Chlorite Zone Research Articles

Monazite Dating of Prograde and Retrograde P–T–d paths in the Barrovian terrane of the Thaya window, Bohemian Massif

Monazite laser ablation–split-stream inductively coupled plasma–mass spectrometry (LASS) was used to date monazite in situ in Barrovian-type micaschists of the Moravian zone in the Thaya window, Bohemian Massif. Petrography and garnet zoning combined with pseudosection modelling show that rocks from staurolite–chlorite, staurolite, kyanite and kyanite–sillimanite zones record burial in the S1 fabric under a moderate geothermal gradient from 4–4·5 kbar and ∼530–540°C to 5 kbar and 570°C, 6–7 kbar and 600–640°C, 7·5–8 kbar and 630–650°C, and 8 kbar and 650°C, respectively. In the kyanite and kyanite–sillimanite zones, garnet rim chemistry and local syntectonic replacement of garnet by sillimanite–biotite aggregates point to re-equilibration at 5·5–6 kbar and 630–650°C in the S2 fabric. Heterogeneously developed retrograde shear zones (S3) are marked by widespread chloritization, but minor chlorite is present in the studied samples. Monazite abundance and size increase with metamorphic grade from 5 µm in the staurolite–chlorite zone to >100 µm in the kyanite and kyanite–sillimanite zones. Irrespective of the monazite-forming reaction, this is interpreted as the onset of limited prograde monazite growth at staurolite grade, and continued prograde monazite growth after the kyanite-in reaction, compatible with conditions of about 5·5 kbar and 570°C and 7·5 kbar and 630°C from pseudosection modelling. Monazite is zoned, showing embayments and sharp boundaries between zones, with low Y in the staurolite zone, high-Y cores and low-Y rims in the kyanite zone, and high-Y cores, a low-Y mantle and a high-Y rim in the sillimanite zone. The 207Pb-corrected 238U/206Pb ages from three samples range from 344 ± 7 to 330 ± 7 Ma, irrespective of metamorphic grade. The dates from monazite inclusions are interpreted as the ages of the staurolite- and kyanite-in reactions along the prograde path at 340 and 337 ± 7 Ma, respectively. The monazite in the matrix (and some inclusions) is interpreted as dating the prograde crystallization at (340–337) ± 7 Ma within the S1 fabric, and then being affected by recrystallization at or down to 332 ± 7 Ma in the S2 and S3 fabrics. The two groups of data, for 340–337 and 332 Ma, are significantly different when only their in-run uncertainties (±1–3 Myr) are compared and indicate a 9 ± 3 Myr period of monazite (re)crystallization. A systematic increase in heavy rare earth element (HREE) content with decreasing monazite age from 344 to 335 Ma is correlated with growth on the prograde P–T path; the drop in HREE of monazite at 335–328 Ma is assigned to recrystallization. The presence of chlorite even in the least retrogressed samples witnesses limited external fluid availability on the retrograde P–T path. Migration of this fluid was probably responsible for heterogeneous fluid-assisted recrystallization and resetting of original prograde monazite, even where included in garnet, staurolite or kyanite. It is suggested that the rocks passed the chlorite-in reaction on the retrograde path at 332 ± 7 Ma. The timing of burial in the Thaya window, a deformed part of the underthrust Brunia microcontinent, was coeval with exhumation of granulites and migmatites of the Moldanubian orogenic root at c. 340 Ma.

Late stage exhumation of HP metamorphic rocks, progressive localization of strain, and changes in transport direction, Sambagawa belt, Japan

New geologic mapping delimits major structures associated with the later stages of exhumation of high-pressure (HP) metamorphic rocks of the Sambagawa belt of central Shikoku. The main sets of structures define an inclined N-dipping wedge, bounded by S-vergent thrust faults below and N-vergent normal faults above. Such faults bound every metamorphic mineral zone of the Sambagawa belt. Faults also occur within metamorphic mineral zones, such as the Asemigawa detachment, which is entirely within the oligoclase–biotite zone, the highest-grade metamorphic zone. This fault is the structurally lowest of the N-vergent normal faults. These D2 structures accommodate the exhumation of the higher-grade rocks relative to the lower-grade rocks within the Sambagawa belt, rather than relative to units bounding the belt, so they are termed an “internal” extrusional wedge. The extrusional wedge is truncated to the east against footwall chlorite zone rocks by the Hamegano out-of-sequence thrust (OOST). The OOST divides the traditional chlorite zone into an upper unit consisting of varicolored schist (L tectonite), and a lower unit consisting of regionally extensive pelitic schist. To the north, the metamorphic mineral zones are stacked over the OOST, forming the Tomisato duplex, and these structures are further modified by the Oboke anticline and Tsuneyama syncline. The evolution of exhumation structures of the Sambagawa suggests progressive localization of strain as exhumation proceeded, as well as a change in movement direction from D1 oblique (orogen-subparallel) exhumation to the D2 structures with dip-parallel (orogen-normal) transport that we describe in detail. The evolution of exhumation structures in the Sambagawa belt differs from that of the HP rocks of the Franciscan Complex of California in that the exhumation of the latter is accommodated entirely by discrete faults and the Franciscan lacks internal normal faults. The internal extrusional wedge structure of the Sambagawa HP rocks resembles that of HP rocks of Leti Island of eastern Indonesia.

Origin of a barite-sulfide ore deposit in the Mykonos intrusion, cyclades: Trace element, isotopic, fluid inclusion and raman spectroscopy evidence

The polymetallic Mykonos vein system in the Cyclades, Greece, consists of 15 tension-gashes filled with barite, quartz, pyrite, sphalerite, chalcopyrite and galena in ca. 13.5Ma, I-type, Mykonos monzogranite. Zones of silica and chlorite–muscovite alteration are associated with the veins and overprint pervasive silicification, phyllic and argillic alteration that affected large parts of the monzogranite. The mineralization cements breccias and consists of an early barite–silica–pyrite–sphalerite–chalcopyrite assemblage followed by later argentiferous galena. A combination of fluid inclusion and stable isotope data suggests that the barite and associated mineralization were deposited from fluids containing 2 to 17wt.% NaCl equivalent, at temperatures of ~225° to 370°C, under a hydrostatic pressure of ≤100bars. The mineralizing fluids boiled and were saturated in H2S and SO2.Calculated δ18OH2O and δDH2O, initial 87Sr/86Sr isotope compositions and the trace and REEs elements contents are consistent with a model in which the mineralizing fluids were derived during alteration of the Mykonos intrusion and subsequently mixed with Miocene seawater. Heterogeneities in the calculated δ34SSO4−2 and δ34SH2S compositions of the ore fluids indicate two distinct sources for sulfur, namely of magmatic and seawater origin, and precipitation due to reduction of the SO4−2 during fluid mixing. The physicochemical conditions of the fluids were pH=5.0 to 6.2, logfS2=−13.8 to −12.5, logfO2=−31.9 to −30.9, logfH2S(g)=−1.9 to −1.7, logfTe2=−7.9 and logα(SO4−2(aq)/H2S(aq))=+2.6 to +5.5. We propose that retrograde mesothermal hydrothermal alteration of the Mykonos monzogranite released barium and silica from the alkali feldspars. Barite was precipitated due to mixing of SO4−2-rich Miocene seawater with the ascending Ba-rich magmatic fluid venting upwards in the pluton.

Mineral chemistry and isotopic composition of magmatic, re-equilibrated and hydrothermal biotites from Darreh-Zar porphyry copper deposit, Kerman (Southeast of Iran)

Abstract The Darreh-Zar porphyry copper in the southeastern part of the Urumieh–Dokhtar magmatic belt developed as a consequence of hydrothermal alteration related to the Darreh-Zar granodioritc porphyry stock intruded into mafic volcanic rocks. Hydrothermal alteration and Cu–Mo mineralization are focused on the Darreh-Zar porphyry pluton belonging to Miocene time and the neighboring basaltic country rocks. A core zone of potassic alteration is enclosed by a peripheral zone of chlorite–sericite, sericitic, argillic and propylitic alteration. Judging by petrographic features as well as chemical composition, the biotites studied display variable size, color, and shape and occur as magmatic (M), re-equilibrated (R) and hydrothermal (H) types. The M and R-type biotites are found in the Darreh-Zar porphyry and the H-type formed during potassic alteration within the Darreh-Zar porphyry and the volcanic host rocks as well. The hydrothermal biotites are characterized by the highest amounts of Al 2 O 3 and K 2 O whereas the magmatic types are hosted by high level of TiO 2 (up to 4.38 wt.%). X Mg values vary between 0.31 and 0.61 which is increased from M to R-biotites then decreased toward low temperature potassic alteration zone. Fluorine and chlorine contents from biotite types also vary between 0.07 to 0.56 wt.% and 0.14 to 0.38 wt.% respectively. These values change to 2.94 to 2.15 wt.% and − 3.6 to − 4.4 wt.% for IV(F) and IV(Cl) respectively. In all types of biotites, positive correlation is observed between X Mg , Al 2 O 3 , Na 2 O + K 2 O + BaO and F but with FeO and Cl it is negative. The TiO 2 content clearly decreases from the M to R and H-type (Average: 3.98, 2.9 and 2.4 wt.% respectively). A direct relation may be noted between Mg# and oxygen fugacity. Log ( f H 2 O/ f HF) and log ( f H 2 O/ f HCl) values range from 6.24 to 5.46 and 4.49 to 4.12 wt.% respectively. Oxygen fugacity increases during formation of re-equilibrated biotites but it decreases in the course of potassic alteration. Crystallization temperatures range from 600° to 650 °C, 550 to 600 °C and 450 to 550 °C for magmatic, re-equilibrated and hydrothermal biotites respectively. The M-biotites from Darreh-Zar have the δD values of − 89 to − 101‰ and the calculated δD values of the aqueous fluids (− 66 to − 78‰) in equilibrium with the biotites are consistent with a large component of magmatic fluid which shows similarities with the biotite originated from the primary mantle source. The δ 18 O also varies between 7.4 to 8.2‰ for fluids in equilibrium with biotites. The obtained M-biotite data from Darreh-Zar porphyry indicates that there is no clear correlation between δD values, Fe and Mg contents but for δD values, Al and Fe 3 + /Fe 2 + some affinities were observed. The isotopic data indicate that M-biotites are formed at high temperature (620 ± 20 °C) from fluids with the highest δ 18 O H 2 O (7.8 ± 0.4‰) in comparison with the biotites from some other copper porphyries.

해남 연-아연 스카른광상의 산상과 지화학적 특성

옥천변성대 서남쪽 화원반도에 위치한 해남광상은 옥천층군의 석회암을 모암으로 이를 포획한 형태로 관입한 백악기 석영반암과의 접촉부를 따라 발달한 연-아연 스카른광상이다. 광석시료의 암석기재학적 특징, 스카른 및 광석광물의 조성, 그리고 관계화성암의 지화학적 연구를 통해 스카른광화작용의 특성을 파악하였다. 스카른은 석류석<TEX>${\pm}$</TEX>휘석<TEX>${\pm}$</TEX>방해석<TEX>${\pm}$</TEX>석영대, 휘석+석류석+석영<TEX>${\pm}$</TEX>방해석대, 방해석+휘석<TEX>${\pm}$</TEX>석류석대, 석영+방해석<TEX>${\pm}$</TEX>휘석대 그리고 방해석<TEX>${\pm}$</TEX>녹니석대 등으로 구분된다. 석류석은 안드라다이트가 주를 이루며 그로슐라와 더불어 누대구조가 발달하기도 하고, 휘석은 관계화성암에서 멀어지면서 Mn-헤덴버자이트에서 투휘석으로 순차적으로 조성이 변한다. 광석광물은 화성암체 가까이에서 황동석이 주를 이루며, 원거리로 갈수록 섬아연석과 방연석이 증가하는 경향을 보인다. 전자현미분석결과 섬아연석은 석류석<TEX>${\pm}$</TEX>휘석<TEX>${\pm}$</TEX>방해석<TEX>${\pm}$</TEX>석영대에서 멀어질수록 FeS가 평균 5.17 mole %, 2.93 mole %, 그리고 0.40 mole %로 점차 감소하는 경향을 보이고, 방연석의 Ag와 Bi의 함량도 석류석<TEX>${\pm}$</TEX>휘석<TEX>${\pm}$</TEX>방해석<TEX>${\pm}$</TEX>석영대에서 평균 0.72 wt.%, 1.62 wt.% 이던 것이 방해석+휘석<TEX>${\pm}$</TEX>석류석대에서는 <0.01 wt.%, 0.11 wt.%로서 감소한다. 이와 같이 해남광상은 스카른 대상분포와 더불어 광석광물의 종류 및 조성변화가 체계적으로 발달하는 특징을 나타낸다. 관계화성암인 석영반암은 Meinert(1995)가 제시한 Zn-스카른 광상보다 다소 분화된 특성을 보이는데, <TEX>$SiO_2$</TEX> 함량은 72.76~75.38 wt.%로서 높은 편에 속하며, 칼크-알칼리 계열로서 과알루미나형에 해당하고, 화산호 환경의 지화학적 특징을 나타낸다. The Haenam Pb-Zn skarn deposit is located at the Hwawon peninsula in the southwestern part of the Ogcheon Metamorphic Belt. The deposit is developed along the contact between limestone of the Ogcheon group and Cretaceous quartz porphyry. Petrography of ore samples, chemical composition of skarn and ore minerals, and geochemistry of the related igneous rocks were investigated to understand the characteristics of the skarn mineralization. Skarn zonation consists of garnet<TEX>${\pm}$</TEX>pyroxene<TEX>${\pm}$</TEX>calcite<TEX>${\pm}$</TEX>quartz zone, pyroxene+garnet+quartz<TEX>${\pm}$</TEX>calcite zone, calcite+pyroxene<TEX>${\pm}$</TEX>garnet zone, quartz+calcite<TEX>${\pm}$</TEX>pyroxene zone, and calcite<TEX>${\pm}$</TEX>chlorite zone in succession toward carbonate rock. Garnet commonly shows zonal texture comprised of andradite and grossular. Pyroxene varies from Mn-hedenbergite to diopside as away from the intrusive rock. Chalcopyrite occurs as major ore mineral near the intrusive rock, and sphalerite and galena tend to increase as going away. Electron probe microanalyses revealed that FeS contents of sphalerite become decreased from 5.17 mole % for garnet<TEX>${\pm}$</TEX>pyroxene<TEX>${\pm}$</TEX>calcite<TEX>${\pm}$</TEX>quartz zone to 2.93 mole %, and to 0.40 mole % for calcite+pyroxene<TEX>${\pm}$</TEX>garnet zone, gradually. Ag and Bi contents also decreased from 0.72 wt.% and 1.62 wt.% to <0.01 wt.% and 0.11 wt.%, respectively. Thus, the Haenam deposit shows systematic variation of species and chemical compositions of ore minerals with skarn zoned texture. The related intrusive rock, quartz porphyry, expresses more differentiated characteristics than Zn-skarn deposit of Meinert(1995), and has relatively high<TEX>$SiO_2$</TEX> concentration of 72.76~75.38 wt.% and shows geochemical features classified as calc-alkaline, peraluminous igneous rock and volcanic arc tectonic setting.

The influence of metamorphic grade on arsenic in metasedimentary bedrock aquifers: A case study from Western New England, USA

Elevated As occurs in many meta-sedimentary bedrock aquifers where elevated bulk-rock As content is one of the primary controls on the concentration of As in groundwater. This study was designed to determine As concentrations in a black shale, black slate and black phyllite sequence that comprises the bedrock aquifer system of the Taconic Mountain region of southwestern Vermont and adjacent New York State. Variability in groundwater As concentrations provides the impetus for this study: 25% of wells in weakly metamorphosed shales and slates (<lower chlorite zone) exceed 10μg/L (133nmol/L) As, yet only 3% of wells in stratigraphically-equivalent phyllites (metamorphosed to ≥upper chlorite zone) exceed 10μg/L As. Geochemical analysis indicates that whole-rock As content is inversely proportional to metamorphic grade, ranging from a mean of 26.9mgkg−1 in low-grade black shales and slates to 13.8mgkg−1 in higher-grade black phyllites. The differences in As concentrations are statistically significant (p<0.03), and Cu, Ni, Pb and Zn are also significantly (p<0.03) depleted in higher-grade phyllites. These differences are attributed to recrystallization of pyrite with increasing metamorphic grade, a process which introduces As and other trace elements into pore fluids, after which the high mobility of As makes it susceptible to be leached out of metapelites. Data from this study and previous research indicates that depletion of As from metapelites tends to occur once the rocks reach upper chlorite zone or lower biotite zone, corresponding to metamorphic temperatures of ~250–350°C. This suggests that, in the absence of subsequent hydrothermal mineralization (e.g. arsenopyrite in late-stage veins), metapelites metamorphosed to upper chlorite zone or higher will be less likely to foster elevated As in groundwater compared to their lower-grade shale and slate counterparts.

Evolution of a volcanogenic hydrothermal system recorded by the behavior of LREE and Eu: Case study of the Key Tuffite at Bracemac–McLeod deposits, Matagami, Canada

The distribution of rare earth elements (REE) in the alteration zones surrounding modern and ancient volcanogenic massive sulfide (VMS) deposits is often characterized by a wide range of variations comprising both gains and losses of light REE (LREE: La to Sm) and europium. The evolution of the hydrothermal system is recorded by mineral assemblages and their chemical composition. To address the behavior of the REE in such a context, the Bracemac–McLeod Archean VMS deposits of the Matagami district are used. There, mineralization was formed mainly by subsea-floor replacement along a tuffaceous marker unit (Key Tuffite) of homogenous andesitic composition, hence providing a single protolith to account for mineral and chemical variations in relation to ore position. We combine whole-rock and mineral REE data in both distal sericite and proximal chlorite alteration surrounding the mineralization. A model, integrating observed textural relationships, geochemical data and current understanding of mineral stabilities, is proposed to explain the redistribution of the REE during the evolution of the VMS hydrothermal system. The distal sericite zone represents the early, low temperature alteration. It is characterized by the destruction of volcanic glass and plagioclase leading to a negative Eu anomaly in the whole-rock analyses and the depletion of LREE. The proximal chlorite zone was formed during the thermal evolution of a fertile VMS hydrothermal system. It is generally characterized by gains of LREE and Eu, although some chloritized samples are depleted in LREE. Petrographic observations suggest that the coprecipitation of allanite with chlorite explains the increase of LREE, whereas a mass balance calculation indicates that the precipitation of apatite (±carbonates) essentially controls the Eu. Local silicification, occurring as pulses during the hydrothermal system life, is also of interest because it could preserve an intermediate alteration state and thus provide additional information on the evolution of the hydrothermal system. The mobility of REE is indicative of specific hydrothermal conditions, which may reflect the efficiency of a mineralizing hydrothermal system. At Matagami, the mobility of LREE and Eu could be used for vectoring in exploration, especially Eu which has an anomalous behavior that extends far beyond the limit of the sulfide zone, into the distal sericitic low-temperature alteration (up to 400m from the ore zones).

Spinel group minerals in metamorphosed ultramafic rocks from Río de Las Tunas belt, Central Andes, Argentina

In the Rio de Las Tunas belt, Central Andes of Argentina, spinel group minerals occur in metaperidotites and in reaction zones developed at the boundary between metaperidotite bodies and their country-rocks. They comprise two types: i) Reddish-brown crystals with compositional zonation characterized by a ferritchromite core surrounded by an inner rim of Cr-magnetite and an outer rim of almost pure magnetite. ii) Green crystals chemically homogeneous with spinel (s.s.) and/or pleonaste compositions. The mineral paragenesis Fo+Srp+Cln+Tr+Fe-Chr and Fo+Cln+Tr+Tlc±Ath+Fe-Chr observed in the samples indicate lower and middle grade amphibolite facies metamorphic conditions. Nonetheless, the paragenesis (green)Spl+En+Fo±Di indicates that granulite facies conditions were also reached at a few localities. Cr-magnetite and magnetite rims in zoned reddish-brown crystals and magnetite rims around green-spinel/pleonaste grains are attributed to a later serpentinization process during retrograde metamorphism. The chemical composition of spinel group minerals in the ultramafic reaction zones is determined by the mineral zone where they crystallize. Green pleonaste occurs in the chlorite zone, ferritchromite predominates in the amphibole zone, whereas Crmagnetite and magnetite are more common in the carbonate zone. The mineral paragenesis of the Rio de Las Tunas metaperidotites together with the chemical characteristics of the spinel group minerals support a clockwise P-T path evolution for the ultramafic protoliths during the Paleozoic regional metamorphic cycle of this area

Multiple methods for regional- to mine-scale targeting, Pataz gold field, northern Peru

Gold production in the Pataz district, northern Peru, is derived from mesothermal veins hosted by the Pataz batholith and basement-hosted epithermal and carbonate–base metal veins. At the regional scale, processing of Advanced Spaceborne Thermal Emission and Reflection Radiometer data can be used to delineate district-scale argillic alteration. One such area extends for tens of kilometres NNW of Vijus in the Maranon Valley. At the southern end of this area, basement-hosted quartz–carbonate–sulfide veins in faults support artisanal gold-mining operations. SEM analyses show that the alteration envelopes around these faults are dominated by illitic clays. These artisanal gold workings highlight the economic potential of the largely unexplored parts of the district-scale argillic alteration zone, further north. At the district scale, paleostress modelling maps areas of low minimum stress during Carboniferous ENE–WSW shortening, based on a new 1:25 000 geological map of the Pataz district. The resulting distribution of low minimum stress is used to predict sites of rock fracture under high fluid pressure, and consequent vein formation. These areas of low minimum stress occupy 11% of the modelled area but contain 50% of the known veins in the Pataz district. Some areas of low minimum stress contain no known veins, and where these are poorly explored or poorly exposed, they are proposed as potential targets for gold exploration. In combination with SEM microanalysis, ASD hyperspectral reflectance analysis of drill core samples shows that visible proximal sericitic alteration around batholith-hosted auriferous veins is predominantly phengitic illite. Automated software interpretation of ASD reflectance spectra using The Spectral Assistant shows that sericite in cryptic alteration distal from auriferous veins varies from mainly illite adjacent to the phengitic illite zone, to more distal muscovite. Reactivation of faults and mineralised vein contacts during the largely Cenozoic Andean orogeny produced chlorite alteration that locally overprints proximal phengitic illite alteration. ASD spectrometry identifies relict phengitic illite in some chloritic alteration zones and thus indicates proximity to mineralised veins at the deposit scale. Elevated pathfinder element concentrations within proximal phengitic illite alteration zones around batholith-hosted veins do not extend more than a few metres beyond the visible alteration envelope. The alkali alteration index [(Rb + Cs)/Th]N is elevated above background levels for up to 15 m beyond the visible sericite alteration zone in one of two holes investigated. In the other hole, both [(Rb + Cs)/Th]N and 3K/Al can be used as a lode-scale vector to gold-bearing veins within broad intersections of visible sericite alteration.

The investigation of the geothermal potential at the Silali volcano, Northern Kenya Rift, using electromagnetic data

The Silali volcano is one among a number of volcanoes found along the axis of the Kenya rift system that has experienced immense volcanism and faulting that formed a huge caldera at its summit. In this study, we investigate the resistivity distribution within the Silali volcano region in order to define a magma chamber that was the source of young lava flows covering the region and to better describe the geologic and structural features or faults around this region that may allow fluid movement. The data reveal a magma chamber beneath the caldera and a northeast trending fault west of the volcano in the Kapedo region. Potential fluid circulation zone within the caldera region are also evident. The depth to the magma chamber is estimated from the deeper low resistivity zone to be about 6km from the surface. A low resistivity zone less than 20ohm-m, at 1–2km depth is interpreted to be a low temperature clay (smectite) alteration zone resulting from fluid circulation as heat is transferred to shallower depth. The resistivity then increases abruptly beneath the inferred low temperature alteration zone and this increase in resistivity is interpreted as a change from clay mineralization to a chlorite-dominated system producing the observed decrease in resistivity. The decrease in conductivity may also imply a decrease in permeability due to decreasing connectivity of conductive fluids. The resistivity changes with depth could be temperature dependent and the inferred chlorite zone may suggest a temperature above approximately 240°C. The deeper low resistivity zone is interpreted as molten rocks and can be interpreted to indicate temperatures above 700°C of the magma body at a depth of 6km.

Paleo-mantle wedge preserved in the Sambagawa high-pressure metamorphic belt and the thickness of forearc continental crust

There are two possible origins for ultramafic rocks in subduction-type high-pressure metamorphic terranes: (1) they are derived from the subducted slab or footwall, or (2) they are derived from the mantle wedge in the hanging wall. The origin of ultramafic rock bodies in a classic high-pressure metamorphic belt, the Sambagawa belt of southwest Japan, is assessed based on wide-ranging field studies covering an area of 23 × 30 km 2 and corresponding to metamorphic pressures of 5–25 kbar. Peridotite and/or serpentinite bodies are common in the higher pressure part of the belt, but no occurrence is known in the low-pressure part (the chlorite zone). If the ultramafic rocks originated in the footwall, they should be metamorphosed together with the subducted material and their distribution should not show any correlation with metamorphic pressure. The restricted distribution of mantle rocks to the high-pressure part is strong evidence for a mantle-wedge origin of the ultramafic blocks. The presence of subducted metasediments surrounding the mantle rocks indicates that the subducted slab can tectonically entrain and transport substantial amount of hanging-wall material to the Earth’s surface. The first appearance of ultramafics occurs within the garnet zone, which has a peak metamorphic pressure of 8.0–9.5 kbar, and the corresponding depth (∼30–35 km) represents the thickness of the forearc continental crust in the Cretaceous Sambagawa subduction zone.

Mass changes during hydrothermal alteration associated with gold mineralization in the Astaneh granitoid rocks, western Iran

The Astaneh pluton, located southwest of Arak city in western Iran, consists of Middle Jurassic granitoids and is widely affected by hydrothermal alteration. Phyllic (sericitic), argillic, chloritic, and propylitic alteration zones were recognized in the area. Field and petrographic observations show that gold mineralization is associated with phyllic zones. It seems that water-rock reaction and mixing with meteoric water were important processes in changing the composition of hydrothermal fluids as well as precipitating ore minerals from the hydrothermal solutions. To understand better the process of mass changes during the hydrothermal evolution of the Astaneh pluton, these alteration zones have been studied in terms of mass transfer and element mobility. Isocon plots quantitatively illustrate that Al 2 O 3 , TiO 2 , and P 2 O 5 were relatively immobile during alteration. Phyllic alteration was accompanied by depletion of Na 2 O, MgO, CaO, and Fe 2 O 3 , and enrichment of SiO 2 and K 2 O. The depletion of Na 2 O and Fe 2 O 3 and enrichment of K 2 O reflect the sericitization of alkali feldspar and the destruction of ferromagnesian minerals. Enrichment of Si in phyllic zones is consistent with silicification, which is a major feature of phyllic alteration. During phyllic alteration, high field strength elements (HFSEs), except Y, were enriched while rare earth elements (REEs) and low field strength elements (LFSEs) were depleted. Argillic zones are characterized by enrichment in SiO 2 , K 2 O and LOI and strong depletion in some other major element oxides, REEs, LFSEs, and HFSEs (except Ga). This indicates that, in argillic zones, primary minerals in the granitoids were completely destroyed and subsequently removed by acidic fluids. In chloritic zones, depletion of SiO 2 , CaO, and Na 2 O, and enrichment of Fe 2 O 3 , MgO, K 2 O, MnO, and LOI reflect alteration of plagioclase and ferromagnesian minerals to sericite and chlorite, respectively. The strong enrichment of REEs in chloritic zones could be due to the stabilization of the ionic complexes, which carried those elements in solution. In chloritic zones, there are notable differences in the behavior of LFSEs and HFSEs. In propylitic zones, the enrichment of Fe 2 O 3 , SiO 2 , MnO, and MgO is likely the result of decomposition of ferromagnesian minerals such as biotite. During propylitic alteration, LFSEs and HFSEs (except Zr and Nb) as well as REEs were leached out. The Astaneh low-sulphidation epithermal system consists of proximal argillic through distal propylitic assemblages. The gold mineralization in the Astaneh, which is typically associated with phyllic hydrothermal zone, occurs as veins containing quartz and sulphide. Gold seems to have been deposited in open-space veins due to boiling of hydrothermal fluids.

Remanent magnetization of maghemitized basalts from Krafla drill cores, NE-Iceland

We investigated the natural remanent magnetization (Jr) of hydrothermally altered basalts from two drill cores KH1 (200 m) and KH3 (400 m) situated at the rim of the Krafla caldera in NE Iceland, where a geothermal field (>150°C) is still active. Low temperature oxidation along with mineral reactions in the chlorite zone (<350°C) is the prevailing cause for the maghemitization and a strong decrease of Jr to occur in our study. Despite a significant decrease of Jr with respect to fresh basalts in surface outcrops of the same area, the stepwise demagnetization analyses of Jr show the presence of a stable magnetic component with the expected inclination of 77° in Iceland. Because the alteration temperature (<350°C) is above the Curie temperatures of most of the original titanomagnetite (40°–350°C), we suggest that a normal direction of remanence is chemically acquired during the low temperature alteration. We observed only one reliable negative inclination at 293.2 m in the KH3 core, which we rather interpret to be acquired during a geomagnetic excursion with reverse polarity than caused by a self-reversal mechanism.

Chrysoprase from Gumigil, Queensland

Chrysoprase from Gumigil, near Marlborough in central east Queensland, occurs in weathered ophiolite of the Marlborough Block, in veins up to 20 cm thick in the lower parts of an approximately 50 m-thick regolith profile. Serpentinised harzburgite saprolite hosting magnesite and chalcedony veins with rare chrysoprase grades upward to a plasmic zone of chalcedony, nontronite and chlorite, with a lateritised zone above consisting of nodular or vermiform ferruginous duricrust, boxwork chalcedony and kaolinite. Gem chrysoprase and the poorer quality so-called ‘wetstone’ are dominantly chalcedonic quartz, coloured by willemseite, which occurs as 30–100 nm-wide zones largely aligned on polycrystalline quartz grain boundaries. Individual willemseite flakes (possibly single crystals) range from 2 to 12 nm thick. The quartz grainsize in chrysoprase is finer than in wetstone, and the willemseite domains in chrysoprase are smaller and less abundant, amounting to about 5% in chrysoprase and 15% in wetstone. Willemseite composition varies slightly between samples, with an average formula (Ni2.7Mg0.3)Si4O10(OH)2.

Metabasites petrology and P-T evolution in the Lesser Himalaya, central Nepal

Petrological study was carried out for the first time on the metabasites of the Lesser Himalaya in central Nepal. The metabasites are mostly tholeiitic basalts emplaced in the elastic sediments as supracrustal dikes and sills, and later metamorphosed together with the host rocks. They contain almost a constant mineral assemblage of Ca-amphiboles + plagioclase + biotite + quartz ± epidote± chlorite + (Fe-Ti oxides). Amphiboles in the form of porphyroblasts show chemical zonation with actinolite/magnesio­homblende cores, tschermakite/ferro-tschermakite rims, and magnesio-hornblende margins. The cores of porphyroblasts are pre-kinematic and were probably formed prior to the Tertiary Himalayan orogeny. The porphyroblast rims and the matrix amphiboles are syn-kinematic and were formed during the Upper Main Central Thrust activity in the Tertiary period. The compositions of both the porphyroblast rims and matrix amphiboles change from actinolite in the chlorite zone to magnesium­ hornblende in the biotite zone and totschermakite/ferro-tschermakite in the garnet zone. The systematic changes in amphibole compositions as well as petrographic characteristics of metabasites confirm the classical concept of increasing metamorphic grade structurally upwards to the Upper Main Central Thrust in the Lesser Himalaya. Application of hornblende-plagioclase thermobarometry shows a coherent prograde P-T path in zoned amphiboles. The cores of amphibole porphyroblasts were formed at average peak temperature of ~540"C and at pressure of ~3 kbar. The porphyroblast rims and matrix amphiboles were recrystallized at average peak temperatures of ~570°C in the biotite zone and ~630°C in the garnet zone at pressure of ~6 kbar. The metabasites petrology is in favor of the tectono-metamorphic models that relate the inverted metamorphism with thrusting along the Upper Main Central Thrust and coeval inversion of isoiliem1S. It is suggested that published amphibole cooling ages from the Nepalese Lesser Himalaya based on simples, homogeneous mineralogy should be reinterpreted in view of the presence of polygenetic amphiboles with heterogeneous composition.

Postsedimentation transformation of triassic terrigenous rocks in West Chukotka as an indicator of folding conditions

Postsedimentation alteration and structural assemblies of the Triassic sedimentary complexes of West Chukotka are discussed. Zoning of the alteration is based on examination of newly formed structural and mineral assemblages, the chemical composition, and the polytypes of clay minerals. Three zones of postsedimentation transformation of sandstones are distinguished: (1) the zone of chlorite, illite, and mixed-layer disordered chlorite-smectite; (2) the zone of illite and chlorite; and (3) the zone of phengite and ferroan chlorite. The grade of postsedimentation transformation and the composition of the newly formed micas are correlated with the cleavage type. The development of two-three types of cleavage leads to the highest degree of rock transformation. The assemblages of clay minerals and the crystal chemistry of the authigenic phengite show that the grade of postsedimentation transformation of the Triassic rocks attains the stage of greenschistfacies metamorphism in the zone of development of two cleavage types. Where the second cleavage is not documented or poorly developed, the rocks remain unmetamorphosed. Evidence is given that postsedimentation transformation of terrigenous rocks in the foldbelt is controlled largely by deformation.

Fission-track zircon ages of psammitic schist from the chlorite zone of the Sambagawa metamorphic belt, central Shikoku, Japan:

We analyzed the fission-track (FT) thermochronology of zircon in two samples of psammitic schist from the chlorite zone of the Sambagawa metamorphic belt, central Shikoku, Japan. The samples were collected from the lower-grade part of the chlorite zone (pumpellyite-actinolite facies). Detrital zircons from a single locality along the Asemi River are completely annealed, yielding FT ages of 92.6±6.2 Ma (1σ, 13 grains), and those from a single locality along the Dozan River are also completely annealed, yielding FT ages of 47.2±3.8 Ma (1σ, 7 grains). Although both samples were collected from within the chlorite zone, the two FT zircon ages are different, possibly because they represent the timing of peak metamorphism in the former sample, and a D3 thermal event (post peak-metamorphism) in the latter sample.

Cr-rich allanite-(Ce) in the serpentinite-metapelite reaction layer in the Sanbagawa belt of Nushima, Hyogo Prefecture, Japan

Cr-rich allanite-(Ce) has been found in a reaction layer between the serpentinite lens and the host metapelite of the Sanbagawa metamorphic belt in Nushima, Hyogo Prefecture, Japan. The reaction layer is divided into talc, tremolite, and chlorite zones in order from the inner to the outer regions of the layer. The Cr-rich allanite-(Ce) occurs as fine and spindle-shaped grains of 30-60 μm in length in the tremolite zone, and coexists with tremolite and subordinate amounts of chlorite, titanite, graphite, and zircon. The Cr-rich allanite-(Ce) is enriched in Cr2O3 (0.2-9.0 wt%) and total iron as FeO (6.0-9.0 wt%), with the following Cr-richest formula: (Ca0.98Na0.01K0.01)1.00(REE0.70Ca0.28Th0.01)0.99(Al1.40Cr0.69Fe2+0.63Fe3+0.11Ti0.09Mg0.07Mn0.03)3.02(Si2.94Al0.06)3.00O12(OH),where REE includes La = 0.17, Ce = 0.33, Pr = 0.04, Nd = 0.13 and Sm = 0.03 per formula unit. The core part of the Cr-rich allanite-(Ce) is partially enriched in TiO2 of up to 2.3 wt%, suggesting the substitution of (Fe2+,Mg)Ti(Fe3+,Al)-2 in the octahedral sites.

飛騨外縁帯糸魚川‐青海地域の地質と変成作用―日本列島地質体最古の沈み込み帯型変成作用と上昇期の加水変成作用―

The geology and petrology of the Itoigawa–Omi area of the Hida Gaien belt have been reexamined with pioneering metamorphic zonal mapping of glaucophane schist facies done by Banno (1958). A geologic study shows that crystalline schists of ca. 262-381 Ma, 450 Ma garnet amphibolite, amphibolite, 520 Ma jadeitite, albitite, and rodingite, reaching 10 km long and 2-5 km wide, occur as tectonic blocks of serpentinite forming the Omi serpentinite melange (OSM). Along the Omi–River of the eastern side of the OSM, chlorite, garnet, and biotite zones of pelitic schists are again recognized across several tectonic blocks of schist, indicating the slightly broken nature of a large metamorphic unit that resulted probably from exhumation of the serpentinite mélange belt. Kfs and chlorite are common throughout pelitic schists along the Omi–River. Thus, biotite- and oligclase-forming reactions may have been K-feldspar + chlorite → muscovite + biotite + quartz + H2O, and muscovite + clinozoisite + quartz → K-feldspar + anorthite + H2O, respectively. Glaucophane described by Banno (1958) in the chlorite zone of the Omi River has been recognized in composite grains consisting of hornblende core, gluacophane mantle, and actinolite rim. In contrast, glaucophane appears in a few samples along with eclogitic assemblages as plurifacial minerals before, during, and after eclogite facies in the pelitic schists of the western side of OSM (EC unit). Thus, the regional metamorphism described along the Omi–river of OSM (EA unit) is not a retrogressive metamorphism of the EC unit, but a regional hydrothermal recrystallization just before or during exhumation in the serpentinite mélange belt.

Alteration and Vein Textures Associated with Gold Mineralization at the Bunikasih Area, Pangalengan, West Java

DOI: 10.17014/ijog.v5i4.108The Bunikasih vein system in the Pangalengan district of West Java is a low-sulfidation, adularia sericite epithermal gold deposit. It is hosted by Late Miocene andesitic volcanic and volcanoclastic rocks occurring in the south western margin of Malabar Volcano complex. Gold ore and alteration minerals related to deposition of gold in Bunikasih deposits superimposed on Late Tertiary-Quaternary andesitic formation that were altered and mineralized by some hydrothermal events. The veins consist almost entirely of quartz, with small amounts of adularia, bladed calcite, pyrite, and gold. Gold ore shoots are vertically restricted and are more continuous horizontally. The veins display complex and multi episodic filling with texture characteristics of open space precipitation such us colloform, lattice bladed, crustiform banding, vugs, breccia, and cockade and comb texture. The presence of bladed calcite and silica pseudomorph after bladed calcite suggests that the hydrothermal fluids boiled. In the Cibaliung section of the area, anomalous gold is related to veins trending northeast - southwest, milky quartz with dark grey to black manganese staining is found intermittently for a length of about 800m. The mineralized andesite ore bodies exhibit broad alteration patterns adjacent to mineralization, passing from fresh rock into anargillic, chlorite zone, and then sericite-silica close to mineralization. An argillic assemblage composed of kaolinite with fine-grained pyrite bulb is present in the upper portions and surrounding of the quartz vein system. The veins range from centimeter to meter in size. Of 24 vein samples collected, gold averages up to 0.3 grams per tone ("g/t"), to a high of 24.6 g/t. The Bunikasih epithermal gold deposit was mined by people for more than 10 years, mainly for the gold ore.