Chlorite Thermometry Research Articles

Geochemistry of the Youngbogari deposit, Republic of Korea: An unusual mesothermal gold-silver deposit of the Youngdong area.

Mesothermal gold-quartz veins of the Youngbogari deposit in the Youngdong area, South Korea, are hosted in faulted shear zones in Precambrian gneiss of the Sobaegsan massif. The Youngbogari deposit shares many features with other metamorphic rock-hosted gold deposits in the area, including a massive single-stage nature of veins and a simple vein mineralogy. However, its ore mineralogy is unique in that (1) arsenopyrite and pyrite occur dominantly with only minor pyrrhotite and (2) electrum grains are very poor in gold content (<40 atom. % Au). A variety of geochemical data indicate that deposition of the mesothermal gold–silver ores of the Youngbogari deposit resulted mainly from cooling of ore fluids accompanying decreasing sulfur fugacity. Fluid inclusion evidence indicates that gold deposition was not related to an early history of CO2 phase separation (at temperatures of 300° to 420°C), but rather was tied to a later history of cooling and dilution of H2O–NaCl ore fluids at temperatures of <300°C. These temperature estimates for gold deposition agree with those based on chlorite thermometry (270° to 320°C) and sulfide mineral assemblages (<290°C). Measured and/or calculated, stable isotope compositions of hydrothermal fluids (δ18OWater = 5.8 to 8.0‰, δDWater = −75 to −54‰) indicate an important role of rock-dominated (possibly magmatic) fluids in the mesothermal gold–silver system. The unusual arsenopyrite pyrite-rich nature of the Youngbogari ores and their anomalously negative δ34S values (−2.9 to −5.1‰), compared to other mesothermal gold deposits of the Youngdong area, are interpreted to reflect a more oxidizing nature of the Youngbogari ore fluids, likely due to less reaction with graphite in wallrock gneiss.

Mélanges in southern Mexico: geochemistry and metamorphism of Las Ollas complex (Guerrero terrane)

Las Ollas complex (LOC) is a subduction complex spatially associated with the early Cretaceous Zihuatanejo-Huetamo subterrane (Guerrero terrane) in southern Mexico. LOC tectonic mélanges compose of a stack of east-dipping, west-vergent tectonic sheets containing blocks of metabasalt, metadolerite, metagabbro, ultramafics, volcaniclastics, quartz-rich sandstone, and chert enveloped in a highly sheared clastic or serpentinitic matrix. Most igneous and igneous-derived metamorphic blocks show geochemical and isotopic features typical of island-arc tholeiitic suites: (i) low TiO2 (0.13 to 0.91%) and Zr (5 to 57 ppm) contents; (ii) high (LFSE/HFSE)N ratios; low LaN/YbN (0.5 to 4) values; and, high εNd(T) (+7.9 to +8.0) ratios. Petrographical and mineral chemistry evidence indicates that blocks underwent early recrystallization under high pressure and low temperature (HP-LT), blueschist facies conditions during subduction. Typical assemblages include blue (sodic through calco-sodic to Na-rich calcic) amphibole + lawsonite ± tremolite ± Mg-chlorite ± white mica ± albite ± quartz. Phase relations and chlorite thermometry suggest temperatures of about 200°-330o C and pressures of 5-7 kbar. It is proposed that sedimentary blocks were generated by in situ remobilization and mixing, whereas igneous blocks most probably derived from the chemically and isotopically identical Zihuatanejo island-arc suite. Our data suggest that LOC represents part of a subduction complex formed by eastward-directed subduction related with the evolution of the early Cretaceous Zihuatanejo island arc.

The metamorphic and alteration history of West Rand Group shales from distal portions of the Witwatersrand Basin

The maximum metamorphic grade in West Rand Group shales from three boreholes to the east and south of the Vredefort Dome is lower greenschist facies. In most samples the peak metamorphic assemblage consists of quartz + muscovite + chlorite, with the phyllosilicates defining a tectonic cleavage. In many of the samples more coarsely crystalline alteration zones that clearly crosscut both bedding and the tectonic cleavage overprint this assemblage. The alteration zones are localized around minor faults and shears where fluid ingress has occurred. Muscovite was not observed in these zones and the cleaved matrix assemblages has been replaced by assemblages consisting of pyrite + quartz ± chlorite ± stilpnomelane ± calcite ± carbonaceous matter ± Mn-garnet ± epidote ± chalcopyrite. In some of the alteration zones stilpnomelane is a very common product and appears to arise through the breakdown of chlorite + muscovite. K and Fe metasomatism are indicated during this reaction, as well as the obvious mobility of S, Cu and C. Assays on small samples indicate gold introduction into the shales during the alteration event. Chlorite thermometry was used to estimate the equilibration temperature of chlorite from the cleaved matrix and the alteration zones. Temperatures close to 300°C are indicated for both chlorite varieties, and for all three boreholes. These results, and the petrographic data, imply that fluid influx post-dated the formation of the tectonic cleavage, but occurred close to the peak of metamorphism. Comparison between the chlorite thermometry data from this study with those from similar studies in the goldfields indicates very similar peak metamorphic grades in both areas, suggesting that these metamorphic conditions may be typical of almost the entire basin. The one exception is obviously the amphibolite facies metamorphism exposed in the northwestern sector of the collar of the Vredefort Dome. Several recent studies have suggested that this metamorphism is representative of a widespread zone of higher grade metamorphism within the central portions of the basin. The results of this study indicate that this is unlikely to be the case and suggest that these amphibolite facies rocks are probably unique to the northern portions of the dome.

Evolution of a kyanite‐bearing belt within a HT‐LP orogen: the case of NW Variscan Iberia

Kyanite replaces andalusite in a belt of Ordovician and Silurian pelitic rocks that form a narrow synform pinched between high‐grade antiforms in NW Variscan Iberia. Kyanite occurs across the belt in Al‐rich, black pelites in assemblages I: kyanite–chloritoid–chlorite–muscovite and II: kyanite–staurolite– chlorite–muscovite. In I, kyanite occurs in the matrix and in kyanite–muscovite aggregates that pseudomorph earlier andalusite porphyroblasts. The aggregates are found across the belt and can still be recognized in assemblage II and even in III: andalusite–staurolite–biotite–muscovite, this latter being a hornfelsic Silurian schist where kyanite is relic and staurolite occurs in the matrix, and is resorbed inside new massive pleochroic andalusite. KFMASH and MnKFMASH pseudosections have been constructed using Thermocalc for Al‐rich and Al‐poorer compositions from the belt. Chloritoid zoning in Al‐rich rocks containing assemblage I, plus chloritoid–chlorite thermometry complemented with garnet–chlorite thermometry in Al‐poorer lithologies, mean that the path is one of increasing pressure and temperature. Conditions prior to assemblage I, with earlier andalusite stable, are those of the andalusite–chloritoid– chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims. The passage from assemblage I to II implies a prograde path within the kyanite field. Assemblage III represents peak conditions, indicating a prograde staurolite‐consuming reaction across a KFMASH field, leading eventually to a locally found andalusite–biotite–muscovite hornfels. The lowest pressure stages are recorded by cordierite–biotite in Al‐poor pelites. Garnet‐bearing MnKFMASH assemblages in Al‐poorer pelites record conditions similar to assemblages II and III. The replacement of andalusite by kyanite in assemblage I is attributed to downdragging of andalusite‐bearing rocks into a synform as testified by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage. Prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the Vivero fault.

A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland

Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H2O-CO2-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V1) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 °C and 0.7–1.8 kbar. Homogeneous quartz δ18O across the batholith (9.5 ± 0.4‰n = 12) suggests V1 precipitation at high temperatures (perhaps 600 °C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V1 indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H2O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 °C), high δD (−18 ± 2‰), low δ18O (0.5–2.0‰) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V2) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H2O-NaCl-CaCl2-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 °C), δD (−17 to −45‰), δ18O (−3 to + 1.2‰), δ13CCO2 (−19 to 0‰) and δ34Ssulphate (13–23‰) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V3). Correlations of salinity, temperature, δD and δ18O are interpreted as the result of mixing of two fluid end-members, one a high-δD (−17 to −8‰), moderate-δ18O (1.2–2.5‰), high-δ13CCO2 (> −4‰), low-δ34Ssulphate (13‰), high-temperature (205–230 °C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-δD (−61 to −45‰), low-δ18O (−5.4 to −3‰), low-δ13C (<−10‰), high-δ34Ssulphate (20–23‰) low-temperature (80–125 °C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V3 veins are late Triassic; the high-δD end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-δD end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated.

Low temperature, low pressure deformation and metamorphism of the Vangorda massive sulphide orebody, Yukon, Canada

The Vangorda orebody is a small stratiform massive sulphide orebody located in Anvil District, Yukon, Canada. The orebody consists of fineto medium-grained semi-massive and massive sulphides with a common sulphide mineralogy of pyrite, pyrrhotite, sphalerite, galena, and minor chalcopyrite. The host rocks and the sulphide lithofacies have been complexly deformed during two phases of deformation (D1 and D2) and associated metamorphism (M1 and M2). The effects of d1 and M1 are penetratively overprinted by D2 and M2. D2 and M2 resulted in tight to isoclinal F2 folding of the orebody, remobilisation of the sulphides, recrystallisation and development of shear zones along the limbs of the F2 folds. Chlorite thermometry and sulphide thermobarometry have been carried out on the host phyllites and on the sulphides. Chlorite was analysed from the S1 and S2 foliations in the phyllites to determine M1 and M2 temperatures, respectively. However, no difference was found between chlorite compositions in these foliations and a mean temperature of 363 °C was calculated from the tetrahedral A1IV occupancy. Arsenopyrite thermometry yielded a comparable mean temperature of 336 °C. Sphalerite inclusions in M2 pyrite porphyroblasts from D2 shear zones were analysed for pressure using the sphalerite + hexagonal pyrrhotite + pyrite barometer. Inclusions were analysed in an attempt to determine if relic m1 sphalerite, and hence pressure signature, was preserved. Inclusion compositions appear to reflect only M2 conditions and yielded a mean pressure of 4.0 kb. Sphalerite + hexagonal pyrrhotite assemblages were analysed from D2 shear zones to determine the M2 pressure using the sphalerite + hexagonal pyrrhotite barometer. These calculations yielded a mean pressure of 6.1 kb. The M2 temperatures and pressures calculated using these calibrations are in good agreement with those estimated from petrogenetic relationships.

Very low‐grade metamorphism of the Taveyanne greywacke, Glarus Alps, Switzerland

ABSTRACTDetailed textural and chemical data for mineral assemblages on a regional scale are presented for the metaandesitic Eocene‐Oligocene Taveyanne greywacke of the Glarus Alps, Eastern Switzerland. Presented data indicate an increase of metamorphic grade from zeolite facies to prehnite‐pumpellyite and pumpellyite‐actinolite facies. Low‐grade outcrops contain laumontite, minor corrensite and pumpellyite (assemblage type 1), whereas outcrops of higher metamorphic grade contain prehnite and two populations of pumpellyite (type 2), prehnite—pumpellyite‐(Al)—white mica (type 3), a single outcrop shows pumpellyite‐actinolite facies (type 4). From the zeolite to prehnite‐pumpellyite/pumpellyite‐actinolite facies there are indications for an increase of the chemical equilibrium domain size for the critical paragenesis from a single detrital grain ≤1 mm) in type 1, to a few millimetres in type 2, and to a whole thin section in type 3. MetamorphicP‐Tconditions were determined by a combination of chlorite thermometry, fluid inclusion and vitrinite reflectance data. Peak temperatures range from 170‐190d̀ C for zeolite facies to 270‐310d̀ C for prehnite‐pumpellyite and pumpellyite‐actinolite facies. For the higher temperature range, pressures of 2‐3 kbar are derived indicating a geothermal gradient of 24‐32d̀ C km‐1. The well‐constrained temperature estimations derived for the assemblages provide a useful test of the different empirical calibrations of chlorite thermometers recently proposed. The best correspondence to the temperatures determined here is for the Cathelineau calibration. In addition, in the lower grade samples differences in textures and calculated temperatures provide a mean to distinguish between detrital and newly formed chlorites.

Structural geology of the Vangorda PbZnAg orebody, Yukon, Canada

The Vangorda PbZnAg orebody is a 7.1 M tonne, polydeformed, polymetamorphosed stratiform massive sulphide orebody in the Anvil mining district, Yukon, Canada. The orebody consists of fine- to medium-grained semi-massive and massive sulphides with a dominant sulphide mineralogy of pyrite, pyrrhotite, sphalerite, galena and minor chalcopyrite. Five sulphide lithofacies have been identified within the orebody; ribbon-banded, carbonaceous, pyritic quartzite; pyritic quartzite; massive pyritic sulphides; baritic, massive pyritic sulphides; and pyrrhotite-sphalerite massive sulphides. The orebody is elongate in a northwest-southeast orientation and plunges shallowly towards the northwest. The host rocks and the various sulphide lithofacies have been complexly deformed during two penetrative phases of deformation (D 1 and D 2, respectively) and associated metamorphism (M 1 and M 2). M 2 metamorphism resulted in extensive recrystallisation and subsequent grain growth of sulphide minerals. M 2 metamorphic grade recorded by arsenopyrite and chlorite thermometry and sphalerite+hexagonal pyrrhotite barometry is of mid-greenschist facies with temperatures of 336° to 363°C, respectively, and a pressure of 4 kb. The earliest structure that can be recognised in the orebody is a penetrative S 1 spaced cleavage that is well-developed in the host phyllites and a differentiated layering in the semi-massive and massive sulphides. However, F 1 folds have not been identified in the orebody, although they do occur within the Anvil District. The S 1 cleavage has been deformed into downward-facing, southwest-verging, recumbent F 2 folds that plunge shallowly northwest-southeast. A penetrative axial planar S 2 crenulation cleavage is developed in the phyllites and ribbon-banded, carbonaceous, pyritic quartzite, but is found only locally in the massive sulphide lithofacies. In the baritic lithofacies, F 2 folds have thickened hinges suggesting remobilisation of the ore. Along the overturned limbs of the F 2 folds and at lithofacies contacts centimetric to metric-scale zones enriched in pyrrhotite, sphalerite and galena are developed. These areas commonly have a significantly reduced grain size, a well-developed sulphide foliation and numerous shear indicators such as winged and rotated clasts. F 2 folds and the S 2 form surfaces are cut by syn- to post-D 2 NW-SE-dipping extensional faults. Minor strike-slip faults cut D 1 and D 2 structures.