Precious-Metal Mineralization and Formation Conditions of the Biche-Kadyr-Oos Epithermal Au-Ag Ore Occurrence (Eastern Sayan, Russia)

: The Biche-Kadyr-Oos epithermal Au-Ag ore occurrence is a prospective object in the Ak-Sug porphyry copper ore cluster (Eastern Sayan) in the northern part of the Central Asian orogenic belt (CAOB). The mineralization consists of gold-sulﬁde-quartz and gold-polysulﬁde-carbonate-quartz veins with argillic zones in the Lower Cambrian volcanic-sedimentary rocks. The origin of the Au-Ag ore occurrence is still debatable. To determine the origin, we examined the mineralogical and geochemical features, conditions of formation, and ﬂuid sources of the Biche-Kadyr-Oos ore. A mineralogical and geochemical investigation outlines three stages of mineral formation: early argillic stage; gold-sulﬁde-quartz stage with pyrite, marcasite, pyrrhotite, arsenopyrite, chalcopyrite, less frequently sphalerite, hessite, gold, and electrum; and late gold-polysulﬁde-carbonate-quartz stage with gold, electrum, Hg-electrum, Se-acanthite, Se-galena, bornite, tennantite, tetrahedrite, hessite, tellurobismuthite, bismuthinite, matildite, jamesonite, ourayite, native Bi, and barite. Fluid inclusion study (thermometry, Raman spectroscopy) in quartz and mineral thermometry (electrum and sphalerite paragenesis) determined that ore veins were formed at P~0.5 kbar from CO 2 -water Na-K-chloride ﬂuid (4.9–9.6 wt % NaCl eqv) and temperatures from 300 to 200 ◦ C (early gold-sulﬁde-quartz veins at 300–230 ◦ C, and late gold-polysulﬁde-carbonate-quartz veins at 290–200 ◦ C) and variations in f O 2 , f S 2 , f Se 2 and f Te 2 . The S isotopic composition in sulﬁdes and δ 34 SH 2 S values of the ﬂuid are +1.3‰ and +4.7‰, respectively, (T = 300–275 ◦ C) indicating magmatic S in ore formation. The oxygen isotope data indicate that during the formation of veins, the magmatic ﬂuid mixed with meteoric water ( δ 18 O ﬂuid is from +3.4 to +6.4‰). The isotopic data that were obtained combined with mineralogical and geochemical features and conditions of ore formation indicate the similarity of Biche-Kadyr-Oos ore occurrence with epithermal Au-Ag deposits of intermediate sulﬁdation (IS) type. The presence of epithermal Au-Ag mineralization of the Biche-Kadyr-Oos IS type in ore cluster of the Ak-Sug Cu-Au-Mo porphyry deposit indicates the existence of a single porphyry-epithermal ore-magmatic system.


Introduction
Porphyry Cu-Au-Mo deposits were identified on the Tuva segment of the Central Asian orogenic belt (CAOB), where the largest are the Ak-Sug and Kyzyk-Chadr deposits in Eastern Tuva, with significant reserves and resources [1,2].The porphyry Cu-Au-Mo deposits are associated with Early-Middle Cambrian calcareous-alkaline porphyry intrusions [3][4][5].
Spatially and genetically, porphyry Cu deposits can be associated with Au-Ag epithermal gold-sulfide and other deposits that vertically succeed each other and form a single porphyry-epithermal ore-magmatic system [6][7][8][9].Prospecting and exploration of new deposits at the periphery of the same ore clusters are of great interest.Many Cu porphyry ore occurrences (Kadyroy, Grebeshkovy, Dashtyg, Biche-Kadyr-Oos, etc.) were identified in the Ak-Sug ore cluster (Eastern Sayan).Biche-Kadyr-Oos veined ore occurrence is one of the potential prospects for native Au.The genesis is still debatable, and mineralogical and geochemical features have not yet been sufficiently examined.
The Biche-Kadyr-Oos occurrence is located in volcanic-sedimentary rocks of the Khamsari Formation (Є 1 ).The diorite and gabbro-diorite intrusions of the Ak-Sug complex (Є 1 ak) cut the volcano-sedimentary rocks.The Biche-Kadyr-Oos ore occurrence is believed to be the result of both VMS and veinlet-disseminated Cu porphyry mineralization [1].It is assumed that the vein zones of this ore occurrence are developed over an undiscovered object of stockwork Au-Mo-Cu-porphyry mineralization (the Ak-Sug deposit type).
The aim of this paper is to examine the mineralogical and geochemical peculiarities and conditions of the formation of the Biche-Kadyr-Oos ore and to identify the oreformational type.

Geological Setting
The Tuva segment is located in the northern part of the CAOB.This region is bordered by the Caledonian structures of the Western Sayan in the west.The Early Baikal and Early Caledonian structures of the Eastern Sayan on the southern boundary of the Siberian craton are in the north and northeast.The N boundary of the Precambrian Tuva-Mongolian microcontinent (the largest tectonic block of the CAOB) is located at the east and southeast borders.The Early Caledonides of the Ozernaya island-arc zone of Western Mongolia are in the south (Figure 1).
The Tuva segment of the CAOB is an accretional and collisional structure formed during the geodynamic evolution and closure of the Paleoasian Ocean [10][11][12].The Eastern Tuva backarc, Sayano-Tuva forearc, and Tannuola-Khamsarin island-arc zones are distinguished within the Tuva segment.The Tannuol-Khamsarin island-arc zone is subdivided into the Khamsarin, Ondum, and Tannuola zones.The geological and tectonic structures in the region developed over a long time in stages of successive changes in geodynamic regimes (oceanic with primitive island-arc complexes ~1000-600 Ma, islandarc-570-518 Ma, accretion-collision-510-450 Ma, etc.) [13].Each stage included particular magmatic and ore formations.
The evolution of the Tuva segment of the CAOB provides the formation of the Upper Proterozoic and Paleozoic rock complexes in the region: island-arc volcano-plutonic assemblages of the ophiolite belts, the sedimentary and basement rocks of the microcontinent [10][11][12].
The Biche-Kadyr-Oos ore occurrence of the Ak-Sug Cu-Au-Mo porphyry ore cluster is located on the Eastern Sayan in the Vendian and Early Cambrian island-arc complexes of the Khamsarin subzone of the Tannuola-Khamsarin island-arc zone formed during the Paleoasian ocean subduction [11].The Kandat fault zone divides the Khamsarin subzone and the Kizir (East Sayan) structural-facial zone.The ore occurrence was discovered in 1963 during a 1:200,000 (1 sm-2 km) geological survey in the Biche-Kadyr-Oos River headwaters [2].Overlapping trough faults (Dashtygoy, Kadyroy, etc.) formed by volcano-sedimentary rocks of the Khamsari Formation (Є 1 ) border the southern flank of the Kandat fault.Massifs and stocks of porphyry granitoid intrusions of the Middle-Upper Paleozoic are localized in the frames of trough faults and along the plicative structures of sublatitudinal, northwestern, and northeastern directions.
The mineralization is characterized by different types of ore mineralization and is controlled by intrusive bodies or tectonic zones.In the Ak-Sug ore cluster, porphyry Cutype deposits and ore occurrences and single epithermal Au-Ag ore occurrences are formed (Figure 1).
The Biche-Kadyr-Oos ore occurrence is confined to the junction area of Precambrian and Early Caledonian structures, in hydrothermally altered volcanic and sedimentary rocks of the Khamsara suite (Є 1 ) with differently oriented tectonic faults of the Cheldezrik Fault (branch of the Kandat fault), intrusive bodies of diorite and gabbro-diorite bodies of the Tannuola complex (Є 1 tn), and small stocks of porphyry granites of the Ak-Sug complex (Є 1 ak).The mineralization is confined to the syn-ore SW tectonic faults (Figures 1 and 2).
Gold mineralization is represented by quartz (more than 20) and quartz-carbonate (more than 15) veins in the volcanic-sedimentary rocks of the Khamsar formation (Є 1 ) over an area of about 1 km 2 .The volcanogenic-sedimentary Khamsar Formation (Є 1 ) is composed of andesite, andesibasalt porphyrites and tuffs, with interlayers of rhyolites, shales, and sandstones.Basalts and andesibasalts consist of plagioclase and pyroxene, which are strongly altered.Pyroxene in porphyritic outcrops is represented by augite, which is most often indeterminate due to complete replacement by secondary minerals.Rhyolites in phenocrysts contain quartz and oligoclase or oligoclase.The thickness of the Lower Khamsarinsky Subformation is more than 1100 m.
According to a geological survey in 1963-1965, the ore occurrence included three veined zones: Zapadnaya, Shirotnaya, and Tsentralnaya.They are located along the NE or SW and are 500 m away from one another.The zones are 5-30 m thick and 150-300 m long.Each zone contains up to 10 ore veins 0.2-4.0m thick and 10-80 m long (Figure 2).
The Shirotnaya veined zone is composed of gold-sulfide-quartz and gold-polysulfidecarbonate-quartz veins up to 10 m thick and up to 150 m long in the view of crushing zones in plagioclase-pyroxene porphyries from 30 to 40 cm to 1 m thick, with a vertical or inclined (70-80 • ) dip to NE-NW.
The Tsentralnaya veined zone is composed of isolated early gold-sulfide-quartz veins deposited in a SW fault zone with a vertical or steep SW dip.The veins are discontinuous along the strike with narrowing and wedges in the flanks.They are from 0.2 to 1.4 m thick and up to 300 m long.Ore mineralization includes pyrite, chalcopyrite, fahlores, bornite, and Au.The veined zone is characterized by intense hydrothermal alteration of host volcano-sedimentary rocks and granite-porphyry intrusion of the Ak-Sug complex (Є 1 ak).
The Zapadnaya veined zone is localized in volcano-sedimentary rocks and less frequently in diorite-porphyries and comprises lens-shaped silicified and pyritized rocks and sometimes veins with chalcopyrite.The zone is up to 200 m long and includes up to 8 lens-like bodies up to 3-4 m thick and up to 50-60 m long.The ore minerals are pyrite and less frequently chalcopyrite.Most likely, the ore veins here are located hypsometrically below the silicified and pyritized rocks.The Zapadnaya veined zone is localized in volcano-sedimentary rocks and less frequently in diorite-porphyries and comprises lens-shaped silicified and pyritized rocks and sometimes veins with chalcopyrite.The zone is up to 200 m long and includes up to 8 lens-like bodies up to 3-4 m thick and up to 50-60 m long.The ore minerals are pyrite Gold-sulfide-quartz and gold-polysulfide-carbonate-quartz veins are accompanied by sheet-shaped bodies of hydrothermally altered quartz-sericite-kaolinite rocks.The Zapadnaya zone of hydrothermal alteration complies with metasomatites formed in the fringes of gold-sulfide-quartz and gold-polysulfide-carbonate-quartz veins of the Shirotnaya and Tsentralnaya zones.In general, hydrothermally altered quartz-kaolinite-sericite rocks form zones 20-50 m thick and up to 400 m long [1].Ore mineralization in veins is disseminated, rarely nest-like.According to [1], the contents of valuable components in mineralized rocks and ore veins are as follows: Cu 1%-1.5%,Zn 1%-1.5%,Ag up to 183 ppm (average value of 25.4 ppm), and Au up to 10 ppm (average value of 2.11 ppm).

Methods
Samples of ores and hydrothermally altered rocks were collected from the surface of the ore zone from outcrops and exploration trenches.We studied polished sections on Olympus BX41 and POLAM P-213M microscopes at Tuvinian Institute for Exploration of Natural Resources SB RAS (Kyzyl, Russia).The chemical composition of minerals was determined using a MIRA 3 LMU SEM (Tescan Orsay Holding, Brno, Czechia) with INCA Energy 450 + XMax 80 and INCA Wave 500 microanalysis systems (Oxford Instruments Nanoanalysis Ltd., Oxford, UK) (Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia).The compositions of native gold and other minerals were examined at the accelerating voltage of 20 kV, an electron beam current of 1.5 nA, and live acquisition time of spectra of 30 s.The following X-rays were selected: K series for Fe, Cu, and Ni, and L series for Pd, Au, Ag, As, Te, Se, Bi, Sb, and Hg.As the standards, we used FeS 2 (on S), PbTe (on Te), PtAs 2 (on As), and the pure elements of Fe, Ni, Cu, Se, Ru, Rh, Pd, Ag, Sn, Sb, Pt, Au, and Bi.
Fluid inclusions were analyzed using a TMS-600 Linkam stage equipped with LinkSystem 32 DV-NC (London, UK) and optical microscope Olympus BX51 (Olympus Corp., Tokyo, Japan).The first melting temperatures of fluid were interpreted using [15,16].The salinity of fluids was identified from the final ice melting temperatures [17].The homogenization temperatures of fluid inclusions are considered as the starting temperatures of the mineral formation process [18].The gas phase composition of fluid inclusions was examined using a Horiba Jobin Yvon LabRam HR800 Raman spectrometer (Glasgow, Scotland) combined with a CCD detector and an Olympus BX51 microscope using a 532 nm Nd:YAG laser.The monochromator was calibrated to the silicon band (520.7 cm −1 ).Measurement data were processed in Statistica 6.1.
The isotopic composition of oxygen in quartz (samples BKO-4 and BKO-5, powder fration 0.5-0.25 mm) was analyzed using laser fluorination in the Geospectr Analytical Center of the Geological Institute SB RAS (Ulan-Ude, Russia) on a FINNIGAN MAT 253 gas mass spectrometer using a double inflow system in the classical version (standard-sample) (analyst V.F.Posokhov).δ 18 O values are given in permil (‰) relative to the SMOW standard (NBS-28) [23].The error of the obtained values did not exceed 0.2 to 0.3‰.
The sulfur isotopic composition in chalcopyrite (powder frations 0.5-0.25 mm) was measured at the Multi-element and Isotope Research Center SB RAS using a Finnigan MAT Delta mass spectrometer in dual-inlet mode (analysts V.N.Reutsky and M.N.Kolbasova, Novosibirsk, Russia).The measurement control was provided by the samples with standard isotopic composition in the range δ 34 S from −15.1‰ to +21.8‰ relative to troilite from Canyon Diablo (CDT), including international ones: NBS-123 (δ 34 S = +17.44)and NBS-127 (δ 34 S = +21.8).The reproducibility of the values of δ 34 S, including sample preparation, was not more than 0.1‰ (2σ).The values of δ 34 S (‰) are given relative to the CDT standard [23].

Mineralogy
Hydrothermally altered quartz-sericite-kaolinite rocks with pyrite (up to 5%) occupied a large area in the ore occurrence, localizing within the tectonic fracture zone of SW trending.They are bleached, sericitized, silicified, and pyritized rocks with relic structures and formed predominantly in exocontacts of ore veins.
Early gold-sulfide-quartz veins (up to 1 m thick and 10 m long) are composed of quartz, pyrite, marcasite, pyrrhotite, arsenopyrite, chalcopyrite, and less frequently sphalerite, hessite, and gold.The ores have spotty, spotty-striped textures, and they have idiomorphic or panidiomorphic grained structures with superimposed cataclastic structures.
Quartz forms fine-grained aggregates and veins.
Pyrite is characterized by spotted clusters and chain-shaped phenocrysts consisting of large (1.7-2 to 5 mm) isometric polygonal grains or fine-grained aggregates with traces of cataclasis.
Marcasite and pyrite form spotted clusters elongated in discontinuous parallel-oriented bands.
Sphalerite (up to 5 mm) is found in quartz, pyrite, and chalcopyrite as a phenocryst of xenomorphic grains (Figure 3).

Mineralogy
Hydrothermally altered quartz-sericite-kaolinite rocks with pyrite (up to 5%) occupied a large area in the ore occurrence, localizing within the tectonic fracture zone of SW trending.They are bleached, sericitized, silicified, and pyritized rocks with relic structures and formed predominantly in exocontacts of ore veins.
Early gold-sulfide-quartz veins (up to 1 m thick and 10 m long) are composed of quartz, pyrite, marcasite, pyrrhotite, arsenopyrite, chalcopyrite, and less frequently sphalerite, hessite, and gold.The ores have spotty, spotty-striped textures, and they have idiomorphic or panidiomorphic grained structures with superimposed cataclastic structures.
Quartz forms fine-grained aggregates and veins.
Pyrite is characterized by spotted clusters and chain-shaped phenocrysts consisting of large (1.7-2 to 5 mm) isometric polygonal grains or fine-grained aggregates with traces of cataclasis.
Marcasite and pyrite form spotted clusters elongated in discontinuous parallel-oriented bands.
Sphalerite (up to 5 mm) is found in quartz, pyrite, and chalcopyrite as a phenocryst of xenomorphic grains (Figure 3).Chalcopyrite (0.01 to 3 mm) often fills the finest cracks in quartz and forms emulsion phenocrysts in sphalerite (Figure 3b).
Hessite, tellurobismuthite, gold, and single grains of the electrum are observed as inclusions (up to 10 µm) in chalcopyrite and associated with sphalerite.
Hessite, tellurobismuthite, gold, and single grains of the electrum are observed as inclusions (up to 10 µm) in chalcopyrite and associated with sphalerite.
Arsenopyrite (up to 2.5 mm), pyrite (up to 1.5 mm), and sphalerite (up to 5 m formed isolated aggregates of isometric or elongated shape in quartz or chalcopy Some sphalerite grains are characterized by Fe admixture up to 15 wt %.Chalcopyrite bornite (up to 5 mm) are present as granular precipitates in quartz.Fahlores of the nantite-tetrahedrite series were found in intergrowths with chalcopyrite.
Galena formed isolated aggregates (up to 0.8 mm) and finest phenocrysts in cha pyrite, pyrite, and carbonate.The boundaries of galena grains are uneven, rectilinear, stepped (see Figure 5).Galena contains impurities of Ag up to 1.85 wt % and Se up wt %.Ni-cobaltite and Co-gersdorffite are located at the boundaries of pyrite and cha pyrite grains and form inclusions in chalcopyrite.
Arsenopyrite (up to 2.5 mm), pyrite (up to 1.5 mm), and sphalerite (up to 5 mm) formed isolated aggregates of isometric or elongated shape in quartz or chalcopyrite.Some sphalerite grains are characterized by Fe admixture up to 15 wt %.Chalcopyrite and bornite (up to 5 mm) are present as granular precipitates in quartz.Fahlores of the tennantite-tetrahedrite series were found in intergrowths with chalcopyrite.
Galena formed isolated aggregates (up to 0.8 mm) and finest phenocrysts in chalcopyrite, pyrite, and carbonate.The boundaries of galena grains are uneven, rectilinear, and stepped (see Figure 5).Galena contains impurities of Ag up to 1.85 wt % and Se up to 5 wt %.Ni-cobaltite and Co-gersdorffite are located at the boundaries of pyrite and chalcopyrite grains and form inclusions in chalcopyrite.
Bismuthite (up to 10 µm), tellurobismuthite (up to 5 µm), and matildite (up to 15 µm) are found as inclusions in chalcopyrite and pyrite and are associated with arsenopyrite, native Au, and electrum (see Figure 5).
Inclusions of matildite and ourayite up to 15 µm in size were found in pyrite.Native Bi (up to 5 µm) formed thin "fines" in quartz.
Small inclusions of hessite (up to 10 µm) of rounded, oval, and twisted shapes were observed in the chalcopyrite associated with gold and the electrum.Se-acanthite (up to 5 µm) with arsenopyrite formed inclusions in chalcopyrite (see Figure 5).
Inclusions of matildite and ourayite up to 15 µm in size were found in pyrite.The compositions of Bi and Te minerals are shown in Table 1.Note.A dash indicates the value was below detection limits.The formulas of hessite are calculated for 2 at., Se-acanthite for 3 at., matildite for 4 at., tellurobismuthite and bismuthinite for 5 at.
Native gold by Ag and Hg content is subdivided (wt %): ( The Ag value in gold is up to 29.93 wt %; in electrum-48.13wt %, Hg-6.52 wt % (Table 2).A single grain of Hg-electrum (20 µm) was found at the contact of pyrite with brecciated arsenopyrite.The gold-polysulfide-carbonate-quartz veins are quantitatively dominated by electrum and low-fineness and medium-fineness gold (Figure 4).

Conditions for the Formation of Ore Mineral Veins
Fluid inclusions were studied in quartz of ore veins to determine the PT conditions of mineral-forming fluid in the Biche-Kadyr-Oos ore occurrence.
We analyzed two-phased (VL) primary and pseudosecondary fluid inclusions in quartz, which are located in the central parts of grains and unrelated to fractures.They are 12-15 µm size (rarely up to 20 µm), round-oval or isometric in shape, sometimes with crystallographic elements.Vapor bubbles occupied up to 15%-20% of the inclusions (Figure 7).

Conditions for the Formation of Ore Mineral Veins
Fluid inclusions were studied in quartz of ore veins to determine the PT conditions of mineral-forming fluid in the Biche-Kadyr-Oos ore occurrence.
We analyzed two-phased (VL) primary and pseudosecondary fluid inclusions in quartz, which are located in the central parts of grains and unrelated to fractures.They are 12-15 µm size (rarely up to 20 µm), round-oval or isometric in shape, sometimes with crystallographic elements.Vapor bubbles occupied up to 15%-20% of the inclusions (Figure 7).The fluid inclusions in quartz of gold-sulfide-quartz veins contain Na-K-chloride fluid with first melting temperatures (from −23 to −24 °C) (Table 3).Fluid inclusions were homogenized at 250-292 °C; the fluid salinity varies from 4.9 to 8.6 wt % NaCl eqv.
The pressure during the formation of the late quartz veins was calculated using a sphalerite geobarometer [20].According to the electrum-sphalerite geothermometer, the pressure during the vein formation was estimated at 0.5 kbar at sphalerite crystallization temperatures equal to 275 °C.3).Fluid inclusions were homogenized at 250-292 • C; the fluid salinity varies from 4.9 to 8.6 wt % NaCl eqv.The fluid inclusions in quartz of gold-polysulfide-carbonate-quartz veins were homogenized at lower temperatures of 225-258 • C. The fluid salinity increases up to 8.2-9.3 wt % NaCl equiv.Raman spectroscopy data indicate that the gas phase consists of CO 2 .
The pressure during the formation of the late quartz veins was calculated using a sphalerite geobarometer [20].According to the electrum-sphalerite geothermometer, the pressure during the vein formation was estimated at 0.5 kbar at sphalerite crystallization temperatures equal to 275 • C.
The δ 18 O value of quartz from early ore veins varies from 11.8 to 13.8‰.According to the fractionation equation [28,29]
The δ 18 O value of quartz from early ore veins varies from 11.8 to 13.8‰.According to the fractionation equation [28,29]
Hydrothermal alteration of rocks is close to mineralized areas and confined to zones of induced fracturing.
The mineral composition of the early gold-sulfide-quartz veins consists of simple sulfides, gold, electrum, and single hessite segregations.
With the pressure correction (0.5 kbar) [37], the fluid inclusion data showed that gold-sulfide-quartz veins were formed at temperatures ranging from 330 to 270 °C due to Na-K chloride fluid with a salinity of 4.9 to 8.6 wt % NaCl eqv.
The late gold-polysulfide-carbonate-quartz veins were formed at temperatures ranging from 290 to 260 °C due to Na-K chloride fluid with salinities from 4.9 to 9.6 wt % NaCl equiv.These data are consistent with mineral geothermometers.Paragenesis of pyrite, pyrrhotite, chalcopyrite, and arsenopyrite from gold-sulfide-quartz veins suggests fS2 from 10 −14.3 to 10 −7.6 at 295 °C.
Hydrothermal alteration of rocks is close to mineralized areas and confined to zones of induced fracturing.
The mineral composition of the early gold-sulfide-quartz veins consists of simple sulfides, gold, electrum, and single hessite segregations.
With the pressure correction (0.5 kbar) [37], the fluid inclusion data showed that goldsulfide-quartz veins were formed at temperatures ranging from 330 to 270 • C due to Na-K chloride fluid with a salinity of 4.9 to 8.6 wt % NaCl eqv.
The late gold-polysulfide-carbonate-quartz veins were formed at temperatures ranging from 290 to 260 • C due to Na-K chloride fluid with salinities from 4.9 to 9.6 wt % NaCl equiv.These data are consistent with mineral geothermometers.Paragenesis of pyrite, pyrrhotite, chalcopyrite, and arsenopyrite from gold-sulfide-quartz veins suggests f S 2 from 10

Figure 6 .
Figure 6.The paragenetic sequence of Biche-Kadyr-Oоs ore occurrence.The line thickness indicates the relative mineral prevalence.

Figure 6 .
Figure 6.The paragenetic sequence of Biche-Kadyr-Oos ore occurrence.The line thickness indicates the relative mineral prevalence.
−14.3 to 10 −7.6 at 295 • C. Paragenesis of native Bi, matildite, Se-acanthite, and other sulfides of gold-polysulfidecarbonate-quartz veins indicates the redox potential changes of the fluid at variations