, Tianjin, China. In-situ LA-MC-ICP-MS U–Pb dating of cassiterite from the Mn–Ag ore was used to determine the geochronology. More information about the instrumental parameters and operating conditions can be found in previous research [
23,
27,
28,
29,
30]. For the cassiterite sample, we used the correction value (K = measured value (164)/“true value” (158)) of 0.96 calculated using the external standard (AY-4) to correct the deviations between the measured and “true” isotopic ratios. The
207Pb/
206Pb and
238U/
206Pb ratios were corrected using the cassiterite external standard, and the calculated ages were determined using Isoplot software [
31].