Geochemistry of coarse quartz sinter overlying an Early Cretaceous Serra Geral quartz andesite flow, Fronteira Oeste Rift, Rio Grande do Sul, Brazil

Abstract Sinter overlying the first quartz andesite flow at the base of the Serra Geral Group is a surface manifestation of intense hydrothermal processes operating in the Paraná Basin during the Early Cretaceous. The coarse quartz sinter from western Rio Grande do Sul state was studied in satellite images, field surveying, optical petrography, scanning electron microscopy, and electron probe microanalyses, including backscattered electron images and chemical analyses of rocks. Quartz forms large crystals (10 cm) because it was either deposited in a dilute aqueous solution or recrystallized from fine-grained sinter. Well-crystallized chamosite — an iron aluminosilicate (Fe-chlorite) — occurs in quartz crystals, partly associated with fractures. The composition of chamosite is akin to that in ore deposit associations. Noble metals in two sinter samples are present in concentrations of 0.1 ppm Ag and 15 ppb Au. The contents of Ba, Bi, Cu, Mo, S, and W are low but are significant. The present description of sinter quartz signals the presence of a major paleo-hotspring field in the Fronteira Oeste Rift, Rio Grande do Sul, Brazil, that mertis further study to fully characterize the extent and metallogenetic endowment (Au-Ag-Cu) of the epithermal province.


INTRODUCTION
The Paraná Volcanic Province (1 million km 2 ) has an extensive and thick (up to 1,700 m) tholeiitic volcanic sequence of basalt (97.5%) and rhyodacite (2.5%) flows encased in the Serra Geral Group.Basalt, quartz andesite, and rhyodacite host the largest world deposits of amethyst and agate geodes, both in the first lava flows and in several stratigraphic positions up to 1,000 m above the first flow.Long considered magmatic in origin by degassing of the lava, the natural history of the geodes started to be elucidated by temperature determination of amethyst crystallization at 50°C (e.g., Juchem 1999).A significant contribution by Gilg et al. (2003) clarified the origin of the mineralizing fluids from the Guarani Aquifer.Extensive alteration of the mineralized basalt by low-temperature minerals was established by Gomes (1996).The origin of the geodic cavity was described by Duarte et al. (2009) as low-temperature ballooning of altered basalt.Lowering the temperature resulted in the partial filling of the geode with valuable minerals.The hydrothermal, epigenetic origin of the geodes was thus established.A pervasive hydrothermal event was recognized in the Serra Geral Group, forming low-temperature minerals (zeolites, clays, and native copper) in the amygdales and fractures during the first hydrothermal event (H1).Hot water continued to be pressurized below the sealed basalt, leading to the explosive injection and effusion (H2) of fluidized sand, which originated in the underlying Guarani Aquifer.A description of the aquifer is in Hirata and Foster (2021).Newly sealed basalt was affected by hot water percolation (H3), leading to the opening and filling of geodes.
The hydrothermal partial alteration of basalt in most flows could evolve in some places to the emission of hot vapor at the paleosurface, forming hot springs in the Early Cretaceous (Hartmann et al. 2021(Hartmann et al. , 2022a)).Volcanism occurred near 134.5 Ma (e.g., Hartmann et al. 2019, Gomes andVasconcelos 2021).Thick soil and forest cover (remnants of the Atlantic Forest) and the focus by researchers on volcanic aspects of the group impeded the finding of paleohot springs.Hydrothermal processes that altered the basaltic lavas in the Cretaceous started to be recognized from the work of Duarte et al. (2009) and Hartmann et al. (2012).Further field work by these authors focused on identifying the paleohot springs.A change in paradigm was required to make possible the discovery of the surficial effusion of hot water and related deposits.The distal imprint of paleodune shape formed rings and arcs in the upper amygdaloidal crust of the Catalán Flow (Hartmann et al. 2021).Additional studies are required to fully characterize the sinter, because of the scientific and potential economic importance of the previous finding by Hartmann et al. (2022a).Silicification, fracturing, and steam venting in paleodunes were evaluated by Hartmann et al. (2022b) and in volcanic rocks by Hartmann et al. (2021).

Geochemistry of coarse quartz sinter overlying an
Early Cretaceous Serra Geral quartz andesite flow, Fronteira Oeste Rift, Rio Grande do Sul, Brazil We selected the sinter from the Macacos Locale for a study of this first identification of paleohot spring activity in the Paraná Volcanic Province.We further characterized the sinter with several techniques, following the initial report by Hartmann et al. (2022a).We integrated field surveying with petrography, scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) analyses, backscattered electron (BSE) imaging, electron probe microanalyzer (EPMA) imaging, and chemical analyses of quartz and chamosite.Sinter was analyzed for major and trace elements.The sinter has chamosite (Fe-chlorite) with quartz (some gypsite and barite), which is significant for the indication of an epithermal Au-Ag-Cu metallogenetic province.Au and Ag contents are in the low range of mineralized epithermal provinces (e.g., Yellowstone; Churchill et al. 2021).The fountain that generated the sinter was active above the first lava flow that covered the active dunes.This is the description of the first window into the surficial hydrothermal processes operative in a large area (> 15,000 km 2 ) of the Serra Geral Group (Fronteira Oeste Rift) during the initial stages of effusion of lavas in the Early Cretaceous.

GEOLOGICAL SETTING
The South American Serra Geral Group occurs mostly in Brazil, and also in Uruguay, Argentina, and Paraguay (Figs. 1A, 1B), with a fragment (8%) drifted to southwestern Africa during rupturing of Gondwana (e.g., Hartmann et al. 2010Hartmann et al. , 2019)).Volcanic rocks are near the top of the Paraná Basin (Zalán et al. 1991), only covered by the Bauru Group sedimentary and volcanic rocks.Sedimentary rocks attained > 200°C in the northeastern part of the basin (Teixeira et al. 2018).The large (1 million km 2 ) volcanic group has been described for its volcanic characteristics, with a tendency to attribute to volcanism all features observed in the field.Intercalated sand layers have been systematically attributed to eolian deposition (e.g., Bertolini et al. 2020Bertolini et al. , 2021)).
The discovery of injection and effusive structures associated with the sand bodies associated with the lavas (Hartmann et al. 2010, Duarte et al. 2020) in the Novo Hamburgo Complex (Fig. 2, Table 1) was integrated with the processes that led to the universal presence of injected sand in the amethyst geode deposits.The initial observation of sand in geodes was made by Bossi and Caggiano (1974) and consolidated with the correct hydrothermal interpretation by Duarte et al. (2009).Hot water was perceived as altering the basalt and opening and filling cavities in the cold (50-150°C) rock.In many outcrops, filled fractures point to shallow levels near the surface; many geodes in the mines have escape channels of fluid above their tops.It seemed reasonable that some of the fractures reached the surface as hot springs and geysers.
A specific search was required.
The Fronteira Oeste Rift (Hartmann and Cerva-Alves 2021) has six flat-lying lava flows distributed in layer-cake stratigraphy (Fig. 2).Sand dikes, sills, and extrudites are common in the rift.Rifting postdated the Serra Geral Group by 10-15 Ma in the region.Both exposed paleodune tops and   The identification of chamosite was initially made on the webmineral site of the International Mineralogical Association (IMA, 2022).The chemical composition of the mineral determined by EPMA led to its only classification as chamosite.A check on the international literature showed no other mineral with a similar composition.

METHODOLOGY
The calculation of chamosite formula followed standard procedures using the results from the EPMA.We used 28 O and applied the sequence of steps from Appendix 1 in Deer et al. (1966).All iron was considered as Fe 2+ .F was calculated separately from O.
The three whole-rock samples (MAC3, MAC4, and MAC6) were analyzed for major and trace elements at GEOSOL, Belo Horizonte, Brazil.Major oxides were analyzed by XRF79Cfusion with lithium tetraborate, analysis by X-ray fluorescence; trace elements by ICM14B -digestion with aqua regia, analysis by ICP OES/ICP MS.Au was analyzed by FAA313 -fire assay AAS, 30 g; and LOI by PHYO1E -loss on ignition by gravimetry at 1,000°C.

RESULTS
Integrated study by Hartmann and Cerva-Alves (2021) and Hartmann et al. (2021), and additional geological survey led to description of elliptical, gray rings and arcs overlying the quartz andesite core.In the studied Macacos Locale, green grasslands cover the core rocks because of humid summer.In cold winter, brown soil is more apparent in the images (Hartmann et al. 2021).Paleodunes are not exposed in the studied region; injected, silicified sandstone occurs as blocks (10-50 cm long) included in the upper amygdaloidal portion of the quartz andesite.In places, sandstone dikes evolve into quartz veins along the same fracture.
In the studied Macacos Locale (Figs. 3A, 3B), the exposed geology is dominated by the Catalán Flow.In large extensions, the massive core is exposed below the grasslands, because the upper amygdaloidal crust was eroded.But erosional remnants of the silicified upper amygdaloidal crust are preserved in rings and arches, as described by Hartmann et al. (2021).Sinter quartz occurs as 0.5-2.0m large blocks (Fig. 4) at the top of the amygdaloidal upper crust of the quartz andesite.The white crystals are large (5-10 cm), locally forming radiating bundles.Large quartz crystals contain associated chamosite inclusions (200 μm long) along fractures (Figs.5A, 5B) (Tables 2 and 3; Fig. 6).The mineral is type I chlorite, trioctahedral, Fe-chlorite in general (Zane and Weiss 1998).EPMA analyses (n = 15; not shown) of quartz yield nearly pure SiO 2 .
Whole-rock composition of sample MAC6 (Table 4) displays high CaO (0.36-0.64 wt.%) and S (0.22-037 wt.%), with no corresponding minerals identified in the studied thin section.Two sinter samples have near 0.1 ppm Ag and 15 ppb Au.The same two samples that display significant noble metal content also have some trace elements above the detection limit -Ba, Bi, Cu, Mo, S, and W. diluted silicon or recrystallization of fine, opal sinter along time ( Jones 2021).A few thousand years seems sufficient for this transformation at 50-150°C.

DISCUSSION
Opal and chalcedony from surficial sinter are commonly transformed with time (> 50,000 years) into fine and coarse quartz (Marcoux et al. 2004).For instance, Holocene sinter from the Yellowstone caldera displays this relationship within a few million years (Kharaka et al. 2000, Tan et al. 2020).The coarse quartz from the Macacos Locale may correspond to the alteration (diagenesis) of surficial chalcedony from Early Cretaceous sinter.
The blocks bear no resemblance to geodes formed within the lava, which are well-known in the volcanic province (e.g., Duarte et al. 2009, Hartmann et al. 2012)    sinter is within the range of the old sinter from Yellowstone (2-19 ppb), whereas the content of Ag (16 and 15 ppm) is much higher than Ag in the old sinter (22-44 ppb).The measured contents of Ba, Bi, Cu, Mo, S, and W are an indication of the epithermal environment of sinter deposition.A test of geological affinity of the chamosite chemistry was made based on the studies by Rivas-Sanchez et al. (2006).The contents of cations from the formula unit were plotted (not shown here) in the following binary and ternary diagrams: Al × Fe × Mg, Al/Si × Mg/Fe, (Fe + Mg + Mn) × Si × Al.The compositions of the analyzed chamosite plot inside or close to the field of "ore" in "continental" environments.This affinity reinforces our interpretation of a fertile Fronteira Oeste Rift for epithermal Au-Ag-Cu deposits.
Additional investigations may locate more outcrops of varied sinter rocks and possibly economic concentrations of Au-Ag-Cu.Sinter at the Macacos Locale overlies the second lava flow of the Serra Geral Group, creating a need to study the local and regional geology to fully characterize the paleohot spring field and its metallogeny.

CONCLUSION
Our integrated study of a coarse quartz sinter overlying the first lava that covered the Botucatu paleodunes shows the high content of Au and Ag (+Ba, Bi, Cu, Mo, S, and W) in the rock and the presence of accessory chamosite (some gypsite and barite).This first description of sinter above a volcanic flow from the Serra Geral Group opens a wide window into scientific and economic studies of the Fronteira Oeste region of southernmost Brazil.We thus report the characterization of sinter, which is commonly indicative of the presence of epithermal deposits of Au-Ag-Cu.

Figure 2 .
Figure 2. Drilling section between Alegrete and Itaqui; drill hole logs projected orthogonally to section line from up to 10 km on either side.
Similar methodology was used as described byHartmann  and Cerva-Alves (2021) andet al. (2022a).Volcanic rock classification followsSantos and Hartmann (2021).Satellite images were interpreted byHartmann et al. (2021)  for the local understanding of the distribution of massive core and elliptical crust remnants of the Catalán Flow.Images were from GoogleEarth, which is based on Landsat images.Field work was done in the Macacos Locale to describe and interpret the previously identified coarse quartz sinter, including sample (n = 3) collection.Samples MAC3, MAC4, and MAC6 were studied with the optical microscope and SEM at the Laboratório de Geologia Isotópica, Centro de Estudos em Petrologia e Geoquímica, Instituto de Geociências, Universidade Federal de Rio Grande do Sul.The SEM is a JEOL JSM-6610LV, equipped with a Bruker XFLASH 5030 energy dispersive X-ray spectrometer.Analytical conditions were 20 kV, spot size 60 μm, working distance 0.7 mm, and counting time 30 s for the EDS analyses.Sample MAC6 was additionally investigated with the SEM and EPMA in a new thin section.Mineral chemistry was acquired with an electron microprobe JEOL JXA-8230 at the Microscopy and Microanalysis Laboratory, Universidade Federal de Ouro Preto.The mineral chemistry was determined on quartz (to confirm identification) and chamosite under conditions of 15 kV accelerating voltage, 20 nA sample current, and 5 μm spot size; ZAF matrix corrections were applied.Counting times on the peaks and background were 10/5 s for all elements.Analytical errors are 0.21-1.43%.Standards used are listed inCerva-Alves et al. (2021).BSE images were made for textural characterization and spot location for chemical analyses.Mineral abbreviations are fromWhitney and Evans (2010).

Figure 3 .
Figure 3. (A) Satellite image of Macacos Locale, showing location of collected sinter samples MAC4, MAC5, and MAC6; gray-colored portions are the silicified upper amygdaloidal crust of the Catalán Flow, which is flat lying (interrupted by creek), two white dots near farm road are animal feeders.(B) Geological map of same area as (A).Brown color corresponds to the massive core of the Catalán Flow, covered with grass and displaying few, small rock blocks.Light gray areas are erosional remnants of the silicified upper amygdaloidal crust of the Catalán Flow.

Figure 4 .
Figure 4. Block of coarse quartz sinter with radiating crystals (indicated by black arrow); quartz partly covered by orange lichen; location of sample MAC6 is shown as red circle.Samples MAC3 and MAC4 were collected within a few meters from this block.Two Catalán quartz andesite samples from the same outcrop were analyzed chemically by Hartmann and Cerva-Alves (2021).
Figure 5. (A and B) BSE images of chamosite crystals contained in coarse quartz and distributed particularly along fractures.EPMA analytical points marked.Dark patches are defects on the thin section.
. Chamosite crystallization in fractures of quartz indicates Fe-rich hot water discharge at the paleosurface.Common in sea-floor sedimentary rocks, chamosite was originated from crystallization of terrestrial weathering or volcanic components (Clement et al. 2020).Hydrothermal alteration of quartz andesite and remobilization of iron are presently considered for the origin of chamosite in the sinter.The content of CaO and S in the sinter may correspond to calcite and pyrite present in the sample.The measured Ag and Au in two chemical analyses of sinter suggest an environment prone to noble metal occurrences, because the concentrations are in the lower range of known epithermal deposits.Examples are the Milestone deposit, Idaho deposit (Fenner et al. 2022), and deposits in the Yellowstone (Churchill et al. 2021).The content of Au (16 and 15 ppb) from the Macacos

Figure 6 .
Figure 6.Simplified classification of chlorite from the Macacos sinter.The analyzed chlorite is chamosite.

Table 1 .
Nomenclature of stratigraphic and other units of the Paraná Basin in the studied Fronteira Oeste region of Brazil in Rio Grande do Sul state.overlyingvolcanicrockswerefracturedintorhomboidsbyforcesrelated to a vertical σ1 ellipsoid (e.g.,Hartmann et al.  2021).Hot water reaching the surface was first identified in the presently studied sinter (Figs.3 and 4)(Hartmann and  Cerva-Alves 2021)and then byHartmann et al. (2022)in a 50-cm large hydrothermal bowl at the top of a paleodune from the Botucatu Formation.
Stratigraphy Novo Hamburgo Complex, 134.5 ± 2 Ma Sand injectites in the Serra Geral Group -dikes, sills, and extrudites Fronteira Oeste Paleohot Spring Field, Early K Extensive occurrence of thousands of hydrothermal bowls and fumaroles at the top of paleodunes and three first lava flows.One silica sinter occurrence Inhanduí Paleodune Field, J-Early K Large (30 × 10 km) dune field aligned NW from Alegrete town, starting near Rio Inhanduí Guaviju Rings and Arcs Field, Early K Extensive (150 × 150 km) distribution of erosional remnants of silicified (sand injectites, quartz veins, and amygdale filling) upper amygdaloidal crust of Catalán Flow 3/8

Table 4 .
Whole-rock chemical analyses of studied sinter.Oxides and S in wt.%, trace elements in ppm, Au in ppb.