Early Retrograde Stage Research Articles

Metamorphism and P-T Evolution of High-Pressure Granulites from the Fuping Complex, North China Craton

Granulite facies rocks provide important keys to evaluating collisional metamorphism in orogenic belts. The mafic granulites of Baoding in the Fuping Complex of the North China Craton occur within the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional orogen. Here, we present results from detailed investigations on newly discovered garnet pyroxenite, garnet two-pyroxene granulite, and garnet-bearing-plagioclase amphibolite using petrographic, mineralogical, geochemical, and zircon U-Pb dating methods. Our results show that the Fuping Complex metamorphic evolution in this study evolved in four stages: prograde (M1), high-pressure granulite facies (M2), granulite facies (M3), and retrograde (M4) stages. The mineral assemblage of the prograde stage (M1) consists of Amp + Pl + Q within garnet cores. The mineral assemblage of high-pressure granulite facies at the peak stage (M2) consists of Gt + Cpx + Pl + Q ± Amp, forming the garnet pyroxenite. The granulite facies stage M3 is characterized by the occurrence of orthopyroxene, with a mineral assemblage of Gt + Cpx + Opx + Amp+ Pl + Q. The early retrograde stage M4-1 includes clinopyroxenes scattered inside amphiboles, following the breakdown of garnet and clinopyroxene. The mineral assemblage of this stage comprises Amp + Pl + Q + Ilm ± Cpx. Later, in the late retrograde stage M4-2, the composition of amphiboles changed to actinolite, and epidote and chlorite started to appear in the matrix. Traditional geothermobarometry yielded P-T conditions of 700~706 °C and 6.0~6.2 kbar for prograde stage M1, 854~920 °C and 13.0~13.8 kbar for high-pressure granulite facies stage M2, 912~939 °C and 8.1~9.9 kbar for M3, 661~784 °C and 3.1~4.4 kbar for M4-1, and 637~638 °C, 1.1~1.3 kbar for M4-2, along a clockwise P-T path with a nearly isothermal decompression (ITD) and slight heating. Zircon LA-ICP-MS U-Pb dating constrains the timing of the high-pressure granulite facies metamorphic event to be between 1.83 and 1.86 Ga. Geochemical features suggest that the protoliths of the high-pressure granulites may have formed in an island arc environment within a convergent margin setting. Together with results from previous studies, our data suggest that the ~1.85 Ga metamorphic age recorded in the Fuping Complex represents a regional metamorphism in the TNCO, associated with the subduction–collision and assembly of the Eastern and Western Blocks of the NCC.

Mineralogy and Ore Characteristics of the Kujang Pb-Zn Skarn Deposit, Sukabumi Regency, West Java

The Kujang Pb-Zn skarn deposit is known to be the newest skarn deposit found in the Sunda-Banda magmatic arc. The skarn orebodies are mostly hosted by limestone which might be the part of the Jampang Formation where dacite porphyry is interpreted to be the ore causative intrusion. Orebodies are mostly found at the contact between marbleized limestone and volcanic rocks and are controlled by NW-SE-trending strike-slip faults. Previous research on the mineralogy and ore characteristics of the Kujang Prospect remains limited on the preliminary studies. Using fieldwork data, petrography, ore microscopy, and assay data from core samples, this study is aimed to characterize the mineralogy of both alteration and ore of the deposit. The alteration of the Kujang Prospect skarn can be divided into 2 phases of alteration i.e. prograde and retrograde. The prograde alteration is characterized by the occurrence of clinopyroxene, wollastonite, and vesuvianite. Garnet is present in very rare amounts. the retrograde alteration is typified by epidote, chlorite, calcite and actinolite. Metalliferous minerals are represented by sphalerite, galena, pyrrhotite, chalcopyrite and pyrite. All ore minerals are formed at the early retrograde stage. the deposit’s average grade is 2.06% Pb, 6.45% Zn and 1.81% Cu.

Overlapping events in the Acoculco geothermal system, Puebla, Mexico

The Acoculco geothermal system records two major hydrothermal events (prograde skarn overlapped by retrograde/”epithermal” to geothermal stages) in association with pre-calderic to calderic events in the eponymous complex, which belongs to the Trans-Mexican Volcanic Belt. The prograde skarn is associated with a hornblende granite (hereby dated at 1.01 ± 0.01 Ma; U-Pb zircon age), whose apex is found at a ∼1600 m depth, and later andesite dykes and sills. Mineralizing fluids in this stage have salinities of fluid inclusions that range between 9.5 wt.% NaCl equiv. and 33 wt.% NaCl and temperatures of homogenization (Th) up to 470 °C that record various process of mineral formation, such as boiling, depressurization, conductive cooling, isothermal mixing, and dilution. The retrograde stage is represented by early “fossil” brines with salinities 1.4 and 7.3 wt.% NaCl equiv. and Th up to 303 °C, and present-day gasothermal fluids that attain stabilized temperatures up to ∼300 °C at a depth of almost 2000 m. Th of early brines are consistently higher than stabilized temperatures at most of the depths that were surveyed, the isotherms that correspond to 250° and 200 °C dropped ca. 500 m from the early retrograde to the gasothermal stage. These features support the hypothesis that the Acoculco geothermal system has entered its senile stage. Despite that, upon a calculated geothermal gradient at ∼150 °C/km for both Th of the early retrograde stage and stabilized temperatures in the EAC-1 well, it is not foreseen a rapid waning of the thermal anomaly and its ultimate cause.Besides the early Ca-silicate assemblages that constitute the skarn, the architecture of alteration assemblages also records the overlapping of shallow on deep hypogene assemblages. The deep hypogene assemblages respond to alkaline to moderately acidic fluids that correspond to the early retrograde (or “epithermal”) stage: propylitic, phyllic and argillic (kaolinite + nontronite + montmorillonite), from bottom to top. Contrastingly, the shallow hypogene assemblages respond to dominantly acidic fluids, and consist of advanced argillic assemblages that may grade upwards into argillic (kaolinite + illite-smectite) or silicic (vuggy or massive to brecciated silica). The shallow hypogene character of advanced argillic assemblages, as due to steam-heated ground environments (as opposed to deep hypogene or magmatic-hydrothermal assemblages), is deduced from (1) the overall tabular shape of their distribution, (2) the mineralogical association and characteristics of rhombohedral alunite within it, and (3) its relationship with overlying silicic alteration that, in turn, do not possess likely characteristics of sinters. Shallow hypogene advanced argillic assemblages are associated with the present-day gasothermal activity due to sealing that was possibly promoted during the early retrograde/”epithermal” activity.

Evolution of Ore-Forming Fluids at Azegour Mo-Cu-W Skarn Deposit, Western High Atlas, Morocco: Evidence from Mineral Chemistry and Fluid Inclusions

The Azegour Mo-Cu-W skarn deposit, located on the northern side of the Western High Atlas, occurs in lower Cambrian volcanic and sedimentary rocks. The mineralizations are linked to the hydrothermal alterations that affected carbonated layers of the lower Cambrian age during the intrusion of the calc-alkaline hyperaluminous Azegour granite. Four stages of the skarn and ore mineral deposition have been identified as follows. Firstly, (i) the early prograde stage and (ii) the late prograde stage. These prograde stages are characterized by anhydrous minerals (wollastonite, garnets, and pyroxenes) associated with scheelite mineralization. Based on mineral chemistry studies, the early prograde stage is dominated by andradite (Ad72.81–97.07) and diopside (Di61.80–50.08) indicating an oxidized skarn; on the other hand, the late prograde stage is characterized by a high portion of grossular (Gr66.88–93.72) and hedenbergite (Hd50.49–86.73) with a small ratio of almandine (Alm2.84–34.99), indicating “strongly reduced” or “moderately reduced” conditions with low f(O2). The next two stages are (iii) the early retrograde stage and (iv) the late retrograde stage, which contain hydrous minerals (vesuvianite, epidote, chlorite, muscovite, and amphibole) associated with sulfide. Fluid inclusions from pyroxene and quartz (prograde skarn stage) display high homogenization temperatures and high to low salinities (468.3 to >600 °C; 2.1 to >73.9 wt% NaCl equiv.). The boiling process formed major scheelite mineralization during prograde skarn development from dominated hydrothermal magmatic fluid solutions. By contrast, fluid inclusions associated with calcite–quartz–sulfide (retrograde skarn stage) record lower homogenization temperatures and low salinities (160 to 358 °C; 2.0 to 11.9 wt% NaCl equiv.). The distribution of the major inclusions types from the two paragenetic stages are along the trend line of fluids mixing in the salinity–homogenization temperature (magmatic water), illustrating the genesis of ore-forming fluid by mixing with fluids of low temperatures and salinities (metamorphic and meteoric waters).

Ultrahigh-pressure metamorphism and P-T-t path of the Yangkou eclogites and granitic gneisses from the central Sulu orogenic belt, eastern China, based on phase equilibria modelling

ABSTRACT Coesite-bearing eclogites and garnet-bearing granitic gneisses from the Yangkou area in the middle portion of Sulu ultrahigh pressure (UHP) metamorphic belt records three stages of metamorphism: I) Prograde to peak stage, II) Early retrograde stage, and III) Late retrograde stage. The prograde stage is recorded in the core to mantle in garnet and Si (3.38–3.43) in phengite in eclogite that has the assemblage of Grt-Omp-Amp-Phg-Lws (+ Qtz + Rt + H2O), and the P-T conditions are constrained at 25–27 kbar and 585–595°C. Stage II is constrained by Xgr (0.48–0.55) in garnet from mantle to rim, XAn (0.009–0.011) from mantle to rim in plagioclase of granitic gneiss, and the P–T conditions are estimated to be 29–35 kbar and 865–990°C with the mineral assemblage of Grt-Bt-Pl-Phg-Liq (+ Qtz/Coe). Stage III is constrained by two P-T ranges: mineral assemblage of Grt-Ep-Bt-Pl-Phg-Liq (+ Qtz) and Xgr (0.45–0.72) from mantle to rim in garnet in granitic gneiss constrained an earlier P-T range of 22–23 kbar and 800–820°C, while Xgr (0.34–0.39) and Xpy (0.16–0.18) from mantle to rim in garnet in eclogite in the assemblage of Grt-Omp-Amp-Phg-LL (+ Qtz + Rt + H2O) constrained a later P-T range of 17–18 kbar and 675–690°C. The formation and evolution of these assemblages have important implications for the generation and differentiation of continental crust through the operation of plate tectonics. The data suggest that both mafic and granitic rocks in the Yangkou area subducted to a depth of more than 115 km and underwent UHP metamorphism. The three stages of metamorphism in the study area are consistent with the regional well-established metamorphic evolution history, from UHP eclogite facies, through HP/HT eclogite facies, to amphibole eclogite facies with the timing of 230, 225, and 195 Ma, respectively.

Formation and evolution of the calcic-magnesian Saheb Fe (Cu) skarn deposit from the Sanandaj-Sirjan Belt, NW Iran: Evidence for multistage boiling in episodes of magnetite saturation

The Saheb Fe (Cu) skarn deposit is located along the northwestern end of the Sanandaj-Sirjan Zone. This deposit occurs over an approximately 2 km distance along the contact zone between the Upper Cretaceous causative granite and the Upper Permian calcareous host rock, with several iron orebodies occurring as high-grade lenticular massive magnetite bodies (with a length of up to 90 m and a width of 25 m).Following the formation of pyroxene hornfels facies, four main stages of skarn formation have been recognized: (I) early prograde stage (type I garnet + clinopyroxene + olivine): fluid inclusions in the type I garnet yielded a high temperature and salinity (average of 506 °C and 57 wt% NaCl equiv.), which may represent the composition of initial magmatic fluids. Higher salinities at this early stage can be associated with orthomagmatic differential fluid exsolution. (II) late prograde stage (type II and III garnets ± clinopyroxene ± disseminated magnetite): the coexistence of vapour-rich and halite-bearing fluid inclusions in type II garnet and their similar homogenization temperatures (420°–480 °C) presumably signify that fluid immiscibility from a supercritical fluid occurred at lithostatic pressures of 300 to 500 bar, corresponding to a depth of 1.5–2.0 km. (III) early retrograde stage (massive magnetite + pyrite + chalcopyrite + epidote + quartz + type I calcite + amphibole + serpentine + talc): hypersaline liquid and low-salinity vapour phases trapped within the epidote, quartz, and type I calcite are coexisting, indicating phase separation of hydrothermal fluids at temperatures of 300°–400 °C and pressures ranging from 310 to >100 bar under hydrostatic conditions, equivalent to a depth of 1.5 to 2.0 km. Hypersaline inclusions in the quartz and type I calcite show Td(NaCl) > Th(L-V), deduced to have been trapped by the over-saturated fluids or under over-pressured conditions before the hydrofracturing (up to 2200 bars). (IV) Late retrograde stage (type II calcite ± quartz ± chlorite), which is characterized by moderate temperature and salinity and non-boiling fluid inclusion assemblage (with average of 172 °C and 17 wt% NaCl equiv.).The first boiling event is documented in the type II garnet of the late prograde stage and provoked the ore-bearing fluids to precipitate the disseminated iron ore. The main deposition of magnetite has plausibly been triggered by the second episode of fluid boiling, recorded in epidote, quartz, and type I calcite during the early retrograde stage. Two boiling events mean producing more fractured and brecciated zones, which can act as conduits for the iron-bearing hydrothermal fluids. This seems to be why there are dozens of isolated magnetite orebodies in the Saheb skarn deposit, which extend over an area of greater than two kilometers (in an E-W direction). Comparably higher salinities even in the type II calcite of the late retrograde stage (25 wt% NaCl equiv.) may reveal that significant mixing of saline magmatic fluids with external, dilute, and cold fluids is almost implausible, despite the presence of brecciated zones. Boiling, therefore, can be pondered as the most salient factor in controlling the ore precipitation at the Saheb Fe (Cu) skarn deposit. However, mixing with meteoric fluids and shifting in pH values could also facilitate the alteration and mineralization.

Mineralogy and fluid characteristics of the Waschleithe Zn skarn – a distal part of the Schwarzenberg mineral system, Erzgebirge, Germany

The Waschleithe skarn is situated in the northern sector of the Schwarzenberg district in the western Erzgebirge (Germany), a district which hosts several large polymetallic skarn deposits. The Waschleithe is a Zn-dominant skarn comprising abundant dark Mn-rich hedenbergite, minor grandite garnet and magnetite related to the prograde stage as well as epidote, quartz, fluorite, amphibole, chlorite, ilvaite and Mn-rich pyroxenoids related to the retrograde stage of skarn formation. Ore minerals are related to the early retrograde stage and include sphalerite, galena, chalcopyrite, pyrite as well as minor amounts of scheelite and cassiterite. Fluid inclusion homogenization temperatures range from ~385 to 360 °C and from ~320 to 285 °C for the prograde and retrograde skarn stage, respectively. Thus, late-stage cooling is proposed as the major controlling factor for sulfide mineral precipitation. Fluid salinities for both stages are very low (<3% eq. w[NaCl]) and all observed fluid inclusion assemblages show homogeneous liquid–vapor ratios. Formation pressures >25 MPa suggest a minimum depth of formation of approximately 3 km. Oxygen fugacity during prograde skarn formation was initially low and increased towards the late prograde stage. The late-Variscan Eibenstock granite is identified as the most likely source of the skarn-forming fluids. The recognition of the Waschleithe skarn as a distal Zn skarn has major implications for the understanding of the Schwarzenberg district mineral system and provides valuable information for exploration targeting.

Magmatic–hydrothermal evolution of the Anjerd Cu skarn deposit, NW Iran: Perspectives on mineral chemistry, fluid inclusions and stable isotopes

The Anjerd Cu skarn deposit is located 40 km north of Ahar, NW Iran, which comprises three different stages of skarn–type mineralization and metasomatic alteration including (i) prograde, (ii) early retrograde and (iii) late retrograde. Prograde stage is identified by garnet as andradite–grossularite and diopsidic pyroxene crystallization, whereas retrograde stage overprinted the prograde skarn and is predominantly composed of tremolite, actinolite, quartz, epidote and chlorite. Metasomatic alteration started with generation and evolution of the mineralizing fluids in the Shayvar batholith, followed by the fluids ascending into fractures of the skarn in the metamorphic zone. Subsequently, retrograde alteration continued to the end of the skarnification process. Based on mineral chemistry studies, garnet is dominated by andradite (27–78 mol. %) and grossularite (20–67 mol. %) compositions and pyroxenes by diopside (65–98 mol. %) and hedenbergite (2–34 mol. %) compositions. Based on detailed petrography studies, three types of fluid inclusions were recognized including two–phase liquid–rich (LV), vapor–rich (VL), and three–phase (LVS) inclusions. The characteristic of the mineralizing fluids during the skarn evolution was changed significantly from early retrograde stage with (i) high temperature (up to 460 °C) and high salinity (up to 39 wt% NaCl equiv.), through late retrograde stage with (ii) moderate to high temperature (334 to 446 °C) and low salinity (0.3 to 0.8 wt% NaCl equiv.), (iii) low to moderate temperature (158 to 362 °C) and moderate salinity (1.7 to 12.3 wt% NaCl equiv.), suggesting that high–temperature and moderately saline fluids are indicative of a magmatic source within the minerals belonging to the prograde skarn, while the fluids with lower temperature and low to moderate salinity indicating a meteoric source for the retrograde skarn assemblage. The δ18O values for prograde garnet and magnetite vary from ∑0 to 8.2‰, whereas δ18O and δD values for retrograde calc–silicates in equilibrium with fluid including epidote range from 6.7 to 9.2‰ and −10.2 to −31.4 and for chlorite range from 1.9 to 2.7‰ and −37.5 to −48.1‰, respectively, pointing out that mixing of magmatic fluids with meteoric water in the retrograde alteration. Moreover, the δ34S values of pyrite and chalcopyrite (−3.6 and +4.9‰), suggesting a magmatic source for sulfur which was largely responsible for the mineralization in the Anjerd deposit.

Ilvaite as a thermodynamic recorder of multistage retrograde alteration in large Galinge skarn Fe deposit, western China

The ilvaite-bearing skarn associations in the Galinge skarn deposit were studied to determine their physicochemical formation conditions. A thermodynamic model setting pressure of 50 MPa (Pf=Ps=50 MPa) was set up to trace the skarn evolution. Petrographic evidence for replacement of garnet and magnetite by ilvaite in the early retrograde stage (Stage I) combined with thermodynamic modeling suggests that the alteration may have occurred at 400–470 °C under moderately high fO2 with ΔlgfO2(HM) ranges from −4 to −4.2. The model is based on a maximum pressure of 50 MPa calculated from magmatic amphibole geobarometer. The continuous breakdown of ilvaite with quartz to form ferro-actinolite and magnetite occur in the late retrograde stage (Stage II). The reactions occurred at 400-440°C under moderate fO2 (ΔlgfO2(HM): −4 to −4.4). In Stage III, the breakdown of ilvaite to form calcite, pyrite and ferroactinolite depends on XCO2 which can be estimated to be in a range of 0.005 to 0.05, and the reaction would occur at higher temperatures with increasing XCO2. Under these conditions, the breakdown occurs at 270–350 °C and low fO2 (ΔlgfO2(HM): up to −5.2). The thermodynamic model for continuous evolution from Stage I to Stage III completely records the conditions of the retrograde alteration, which is inconsistent with the thermobarometry imprints of fluid inclusions. Therefore, the petrography and phase relations of ilvaite are useful indicators of reaction conditions in various skarn deposit types.

Thermal history of Early Jurassic eclogite facies metamorphism in the Nagaland Ophiolite Complex, NE India: New insights into pre-Cretaceous subduction channel tectonics within the Neo-Tethys

Eclogite from two locations in a fossil accretionary complex now exposed in Nagaland, NE India, at the northern end of the Indo-Myanmar ranges, provides the oldest evidence for Neo-Tethyan subduction along the Yarlung-Tsangpo suture zone. Metamorphic texture, mineral composition, peak metamorphic P-T estimates, reconstructed metamorphic P-T paths, and U–Pb zircon ages from three eclogite samples collected near Mokie and Thewati villages collectively reveal snapshots of a c. 205–172 Ma subduction burial-exhumation cycle in the eastern Neo-Tethys. The Thewati eclogite records a clockwise metamorphic P-T path involving: (1) prograde metamorphism traversing the epidote blueschist facies and culminated in peak eclogite facies conditions at 25–28 kbar and ~650 °C; (2) an early retrograde stage involving decompression with cooling in the eclogite facies at ~18.3 kbar and 630 °C; followed by (3) cooling through the epidote blueschist, transitional lawsonite blueschist and greenschist facies to ~6 kbar and 300 °C. Circa 189–185 Ma peak metamorphism is inferred for the Mokie and Thewati eclogites, and was accompanied by an apparently low thermal gradient (~7–8 °C/km). Such conditions and the clockwise P-T path are associated with a tectonic scenario involving cold mature subduction within the Neo-Tethys in Nagaland. This is thus the first comprehensive dataset of an Early Jurassic subduction channel (with respect to present geographic co-ordinates) for the eastern margin of the Neo-Tethyan suture. The Nagaland samples offer a critical record of a pre-Cretaceous subduction system within the Neo-Tethys, which incorporated an Early-Middle Jurassic Andean-type convergent margin extending along the southern Eurasian margin from Pakistan to the Indo-Myanmar Ranges.

Multiple-stage tungsten mineralization in the Silurian Jiepai W skarn deposit, South China: Insights from cathodoluminescence images, trace elements, and fluid inclusions of scheelite

The Jiepai deposit, located in the northeastern part of the Miao’ershan-Yuechengling batholith, South China, is characterized by predominant skarn mineralization that formed near the contact between the Silurian biotite granites and the Upper Cambrian carbonate rocks. Jiepai scheelites are recognized as two main generations based on their occurrences, cathodoluminescence (CL) signatures, and REE systematics. Prograde skarn-hosted scheelite I has uniformly dark CL characteristics and LREE-enriched patterns. Retrograde skarn-hosted scheelite IIa is homogeneously dark under CL, and has a slightly MREE-enriched pattern, whereas quartz vein-hosted scheelite IIb is delineated by fine oscillatory CL zones and a relatively flat REE pattern. Additionally, the CL images reveal that both scheelite I and II were modified by later-stage hydrothermal overprints, which occur as ubiquitous overgrowth rims and cutting veins with bright CL signatures, low REE, but similar REE patterns to their precursors. The compositional variations and fluid inclusions in Jiepai scheelites identify episodic pulses of hydrothermal fluids with different properties. The ore-forming fluids with high Nb/Ta, but low Nb, Ta and REY in the early retrograde stage are featured by moderate to high temperature and low to moderate salinity. Under relatively oxidizing conditions, they interacted with the host rock and underwent boiling to precipitate scheelite I. By contrast, the fluids with higher Nb, Ta and REY, but lower Nb/Ta in the late retrograde stage have slightly lower temperature and higher salinity. We therefore speculate that scheelite II may deposit from later magmatic fluids that have experienced continued boiling in more reducing environment.

Metamorphic evolution of a newly identified Mesoproterozoic oceanic slice in the Yuka terrane and its implications for a multi‐cyclic orogenic history of the North Qaidam UHPM belt

AbstractThe North Qaidam Orogenic Belt (NQOB), lying at the northern margin of the Tibet Plateau, records two orogenic cycles: A Proterozoic cycle related to the amalgamation and breakup of the supercontinent Rodinia, and an Early Palaeozoic cycle including oceanic subduction and continental deep subduction. At present, the only information about the Proterozoic cycle is the concurrent c. 1,000–900 Ma magmatic and metamorphic events, which limited the understanding of the Proterozoic evolution of NQOB and the relationship between the Qaidam Block and other Rodinia fragments. In this study, a kyanite‐bearing eclogite was identified in Yuka terrane. It has positive‐slope chondrite‐normalized rare earth element distribution patterns, similar to present‐day N‐MORB. LA–ICP–MS zircon U–Pb dating obtained a protolith age of 1,273 Ma and an eclogite facies metamorphic age of 437 Ma, which is similar to the continental deep subduction age of the Yuka terrane. Zircon Lu–Hf analysis show that the magmatic zircon cores have high εHf(t) of 8.36–15.98 and TDM1 of 1,450–1,131 Ma (M = 1,303 ± 55 Ma, consistent with its protolith age within error), indicating a juvenile crust protolith of the eclogite. The MORB‐like whole‐rock composition and zircon U–Pb and Lu–Hf analysis indicate that the protolith of the kyanite‐bearing eclogite was a Mesoproterozoic oceanic slice. P–T pseudosection analysis shows that the kyanite‐bearing eclogite experienced four metamorphic stages: (1) a prograde stage with the assemblage garnet+omphacite+talc+lawsonite+phengite+quartz at 22.4–23.2 kbar and 585°C; (2) a peak stage with the assemblage garnet+omphacite+lawsonite+phengite+coesite at 32.5 kbar and 670°C; (3) an early retrograde stage with the assemblage garnet+omphacite+kyanite+phengite+coesite/quartz±lawsonite at 27.1–30.0 kbar and 670–690°C; and (4) a late retrograde stage with the assemblage garnet+omphacite+epidote+hornblende+phengite+quartz at &lt;18.0 kbar. The established clockwise P–T path is similar with other continental‐type eclogites in this area. On the basis of the geochemical and geochronological data, as well as the P–T path, we suggest that the protolith of the kyanite‐bearing eclogite was emplaced in the active margin of the Qaidam Block during the assembly of Rodinia and underwent continental deep subduction in the Early Palaeozoic. We conclude that (1) the Qaidam Block participated in the assembly of the Rodinia supercontinent. It was situated at or proximal to the margin of the supercontinent and probably close to India, east Antarctica and Tarim; and (2) both Mesoproterozoic and Early Palaeozoic oceanic crust slices occur in the NQOB. Thus, special caution is needed when using the metamorphic ages of oceanic affinity eclogites without protolith ages to constrain the evolution history of the North Qaidam UHPM belt.

Geochemistry of magnetite from theHuanggangskarn iron–tin polymetallic deposit in the southernGreat Xing'an Range, NE China

The Huanggang iron–tin polymetallic skarn deposit is located in the southern Great Xing'an Range. According to the ore types and mineral assemblages, the paragenetic sequence of the Huanggang deposit can be divided into three stages, and these magnetite grains were mainly formed at the retrograde and sulphide skarn stages. The magnetite (sample HG12‐13) from the early retrograde stage is represented by fine‐grained magnetite cutting across coarse‐grained magnetite surrounded by quartz and calcite. The magnetite (sample HG12‐73) from the late retrograde stage is locally replaced by haematite along the margin or interior and is surrounded by calcite. The magnetite (sample HG12‐88) from the early sulphide stage is characterized by obvious core–rim textural features. The magnetite (sample HG‐62) from the late sulphide stage is featured by zone‐like magnetite occurring along the margin and interior of the primary magnetite.Laser ablation inductively coupled plasma mass spectrometry was used to obtain trace element concentrations of magnetite from the different mineralization stages in order to better understand the geochemical variations in the ore‐forming process. Some magnetite grains have abnormally high Mg, Al, K, Cu, Zn, and Sn due to the presence of numerous inclusions (e.g., chlorite, sylvite, chalcopyrite, sphalerite, and cassiterite). In general, magnetite grains from the different mineralization stages demonstrate similar bulk continental crust normalized trace elements patterns, suggesting that they share a similar origin. The increasing Si + Al/Mg + Mn ratios and decreasing Mg + Mn contents for magnetite show an increasing fluid–rock ratio from the retrograde to sulphide stage. Co contents of magnetite decrease abruptly from the retrograde stage to the sulphide stage, whereas Mn contents show the reverse trend, which is affected by minerals coprecipitating with magnetite. A zoned magnetite from the sulphide stage shows decreasing Ti and V contents from core to outer rim; the variation of Ti and V may be related to the temperature or oxygen fugacity. Magnetite compositions from the Huanggang deposit are similar to those from Fe, Cu‐polymetallic, or other skarn deposits, but display more variable compositions than previously presented. Our study demonstrates that the evolution of magnetite in the skarn deposit and trace element of magnetite could be a powerful tool in determining the origin of skarn iron deposit.

مطالعه کانی شناسی و فرآیندهای اسکارنی شدن در اسکارن مس- آهن آوان، شمالخاور خاروانا، شمال باختر ایران

اسکارن مس- آهن آوان در نتیجه نفوذ باتولیت گرانیتوئیدی قره داغ به‌ داخل سنگهای کربناته ناخالص کرتاسه بالایی تشکیل شده است. اسکارن آوان متشکل از پهنه های مرمر، اگزواسکارن و اندواسکارن بوده و بخش اصلی آن پهنه اگزواسکارن، است. بررسیهای کانی شناسی بیانگر آن است که پهنه اگزواسکارن شامل زیرپهنه های گارنت اسکارن، پیروکسن- گارنت اسکارن و کانه اسکارن است. مهمترین کانی کالک سیلیکاته بی آب در پهنه اگزواسکارن، گارنت و ترکیب آن اوگراندیتی (Ad53-89) است. برخی بلورهای گارنت، منطقه‌بندی دارد و ترکیب آنها به طرف حاشیه‌ها به آندرادیت خالص (Ad99) میل می‌کند. کلینوپیروکسن موجود در زیرپهنه پیروکسن- گارنت اسکارن از نوع دیوپسید (Di75-96) است. زیرپهنه کانه اسکارن از کانه‌های مگنتیت، هماتیت، پیریت، کالکوپیریت، بورنیت، مالاکیت و گوتیت تشکیل شده است. وجود میزان محدود ولاستونیت در مجموعه کانی شناسی اسکارن آوان، هم‌رشدی بلورهای گارنت و کلینوپیروکسن و نبود هاله واکنشی بین این دو کانی و نبود بافتهای جانشینی نشان‌دهنده آن است که این کانیها به‌صورت هم‌زمان در محدوده دمایی C°600- 430 و فوگاسیته اکسیژن بیشتر از 10-26 تشکیل شده‌اند.

Geology, fluid inclusion and stable isotopes (O, S) of the Hetaoping distal skarn Zn-Pb deposit, northern Baoshan block, SW China

The Hetaoping zinc–lead deposit is located in the northern Baoshan block, Sanjiang region, SW China. The ore deposit comprises massive orebodies in the lower part and lenticular and vein-like orebodies in the upper part, both of which are hosted in the marbleized Upper Cambrian limestone and slate of the Hetaoping Formation. Three mineralization stages of Hetaoping skarn system have been recognized based on petrographic observation, which are pre-ore stage (pyroxene–garnet–actinolite–epidote–magnetite), syn-ore stage (sulfides–quartz–calcite–fluorite), and post-ore stage (calcite–quartz–chlorite). Andradite and hedenbergite are dominant in pre-ore garnet and pyroxene, respectively. Ore minerals consist of mainly pyrite, sphalerite, chalcopyrite, bornite and galena. Three types of fluid inclusions have been identified in Hetaoping, including primary two-phase (A type), primary three-phase (B type) and secondary two-phase (C type) inclusions. Based on fluid inclusion microthermometric study, the fluids forming the Hetaoping skarn minerals and sulfides evolved from high-moderate temperature (255–498°C) and low-moderate salinity (5.0–18.0wt.% NaCl equiv) in pre-ore stage, through moderate-low temperature (152–325°C) and low salinity (0.4–14.2wt.% NaCl equiv) in syn-ore stage, to low temperature (109–205°C) and low salinity (0.9–10.0wt.% NaCl equiv) in post-ore stage. The sulfide δ34S values range from 3.7 to 7.1‰ (mean=5.2‰, n=29), indicative of a dominantly magmatic sulfur origin. Silicate and carbonate oxygen isotopes give calculated δ18OH2O ranges of 3.9–11.1‰ in prograde stage, −0.9 to 4.6‰ in early retrograde stage, and −1.3 to 2.9‰ in late retrograde stage (syn-ore stage), The oxygen isotope data reveal that the prograde fluid in Hetaoping could be primarily magmatic, which has been mixed significantly with meteoric water in the late retrograde stage. Such a fluid mixing process is considered to be a key factor controlling ore precipitation.

Ruby–sapphire–spinel mineralization in marble of the middle and southern Urals: Geology, mineralogy, and genesis

Ruby and spinel occurrences hosted in marble on the eastern slope of the Urals are considered. Ruby- and spinel-bearing marble is a specific rock in granite-gneiss complexes of the East Ural Megazone, which formed at the Late Paleozoic collision stage of the evolution of the Urals. Organogenic marine limestone is the protolith of the marble. No relict sedimentary bedding has been retained in the marble. The observed banding is a secondary phenomenon related to crystallization and is controlled by flow cleavage. Magnesian metasomatism of limestone with the formation of fine-grained dolomite enriched in Cr, V, Ti, Mn, Cu, Zn, Ga, and REE took place at the prograde stage of metamorphism. Dedolomitization of rocks with the formation of background calcite marble also developed at the prograde stage. Mg-calcite marble with spinel and ruby of the first type formed in the metamorphic fluid circulation zone. Magnesian metasomatism with the formation of bicarbonate marble with ruby, pink sapphire, and spinel of the second type developed at the early retrograde stage. The formation of mica-bearing mineralized zones with corundum and spinel of the third type controlled by cleavage fractures is related to the pneumatolytic–hydrothermal stage. The data on ruby-bearing marble in the Urals may be used for forecasting and prospecting of ruby and sapphire deposits hosted in marble worldwide.

Geology and characteristics of Pb-Zn-Cu-Ag skarn deposit at Ruwai, Lamandau Regency, Central Kalimantan

This study is dealing with geology and characteristics of mineralogy, geochemistry and physicochemical conditions of hydrothermal fluid responsible for the formation of skarn Pb-Zn-Cu-Ag deposit at Ruwai, Lamandau Regency, Central Kalimantan. The formation of Ruwai skarn is genetically associated with calcareous rocks consisting of limestone and siltstone (derived from marl?) and controlled by NNE-SSW-trending strike slip faults and localized along N 70° E-trending thrust fault, which also acts as contact zone between sedimentary and volcanic rocks in the area. Ruwai skarn is mineralogically characterized by prograde alteration (garnet and clino-pyroxene) and retrograde alteration (epidote, chlorite, calcite and sericite). Ore mineralization is characterized by sphalerite, galena, chalcopyrite and Ag-sulphides (particularly acanthite and argentite), which formed at early retrograde stage. Geochemically, SiO2 is enriched and CaO is depleted in limestone, consistent with silicic alteration (quartz and calc-silicate) and decarbonatization of the wallrock. The measured reserves of the deposit are 2,297,185 tonnes at average grades of 14.98 % Zn, 6.44 % Pb, 2.49 % Cu and 370.87 g/t Ag. Ruwai skarn orebody originated at moderate temperature of 250-266 °C and low salinity of 0.3-0.5 wt.% NaCl eq. The late retrograde stage formed at low temperature of 190-220 °C and low salinity of ~0.35 wt.% NaCl eq., which was influenced by meteoric water incursion at the late stage of the Ruwai Pb-Zn-Cu-Ag skarn formation.&#x0D; &#x0D; Keywords: Geology, skarn, mineralogy, geochemistry, Ruwai, Central Kalimantan

Geology and Characteristics of Pb-Zn-Cu-Ag Skarn Deposit at Ruwai, Lamandau Regency, Central Kalimantan

DOI: 10.17014/ijog.v6i4.126This study is dealing with geology and characteristics of mineralogy, geochemistry, and physicochemical conditions of hydrothermal fluid responsible for the formation of skarn Pb-Zn-Cu-Ag deposit at Ruwai, Lamandau Regency, Central Kalimantan. The formation of Ruwai skarn is genetically associated with calcareous rocks consisting of limestone and siltstone (derived from marl?) controlled by NNE-SSW-trending strike slip faults. It is localized along N 70° E-trending thrust fault, which also acts as the contact zone between sedimentary and volcanic rocks in the area. The Ruwai skarn is mineralogically characterized by prograde alteration comprising garnet (andradite) and clino-pyroxene (wollastonite), and retrograde alteration composed of epidote, chlorite, calcite, and sericite. Ore mineralization is typified by sphalerite, galena, and chalcopyrite, formed at early retrograde stage. Galena is typically enriched in silver up to 0.45 wt % and bismuth of about 1 wt %. No Ag-sulphides are identified within the ore body. Geochemically, SiO is enriched and CaO is depleted in limestone, consistent with silicic alteration (quartz and calc-silicate) and decarbonatization of the wallrock. The measured resources of the deposit are 2,297,185 tonnes at average grades of 14.98 % Zn, 6.44% Pb, 2.49 % Cu, and 370.87 g/t Ag. Ruwai skarn orebody was originated at moderate temperatures of 250 - 266 °C and low salinity of 0.3 - 0.5 wt.% NaCl eq. The late retrograde stage was formed at low temperature of 190 - 220 °C and low salinity of ~0.35 wt.% NaCl eq., which was influenced by meteoric water incursion at the late stage of the Ruwai Pb-Zn-Cu-Ag skarn formation.

Fluid inclusions in quartz related to subsequent stages of foliation development during a single metamorphic cycle (Schneeberg Fault Zone, Eastern Alps, Austria)

Macro- and micro-scale structures, fluid inclusions, quartz rheology and host mineral chemistry were used to establish the P– T– d evolutionary path of monometamorphic metapelites of the Schneeberg Fault Zone. Four selected quartz generations indicate formation conditions during stages of prograde and retrograde metamorphism. Earliest foliation development S1 is outlined by quartz foam microstructures that are armored within foliation-parallel garnet layers which reflect individual growth characteristics during the prograde and early retrograde stage. The prograde path is linked with garnet growth at low differential stress up to peak pressure conditions of ca. 10 kbar. Final garnet growth during increasing temperatures and WNW-directed shearing up to ≤ 600 °C predates exhumation along an isothermal decompressional path and increasing differential stress. Exhumation was accompanied by the formation of younger quartz generations and the formation of a second foliation S2 that was incorporated into intense folding. Minimum pressure condition for S2 is about 4.5 kbar resulting from the gradient of fluid inclusion density isochores estimated from S2-related pressure shadows in combination with the temperature range for dislocation creep in quartz aggregates between 500 and 600 °C. The dominance of carbonic fluid inclusions indicates decarbonation associated with H 2O leakage as a main process during Alpine monometamorphism in the metasedimentary host rocks. Aqueous fluid inclusions are arranged along late fluid planes and reflect a wide range in densities with variable salinities. This study corroborates the successful approach of combining mineral growth, fluid inclusions and macro- to micro-scale deformation stages to constrain a prograde to retrograde growth history of metamorphic minerals like garnet and quartz even though these underwent polyphase deformation and upper amphibolite facies metamorphism. It supports previous arguments about low differential stress at high-pressure conditions and allows speculations about the location of the metasediments of the Schneeberg Fault Zone nearby the low strength plate interface during late stage of Cretaceous subduction and subsequent early exhumation.

Magmatic-dominated fluid evolution in the Jurassic Nambija gold skarn deposits (southeastern Ecuador)

The Jurassic (approximately 145 Ma) Nambija oxidized gold skarns are hosted by the Triassic volcanosedimentary Piuntza unit in the sub-Andean zone of southeastern Ecuador. The skarns consist dominantly of granditic garnet (Ad20–98) with subordinate pyroxene (Di46–92Hd17–42Jo0–19) and epidote and are spatially associated with porphyritic quartz-diorite to granodiorite intrusions. Endoskarn is developed at the intrusion margins and grades inwards into a potassic alteration zone. Exoskarn has an outer K- and Na-enriched zone in the volcanosedimentary unit. Gold mineralization is associated with the weakly developed retrograde alteration of the exoskarn and occurs mainly in sulfide-poor vugs and milky quartz veins and veinlets in association with hematite. Fluid inclusion data for the main part of the prograde stage indicate the coexistence of high-temperature (500°C to >600°C), high-salinity (up to 65 wt.% eq. NaCl), and moderate- to low-salinity aqueous-carbonic fluids interpreted to have been trapped at pressures around 100–120 MPa, corresponding to about 4-km depth. Lower-temperature (510–300°C) and moderate- to low-salinity (23–2 wt.% eq. NaCl) aqueous fluids are recorded in garnet and epidote of the end of the prograde stage. The microthermometric data (Th from 513°C to 318°C and salinity from 1.0 to 23 wt.% eq. NaCl) and δ18O values between 6.2‰ and 11.5‰ for gold-bearing milky quartz from the retrograde stage suggest that the ore-forming fluid was dominantly magmatic. Pressures during the early retrograde stage were in the range of 50–100 MPa, in line with the evidence for CO2 effervescence and probable local boiling. The dominance of magmatic low-saline to moderately saline oxidizing fluids during the retrograde stage is consistent with the depth of the skarn system, which could have delayed the ingression of external fluids until relatively low temperatures were reached. The resulting low water-to-rock ratios explain the weak retrograde alteration and the compositional variability of chlorite, essentially controlled by host rock compositions. Gold was precipitated at this stage as a result of cooling and pH increase related to CO2 effervescence, which both result in destabilization of gold-bearing chloride complexes. Significant ingression of external fluids took place after gold deposition only, as recorded by δ18O values of 0.4‰ to 6.2‰ for fluids depositing quartz (below 350°C) in sulfide-rich barren veins. Low-temperature (<300°C) meteoric fluids (δ18Owater between −10.0‰ and −2.0‰) are responsible for the precipitation of late comb quartz and calcite in cavities and veins and indicate mixing with cooler fluids of higher salinities (about 100°C and 25 wt.% eq. NaCl). The latter are similar to low-temperature fluids (202–74.5°C) with δ18O values of −0.5‰ to 3.1‰ and salinities in the range of 21.1 to 17.3 wt.% eq. CaCl2, trapped in calcite of late veins and interpreted as basinal brines. Nambija represents a deep equivalent of the oxidized gold skarn class, the presence of CO2 in the fluids being partly a consequence of the relatively deep setting at about 4-km depth. As in other Au-bearing skarn deposits, not only the prograde stage but also the gold-precipitating retrograde stage is dominated by fluids of magmatic origin.