Isotopic study of the Pb-Zn (Cu-Ag) Santa Maria Deposit, Caçapava do Sul Region, Rio Grande do Sul, Brazil

The Camaquã Mines, in the central-southern region of Rio Grande do Sul State, Brazil, contain the largest deposits of base metals (Camaquã, Cu; Santa Maria, Pb-Zn) in the Camaquã Basin. The host rocks are consisted of sandstones and conglomerates of the Santa Bárbara Group and interbedded volcanic rocks of undefined stratigraphic position. Peperitic features were identified in a mineralized trachytic sill. An age of 565 ± 5 Ma (U-Pb LA-MC-ICP-MS) was obtained for another trachytic rock, which allowed for the identification, for the first time, of the Acampamento Velho Formation in the Camaquã Mines region. Stratigraphic relations demonstrate that both rocks represent this magmatism. Data from Pb isotopes and spatial relationship observed in the field indicate a possible genetic link between Acampamento Velho Formation and Camaquã mineralization. Pb isotopic data also suggest a predominantly older crustal source (1.07 Ga model age) for Pb and possibly for the other metals, with intermediate or mixed origin, suggesting that magmatic-hydrothermal fluids leached the metals from the percolated rocks of the basement.


INTRODUCTION
Camaquã Mines correspond to the main deposits of base metals of the Camaquã Basin, the Camaquã (Cu, Au-Ag) and Santa Maria (Pb-Zn, Cu-Ag), which are settled in conglomerates and sandstones of the Seival and Rincão dos Mouras formations, that belong to the Santa Bárbara Group (Fambrini 2003). They are located near the city of Caçapava do Sul, in the central-southern region of the state of Rio Grande do Sul, and their ore reserves correspond to ca. 30 million tons, with an average content of 1.05% Cu (Camaquã deposit, 24 million tons already mined), and ca. 33 million tons with an average content of 1.44% Pb and 1.06% Zn (Santa Maria deposit) (Teixeira et al. 1978, Badi 1987. The Camaquã deposit is constituted by the already exhausted Uruguay (open-pit) and São Luiz (underground) mines. It has been known since 1865, when it was discovered by Englishmen, who were mining gold in the Lavras do Sul region. Mining activities developed intermittently, mainly between 1950 and 1996 (Teixeira et al. 1978, Laux and Lindenmayer 2000, Remus et al. 2000. In this deposit, the ore consists of Cu and Cu-Fe sulfides (chalcopyrite, bornite, chalcocite, and pyrite) associated with a gangue of quartz, hematite, chlorite, carbonate, and barite Gonzalez 1988, Ronchi et al. 2000).
The Pb-Zn (Cu-Ag) Santa Maria deposit, located 3 km SW from the Uruguay and São Luiz mines, was discovered in 1978 (Licht 1980). According to Badi and Gonzalez (1988) and Remus et al. (2000), the ore is formed by Pb-Zn sulfides (galena and sphalerite) and smaller amounts of Cu-Fe sulfides (pyrite, chalcopyrite, bornite, and chalcocite) in a gangue of chlorite, adularia, carbonate, and baryte.
The Acampamento Velho magmatism corresponds to a bimodal plutonic-volcanic sodic alkaline silica-saturated association that developed in the post-collisional stage of the Brasiliano orogeny (Bitencourt and Nardi 2000, Almeida et al. 2002, Wildner et al. 2002. It has already been identified in several locations in the Camaquã Basin, including the Ramada (Sommer et al. 2005, Matté et al. 2016, Taquarembó, and Bom Jardim plateaus ( Janikian et al. 2012), the Bugio (Almeida et al. 2012), Tupanci, and Picados Hills , and Palma (Vedana et al. 2017). However, there is no record so far of its occurrence in the region of the Camaquã Mines.
The genesis of the Camaquã Mines has been the subject of a long scientific debate, and the prevailing idea is that it has an epithermal or distal hydrothermal-magmatic origin (Bettencourt 1972, Beckel 1992, Remus et al. 2000, Ronchi et al. 2000, Laux et al. 2005, Renac et al. 2014.
This work presents new petrographic, geochronological and isotopic data about both trachytes interbedded with the rocks of the Santa Bárbara Group and the Pb-Zn mineralization from the Santa Maria deposit. Based on these data, a possible genetic relationship between this magmatism and mineralization is discussed.

REGIONAL GEOLOGY
The Camaquã Basin rests discordantly on the igneous and metamorphic terrains of the Sul-Riograndense Shield, which registers collisional and accretionary events of the Paleoproterozoic (Trans-Amazonian; 2.2-1.9 Ga) and Neoproterozoic (Brasiliano;, that led to the amalgamation of the southwestern part of the Gondwana (e.g., Babinski et al. 1996). The evolution of the Brasiliano orogeny in the southern region of Brazil resulted in a collision between the Rio de La Plata (western) and Kalahari (eastern) cratons, which led to the formation of the Dom Feliciano Belt (Fernandes et al. 1992).
Camaquã Basin thickness exceeds 6,000 m and the contacts between its units are defined by angular or erosive disconformities resulting from several episodes of tectonic subsidence , Fambrini et al. 2005. It is filled by the sedimentary and volcano-sedimentary succession of the Camaquã Supergroup (Ediacaran-Eopaleozoic), which was defined by Fragoso-Cesar et al. (2003). The basin was subdivided as follows, from base to top: the Maricá Group, Bom Jardim Group, Acampamento Velho Formation, Santa Bárbara Group and Guaritas Group (Fig. 1).
Bom Jardim Group consists of a volcanic (andesites), pyroclastic and sedimentary rocks succession with estimated thickness of 4,000 m . It represents, essentially, delta depositional systems. The Shoshonite Association of Lavras do Sul, part of this group, is composed by Hilario Formation volcanic rocks and by Lavras Granite Complex central portion granitoids (Nardi and Lima 1985). An age of 594 ± 4 Ma (U-Pb zircon method) was obtained for the Lavras Granite (Remus et al. 2000).
Different proposals have been presented for the formation and evolution of this basin (Brito Neves and Cordani 1991, Fernandes et al. 1992, Fragoso-Cesar et al. 2000, 2003. The dominant interpretation, however, is that a retroarc basin, related to the late to post-orogenic stages of the Brasiliano cycle, evolved into a rift basin in its final stages (Gresse et al. 1996, Paim et al. 2000, Almeida et al. 2012, Oliveira et al. 2014.

GEOLOGY OF CAMAQUÃ MINES
The Santa Bárbara Group (Fambrini 2003), which is correlated to the Arroio dos Nobres Formation (Ribeiro et al. 1966), encloses most of the sedimentary rocks in the region of the Camaquã Mines. According to Teixeira et al. (1978), the magmatism observed in this area corresponds to amygdaloid andesitic volcanic rocks of a brown color, which occur as intercalations or are associated with a regional fault set in the NE direction, and diabase dikes of the Serra Geral Formation, along the NW direction. Teixeira et al. (1978) subdivided the Vargas Member of the Arroio dos Nobres Formation in the region of the Camaquã Mines into the following five lithostratigraphic units: Lower Sandstone, Lower Conglomerate, Middle Sandstone, Upper Conglomerate and Upper Sandstone. Fambrini et al. (2005) applied the stratigraphic redefinition of the Santa Bárbara Group proposed by Fambrini (2003) to this region and identified the following formations (from base to top): Passo da Capela, Seival, Rincão dos Mouras and João Dias. A combination of these two stratigraphic classifications is presented on the geologic map of Figure 2.
The Camaquã deposit consists of massive veins, stockworks, and disseminated ore containing Cu and Cu-Fe sulfides (chalcopyrite, bornite, chalcocite, and pyrite) associated with gangue composed mainly of quartz, hematite, chlorite, carbonate, and baryte; traces of Au and Ag are found in the crystalline structure of the sulfides Gonzalez 1988, Ronchi et al. 2000). The Santa Maria deposit (Pb-Zn, Cu-Ag) comprises three mineralized areas (Fig. 2), which contain disseminations, veins, veinlets and stringers, as well as localized massive ore composed of galena and sphalerite, with lower amounts of pyrite, chalcopyrite, bornite, chalcocite, and native copper and silver.
According to Laux and Lindenmayer (2000), the hydrothermal alteration observed in the Uruguay and São Luiz mines is subordinate, being limited in extension to only a few meters from the veins with which it is associated. The hydrothermal minerals are product of chloritization, silicification, and sericitization processes, as well as argilization and sulphidation. The characteristic alteration processes of the Santa Maria deposit correspond to chloritization, sericitization, carbonation, and feldspatization (Remus et al. 2000).

METHODOLOGY
In this study, drill core and outcrop samples (provided by the Nexa Resources Company) were obtained from trachytes and host rocks of the mineralization, as well as from country rocks that occur in the region of Camaquã Mines. Polished thin sections were made and described by transmitted and reflected light microscopy at the Institute of Geosciences, Universidade Federal do Pará (IG-UFPA). A Scanning Electron Microscope (SEM) (ZEISS, LEO 1430 model) coupled to an Energy Dispersive X-Ray Spectrometer (EDS) (Sirus SD) of the Microanalysis Laboratory, IG-UFPA, and a SEM ZEISS EVO LS 15 coupled to a EDS X-Act SDD 10 mm 2 (Oxford), from the Superintendence of Belém of the Brazilian Geology Survey (Companhia de Pesquisa de Recursos Minerais -CPRM), were used to complement the petrographic and minerographic descriptions. Additionally, some of these thin sections were subjected to the double staining process (alizarin + potassium ferrocyanide in diluted HCl) to identify the different carbonate phases.
Analyses by X-ray powder diffraction (XRD) were performed using the X'Pert MPD-PRO PANalytical instrument, which is equipped with a Co anode (λ = 1.7903), of the X-ray Laboratory, IG-UFPA, at intervals from 4° to 74° (2θ) in total powder, using 5 to 10 g of previously pulverized aliquots. The results were processed using the Match! software and the search for reference standards was performed in the Crystallography Open Database (COD).
The experimental procedure for the Pb analysis in sphalerite followed the techniques developed by Manhès (1982). The leaching techniques were adapted to the conditions of the Pará-Iso Isotopic Geology Laboratory, IG-UFPA, and are described in detail in Galarza and Macambira (2002). However, in this study, the chromatographic separation of Pb was performed with ion exchange resin (Eichrom ® Sr 50-100 μm) conditioned with HCl (2N).
Less than 0.5 mg of galena was used in the total dissolution procedure, with a chemical attack made with HBr (8N) + HNO 3 (13N) + H 3 PO 4 . Then, the sample residue was deposited in a simple rhenium filament for analysis. In both cases of Pb analysis, the final concentrate was analyzed in a thermal ionization mass spectrometer (TIMS) (Thermo-Fischer, Triton Plus model) at the Pará-Iso Laboratory, IG-UFPA. The Pb model age was obtained by averaging the galena compositions and the less radiogenic sphalerite leachates, adopting a value of μ = 9.74. Regression, age calculations, and isotopic data presented on Pb-Pb diagrams were performed using the Isoplot v.3.68 (Ludwig 2008).
The U-Pb method through LA-MC-ICP-MS in zircon was used to determine the age of crystallization of the trachyte rock. After the experimental procedure for crystal separation, polished sections were prepared, and cathodoluminescence (Mono-CL) images were obtained at the Microanalysis Laboratory, IG-UFPA. In the final stage, these sections were taken to the Pará-Iso Laboratory and analyzed, along with the GJ-1 standard, in the MC-ICP-MS Neptune (Thermo Finnigan) coupled with a Nd:YAG LSX-213 G2 CETAC laser microprobe. The cup configuration, arranged to simultaneously measure U, Th, Pb, and Hg, was the following: M3 = 202 Hg, M4 = 204 (Hg+Pb), L4 = 206 Pb, M6 = 207 Pb, L3 = 208 Pb, H2 = 232 Th, and H4 = 238 U. The Inductively Coupled Plasma (ICP) configurations were: radio frequency power = 1,200-1,300 W; cool gas flow rate = 16 L/min (Ar); auxiliary gas flow rate = 0.7-1.0 L/min (Ar); sample gas flow rate = 1.0-1.3 L/min. The laser setup was: energy = 4-5 J/cm 2 , frequency = 10 Hz, spot size = 25 μm, and helium gas flow = 450-500 mL/min. For the calculations of the isotopic ratios ( 206 Pb/ 238 U, 207 Pb/ 235 U, and 207 Pb/ 206 Pb) and consequently of the ages, the values were corrected from background and 204 Hg interference on the 204 Pb.

Figure 2.
Geologic map of the Camaquã Mines region with localization of the drill holes selected for the study. The symbology used for representing the drill holes refers to the region in which the samples were collected, as follows: A1 (Area 1), A2 (Area 2), A3 (Area 3), and CL (Claudino farm). CL also represents outcrop samples.
For the correction of common lead contribution, the Stacey and Kramers (1975) terrestrial Pb evolution model has been used. Regression, age calculations, and data presentation in the Concordia diagram were made with Isoplot (v.3.68). A detailed description of the experimental protocol of the U-Pb methodology in zircon by LA-MC-ICP-MS at the Pará-Iso Laboratory can be found in Milhomem Neto and Lafon (2019).

PETROGRAPHY
Four drill holes and one outcrop were selected (Fig. 2) for the study of the trachytic rocks, mineralization, and its country rocks.

Seival Formation
The Seival Formation is predominantly formed by fine to medium-grained sandstones classified as arkoses, which can present rhythmic intercalation with coarse siltstones of similar mineral composition, and more rarely argillites, in addition to subordinate conglomeratic sandstones.
These rocks are red, orange, or white, and have local brecciation, with a moderate to intense degree of oxidation in non-mineralized areas. Sandstones and siltstones are well selected, with flat stratification/lamination and more rarely tabular cross-stratification (in some cases with clay films in the foresets). They consist of angular to rounded grains of quartz, orthoclase, microcline and muscovite, and fragments of volcanic, granitic, schist, sandstone, slate or shale rocks. Biotite, zircon, rutile, magnetite, and apatite occur as trace minerals. Porosity is low (1%) to absent, and both intergranular space and fractures are filled with carbonate and/or illite, which partially replace quartz, muscovite and rock fragments, and more intensely replace orthoclase and microcline. Illite partially replaces carbonate, and both occur in mineralized and altered zones.

Rincão dos Mouras Formation
The Rincão dos Mouras Formation consists primarily of reddish to whitish polymictic conglomerates with sub-rounded to sub-angular pebbles to boulders of gneiss, granitoid, mafic volcanic, sandstone, quartz, and potassium feldspar composition. Locally, interbedded with the conglomerates are levels of coarse to conglomeratic laminated sandstone, dark red laminated siltstone or argillite.
This unit presents inverse gradation (characterized by the predominance of conglomerate at the top and feldspathic conglomeratic sandstone at the base), localized fracturing, and occurs in the upper part of the Area 3 drill hole (BRSMSMDD 0008), reaching a maximum depth of 60 m. The degree of oxidation in this unit is lower than that observed in the Seival Formation. Rutile, zircon, and muscovite are accessory minerals, while carbonate and/or illite fill the intergranular spaces and fractures, and replace the rock grains, as in the Seival Formation.

Acampamento Velho Formation
In the study samples, the Acampamento Velho Formation is represented by two trachytic rocks that occur interbedded with sedimentary rocks, with one in the Seival Formation and the other in the Rincão dos Mouras Formation. The drill holes and outcrop samples selected for this study (Fig. 2) represent such occurrences. One of these drill holes (PCSM79-14) is in Area 1 of the Santa Maria deposit, about 5 km SW of the Camaquã deposit, and another is in the Claudino farm (drill hole FSM 81), about 4 km NE of the Camaquã deposit, where trachyte outcrop samples were also collected.
The trachyte observed in the Seival Formation occurs along about 3.5 m of the PCSM79-14 drill hole from an approximate depth of 131 m. It corresponds to orange to brownish rocks with fine granulation composed essentially of prismatic to tabular crystals of K-feldspar (orthoclase dominantly; minor sanidine) and plagioclase (subordinately) of variable size (0.1-0.4 mm) immersed in brown volcanic glass with variable shades and K-feldspar microliths. It is also characterized by the presence of vesicles, veinlets, and an intense degree of alteration. Iron-magnesium minerals (pyroxene and amphibole) have been entirely replaced. Rutile and apatite crystals have a widespread occurrence.
The observed alterations correspond to ilitization, oxidation, and chloritization, which are associated with the color variation of the rock from orange (when ilitization is predominant) to brown (when oxidation and chloritization prevail).
The vesicles (submillimetric to centimetric) are rounded or elongated, oriented, and filled by gangue minerals of illite, ankerite, siderite, quartz, chalcedony, iron-aluminum-celadonite, and ore minerals of galena, sphalerite, and chalcocite. The veinlets (submillimetric to centimetric) are composed of the same minerals, with a predominance of galena. The vesicles are larger and more frequent at the base, where quartz and chalcedony are predominant, while the top is mineralized and has more veinlets.
The igneous clasts are concentrated in a thick zone of ca. 1 cm parallel to trachytic contact, but also occur scattered upward up to some centimeters (Fig. 3C). The association of these features did allow the characterization of a fluidal or globular peperite (Busby-Spera and White 1987, Skilling et al. 2002, Martin andNémeth 2007) on top contact of a sill (Allen 1992).
The origin of such peperite is probably related to a sediment fluidization process promoted by intense pore water heating due to magmatic activity, which would have caused the mixing of sediment and subvolcanic clasts (Busby-Spera and White 1987, Skilling et al. 2002, Martin andNémeth 2007).
Samples of the trachyte that occur in association with the Rincão dos Mouras Formation were collected both at outcrop and drill hole ( Fig. 2; FSM 81; about 20 m depth). They correspond to a porphyritic, intersertal, vesicular/ amygdaloidal, slightly fractured and very altered rock with a dark gray color, whose porphyries are subhedral columnar crystals of K-feldspar (orthoclase and sanidine; minor plagioclase) with a size varying between 0.7 and 1.2 mm. The matrix consists primarily of K-feldspar laths with a small quantity of volcanic glass between them, in addition to rutile, zircon, and skeletal ilmenite, which occur as accessory minerals. Iron-magnesium minerals (pyroxene and amphibole) have been entirely replaced in this case too. Chalcopyrite occurs as disseminated crystals throughout the rock.
The vesicles are millimetric and filled with aggregates of tiny crystals of quartz, radiating fibrous zeolite crystals, hematite, carbonate (calcite, ankerite and siderite), and chlorite (chamoisite). Quartz and hematite form millimetric to centimetric veinlets.
K-feldspar (and possibly plagioclase) is intensely albitized, being replaced still by calcite, siderite, ankerite, chamoisite, and hematite. Volcanic glass is also chloritized and carbonated and has an intense degree of oxidation (hematite and ulvospinel).
The hematization intensity decreases with depth. In regions close to the surface, transformed considerably to hematite, K-feldspar is more carbonated (calcite), all vesicles are filled and zeolites are more developed. Additionally, quartz is only observed in the vesicles in these regions, filling the largest ones.  It was not possible to observe the contact relations of this rock, which hampered the characterization of its placement process. However, field and stratigraphic relations attest to its concordant feature, suggesting it is a sill or a lava flow.

Santa Maria mineralization
Santa Maria ore occurs in all stratigraphic units described so far. The Seival Formation is the main host of the Pb-Zn (Cu-Ag) sulfides (Fig. 2). Galena and sphalerite predominate in relation to pyrite, chalcopyrite, bornite, chalcocite, and native copper and silver. Galena occurs disseminated among rock grains and in K-feldspar cavities, or as massive veins filling fractures/breccias, in which can replace ferroan carbonates. In the Rincão dos Mouras Formation, the Pb-Zn ore consists mainly of galena, sphalerite, and pyrite. Galena presents a similar mode of occurrence to the one of Seival Formation. Sphalerite occurs disseminated and can present intergrowth with galena. In the Area 1 trachytic rock (Acampamento Velho Formation), galena is also the dominant ore mineral, occurring in the vesicles and veinlets. It can present intergrowth with sphalerite, chalcocite, and bornite. Covelite replaces partially to chalcocite. Chalcopyrite and pyrite are disseminated. Claudino farm trachyte, on the other hand, is a barren rock, containing only chalcopyrite as disseminated crystals.

U-Pb GEOCRONOLOGY IN ZIRCON
The crystallization age of the trachytic rock that is interbedded with the rocks of the Rincão dos Mouras Formation was determined by the U-Pb method by LA-MC-ICP-MS using the DMK-CL sample (which includes samples from drill core and outcrop). The analyzed zircon crystals can be morphologically classified into two groups (Fig. 4): • short prismatic crystals (3:2 length/width ratio; 150 μm in length) and prismatic crystals (2:1 length/width ratio; 200 μm in length), with oscillatory zoning; the core can be homogeneous in some cases (Figs. 4A, 4B, 4C and 4D); • elongated prismatic (3:1 length/width ratio; 200 μm in length) or elongated crystals (4:1 length/width ratio; 200 μm in length) with only rims partially zoned and a complex (Fig. 4E) or homogeneous (Fig. 4F) core.
Fractures and inclusions are rare. The Th/U ratios between 0.66 and 1.33 of these crystals, their characteristic oscillatory zoning (in most cases) and their dominantly prismatic feature, attest of their magmatic origin (Corfu et al. 2003, Xiang et al. 2011). Thirty-one analytical points were performed for 30 crystals (Tab. 1; three were discarded for having a high common Pb content) with a variable size of 75-200 μm. Based on the results, the crystals were grouped into three distinct age sets (Fig. 5). The oldest crystals were from Paleoproterozoic, including a grain with 2,357 ± 25 Ma and two grains with a concordant age of 2,035 ± 25 Ma (MSWD = 0.57). One grain presented a Mesoproterozoic age of 1,149 ± 36 Ma. The majority of crystals (n = 17) were from the Neoproterozoic, with ages ranging from 838 to 540 Ma. Within this group, the following concordant ages were obtained: 768 ± 15 Ma (MSWD = 0.43; two crystals), 675 ± 10 Ma (MSWD = 0.30; three crystals), 625 ± 9 Ma (MSWD = 0.45; three crystals), and 565 ± 5 Ma (MSWD = 0.037; nine crystals). There were also seven crystals represented by discordant points, possibly owing to the loss of radiogenic Pb.
The lowest concordant Neoproterozoic age (565 ± 5 Ma) is considered to be the trachyte crystallization age (Fig. 6), while older crystals are interpreted as xenocrystals inherited from basement or sedimentary country rocks. The crystals that define the age of the dated rock have morphological characteristics typical of group 1, dominantly. The inherited zircon crystals have a more variable morphology and can be classified into any of the two groups.
Similar ages to the one obtained for the analyzed trachyte rock, attributed to the Acampamento Velho Formation, characterize the magmatism of several locations in the Camaquã Basin (Tab. 2). However, there are no similar records for the region of the Camaquã Mines. Therefore, this study is the first to present the record of the Acampamento Velho Formation in this region.

Pb ISOTOPIC COMPOSITION OF SULFIDES
Galena samples from the three mineralized areas (one per area) and two sphalerite samples from Area 3 of the Santa Maria deposit were analyzed and plotted on a Pb isotope diagram ( Fig. 7; Tab. 3). The galena samples were treated by the total dissolution method, while the sphalerite samples were treated by sequential leaching.
The isotopic composition of Pb obtained by averaging the galena compositions and the lowest isotopic ratios of sphalerite leachates indicate a primitive source for the Santa Maria ore, according to the growth curve of Stacey and Kramers (1975), regarding an estimated age between 500-600 My ago for this deposit, with a model age of 1.07 Ga (μ = 9.74).
The linearity observed by Remus et al. (2000) in the isotopic diagram (Fig. 7) for the sulfides of Camaquã and Santa Maria deposits establishes a correlation between the mineralizing fluids of Cu (Au-Ag) and Pb-Zn (Cu-Ag) of the Camaquã Mines, suggesting a same age for both deposits and an essentially homogeneous source. A minimal interaction with the wall rocks, however, can be admitted to explain the small scatter presented by the isotopic compositions. The subparallel adjustment of this set of isotopic data with the reference isochrone of the Acampamento Velho Formation at Camaquã Mines indicates these deposits probably formed at 565 My ago.
The isotopic data of the analyzed sulfides in this work were plotted onto the Zartman and Doe (1981) diagrams, falling on the Orogen evolution curve (Fig. 8), which suggest  Pb) s (c = common; s = sample); b Th/U ratios and amount of Pb, Th, and U (in ppm) are calculated relative to the GJ-1 reference zircon; c corrected for the analytical "blanc" (background) and Pb/U internal fractionation, and normalized to GJ-1 reference zircon (ID-TIMS Pb/ 235 U ratios; e corrected for mass-bias by normalization to GJ-1 reference zircon and common Pb using the model of Stacey and Kramers (1975) (Fig. 9), the data plot between Mantle, Orogen and Lower Crust curves, being mostly nearer to the latter, which points out to a high Th/U source contribution. It is also possible to observe that only the 206 Pb/ 204 Pb isotopic ratios had a significant variation, highlighting the homogeneity of the 208 Pb/ 204 Pb ratios, as well as the consistency between the present isotopic data and those from Remus et al. (2000) for galena and sphalerite.

DISCUSSION AND CONCLUSIONS
The age of 565 ± 5 Ma obtained for the trachytic DMK-CL sample allowed us to identify, in an unprecedented way, the occurrence of the Acampamento Velho Formation in the region of the Camaquã Mines.
The two trachytic rocks (one in PCSM79-14 drill hole, at Area 1, and the other in FSM 81 drill hole, at Claudino Farm) have similar mineral composition and both present evidence of contemporary genesis with the sedimentary country rocks. The peperitic features found on the mineralized subvolcanic trachyte sill upper contact, at Area 1 of the Santa Maria deposit, indicate magma-wet sediment interaction. The age obtained for the DMK-CL sample (565 ± 5 Ma), located at the Claudino farm, is equivalent to the maximum depositional age of the Santa Barbara Group (566 ± 7 Ma; Bicca et al. 2013;568 ± 6 Ma;Oliveira et al. 2014), determined from the dating of detrital zircon crystals. This is probably due to the fact that trachyte intruded wet sediments as a subvolcanic sill, like Area 1 trachytic rock, or means it is formed as a lava flow during a rapid succession of sedimentation and magmatic events. The concordant character of this rock with its wall rocks is in agreement with both hypotheses.
From this contemporaneity relationship, it is possible to infer an approximate age for the Area 1 trachyte sill rock. Such rock occurs in a lower stratigraphic level (Seival Formation) relatively to Claudino farm rock (Rincão dos Mouras Formation). Thus, its age should necessarily be between DMK-CL sample age and the maximum depositional age of the Santa Barbara Group. As these ages are equivalent, Area 1 trachyte sill has approximately the same age as the Claudino farm trachyte. Therefore, the Acampamento Velho magmatism in the region of the Camaquã Mines is represented by both rocks.
The fact that galena and sphalerite have, in essence, the same Pb isotopic composition suggests a homogeneous source for Table 3. Pb isotopic ratios obtained for the sulfides from the Santa Maria deposit. Similar data from Remus et al. (2000) are also presented, for which the total analytical error for each Pb isotopic ratio corresponds to ± 0.15%. the several ore bodies in the Santa Maria deposit. The small scatter these sulfides present can result of a minimal contamination of the ore fluid by the wall rocks. The isotopic data of ore samples from the Camaquã (Cu; Au-Ag) and Santa Maria (Pb-Zn; Cu-Ag) deposits form a linear trend. While this trend does not correspond to a true isochron, possibly due to the contribution of old radiogenic Pb derived from U-Th-rich inclusions within ore pyrites (Remus et al. 2000), it suggests a simultaneous genesis and, again, an essentially homogeneous isotopic system for the Camaquã Mines deposits.   Remus et al. (2011) studied Crespos Cu-sulphide ore deposit, hosted by Acampamento Velho Formation rhyolites, and observed the Pb isotopic composition of this mineralization is similar to the least radiogenic compositions of Santa Maria, Camaquã, and Cerro dos Martins deposits. Based on these and other data, the authors did conclude these deposits are genetically related to the Acampamento Velho Formation magmatism, developed about 545 ± 13 Ma (Rb-Sr; Almeida et al. 1996).

Sample
These authors' conclusion was reinforced in this work by the spatial association observed in the field between trachytic rocks and mineralization, both related to the same hydrothermal process, as well as by the genetic association between this alkaline magmatism and mineralization pointed out by the subparallel nature between the linear arrangement formed by the Pb isotopic compositions of the Camaquã and Santa Maria sulfides and the reference isochrone of 565 Ma for the Acampamento Velho Formation (this work). However, as the isochrone inclination is not sensible enough to distinguish possible different ages to these ore deposits, Acampamento Velho Formation is seen here as only a possible magmatic candidate to be genetically associated to these deposits. Further studies are necessary to elucidate this topic.
Isotopic data also suggest a dominantly crustal and ancient basement source for Pb, with a model age of 1.07 Ga. Although rocks of such an age are unknown in the Camaquã Basin and its basement, they possibly occur at greater depths. Similar ages were obtained in inherited detrital zircon crystals (1.2 to 1.0 Ga) in the Sul-Riograndense Shield, whose source has not yet been identified (Remus et al. 1999, Hartmann et al. 2016). An igneous/metamorphic old rock with galena was probably the lead source for the Camaquã and Santa Maria deposits.
Galena and sphalerite Pb isotopic ratios of the Santa Maria deposit fall along the Orogen curve of Zartman and Doe (1981), which indicates an intermediate or mixed origin for these sulfides. These data also plot between Mantle, Orogen, and Lower Crust curves, being mostly nearer to the latter, in the thorogenic diagram. This points out to a lower crust contribution to the ore source rock magma.
From the data presented so far, it's possible to conclude that magmatic-hydrothermal fluids, possibly from the Acampamento Velho magmatism at the Camaquã Mines region, leached metals from an igneous or metamorphic deep old rock, with little wall rock interaction during its ascendant movement. Another valid hypothesis is that this ancient rock not only provided the metals, but also gave origin, by partial melting, to the parental magma of the trachytic rocks. In one way or another, the isotopic homogeneity observed in the sulfides is explained.