Epidote Minerals Research Articles

New data on REE and rare-metal mineralization in pegmatites of the Slyudyanogorsk muscovite deposit in the Southern Urals

The Slyudyangorsk muscovite deposit in the southern Urals was explored and mined in 1926–1957. By the mid-1950s, 104 veins of quartz–feldspar pegmatites including 21 muscovite-bearing veins have been found. Pegmatites with giant black Y-bearing epidote crystals are crosscut by veins with giant muscovite crystals, which, in turn, are intersected by veins of two-mica–quartz–two-feldspar pegmatites with rare-metal and REE mineralization. Microprobe data on compositions of complex Ti–Ta–Nb oxides [fergusonite-(Y), samarskite-(Y), euxenite-(Y), polycrase-(Y), columbite-(Fe), pyrochlore supergroup] are characterized, as well as of uraninite, ilmenorutile, scheelite, Y-bearing epidote, certain sulfides and rock-forming minerals from the Slyudyanogorsk deposit. The morphology and interrelation of minerals indicate that they are the result of crystal growth in cavities rather than of metasomatic replacement of gneisses, as has been suggested earlier. Thus, it is more promising for rare-metal and REE minerals in the Slyudorudnik area to be found in igneous rocks (granitic muscovite–quartz–feldspar pegmatites with the Nb–Ta–Ti–Y–U–W–Mo mineralization) than in metasomatic rocks.

A metamorphic mineral source for tungsten in the turbidite-hosted orogenic gold deposits of the Otago Schist, New Zealand

The orogenic gold deposits of the Otago Schist, New Zealand, are enriched in a variety of trace elements including Au, As, Ag, Hg, W and Sb. We combine laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) traverses and images to show that detrital rutile is the most important host mineral for W in the subgreenschist facies rocks. Furthermore, the prograde metamorphic recrystallisation of detrital rutile to titanite releases significant amounts of W (potentially 0.41 g/tonne of rock). Scheelite development closely follows the progression of this W-liberating reaction. Scheelite micrograins form early within the fabric of the rock evolving to locally and regionally sourced scheelite-bearing veins. Scheelite from syn-metamorphic veins at Fiddlers Flat and Lake Hāwea shows distinct differences in composition compared with scheelite from late-metamorphic veins at the Macraes Mine, the latter of which is enriched in REEs, Y and Sr. We suggest that the scheelite at Macraes became enriched due to the liberation of these elements during alteration of the Ca-silicate minerals epidote and titanite by the ore-forming fluid. These results are supportive of recent models for orogenic gold mineralisation in the Otago Schist, whereby prograde metamorphic recrystallisation of diagenetic or detrital metal-rich mineral phases (pyrite to pyrrhotite: Au, As, Ag, Hg and Sb; rutile to titanite: W) releases significant amounts of metals into the concurrently developing metamorphic fluids that can be subsequently focussed into regional structures and form significant tungsten-bearing orogenic gold deposits.

U–Pb zircon, Re–Os molybdenite geochronology and Rb–Sr geochemistry from the Xiaobaishitou W (–Mo) deposit: Implications for Triassic tectonic setting in eastern Tianshan, NW China

The Xiaobaishitou W (–Mo) deposit is located in the eastern segment of the Central Tianshan, northwestern China. The deposit represents a skarn system distributed in the contact zones of biotite granite and crystalline limestone of the Mesoproterozoic Kawabulag Group. The Xiaobaishitou deposit is characterized by a typical calc-silicate mineralogy dominated by garnet, diopside and wollastonite, with minor epidote, tremolite, actinolite, chlorite, quartz, fluorite and calcite. The prograde and retrograde skarns are characterized by garnet–clinopyroxene–wollastonite and epidote–tremolite–actinolite–chlorite, respectively, intruded and replaced by mineral assemblages of scheelite–cassiterite–magnetite, quartz–sulfides and calcite–quartz–fluorite in younger order.Six molybdenite samples from the deposit yielded Re−Os isotope model ages ranging from 239.7±3.6Ma to 251.4±3.6Ma. The zircon crystals from biotite granite and Mo-mineralized granite yield weighted 206Pb/238U age of 242±1.7 and 240.5±2.1Ma, respectively. Both the zircon U−Pb and the molybdenite Re−Os ages obtained in this study fall in a narrow span of 242–240Ma, which suggest that the Xiaobaishitou W (–Mo) system was formed in the Triassic. The Re contents of the molybdenites range from 40.33 to 64.67ppm, suggesting that the ore-forming materials were derived mainly from continental crust together with the involvement of minor mantle components. Combined with the 87Sr/86Sr ratios of tungsten-bearing quartz veins from other studies, which scatter between 0.707153 and 0.709877, demonstrating mixing between two end-member isotopic compositions of crust and mantle. It can be concluded that the Indosinian Xiaobaishitou deposit was formed in a tectonic transition from collisional crust shortening and thickening to post-collisional extension and thinning.

Microthermometric and stable isotopic (O and H) characteristics of fluid inclusions in the porphyry related Çöpler (İliç - Erzincan) gold deposit, central eastern Turkey

Abstract The Çöpler gold deposit occurs within the stockwork of quartz hosted by the Çöpler granitoid (Eosen) and by surrounding metasediments of Keban metamorphic (Late Paleozoic - Early Mesozoic) and the Munzur limestones (Late Carboniferous - Early Cretaceous). Native gold accompanied by small amounts of chalcopyrite, pyrite, magnetite, maghemite, hematite, fahlerz, marcasite, bornite, galena, sphalerite, specular hematite, goethite, lepidochrosite and bravoitic pyrite within the stockwork ore veinlets. In addition, epidote (pistazite - zoisite), garnet, scapolite, chlorite, tremolite/actinolite, muscovite and opaque minerals were determined within the veinlets occurred in skarn zones. The study of fluid inclusions in quartz veinlets showed that the hydrothermal fluids contain CaCl2, MgCl2 and NaCl and the salinities of the two phases (L+V) inclusions range from 1.7 to 20.6% NaCl equivalent. Salinity values up to 44% were determined within the halite bearing three phases inclusions. Their homogenization temperature values have a wide range from 145.0 to 380.0°C, indicative of catathermal/hypothermal to epithermal conditions. The δ 18O and δD values of the fluid inclusion waters from the Çöpler granitoid correspond to those assigned to Primary Magmatic Water, those from the metasediments of Keban metamorphics fall outside of the Primary Magmatic and are within the Metamorphic Water field. A sample from a quartz vein within the skarn zone hosted by the Munzur limestones has a particularly low δD value. The results suggest that fluids derived from the granitoids were mixed with those derived from the metasediments of Keban metamorphics and the the Munzur limestones and resulting in quartz veinlets in these lithologies and the formation of stockwork ores. In view of the occurrence, the features described and processes envisaged for this study area may be applicable in similar settings.

Metamorphic evolution of relict lawsonite‐bearing eclogites from the (U) HP metamorphic belt in the Chinese southwestern Tianshan

AbstractIn the Chinese southwestern Tianshan (U)HP belt, former lawsonite presence has been predicted for many (U)HP metamorphic eclogites, but only a very few lawsonite grains have been found so far. We discovered armoured lawsonite relicts included in quartz, which, on its part, is enclosed in porphyroblastic garnet in an epidote eclogite H711‐14 and a paragonite eclogite H711‐29. H711‐14 is mainly composed of garnet, omphacite, epidote and titanite, with minor quartz, paragonite and secondary barroisite and glaucophane. Coarse‐grained titanite occasionally occurs in millimetre‐wide veins in equilibrium with epidote and omphacite, and relict rutile is only preserved as inclusions in matrix titanite and garnet. H711‐29 shows the mineral assemblage of garnet, omphacite, glaucophane, paragonite, quartz, dolomite, rutile and minor epidote. Dolomite and rutile are commonly rimed by secondary calcite and titanite respectively. Porphyroblastic garnet in both eclogites is compositionally zoned and exhibits an inclusion‐rich core overgrown by an inclusion‐poor rim. Phase equilibria modelling predicts that garnet cores formed at the P‐peak (490–505 °C and 23–25.5 kbar) and coexisted with the lawsonite eclogite facies assemblage of omphacite + glaucophane + lawsonite + quartz. Garnet rims (550–570 °C and ~20 kbar) grew subsequently during a post‐peak epidote eclogite facies metamorphism and coexisted with omphacite + quartz ± glaucophane ± epidote ± paragonite. The results confirm the former presence of a cold subduction zone environment in the Chinese southwestern Tianshan. The P–T evolution of the eclogites is characterized by a clockwise P–T path with a heating stage during early exhumation (thermal relaxation). The preservation of lawsonite in these eclogites is attributed to isolation from the matrix by quartz and rigid garnet, which should be considered as a new type of lawsonite preservation in eclogites. The complete rutile–titanite transition in H711‐14 took place in the epidote eclogite facies stage in the presence of an extremely CO2‐poor fluid with X(CO2) [CO2/(CO2 + H2O) in the fluid] <<0.008. In contrast, the incomplete rutile–titanite transition in H711‐29 may have occurred after the epidote eclogite facies stage and the presence of dolomite reflects a higher X(CO2) (>0.01) in the coexisting fluid at the epidote eclogite facies stage.

대현 금-은광상의 모암변질에 따른 원소분산

대현 금-은광상은 Cambro-Ordovician 석회암질 대리암내에 발달된 NE 방향성의 단열대를 따라 형성된 2개의 열수성 석영맥으로 구성된다. 모암변질시 원소 분산과 이득/손실을 알아보기 위해 모암, 열수변질대 및 금-은 광맥에서 시료를 채취하였다. 모암변질시 뚜렷한 변질대가 관찰되지 않으며 구성광물은 주로 방해석, 돌로마이트, 석영과 소량 녹염석 등이다. 이 광상의 광석광물은 유비철석, 자류철석, 황철석, 섬아연석, 황석석, 황동석, 방연석, 에렉트럼, 자연 창연 및 은 광물 등이 산출된다. 분석된 자료를 기초로, 모암은 주로 <TEX>$SiO_2$</TEX>, CaO 및 <TEX>$CO_2$</TEX>로 구성되며 소량 <TEX>$Al_2O_3$</TEX>, <TEX>$Fe_2O_3(T)$</TEX> 및 MgO 등이 함유되어 있다. 광체로부터 모암으로 감에 따라 <TEX>$SiO_2$</TEX>, <TEX>$Fe_2O_3(T)$</TEX>, MgO, CaO 및 <TEX>$CO_2$</TEX> 함량은 변화 폭이 크게 관찰된다. 모암변질시 모암의 물리-화학적 물성 때문에 원소 분산은 현저히 관찰되지 않으며 단지 광체 주변부에서만 관찰된다. 모암변질시 이득원소들은 <TEX>$Fe_2O_3(T)$</TEX>, 총 S, Ag, As, Bi, Cd, Cu, Ni, Pb, Sb, Sn, W 및 Zn이고 손실원소들은 <TEX>$SiO_2$</TEX>, MnO, MgO, CaO. <TEX>$CO_2$</TEX> 및 Sr 이다. 따라서 우리의 결과물은 황강리 광화대내 석회암질 대리암을 모암으로 갖는 광상들의 지화학탐사시 활용될 수 있을 것이다. The Daehyun gold-silver deposit consists of two hydrothermal quartz veins that fill NE-trending fractures in the Cambro-Ordovician calcitic marble. I have sampled wallrock, hydrothermaly-altered rock and gold-silver ore vein to study the element dispersion and element gain/loss during wallrock alteration. The hydrothermal alteration doesn't remarkably recognized at this deposit and consists of mainly calcite, dolomite, quartz and minor epidote. The ore minerals composed of arsenopyrite, pyrrhotite, pyrite, sphalerite, stannite, chalcopyrite, galena, electrum, native bismuth and silver-bearing mineral. Based on analyzed data, the chemical composition of wallrock consists of mainly <TEX>$SiO_2$</TEX>, CaO, <TEX>$CO_2$</TEX> with amounts of <TEX>$Al_2O_3$</TEX>, <TEX>$Fe_2O_3(T)$</TEX> and MgO. The contents of <TEX>$SiO_2$</TEX>, <TEX>$Fe_2O_3(T)$</TEX>, MgO, CaO and <TEX>$CO_2$</TEX> vary significantly with distance from ore vein. The element dispersion doesn't remarkably recognized during wallrock alteration and only occurs near the ore vein margin because of physical and chemical properties of wallrock. Remarkable gain elements during wallrock alteration are <TEX>$Fe_2O_3(T)$</TEX>, total S, Ag, As, Bi, Cd, Cu, Ni, Pb, Sb, Sn, W and Zn. Remarkable loss elements are <TEX>$SiO_2$</TEX>, MnO, MgO, CaO. <TEX>$CO_2$</TEX> and Sr. Therefore, Our result may be used when geochemical exploration carry out at deposits hosted calcitic marble in the Hwanggangri metallogenic district.

Evolution of a very deeply subducted metasediment from As Sifah, northeastern coast of Oman

Near the coastal village of As Sifah, NE Oman, eclogite-facies rocks occur in the Saih Hatat window. We investigated a metapelite from this area, which is composed of mm-sized garnet and greenish phengite and minor epidote, blue amphibole, paragonite, albite, quartz, rutile, opaque phases, barite, and carbonate. Garnet exhibits a chemical zonation with Gro17Alm66Pyr6Spe11, Gro22Alm72.5Pyr5Spe0.5, and Gro25Alm65Pyr8Spe2 as inner core, mantle and outermost rim compositions. Inner portions of phengite have Si contents of up to 3.6 per formula unit (pfu), whereas rims are poorer in Si (3.2–3.4 pfu). We constructed a P-T pseudosection in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O for the bulk-rock composition of the studied metapelite and contoured it by isopleths of various parameters such as the molar fractions of garnet components. Based on this contouring a P-T path was derived that starts at ultrahigh-pressure conditions. Garnet began to form at 25kbar and 490°C. Subsequently, temperatures increased and pressures decreased to finally reach P-T conditions of 13kbar and 565°C at which low-Si phengite, the outermost rim of garnet, Na-amphibole, epidote, quartz, magnetite, and rutile were in equilibrium. The P-T path is related to events in a subduction channel where the top of subducted oceanic crust, including the studied metasediments, was involved in an upwards-directed mass flow, resulting in the release of about 3wt.% H2O by garnet formation from hydrous minerals such as chlorite and lawsonite. In order to get hints at the interaction of such hydrous fluids, we have analyzed the trace and minor elements in phengite. The contents of B, Rb, Cs, and Tl (20, 397, 6.7 and 1.7ppm, respectively) are nearly constant over the entire Si range of potassic white mica. In contrast, the contents of Ba and Sr increase from 900 and 0.5 to 10500 and 14ppm, respectively, with decreasing Si content in phengite. We hypothesize that this result reflects early leaching of mobile elements during subduction and the later approach of the rock to a barite deposit within a melange ascending in the subduction channel.

The Caraguataí syenitic suite, a ca. 2.7Ga-old alkaline magmatism (petrology, geochemistry and U–Pb zircon ages). Southern Gavião block (São Francisco Craton), Brazil

Abstract The Gaviao Block comprises amphibolite- and granulite-facies gneisses and migmatites of tonalitic, granodioritic and granitic compositions and supracrustal sequences including volcanosedimentary layers metamorphosed up to the amphibolite facies. In the region of Abaira-Jussiape (BA), two main igneous suites, called Caraguatai and Jussiape, are exposed in the core of an anticline. The Caraguatai suite encompasses alkali-feldspar granites, syenites and quartz syenites that contain biotite, magnetite/hematite, apatite, titanite, hastingsite/pargasite and zircon as accessory minerals that were adjusted to the amphibolite facies. White mica and epidote minerals are related to retrograde greenschist facies. These rocks were deformed in dextral to reverse-dextral shear zones, giving origin to protomylonites and augen-mesomylonites to ultramylonites. The ultramylonites have a prominent banding parallel to the main foliation of the rocks. Lithogeochemical studies revealed subalkaline to alkaline, metaluminous to peraluminous, Fe-rich protolith for instead of to these rocks associated with A2-type magmatism and partial melting of igneous continental crust. In situ U–Pb zircon dating using the Laser Ablation ICPMS method was carried out for five samples of the Caraguatai alkaline suite. The ages obtained for an isotropic syenite (SCP 1470: 2680 ± 24 Ma), a foliated syenite (SCP 2035: 2703 ± 11 Ma), a syenitic augen gneiss (SCP 2017: 2706 ± 34 Ma) and two ultramylonitic syenitic banded gneisses (SCP 1446: 2711 ± 34 Ma and SCP 1809: 2698 ± 10 Ma) fall in the same range. The average of the 62 concordant analyses obtained from the five samples allows to determine a mean 207 Pb/ 206 Pb age of 2696 ± 5 Ma (±2σ) interpreted as that of the alkaline plutonism. The geochronologic data obtained up to now have not helped to constrain an age for the metamorphism that affected the study area. The A2 type of magmatism, instead of and the T DM model ages (ca. 3.2–3.8 Ga) and the corresponding negative e(t) values (−4 to −6), suggest that the alkaline magmas are mainly derived from partial melting of Paleoarchean gneisses in an intraplate setting similar to those from the surrounding Aracatu region. The isotopic data obtained in this work together with published data suggest that in the southern sector of the Gaviao Block an important event of crustal recycling occurred ca. 2.7 Ga ago. Similar ages have been found in other parts of the Sao Francisco Craton.

Hydrothermal Alteration and Veins at the Epithermal Au-Ag Deposits and Prospects of the Waitekauri Area, Hauraki Goldfield, New Zealand

The Waitekauri area of the Hauraki goldfield, New Zealand, contains several adularia-sericite epithermal Au-Ag deposits and prospects. From west to east, the area contains the Sovereign, Jubilee, Scimitar, Scotia, Teutonic, and Jasper Creek deposits and prospects, which are hosted by andesitic and dacitic flows, breccias, and localized pyroclastic and air fall deposits. Drill core reveals spatial and temporal zonation of alteration and vein minerals along a 3-km-long composite cross section through the area. Most host rocks are intensely altered, with 100 percent of the igneous minerals replaced by hydrothermal minerals, although the alteration intensity becomes more variable and weaker toward the east. Alteration minerals include quartz, adularia, albite, chlorite, pyrite, illite, interstratified illite-smectite, smectite, calcite, hematite, and minor epidote. Many of these minerals have zoned distributions; adularia is widespread at Sovereign, but is restricted to shallow levels at both Scotia and Jasper Creek. Albite occurs in a discrete zone below adularia at Scotia, and minor epidote is restricted to Sovereign and Jubilee. Illite occurs throughout Sovereign and Jubilee and at the western margin of Scotia and Scimitar, where it grades eastward into interstratified illite-smectite and smectite at Teutonic and Jasper Creek. Veins are typically less than 10 cm wide, but have diverse mineralogy with zoned distributions. Quartz veins dominate at Sovereign and Jubilee, whereas calcite veins are more abundant at Scotia, Scimitar, and Jasper Creek. Laumontite occurs at Scotia and locally at Scimitar, whereas veins of clinoptilolite and mordenite ± calcite occur at Jasper Creek and stilbite veins occur at Teutonic. Fluid inclusions in quartz and calcite homogenized between 132° and 310°C and trapped a dilute solution with an apparent salinity of less than 2.6 wt percent NaCl equiv. Homogenization temperatures are highest at Sovereign (avg. 241°C), Jubilee (avg 239°C), and Scimitar (avg 236°C), lower at Scotia (avg 204°C) and lowest at Teutonic (avg 168 °C) and Jasper Creek (avg 162°C). Estimated positions of the paleowater table above Sovereign, Jubilee, Scimitar, Scotia, Jasper Creekg and Teutonic relative to present elevations was at least 690, 750, 575, 450, 225, and 150 m above sea level, respectively; the deposits and prospects, therefore, span a 600-m vertical interval. Individual deposits and prospects have undergoen at least 35 to more than 455 m of erosion with the greatest erosion to the west. Alteration intensity, alteration and vein mineral distributions, and fluid inclusion microthermometry are interpreted to indicate that Sovereign and Jubilee formed at relatively high temperatures, whereas Teutonic and Jasper Creek formed at relatively cooler temperatures. Several hydrologic reconstructions are possible, including (1) a single hydrothermal system with an inclined water table and significant lateral outflow to the east, or 2) a single low-relief hydrothermal system with a flat-lying water table that has subsequently been displaced by postmineral faults or tilted approximately 10° to the east. Regardless of the preferred reconstruction, the Sovereign and Jubilee deposits appear to have formed in the main zone of fluid upflow, whereas the Teutonic and Jasper Creek prospects appear to have formed toward the margin. Moreover, the greatest erosion has occurred at the Jubilee and Sovereign deposits (~300–400 m), and these may represent the roots of a more extensive vein network that has largely been eroded.

The three partners of metamorphic petrology

Study of the evolution of the Earth’s crust advanced greatly in the last half-century through the dynamic collaboration of three equally contributing partners: field observations, together with microanalysis of mineral assemblages, experimental petrology, and stability calculations based on systematically measured and compiled thermodynamic properties of minerals. In addition to providing the initial inspiration for experimental and theoretical studies of petrogenesis, quantitative field observations have had an important feedback relationship with the indoor approaches. It has been demonstrated that physico-chemical properties of mineral systems can, in favorable circumstances, be inferred from purely geologic criteria, and, in some cases, field criteria have been used to challenge the validity of existing experimental or thermodynamic data, leading to reinvestigations and revisions. The “geo-experimental” method may be particularly useful to infer phase equilibrium relations among minerals where experimental reactions are prohibitively slow at geologically reasonable temperatures or where fluid fluxing of mineral reactions is necessarily limited, as in investigations concerning the low-H2O-activity deep-crustal rocks, the granulites. Geo-experimental phase equilibrium diagrams, constructed largely from quantitative petrographic observations, have been successful in deducing polymorphism, order-disorder, and solid solution properties of such mineral systems as the aluminum silicate minerals, feldspars, epidote minerals, scapolites, and cordierite. The last minerals are particularly important in that they may retain a record of volatile components of an intergranular mineralizing fluid. Knowledge of the important effects of mixed volatile fluids has been inhibited by lack of experimental data on devolatilization, ion exchange, redox, sulfidation, and fluid-complexing reactions of the volatile-bearing minerals biotite, amphibole, apatite, scapolite, cordierite, carbonates, and sulfides with the major minerals of rocks of the deeper interior. A concerted approach among the experimental, thermodynamic, and petrographic disciplines is necessary to make progress on the still poorly understood role of volatiles in shaping the crust. The relatively young science of fluid inclusion analysis will have an increasingly important role in the interdisciplinary effort to interpret the evolution of the continental crust and its underpinnings.

Petrological Constraints on Crystallization Conditions of Mesoarchean Sanukitoid Rocks, Southeastern Amazonian Craton, Brazil

We report petrological and geochemical data for the 2*87 Ga Rio Maria sanukitoid granodiorite and associated rocks from Mesoarchean granite-greenstone terranes of the eastern Amazonian craton, Brazil. The dominant rocks have granodiorite to subordinate monzogranitic compositions, with minor proportions of intermediate quartz diorites or quartz monzodiorites, in addition to mafic endmembers occurring as layered rocks or as enclaves. The mineral assemblage is dominated by amphibole-plagioclase-biotite and epidote minerals, all of inferred magmatic origin, pyroxenes being notably absent. Textural and compositional criteria indicate that amphibole is a principal mineral on the liquidus of all the Rio Maria rocks. Crystallization conditions have been derived from a comparison between natural phase assemblages, proportions and compositions and experimental studies carried out on similar magma compositions. The comparison shows that the parental magmas were water-rich, with more than 7 wt % dissolved H2O, with crystallization temperatures in the range 950-680°C. The Mg/(Mg + Fe) ratios of both amphibole and biotite indicate fO2 conditions in the range NNO + 0*5 to NNO + 2*5 (where NNO is nickel-nickel oxide buffer), therefore pointing to both water-rich and oxidizing conditions for sanukitoid magmas. Amphibole compositions indicate emplacement at around 200 MPa, and record a high-pressure stage of magma crystallization around 600-900 MPa. Sanukitoid magmas share two of the principal characteristics of modern arc magmas, elevated redox state and volatile contents, which suggest that they may have formed in a geodynamic environment broadly similar to present-day subduction zones.

Hydrothermal minerals record CO2 partial pressures in the Reykjanes geothermal system, Iceland

The Reykjanes Peninsula in southwest Iceland is the landward extension of the Mid-Atlantic Ridge spreading center. At present seawater penetrates the coastal Reykjanes geothermal system at depth, mixes with magmatic volatiles and reacts with the basaltic host rock to form secondary geothermal minerals in progressively higher-grade mineral alteration zones with depth where highest recorded temperatures are ~320°C. Within the epidote-chlorite and portions of the epidote-actinolite zones of alteration, epidote-prehnite-calcite-quartz-fluid constitutes a quadravariant assemblage that, under conditions of specified temperature, pressure, and activity of H2O allows prediction of geothermal fluid PCO2 as a function of the composition of the solid solution minerals epidote or prehnite. This assemblage is most common at temperatures >250°C and 600m between the reference temperatures of 275°C and 310°C range from ~1 to ~6.5 bars, when only epidote, prehnite and quartz are observed. When all members of the assemblage are present, including calcite, the calculated range in PCO2 is from ~0.5 to 6 bars. If calcite is absent from the assemblage, the computation of PCO2 using equilibria for reaction 1 provides the maximum value of PCO2 at which calcite will not be present. Actual PCO2 values of geothermal fluids from the Reykjanes system were derived from analytical data on liquid and vapor samples collected at the surface using both the WATCH and SOLVEQ speciation programs. The geothermal fluids at reference temperature between 275°C and 310°C have PCO2 concentrations ranging from 1.3 bars to 4.0 bars. The calculated PCO2 values based on epidote compositions are in close agreement with present-day measured CO2 in the Reykjanes geothermal system fluids. 74% of the calculated PCO2 values based on epidote compositions where the complete epidote-prehnite-calcite-quartz assemblage is observed fall within the range of measured present-day fluids, while 62% of the calculated PCO2 values fall within the range when calcite is not present in the assemblage. Therefore, our method for calculating fluid PCO2 is proven quite reliable when all four index minerals are present. Additionally, if only epidote, prehnite and quartz are observed, our method still serves as a moderately accurate predictive proxy for fluid PCO2 composition in the Reykjanes geothermal system. Ultimately, the correlation between measured and predicted fluid compositions, provides insight into future abilities to characterize spatial and temporal concentrations of CO2 in active and fossil hydrothermal and low-grade metamorphic environments in mafic lithologies based on compositional variations and paragenesis of hydrothermal minerals. In addition, this study will aid in understanding the nature of reactions that involve natural sequestration of CO2 derived from magmatic degassing, and injection of industrial CO2-rich fluids within hydrothermal environments in basaltic rocks.

Petrografia, química mineral e processos hidrotermais associados ao depósito de ouro São Jorge, Província Aurífera do Tapajós, Cráton Amazônico

The Paleoproterozoic São Jorge gold deposit is situated in the Tapajós Gold Province of the Amazonian Craton. The deposit is hosted by an amphibole biotite monzogranite, composed of quartz, potassic feldspar, plagioclase, amphibole, biotite, titanite and opaques. Four dominant mineral assemblages were recognized in the deposit. Assemblage 1 was formed during the magmatic stage and is characterized by the presence of amphibole and andesine-oligoclase. Assemblage 2 shows the total replacement of the amphibole and intense saussuritization of the primary plagioclase, epidote is a ubiquitous phase and biotite is partially chloritized. Assemblages 3 and 4 are related to the hydrothermal processes, which generated the sulfide and gold mineralization. Assemblage 3 is dominated by chlorite and albitic plagioclase with subordinate white micas and, sometimes, biotite. Assemblage 4 is dominated by white micas, carbonates and pyrite. It is a result of a phyllic alteration, with associated carbonatization. Chlorite geothermometer suggests temperatures of 300±40 °C for the 3 and 4 assemblages. Al in hornblende geobarometer indicates low pressures, around 1 kbar for the crystallization of the mineralized granites. Oxidizing conditions, above NNO, prevailed during the genesis of the deposit. The hydrothermal assemblages of São Jorge differ from those described at Joel and Davi and are not dominated by epidote minerals as registered in other areas of the Tapajós province. A porphyry model or intrusion-related model are best adapted for the São Jorge deposit. The latter has similarities with the Serrinha deposit in the Juruena province and Batalha in the Tapajós province but it has more strong analogies with the Volta Grande hydrothermal system in southern Brazil.

Petrografia, química mineral e processos hidrotermais associados ao depósito de ouro São Jorge, Província Aurífera do Tapajós, Cráton Amazônico

The Paleoproterozoic Sao Jorge gold deposit is situated in the Tapajos Gold Province of the Amazonian Craton. The deposit is hosted by an amphibole biotite monzogranite, composed of quartz, potassic feldspar, plagioclase, amphibole, biotite, titanite and opaques. Four dominant mineral assemblages were recognized in the deposit. Assemblage 1 was formed during the magmatic stage and is characterized by the presence of amphibole and andesine-oligoclase. Assemblage 2 shows the total replacement of the amphibole and intense saussuritization of the primary plagioclase, epidote is a ubiquitous phase and biotite is partially chloritized. Assemblages 3 and 4 are related to the hydrothermal processes, which generated the sulfide and gold mineralization. Assemblage 3 is dominated by chlorite and albitic plagioclase with subordinate white micas and, sometimes, biotite. Assemblage 4 is dominated by white micas, carbonates and pyrite. It is a result of a phyllic alteration, with associated carbonatization. Chlorite geothermometer suggests temperatures of 300±40 °C for the 3 and 4 assemblages. Al in hornblende geobarometer indicates low pressures, around 1 kbar for the crystallization of the mineralized granites. Oxidizing conditions, above NNO, prevailed during the genesis of the deposit. The hydrothermal assemblages of Sao Jorge differ from those described at Joel and Davi and are not dominated by epidote minerals as registered in other areas of the Tapajos province. A porphyry model or intrusion-related model are best adapted for the Sao Jorge deposit. The latter has similarities with the Serrinha deposit in the Juruena province and Batalha in the Tapajos province but it has more strong analogies with the Volta Grande hydrothermal system in southern Brazil.

Trace element-enriched fluids released during slab dehydration: Implications for oceanic slab-mantle wedge transfer

Mafic blueschists of the Tianshan (NW China) display an interconnected network of eclogite-facies veins derived by prograde blueschist dehydration. The eclogite-facies vein-network in the Tianshan blueschist was found to be the product of hydrofracturing induced by fluids released by the breakdown of glaucophane, paragonite and epidote minerals during blueschist–eclogite transition and, thus represent former fluid pathways within a Paleozoic subduction zone. The veins are predominantly composed of omphacite fibers with minor quartz, calcite, and apatite. The transition from blueschist- to eclogite-facies parageneses occurs as “dehydration” halos along some of these veins. The fluids are interpreted to have been derived from the host blueschist as a result of dehydration reactions at peak metamorphic conditions of 480–600 °C and 18–21 kbar (Gao and Klemd, 2001 and John et al., 2006). The low in trace element fluid caused a strong mobilization of LILE, REE, and high field strength elements (HFSE) in those parts of the host rock with which the passing fluid reacted (John et al., 2006). Hence indicating that so-called immobile elements can be mobilized by reactive flow. Furthermore, field evidence shows that rutile occurs in the form of needle-like segregations in an eclogite boudin and as prismatic crystals in an omphacite-bearing vein cross-cutting foliated host eclogites. Textural and geochemical studies indicate that titanium, niobium and tantalum can be mobilized and transported by fluids which were liberated by means of dehydration of the subduction oceanic crust. Vein-rutile precipitation occurred at sites where evidence for an increasing oxygen fugacity in the acting fluid is found, causing a sudden depletion of the HFSE concentration in the fluid. Such a fluid may act as agent for the subduction component seen in the distinct chemistry of normal island-arc magmas.

Hydrothermal syntheses of epidote and piemontites on the join Ca2Al2Fe3+Si3O12(OH)-Ca2Al2Mn3+Si3O12(OH) at relatively low pressures of 200-400 MPa

Syntheses of epidote minerals on the join Ca2Al2Fe3+Si3O12(OH)-Ca2Al2Mn3+Si3O12(OH) were conducted using standard cold-seal pressure vessels and solid oxygen buffering techniques at 200-400 MPa and 500 °C. Two types of starting materials were used: (1) oxide mixtures (type 1) + excess H2O, and (2) mixtures of synthetic CaFe3+AlSiO6-pyroxene and oxides (type 2) + excess H2O. The compositions of Ca2Al2Fe3+qMn3+1-qSi3O12.5 starting materials were q = 1.0, 0.9, 0.5 and 0.25 for the type 1, and q = 1.0, 0.75, 0.5 and 0.25 for the type 2. Mn3+-free epidote was not crystallized from the type 1 starting materials, even at 400 MPa, whereas Mn3+-Fe3+-Al piemontites were produced at 300 MPa, but they were along with some associated minerals. Recrystallization of hematite during runs is a major problem when using oxide mixture starting materials. However, recrystallization of hematite was suppressed by using the type 2 starting materials, and Mn-free epidote and Mn3+-Fe3+-Al piemontite were synthesized as almost single phases at 380 MPa and at 200-370 MPa, respectively. The chemical compositions of the synthetic Mn3+-free epidote and Mn3+-Fe3+-Al piemontites formed were generally close to those of the starting materials, except that Fe3+ contents tended to be slightly lower. Pressure, temperature and oxygen fugacity have been considered to be the main factors controlling the occurrence of epidote minerals. However, this study demonstrates the importance of Fe3+-source material. Although the results of previous synthesis experiments of epidote minerals have not necessarily been compatible with those of stability experiments and the natural occurrences of epidote minerals, the syntheses here succeeded in producing nearly single phase epidotes with Ca2Al2(Fe3+,Mn3+)Si3O12(OH) at relatively low pressures less than 400 MPa. This aids the understanding of the varied occurrence of natural epidote minerals.

Iron oxide - copper - gold-type and related deposits in the Manitou Lake area, eastern Grenville Province, Quebec: variations in setting, composition, and style

The Manitou Lake area (Kwyjibo and Lac Marmont sectors), located in Quebec's eastern Grenville Province, contains magnetite-rich deposits with variable morphological, mineralogical, and chemical characteristics. Most Kwyjibo sector deposits are rich in Cu, rare-earth elements (REE), Y, P, F, and Ag and are anomalous in Th, U, Mo, W, Zr, and Au, and Lac Marmont sector deposits are commonly poor in these elements. Deposits occur in or are closely associated with 1175–1168 Ma leucogranite. They contain combinations of magnetite, clinopyroxene, blue–green hornblende, titanite, apatite, fluorite, quartz, biotite, andradite, epidote, albite, hematite, sulfides (chalcopyrite, pyrite, pyrrhotite, molybdenite, sphalerite), ilmenite, allanite, and other REE-bearing minerals. Veins and breccias are common. Most of the magnetite mineralization was preceded by potassic metasomatism (microcline) and was followed by most of the sulfides and radioactive minerals. Nearby sulfide-dominant deposits may be related. The deposits were formed by metasomatic replacement and fracture filling from hydrothermal fluids of variable composition, which were probably channeled in major, active faults. Oxygen-isotope data from magnetite-rich rocks suggest that fluids were predominantly magmatic and (or) metamorphic and that, locally, mixing with cooler meteoric water may have facilitated precipitation of sulfides and rare-metal minerals. Titanites in mineralized rock have been dated at 972 ± 5 Ma, but most magnetite may be older. Mineralization was syn- to post-tectonic and occurred in an orogenic to orogenic-collapse setting. The Cu–REE–Y-rich deposits are similar to iron oxide – copper – gold (IOCG) Olympic Dam type deposits, and copper- and rare-metals-poor occurrences resemble magnetite ± apatite Kiruna-type deposits.

Fluid Inclusions in Epidote Minerals and Fluid Development in Epidote-Bearing Rocks

The widespread occurrence of epidote minerals in metamorphic and igneous rocks as well as in many ore deposit types makes it a promising candidate for fluid inclusion studies. Apart from high- to very high-temperature and low- to intermediate-pressure conditions, epidote minerals are stable over a wide range of pressure and temperature in the continental and oceanic crust (e.g., Poli and Schmidt 1998). Yet fluid inclusion studies on epidote minerals are surprisingly scarce, even in fluid-saturated environments like certain vein-type deposits or hydrothermal-volcanic vugs and druses. For example, epidote minerals are not mentioned in the subject index of Roedder’s (1984) outstanding summary and review of fluid inclusion studies and occurrences, which lists more than sixty different host minerals for fluid inclusions. Nonetheless, more recent studies showed fluid inclusions in epidote minerals to be the only direct witness of the physiochemical and compositional fluid evolution during certain geodynamic processes mainly found in fossil geothermal systems, ore deposits and high-pressure to ultra-high pressure rocks. The aim of this review is to outline and summarize some aspects and interpretations of geodynamic processes, which are based on temperature ( T ), pressure ( P ), molar volume ( V ) and composition ( X ) data from fluid inclusions in epidote minerals as well as associated host minerals from various geological environments. The review starts with a chapter on some typical mixed volatile solid-fluid equilibria involving epidote minerals, which are relevant for the here discussed environments. This is followed by a short introduction into the basic concepts of fluid inclusion research and the role of epidote minerals. The next section covers fluid inclusion studies on epidote minerals from active and fossil geothermal systems as well as low-grade metamorphic rocks and constraints on the P - T - X properties of the fluids present in these systems. This is followed by a short introduction …

Spectroscopy of Epidote Minerals

Numerous spectroscopic techniques have been applied to the epidote minerals to characterize their structure and crystal chemistry. The substitution of transition metal ions Mn, Cr, and V, besides Fe, in the different crystallographic sites of epidote minerals and with different valence states has been studied by optical absorption spectroscopy. These studies mainly focused on the determination of (i) the site preferences of the different transition metal ions within the epidote minerals (e.g., Burns and Strens 1967; Tsang and Ghose 1971), (ii) the physical and structural characteristics of these sites as a function of composition, temperature and/or pressure (e.g., Taran and Langer 2000; Langer et al. 2002), (iii) their crystal field stabilization energy (e.g., Burns and Strens 1967; Langer et al. 2002), and (iv) the cause of the color and pleochroism in some epidote minerals (e.g., Faye and Nickel 1971). Major topics of infrared spectroscopic studies have been the proton environment and its changes with composition, temperature, and pressure (e.g., Langer and Raith 1974; Winkler et al. 1989; Della Ventura et al. 1996; Liebscher et al. 2002) and the phase transition within the orthorhombic solid solution series (e.g., Liebscher and Gottschalk 2004). Mossbauer spectroscopy has been used (i) to resolve the valence state of Fe in the different epidote minerals and its site location (e.g., Dollase 1973; Kartashov et al. 2002) and (ii) to study the intracrystalline Al-Fe partitioning between the different octahedral sites and the kinetic of this ordering process (e.g., Patrier et al. 1991; Fehr and Heuss-Asbichler 1997). This chapter reviews the different spectroscopic studies and techniques applied to epidote minerals with emphasis given to the crystal chemical results. An in-depth presentation and discussion of the different spectroscopic techniques and their theoretical framework is beyond the scope of this chapter. …

Epidote Minerals in High P/T Metamorphic Terranes: Subduction Zone and High- to Ultrahigh-Pressure Metamorphism

Epidote minerals—the monoclinic epidote group minerals together with the orthorhombic polymorph zoisite—are important Ca-Al-silicates in many metabasites, metapelites and metacherts that are characterized by high P / T ratios. Such high P / T ratios are typical for subduction zones and the high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism during continent-continent collisions (e.g., Liou 1973, 1993). All of these P-T conditions can be described by geothermal gradients between 5 and 20°C/km, that therefore provide a rough framework for the P-T conditions covered by this review (Fig. 1⇓). Depending on the actual thermal structure of a subduction zone, the subducting plate will encounter subgreenschist, greenschist, blueschist, epidote-amphibolite, amphibolite, HP granulite, and/or eclogite facies conditions during its travel down into the mantle (Fig. 1⇓). The P-T regime of the eclogite facies can further be subdivided into amphibole eclogite, epidote eclogite, lawsonite eclogite, and dry eclogite facies (Fig. 1⇓). HP metamorphism refers to metamorphic pressure in excess of ~1.0 GPa and includes parts of the blueschist, epidote-amphibolite, and HP granulite facies as well as the eclogite facies (Fig. 1⇓). UHP refers to the metamorphism of crustal rocks (both continental and oceanic) at P high enough to crystallize the index minerals coesite and/or diamond. HP and UHP metamorphism are separated conveniently by the quartz-coesite equilibrium which implies a minimum P > 2.7 GPa at T > 600°C for UHP metamorphism (Fig. 1⇓). The equilibrium boundary for the graphite-diamond transition can be used to further subdivide the UHP region into diamond-grade and coesite-grade. The stability of coesite and other UHP minerals in a metamorphic regime requires abnormally low temperatures at depths greater than 100 km. Such environments can be attained only by the subduction of cold oceanic crust-capped lithosphere ± pelagic sediments or of continental crust. Figure 1. P-T regimes of UHP …