Mineralogy and Metallogenesis of the Sanbao Mn–Ag (Zn-Pb) Deposit in the Laojunshan Ore District, SE Yunnan Province, China

The Sanbao Mn–Ag (Zn-Pb) deposit located in the Laojunshan ore district is one of the most important deposits that has produced most Ag and Mn metals in southeastern Yunnan Province, China. Few studies are available concerning the distribution and mineralization of Ag, restricting further resource exploration. In this study, detailed mineralogy, chronology, and geochemistry are examined with the aim of revealing Ag occurrence and its associated primary base-metal and supergene mineralization. Results show that manganite and romanèchite are the major Ag-bearing minerals. Cassiterite from the Mn–Ag ores yielded a U–Pb age of 436 ± 17 Ma, consistent with the Caledonian age of the Nanwenhe granitic pluton. Combined with other geochemical proxies (Zn-Pb-Cu-Sn), the Sanbao Mn–Ag deposit may originally be of magmatic hydrothermal origin, rather than sedimentary. The Ag-rich (Zn-Pb (Sn)-bearing) ore-forming fluids generated during the intrusion of the granite flowed through fractures and overprinted the earlier Mn mineralization. Secondary Ag (and possibly other base-metals) enrichment occurred through later supergene weathering and oxidation.


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.  [20]).
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 (Figures 2a and 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.

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 (Figures 3-5). 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. . Specimens of Mn-Ag ore from the Sanbao deposit: (a) rhodochrosite is the main mineral composition of pre-ore (primary ore); (b) typical vein structure in rhodochrosite; (c) pyritization with chalcopyrite in rhodochrosite; (d) partial oxidized syn-ore mainly is composed of diopside and has a porous texture; (e) there are many pores (voids), even cracks, and loose texture in partial oxidized syn-ore; (f) unconsolidated and powdery texture in the post-ore (strong oxidized-ore).

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.%).

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].  [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 207 Pb/ 206 Pb and 238 U/ 206 Pb ratios were corrected using the cassiterite external standard, and the calculated ages were determined using Isoplot software [31].

Results
Two samples (sby-28 and sby-03) were selected for EPMA analysis. The EPMA results show that the main Ag-bearing minerals are manganite and romanèchite. Results of manganite and romanèchite from electron probe X-ray microanalysis are listed in Tables 2 and 3.
The Ag, trace element, and REE (Rare Earth Elements) compositions are reported in Tables 4 and 5     The cassiterite from the Sanbao Mn-Ag ores is light to dark brown, mostly euhedral to subhedral under the observation of an optical microscope. The U-Pb data for the cassiterite (sby-03) are summarized in Table 6. The external standard (AY-4) yielded an isochron age of 164 ± 12 Ma, which is consistent with the "standard" age of 158.2 ± 0.4 Ma, within the margin of error. The cassiterite sample (sby-03) yielded an isochron age of 436 ± 17 Ma.

Element Variations
A positive correlation between Ag and Mn was found in the Mn-Ag ore ( Figures 5 and 6a), indicating that Ag may co-exist in the Mn minerals. Furthermore, there were two types of correlations with respect to Ag and Ba content as shown in Figure 6b. The first tendency showed a positive correlation between Ag and Ba, indicating Ag may co-exist in a Ba-bearing mineral [32]. This mineral was determined by EPMA to be romanèchite, where a positive correlation between Ag and Ba content was also found (Figure 6d). The second tendency showed no correlation between Ag and Ba (Figure 6b), indicating that another Ag-bearing mineral may occur. This mineral was determined by means of EPMA to be manganite, and a positive correlation between the Ag and K content was found in manganite (Figure 6c).

Occurrence of Ag in Mn Minerals
According to the EPMA results, the major Ag-bearing minerals are manganite and romanèchite.

Manganite
The morphology of manganite is primarily striature (Figure 7a,b), massive (Figure 7c), and acicular ( Figure 8a). Manganite in nature is not a homogenous mineral but usually contains a small amount of impurities [33]. Although the total wt.% is less than 100%, the observed data variance between test points is small enough to demonstrate the data's overall reliability (Table 2). Based on average values, this Ag-bearing mineral primarily contains Mn, Fe, K, and Zn, giving an empirical formula of (Mn 0.90 Fe 0.06 K 0.02 Zn 0.01 ) Σ0.99 O(OH) and the idealized formula MnO(OH) [34]. Manganite is formed in conditions of insufficient oxidation [35]. Additionally, cluster crystal manganite is often associated with barite and calcite in a low-temperature vein [36,37]. Some calcite accreted with manganite was found in the Sanbao Mn-Ag deposit. However, manganite generated from sedimentary manganese deposit often becomes massive or oolitic. Manganite from Sanbao is a transitional mineral between Mn 4+ (e.g., pyrolusite) and Mn 2+ mineral (e.g., rhodochrosite) [38].
Ag is evenly distributed throughout the Ag-bearing manganite in the deposit. Different geochemical properties of Ag and Mn lead to Ag only occupying a crystal lattice defect instead of replacing Mn in manganite. Furthermore, a positive correlation between Ag and K results from their easy absorption by manganite with a porous texture. Zn occurs in the columnar and dark-fringed manganite (Figure 8b). Columnar manganite contains a small amount of silver and platy manganite does not contain Ag (Figure 9a).

Romanèchite
Romanèchite is one of several naturally occurring manganese oxides with a tunnel structure [39,40]. Romanèchite rarely occurs as sizable single crystals, but is intergrown with other minerals, commonly at the unit-cell level [41,42]. Its structure is closely related to those of hollandite and todorokite, and to their many derivative structures [41]. The lower-valence Mn is segregated into the Mn sites at the edges of the triple chains of romanèchite [43,44]. Burns et al. [44] also proposed a similar segregation in the analogous sites of todorokite. Additionally, such an ordering of lower-valence Mn was found for a related material (Rb 0.27 MnO 2 ) [45]. Structure refinements of some hollandite structures and Rb 0.27 MnO 2 indicate that the lower-valence Mn cation is Mn 3+ rather than Mn 2+ [45,46]. If there were no tunnel cations in the manganese oxide tunnel structures, their ideal formula would be Mn 4+ O 2 . Based on the above analysis, the MnO is distributed to both Mn 4+ and Mn 3+ , and the standardized crystal-chemical formula of romanèchite is defined as (Ba,H 2 O) 2 (Mn 4+ ,Mn 3+ ) 5 O 10 .
Various forms of romanèchite were observed in the Sanbao Mn-Ag deposit, including idiomorphic (Figure 10a), irregular shape (Figure 10b), colloidal (Figure 10c), and massive (Figure 10d). Ag-rich romanèchite coexisting with goethite was found (Figure 9b). The form of the goethite is crystalline and acicular (Figure 9c,d). Mn is generally replaced by Fe, Al, and V; Ba is replaced by Ca and Na [32,43,47,48]. The hypidiomorphic romanèchite contained sphalerite (Figure 10d). According to the EPMA results (Table 3) As seen from the EPMA BSE image, Ag is uniformly distributed in romanèchite with massive texture. Furthermore, independent Ag minerals were not found. Therefore, Ba is replaced by Ag in romanèchite ( Figure 11).

Geochronology and Ore Genesis
The external standard (AY-4) yielded a Tera-Wasserburg age of 164 ± 12 Ma (Figure 12d), which is consistent with the "standard" age of 158.2 ± 0.4 Ma, within the margin of error [49]. This indicates that the determined age represents true values. The corrected isotopic ratios of the cassiterite sample sby-03 collected from the Sanbao deposit yielded a robust age of 436 ± 17 Ma (Figure 12a-c, Table 6), which is consistent with the age of the Nanwenhe granitic gneiss [10,20,50]. The Xinzhai tin deposit located near the Sanbao deposit has been dated on cassiterite and yielded an age of 419.1 ± 6.7 Ma [23], which is in agreement with the age of the Sanbao deposit within the range of permitted error. Furthermore, the origin of the Xinzhai tin deposit is closely related to the Caledonian Nanwenhe granitic gneiss [23]. Therefore, the ore genesis of the Sanbao deposit may be related to the Caledonian granitic pluton. "Primary ore (rhodochrosite)" and wall rock from the Sanbao Mn-Ag deposit are mainly plotted in the field of fossil Fe-Mn hydrothermal district and of oceanic sediment, respectively ( Figure 13, Table 4). The intense positive Eu anomaly of the primary ore is different from the Eu negative anomaly of the wall rock ( Figure 14a, Table 5). The La/Ce value (0.51-0.68) of rhodochrosite is near to the hydrothermal sediments value (La/Ce = 1 [51]), but much lower than that measured in sea water (Figure 14b). These results may indicate that primary ore may be of hydrothermal origin rather than sedimentary [52]. In addition, Ag was mainly enriched in the oxidized ore. Accessory minerals, such as cassiterite, rutile and zircon, were also found in the oxidized ore by EPMA. Figure 13. Trace element characteristics of the Sanbao Mn-Ag ore [53].

Possible Genetic Model
According to the mineralogy and metallogenesis of the Sanbao Mn-Ag deposit, the current study provides important evidence for collating and stipulating the characteristics of the multi-period and multi-phase petrogenesis-mineralization of Sanbao Mn-Ag ores. The metallogenetic process of the manganese and silver can be divided into four stages: (1) Cambrian: sedimentary strata with a little Ag formed in the Sanbao area, but it has not yet become an economically viable mineral deposit. Thus, the sedimentary strata were treated as an ore source-bed. Additionally, the Tianpeng Formation was an essential part of the sedimentary strata. Rhodochrosite is mainly hosted in the Tianpeng Formation in the Sanbao Mn-Ag deposit (Figure 15a). (2) Cambrian to Silurian: the Tianpeng Formation with a small amount of Ag is favorable ore-source rock and useful ore-preserving wall rock ( Table 4). The primary weathering and oxidation of the Tianpeng Formation may supply some ore-forming materials and enrich Ag and Mn. This process contributed directly to forming the oxidized orebody ( Figure 15b). Mn exists mainly in the rhodochrosite and manganoan calcite phases in the ore source-bed. Rhodochrosite mineralization contains a small amount of Ag (and base-metals). Mn and Ag are easily leached and enriched through long-term weathering and oxidation (supergene). In the process, most rhodochrosite gradually changed into manganite and romanèchite. As a result, the ore texture became loose and porous and beneficial for Ag entrance. This phenomenon agrees with the fact that Ag primarily occurs in manganite and romanèchite. (3) Silurian: the Caledonian Orogeny is an important tectonic and metallogenetic event in the geological evolution of the Laojunshan ore district [23]. The age of 436 ± 17 Ma, obtained by dating cassiterite from the Mn-Ag ore, is consistent with the emplacement age of the Caledonian Nanwenhe granite and the metallogenic age of the Xinzhai tin deposit [23]. Therefore, the geological evolution and metallogenic dynamics of the Sanbao deposit likely have a close relationship with the Caledonian granite intrusion (Figure 15c). The overprinting from the Caledonian granitic pluton intrusion cannot be ignored [20]. The Ag-rich ore-forming base-metal bearing fluids developed and mineralized along fractures while overprinting the primary and oxidized mineralized layers during the intrusion of granite. The overprinting also drove some Ag into the Mn-Ag ore body. (4) Later Silurian: secondary Ag enrichment (remobilization) occurred during later weathering and oxidation (Figure 15d), eventually evolving into the present Sanbao Mn-Ag deposit.
The above analysis suggests that the Sanbao deposit is characterized by multi-stage mineralization ( Figure 15). First, a sedimentary stage occurred during the Cambrian Period and a so-called "primary ore source-bed" formed ( Figure 15a) [8]. Second, the primary ore source-bed suffered weathering and oxidation after the Cambrian (Figure 15b). Third, large-scale magma activity occurred at 418-442 Ma (i.e., the Silurian) and produced Caledonian Nanwenhe granite [20]. Ag (with Zn-Pb (Sn)) was further enriched by the overprinting of the Caledonian Nanwenhe granite (Figure 15c). This shows that the Ag mineralization of the Sanbao deposit is likely related to Caledonian Nanwenhe granite rather than Yanshanian Laojunshan granite [55,56]. Lastly, secondary Ag enrichment occurred during later supergene weathering and oxidation, eventually evolving into the present Sanbao deposit (Figure 15d).

Conclusions
Based on the results presented in this study, the following conclusions may be drawn: (1) Manganite and romanèchite are the major Ag-bearing minerals of the Sanbao Mn-Ag deposit. The Ag-bearing manganite primarily contains Mn, Fe, K, and Zn giving an empirical formula of ( (2) The corrected isotopic ratios of the cassiterite sample sby-03 collected from the Sanbao deposit yielded an age of 436 ± 17 Ma, which is consistent with the age of the Nanwenhe granitic gneiss and the mineralization age of the Xinzhai tin deposit in the Laojunshan ore district. The origin of the Xinzhai tin deposit is closely related to the Caledonian Nanwenhe granitic gneiss. This therefore implied that the ore genesis of the Sanbao deposit may be related to the Caledonian granitic pluton. Combined with other geochemical proxies (Zn-Pb (Sn)), the primary Sanbao Mn-Ag deposit may be of magmatic hydrothermal origin (skarn-related) rather than sedimentary.
(3) Based on newly obtained data on mineralogy and metallogeny of the Sanbao deposits, a possible genetic model is established. Firstly, the so-called "primary ore source-bed" formed in the Cambrian. Secondly, the primary ore source-bed suffered weathering and oxidation after the Cambrian. Thirdly, large-scale magma activity occurred at 418-442 Ma (i.e., the Silurian) and produced Caledonian Nanwenhe granite. Ag (with Zn-Pb (Sn)) was further enriched by the overprinting of the Caledonian Nanwenhe granite. Lastly, secondary Ag enrichment occurred during later supergene weathering and oxidation, eventually evolving into the present Sanbao deposit.