Subcalcic Garnets Research Articles

Thermobarometry of diamond inclusions: Mantle structure and evolution beneath Archean cratons and mobile belts worldwide

Thermobarometric calculations for mineral inclusions in diamonds provide a systematic comparison of P-T-fO2 conditions for different cratons and mobile belts worldwide, using a database of 4440 mineral EPMA analyses. Beneath all cratons, the cold branch of the mantle geotherm (35-32 mWm−2) relates to PT estimates for sub-calcic garnets and rare omphacitic diamond inclusions, referring to major continental growth events in Archean. High-temperature plume-related geotherms are common in Proterozoic kimberlites such as Premier, Mesozoic-Roberts Victor etc. and are common in Slave and Siberian cratons. Eclogitic and pyroxenitic pyroxenes and garnets and Fe- and Ca-enriched pyrope diamond inclusions prevail in mobile belts like Limpopo, Magondi, Ural, Khapchan and in the marginal parts of cratons (Kimberly in Australia) They refer to diamondiferous eclogite-pyroxenite lens in the mididdle parts of lithospheric mantle. Eclogitic garnets and clinopyroxenes show trends of joint decreasing Mg# and pressures due to differentiation of rising melts. Dunitic Ca pyropes giving high-pressure estimates prevail in the central parts of cratons. The PT for the chromites reflect conditions just above the lithosphere-asthenosphere boundary formed due to interaction with the hydrous plume protokimberlite melts. Archean diamond Ca-enrich pyropes inclusions from Wawa province Canada give low-temperature conditions. Inclusions from younger kimberlites in Superior and Slave, Siberian and East European cratons show complex high-temperature geotherms due to plumes influence. Peridotite garnets beneath the Amazonian craton indicate complex layering in the lithosphere base and a pyroxene layer in the middle part of mantle lithosphere. Diamond inclusions from the Kimberley Craton of Australia show the greatest variations in the temperatures and composition. The mobile belts and marginal parts of cratons are favorable for the finding of the abundant colored diamonds. The huge plume influence for hydrated and eclogite-rich mantle like in Premier, Venetia and other kimberlites brings to the formation of giant diamonds. Diamond inclusion reveal the huge variations of PT conditions formed by different agents due to long complex stories of craton deciphered using thermobarometry and trace element and isotopic geochemistry.

Helvine Mineralization in Devonian-Carboniferous Skarns of the Oriental Pampean Ranges, Córdoba, Argentina: Mineralogy and Nature of Ore-Forming Fluids

AbstractHelvine [Be3Mn2+4(SiO4)3S] occurs in less than one meter-sized spotty concentrations in some calcic skarns of Córdoba province, Argentina. The local geology, mineral chemistry, paragenetic mineral associations, and the evolution of helvine mineralizing fluids were studied from two selected localities (i.e., the Chingolo scheelite mine and Casa la Plata). Helvine from the Chingolo scheelite mine occurs in idiomorphic crystals up to 15 cm long, partially intergrown with spessartine-rich garnet, and in part or totally included in vug-filling spar calcite in the prograde garnet-vesuvianite skarn zone developed between replaced aplite-pegmatite dikes and calcic phlogopite-bearing marbles. Helvine from Casa la Plata occurs abundantly in vein-like, fluorite-rich garnet-vesuvianite skarn associations, where tetrahedrons up to 2 cm long occur preferentially included in fluorite in an illite-sericite-chlorite strongly replaced schist. The composition of helvine from both localities does not differ from other worldwide known compositions. Associated garnet from both localities is enriched in molar subcalcic garnet, largely as spessartine. Clinochlore is a conspicuous phase that occurs both as late infills and/or replacing phases in both the Chingolo scheelite mine and Casa la Plata. Primary fluid inclusions from helvine of both localities suggest that helvine deposited from moderate to high temperature and salinity aqueous fluids of likely magmatic origin. In both localities a late influx of gas-rich, CO2-bearing, moderate temperature, and moderate to low salinity fluid was trapped as secondary fluid inclusions. At the Chingolo scheelite mine, CO2-bearing fluids likely originated from decarbonation along the skarnification reaction front distanced from the prograde zone at lower temperature. In both studied areas, the latest trapped secondary fluids were of lower temperature and lower salinity. Pressure-corrected homogenization temperatures between 1.8 and 2.0 kbar sustain trapping temperatures for primary fluid inclusions within the 510–610 °C range for both localities. In the Chingolo scheelite mine, helvine formed at the contact zone between Ordovician aplite-pegmatites and Cambrian marbles that differentially reacted with infiltrated distal metasomatic-hydrothermal Be-bearing fluids fractionated from evolved granitic facies and/or pegmatites of the neighboring Devonian to Carboniferous Achala Batholith. Calculated δ18OH2O in equilibrium with garnet at ∼575 °C yielded δ18OH2O = 11.1 or 12.2‰. These heavy δ18OH2O values may derive from a magmatic fluid source possibly enriched in 18O from marbles or any other metasedimentary country rock during skarnification. This interpretation is supported by δ18OH2O values (8.3 to 10.1‰) and heavy δD values (−21.6 to −20.6‰) from retrograde epidote of other neighboring scheelite mines related to the Chingolo scheelite mine. The presence of beryl partially replaced by bertrandite + K-feldspar, non-paragenetically associated with helvine, confirms that early prograde crystallization conditions switched from a high temperature, subaluminous environment to a lower temperature, aluminous and increasingly acidic environment toward the retrograde skarn stage. At Casa la Plata, Cambrian schists and marble-amphibolite were also skarnified after the circulation of Be-bearing fluids derived from an epizonal Carboniferous granite (Capilla del Monte pluton) and its pegmatite dike swarm, which share parental links with the Achalian magmatism. The major Be supply for both studied localities should be attributed to Devonian and Carboniferous postorogenic to extensional peraluminous A-type granitic magmatism, which is the major source of beryllium not only for the Pampean Ranges but for the whole country.

Occurrence of dravitic tourmaline in a diamond-bearing breccia: a possible lamproite deposit in the Alto Paranaíba Igneous Province

The mineral chemistry of diamond indicator minerals is used to trace mineralizations in primary rocks such as lamproites/kimberlites and in secondary deposits. Subcalcic garnets and magnesian ilmenites are both minerals associated with the diamond potential described in many deposits in the Alto Paranaíba Igneous Province where several kimberlitic pipes and alluvial deposits occur. Our study was conducted on the Romaria diamond deposit of (Brazil), where garnet, ilmenite and tourmaline grains were recovered from a diamon-bearing breccia were analyzed under electron microprobe. The garnet composition is compatible with G10, G9 and G5 types. Ilmenite grains are Mg- to Mn-rich. Nearly 80% of the tourmaline grains are dravite-type. This association of dravite tourmaline with diamond in rocks of lamproitic affinity may be common, as evidenced in the Argyle, Ellendale, Prairie Creek, Sask, Presidente Olegário and Ymi-1 pipes. We therefore suggest a possible lamproite origin for the studied diamond deposit.

Mineral Chemistry and U-Pb Garnet Geochronology of Strongly Reduced Tungsten Skarns at the Pampa de Olaen Mining district, Córdoba, Argentina

Los Guindos scheelite (±Zn, Bi, Sn, Ag) skarn presents mineral assemblages and a mineral chemistry similar to other worldwide strongly reduced W skarn deposits. Its reduced nature is defined based on the predominance of subcalcic garnets, Mo-free scheelite and absence of magnetite. Both the prograde and retrograde stages are evident at Los Guindos scheelite skarn. The prograde skarn is characterized by three zones: A zone I of garnet + helvine (Gr57Sps24Ad19Alm8 - Sps50Alm24Gr22Ad3; Grt + Hlv); a zone II of clinopyroxene + garnet (Di67Hd24Jo9 + Gr66Sps19Ad12Alm3; Cpx + Grt) and a zone III of garnet + vesuvianite (Gr73Ad22Sps3Alm2 - Gr58Sps22Ad10Alm9; Grt + Ves). Retrograde skarn is mainly represented by epidote - actinolite and minor F-rich actinolite (0.663 apfu of F) – potassium feldspar - chlorite (chamosite/clinochlore: ∼ 50/50) – muscovite – calcite - quartz. A hydrothermal stage developed in temporal continuity with retrograde skarn formed variable infilling associations of the following species: epidote – actinolite – scheelite – fluorite – calcite – quartz – sphalerite and chlorite. Scheelite mineralization process was triggered by an increase of Ca released during retrograde skarn replacements and was deposited during the following hydrothermal infilling stage. Other than sphalerite, minor bismuthinite and tetradymite, andorite, lillianite, gustavite, matildite and kësterite occur as hydrothermal associations after scheelite deposition. Scheelite-free reaction skarn preceding scheelite skarns was observed. Geobarometric calculations in this reaction skarn suggests an initial confining pressure of 2.5 kbar for the Los Guindos scheelite skarns. This pressure matches the estimated emplacement pressure of the Devonian-Carboniferous Achala batholith reported by previous authors. Geochemical correlation analyses suggest that this magmatism may have contributed mineralizing fluids channeled through regional structures and lithological contacts, causing infiltration metasomatism that originated scheelite (±Zn, Bi, Sn, Ag) mineralization in Cambrian and Ordovician country rocks. U-Pb analyses (LA-ICP-MS) of garnet in the Los Guindos scheelite skarn gave an age of 361 ± 11 Ma representing the age of the prograde stage of scheelite skarns and it should be framed within the Devonian-Carboniferous Metallogenic Epoch.

Two billion years of episodic and simultaneous websteritic and eclogitic diamond formation beneath the Orapa kimberlite cluster, Botswana

The Sm–Nd isotope systematics and geochemistry of eclogitic, websteritic and peridotitic garnet and clinopyroxene inclusions together with characteristics of their corresponding diamond hosts are presented for the Letlhakane mine, Botswana. These data are supplemented with new inclusion data from the nearby (20–30 km) Orapa and Damtshaa mines to evaluate the nature and scale of diamond-forming processes beneath the NW part of the Kalahari Craton and to provide insight into the evolution of the deep carbon cycle. The Sm–Nd isotope compositions of the diamond inclusions indicate five well-defined, discrete eclogitic and websteritic diamond-forming events in the Orapa kimberlite cluster at 220 ± 80 Ma, 746 ± 100 Ma, 1110 ± 64 Ma, 1698 ± 280 Ma and 2341 ± 21 Ma. In addition, two poorly constrained events suggest ancient eclogitic (> 2700 Ma) and recent eclogitic and websteritic diamond formation (< 140 Ma). Together with sub-calcic garnets from two harzburgitic diamonds that have Archaean Nd mantle model ages (TCHUR) between 2.86 and 3.38 Ga, the diamonds studied here span almost the entire temporal evolution of the SCLM of the Kalahari Craton. The new data demonstrate, for the first time, that diamond formation occurs simultaneously and episodically in different parageneses, reflecting metasomatism of the compositionally heterogeneous SCLM beneath the area (~ 200 km2). Diamond formation can be directly related to major tectono-magmatic events that impacted the Kalahari Craton such as crustal accretion, continental breakup and large igneous provinces. Compositions of dated inclusions, in combination with marked variations in the carbon and nitrogen isotope compositions of the host diamonds, record mixing arrays between a minimum of three components (A: peridotitic mantle; B: eclogites dominated by mafic material; C: eclogites that include recycled sedimentary material). Diamond formation appears dominated by local fluid–rock interactions involving different protoliths in the SCLM. Redistribution of carbon during fluid–rock interactions generally masks any potential temporal changes of the deep carbon cycle.

Kelyphite Rims on Garnets of Contrast Parageneses in Mantle Xenoliths from the Udachnaya-East Kimberlite Pipe (Yakutia)

A new classification of kelyphitic rims on garnets from xenoliths of peridotitic and eclogitic parageneses of the mantle section under the Udachnaya-East kimberlite pipe (Yakutia) is presented. Five types of rims are identified: Rim1 develops between garnet and olivine/pyroxene (or rim2) and is composed of high-alumina pyroxenes, spinel, phlogopite; rim2, the coarse grain part of rim1, is located between rim1 and olivine/pyroxene, and mainly consists of phlogopite and less aluminous larger pyroxenes and spinel; rim3 develops between garnet and kimberlite, and presents with phlogopite and Fe-Ti spinel; rim4 sometimes presents instead of rim1/rim2 and consists of zoned high-Cr phlogopite with rare fine grains of chromium spinel; rim5, a “pocket” between garnet and rim1, is represented by microcrystalline aggregates of clinopyroxene, mica, spinel, calcite, and feldspar in different variations. Rims 1, 2, and 3 are typical for garnets of all studied parageneses. Rims 4 and 5 develop on high-Cr subcalcic garnets of the most depleted peridotites. Reactions of the formation of all types of rims are given in the article. Each type of kelyphite demonstrates a clear enrichment with a certain component: Rim1—MgO and alkalis; rim2—TiO2; rim3—FeO and TiO2; rim4—Cr2O3; and rim5—CaO, suggesting the multistage injection of different components by mantle fluid.

Garnet Geochemistry of Reduced Skarn System: Implications for Fluid Evolution and Skarn Formation of the Zhuxiling W (Mo) Deposit, China

A newly discovered tungsten ore district containing more than 300,000 tons of WO3 in southern Anhui Province has attracted great attention. The Zhuxiling W (Mo) deposit in the district is dominated by skarn tungsten mineralization. This paper conducted in suit EPMA and LA-ICPMS spot and mapping analysis of the skarn mineral garnet to reveal the evolution of fluids, metasomatic dynamics, and formation conditions of skarn. Two generations of garnet have been identified for Zhuxiling W (Mo) skarn: 1) Gt-I generation garnet is isotropic, Al-rich grossular without zoning. As a further subdivision, Gt-IB garnet (Adr19-46Grs49-77 (Sps+Pyr+Alm)4-5) contains significantly high content of Ti and Mn compared with Gt-IA garnet (Adr3-42Grs53-96 (Sps+Pyr+Alm)1-5). 2) Gt-II generation garnet is anisotropic, Fe-rich andradite with oscillatory zoning. Gt-II garnet displays compositional changes with a decrease of Fe and an increase of Mn from proximal skarn (Gt-IIA) to distal skarn (Gt-IIB) with the presence of subcalcic garnet for Gt-IIB type (Sps+Pyr+Alm = 56–68). The presence of pyrrhotite associated with subcalcic garnet indicates a relatively reduced skarn system. Gt-I grossular is overall enriched in Cr, Zr, Y, Nb, and Ta compared with the Gt-II andradite, and both W and Sn strongly favor Fe-rich garnet compared with Al-rich garnet. Gt-IA grossular garnet presents a REE trend with an upward-facing parabola peaking at Pr and Nd in contrast to low and flat HREE, and Gt-IB grossular garnet has a distinct REE pattern with enriched HREE. Gt-IIA andradite garnet displays a right-dipping REE pattern (enriched LREE and depleted HREE) with a prominent positive Eu anomaly (Eu/Eu* = 3.6–15.3). In contrast, Gt-IIB andradite garnet shows depleted LREE and enriched HREE with a weak positive Eu anomaly (Eu/Eu* = 0–6.0). The incorporation and fractionation of REE in garnet are collectively controlled by crystal chemistry and extrinsic factors, such as P–T–X conditions of fluids, fluid/rock ratios, and mineral growth kinetics. Major and trace elements of two generations of garnet combined with optical and textural characteristics suggest that Gt-I Al-rich grossular garnets grow slowly through diffusive metasomatism under a closed system, whereas Gt-II Fe-rich andradite represent rapid growth garnet formed by the infiltration metasomatism of magmatic fluids in an open system. The Mn-rich garnet implies active fluid–rock interaction with Mn-rich dolomitic limestone of the Lantian Group in the district.

Thermobarometry and Geochemistry of Mantle Xenoliths from Zapolyarnaya Pipe, Upper Muna Field, Yakutia: Implications for Mantle Layering, Interaction with Plume Melts and Diamond Grade

Minerals from mantle xenoliths in the Zapolyarnaya pipe in the Upper Muna field, Russia and from mineral separates from other large diamondiferous kimberlite pipes in this field (Deimos, Novinka and Komsomolskaya-Magnitnaya) were studied with EPMA and LA-ICP-MS. All pipes contain very high proportions of sub-calcic garnets. Zapolyarnaya contains mainly dunitic xenoliths with veinlets of garnets, phlogopites and Fe-rich pyroxenes similar in composition to those from sheared peridotites. PT estimates for the clinopyroxenes trace the convective inflection of the geotherm (40–45 mW·m−2) to 8 GPa, inflected at 6 GPa and overlapping with PT estimates for ilmenites derived from protokimberlites. The Upper Muna mantle lithosphere includes dunite channels from 8 to 2 GPa, which were favorable for melt movement. The primary layering deduced from the fluctuations of CaO in garnets was smoothed by the refertilization events, which formed additional pyroxenes. Clinopyroxenes from the Novinka and Komsomolskaya-Magnitnaya pipes show a more linear geotherm and three branches in the P-Fe# plot from the lithosphere base to the Moho, suggesting several episodes of pervasive melt percolation. Clinopyroxenes from Zapolyarnaya are divided into four groups according to thermobarometry and trace element patterns, which show a stepwise increase of REE and incompatible elements. Lower pressure groups including dunitic garnets have elevated REE with peaks in Rb, Th, Nb, Sr, Zr, and U, suggesting mixing of the parental protokimberlitic melts with partially melted metasomatic veins of ancient subduction origin. At least two stages of melt percolation formed the inclined PT paths: (1) an ancient garnet semi-advective geotherm (35–45 mW·m−2) formed by volatile-rich melts during the major late Archean event of lithosphere growth; and (2) a hotter megacrystic PT path (Cpx-Ilm) formed by feeding systems for kimberlite eruptions (40–45 mW·m−2). Ilmenite PT estimates trace three separate PT trajectories, suggesting a multistage process associated with metasomatism and formation of the Cpx-Phl veinlets in dunites. Heating associated with intrusions of protokimberlite caused reactivation of the mantle metasomatites rich in H2O and alkali metals and possibly favored the growth of large megacrystalline diamonds.

Aportes a la metalogénesis del basamento proterozoico superior-cámbrico de las Sierras Pampeanas Orientales: mineralogía, termometría e isótopos de azufre del skarn Malagueño (Cu- Fe±Zn), Sierra Chica, Córdoba, Argentina

On-going mining operations in a marble quarry (Cantera Centro) from Malagueño, Sierra Chica de Córdoba, Argentina, have unearthed veins, veinlets and lenses of sulfides (pyrrhotite&gt;pyrite≅&#x0D; chalcopyrite&gt;&gt;sphalerite). These veins and lenses are up to 0.3 m thick and 2-3 m long, although intermittently can extend about a hundred meters. They are associated with skarns. The metasedimentary host sequence, largely composed of gneisses, amphibolites and marbles, was intruded by amphibolic metagabbro and metadiorite dykes, metatonalite plutons and alkali-feldspar metagranites; the whole complex was metamorphosed into medium to high amphibolite facies and strongly deformed as a result of the a regional event M2-D2/D3 that affected the Neoproterozoic-Cambrian basement during the Pampean orogeny. Except for gneisses, all the other metamorphic lithologies register evidence of differential alteration into skarn, although the process was preferentially developed on marbles, amphibolites and metagranite, and to a lesser exten on mafic and mesosilicic dykes. The metasomatic rocks are characterized by a garnet&gt;&gt;pyroxene skarn (Grs27-48Adr22-34Alm15-27Sps9-21), formed after the replacement of the metagranite; a pyroxene-rich skarn (Hd42-63Di32-50Jo3-5) developed after para-amphibolite, and a garnet (Adr54-71Grs22-40Alm4-7Sps1-2) (±wollastonite) skarn that replaced a calcic marble. The dykes show poor metasomatic replacement and lack sulfides of metasomatichydrothermal origin. Retrograde mineral associations include hastingsite, ferroactinolite, epidote, clinozoisite, sericite, plagioclase (An18), chamosite and calcite. The sulfide mineralization is paragenetically associated with late-stage, infilling skarn-hydrothermal minerals that were sequentially deposited as: calcite→clinochlore→quartz→pyrite→pyrrhotite→chalcopyrite+sphalerite; these phases occur as veins and veinlets within the garnetpyroxene skarn, and as massive pyrrhotite lenses in the piroxene-rich skarn. Microthermometric data from fluid inclusions in sulfide related calcite, together with the geothermometric data from the Fe/Mg ratios in clinochlore and the phase equilibria data from intergrown high and low T ºC pyrrhotite phases, all constrain the infilling gangue phases and sulfide crystallization temperature within the ~360 °C to 250 °C range; the gradual termal decrease is in agreement with the temporal depositional sequence of the infilling phases. Fluid inclusion petrographic data and salinity estimations suggest that sulfide precipitation was triggered by boiling, from a fluid of moderate to high salinity (~14.5 to 33.5 wt% eq. NaCl). Evidence that the fluid evolved under dominantly reducing conditions are the high Fe+2/(Fe+2+Fe+3) ratios and molar proportions of subcalcic garnet (Alm+Sps) in garnet-pyroxene skarn, the presence of fluid inclusion sulfide daughter crystals in calcite, the high H2S/SO42- ratios in the sulfide-bearing fluid and the presence of pyrrhotite among the sulfide phases. Disseminated primary chalcopyrite and pyrite in metagabbro-diorite dykes (Cu ~300 ppm) and the high contents of Cu in amphibolite (~900 ppm) suggest that these protolithic lithologies were the probable sources of metals (Cu&gt;&gt;Zn±Ag). Values of δ34SΣfluid between ~-4 to +1.4‰ obtained from the fractionations factors of pyrite, pyrrhotite and chalcopyrite within the thermal range 350-150 ºC, indicate a magmatic source for sulfur, likely provided by leaching or desulfidation of primary sulfides of the metagabbro-diorite dykes. Metals and sulfur supplied by these dykes and amphibolite would have been redistributed in the skarn after the circulation of the metasomatic-hydrothermal fluids. Skarn bodies would have formed by infiltration of deep metasomatic fluids and fluid-rock interaction which affected lithologies of the Upper Proterozoic-Cambrian metamorphic basement. Fluids could have derived from hidden Cambrian intrusives, or from the surrounding regional migmatization, channeled along lithological contacts and faults/fractures. Field setting, textural and mineralogical evidence of sulfide mineralization in the skarn assemblage of Cantera Centro, suggest a Cambrian age associated to the post-deformational stage of the Pampean Orogeny.

Mineral chemistry, petrogenesis and evolution of the Ghorveh-Seranjic skarn, Northern Sanandaj Sirjan Zone, Iran

The Ghorveh-Seranjic (GS) skarn is located in the northern part of the Sanandaj-Sirjan zone, NW Iran, which is part of Alpine-Himalaya orogenic belt. The GS metamorphic complex is the oldest unit in the GS area composed of marble, dolomitic marble, greenschist, and amphibolite of Early Jurassic age. The complex is intruded by NW-SE trending Late Jurassic peraluminous granitoids, which caused contact metamorphism and resulted in the development of skarn, hornfels and crystallization of marble. The skarn is showing distinct textural, mineralogical and geochemical zonation. At least four stages of skarn development have been recognized; stage I, clinopyroxene+garnet±vesuvianite±quartz±calcite±scheelite± pyrrhotite; stage II, garnet+clinopyroxene +vesuvianite+scheelite±apatite+calcite±pyrrhotite; stage III, amphibole+vesuvianite+epidote+chlorite±quartz±calcite±pyrrhotite±pyrite±chalcopyrite and stage IV, quartz+calcite±amphibole±epidote±chlorite±pyrite±chalcopyrite. Scheelite occurs in stages 1 and 2 together with garnet and clinopyroxene, and its abundance slightly increases with vesuvianite growth. In general, mineral chemistry of the GS skarn shows enrichment in Ca, Al and Mg. Two types of garnet, clinopyroxene and vesuvianite are identified in the prograde stage within the GS skarn. Variable Mg:Mn:Fe proportions in clinopyroxene of the early prograde stage suggest formation from a relatively homogeneous F-rich volatile phase. Mineralogical documentation of the GS skarn indicates that presence of the F-rich volatile phase affected zoning patterns and mineral abundances. Addition of fluorine increases the solubility of Al in the hydrothermal fluid by forming strong Al-F complexes, causing an increase in vesuvianite instead of clinopyroxene during the late prograde substage, resulting in high garnet/clinopyroxene ratios. The presence of granditic (grossular-andradite) and subcalcic (grossular-almandine-spessartine) garnet during the skarn evolution suggests variable Fe/Mn and Fe2+/Fe3+ ratios during the prograde stage of the skarn formation. Subcalcic garnet formed in a relatively reduced environment compared to the granditic garnet. Paragenetic reconstructions indicate that clinopyroxene, garnet and scheelite grew together during the prograde stage. These minerals were stable and coexisted at temperatures between 580 °C and 400 °C and at a logfO2 = −18 to −28.

Metamorphism and Skarn Mineralisation in the Cobar Basin: Implications for Exploration

A metamorphic map of the Siluro-Devonian Cobar Basin highlights zones of high heat (hornblende hornfels facies) overprinting cool (sub-greenschist facies) southern basin sequences. Contrary to current models for mineralisation in the Cobar Basin, which involve metals/fluids derived from basement during regional metamorphism and basin inversion, these zones of localised, large thermal contrast imply proximal, albeit blind, magmatic heat sources. Mineralisation in the main northern Cobar mineral field is associated with greenschist facies high-strain zones and is linked with fluids that exploited major fault systems and regional lithological contacts. Correlative mineralisation along these same fault systems and lithological contacts at Hera and Nymagee orebodies to the south, is associated with hornblende hornfels facies skarn alteration and Au-Ag-Cu-Zn-Pb mineralisation. Petrographic studies at Hera reveal orebody-scale mineralogical zonation from southern garnet-rich, central pyroxene-rich and northern anorthite-tremolite-rich skarn (with remnant carbonate blocks/clasts). Chemical composition of skarn minerals, including sub-calcic garnet (Mn-Ca-rich), low-Mo scheelite, Mn-enriched diopside/tremolite, zoisite and anorthite are consistent with the Hera-Nymagee orebodies representing reduced, low XCO2 distal Zn(W-Cu-Au)-skarns. Stable isotope (H-O-S) data from the Hera and Nymagee orebodies are consistent with magmatic water/sulfur sources, with formational water/sulfur also represented. A distal skarn origin for mineralisation in the southeastern Cobar Basin and the distinction of zones of elevated hydrothermal heat reflect a relatively intrusion proximal southern mineral system, while correlative deposits in the main Cobar mineral field to the north are hosted in lower temperature hydrothermal zones and likely reflect relatively intrusion-distal parts of the same magmatically driven system. Zn-rich skarns akin to the Hera-Nymagee orebodies generally occur in the distal portions of major magmatic/hydrothermal systems. Thus, the absence of near-orebody intrusions implies the mineral systems of the southeastern Cobar Basin may not have been traced to their ends and the basin is immature with respect to exploration for skarn and intrusion-related mineral systems.

Mineralogical and metasomatic evolution of the Jurassic Baoshan scheelite skarn deposit, Nanling, South China

The Baoshan scheelite deposit, located in the Nanling region, South China, is a skarn-type tungsten ore deposit. Orebodies occur in the contact zone between Carboniferous limestone and Baoshan granitic pluton. The pluton consists mostly of coarse-grained, porphyritic, and fine-grained biotite granites, which have zircon U-Pb ages of 172.4 ± 0.3 Ma, 166.6 ± 0.3 Ma, and 156.6 ± 1.4 Ma, respectively. These rocks have similar geochemical characteristics with high SiO2, total alkalis, high differentiation indexes and high Rb/Sr ratios of S-type granites formed by partial melting of upper continental crust. The porphyritic granites display the highest degree of fractionation, and are most closely associated with the formation of skarn ores.The skarns are zoned mineralogically outward from garnet, through clinopyroxene, to wollastonite, and overprinted by retrograde minerals. Three stages of skarnization are recognized: Early prograde, late prograde and retrograde. The early prograde stage is mainly composed of Al-rich garnet (Grs60–76) and Mg-rich clinopyroxene (Di41–62) with subordinate Mo-rich scheelite (Sch59–86), all of which are typically overgrown by late prograde assemblages of Fe-rich garnet (And67–95), Fe-rich clinopyroxene (Hd81–87), and Mo-depleted scheelite (Sch97–98). The retrograde stage is characterized by Al-rich garnet (Grs40–66) and pure scheelite (Sch100), accompanied by hydrous and sulfide minerals. Minor subcalcic garnet with magnetite sometimes occurs in the limited endoskarns. Fluid inclusions of the early prograde minerals homogenize at 390.5 to >550 °C, and one inclusion in wollastonite has a salinity of 13.0 equiv. wt% NaCl. Inclusions in minerals of the late prograde and retrograde stages have homogenization temperatures of 196.9–377.4 °C and 160.4–291.4 °C, respectively; their salinities 2.4–19.0 equiv. wt% NaCl and 1.4–11.6 equiv. wt% NaCl, respectively. Accordingly, the high-temperature and moderate- to high-salinity fluids in the early skarn stages are typical of magmatic fluids originating granites, whereas the lower temperature and variable salinity in the later stages suggest mixing with meteoric water.

An exsolution origin for Archean mantle garnet

It is well established that the cratonic subcontinental lithospheric mantle (C-SCLM) represents a residue of extensively melted peridotite. The widespread occurrence of garnet in C-SCLM remains a paradox because experiments show that it should be exhausted beyond similar to 20% melting. It has been suggested that garnet may have formed by exsolution from Al-rich orthopyroxene; however, the few documented examples of garnet exsolution in cratonic samples are exotic and do not afford a direct link to garnet in granular harzburgite. We report crystallographic, petrographic, and chemical data for an exceptionally well preserved orthopyroxene megacryst containing garnet lamellae, juxtaposed against granular harzburgite. Garnet lamellae are homogeneously distributed within the host orthopyroxene and occur at an orientation that is unrelated to orthopyroxene cleavage, strongly indicating that they formed by exsolution. Garnet lamellae are subcalcic Cr-pyrope, and the orthopyroxene host is high-Mg enstatite; these phases equilibrated at 4.4 GPa and 975 degrees C. The reconstructed precursor is a high-Al enstatite that formed at higher pressure and temperature conditions of similar to 6 GPa and 1750 degrees C. The megacryst shows evidence for disintegrating into granular peridotite, and garnet and orthopyroxene within the granular peridotite are texturally and chemically identical to equivalent phases in the megacryst. Collectively, this evidence supports a common origin for the granular and exsolved portions of the sample. We hypothesize that high-Al enstatite was a common phase in the C-SCLM and that exsolution during cooling and stabilization of the C-SCLM could be the origin of most subcalcic garnets in depleted peridotites.

Evolution of the Mo-rich scheelite skarn mineralization at Kozbudaklar, Western Anatolia, Turkey: Evidence from mineral chemistry and fluid inclusions

The Kozbudaklar scheelite skarn deposit in the Tavşanlı Zone, located approximately 22km southeast of Bursa, is hosted by the Triassic calcic İnönü Marble and Eocene Topuk Pluton. At least four stages have been recognized through skarn evolution. Scheelite skarn distributed close to the Topuk Pluton occurred during the early (stage 1) and late (stage 2) prograde substages. The early prograde endo and exoskarn are composed of hedenbergite (Hd96Joh4)–plagioclase (An55–64) and hedenbergite (Hd61–94Joh4–7), accompanied by calcic garnet (Grs38–94Sps1–5Alm0) and scheelite (Pow1–6). The second stage represents a relatively oxidized mineralogy dominated by diopside (Hd16–48Joh0–9), subcalcic garnet (Grs24–92Sps0–11Alm0–31) and scheelite (Pow7–32). The stage 3 and 4 mineral assemblages are characterized by few hydrous minerals in the retrograde stage and intense fracturing.Fluid inclusions from skarn rocks are indicative of multiple fluid events: (1) low-moderate salinity (5–16wt.%NaCl equiv.) inclusions homogenized dominantly by a high-temperature (308°C to >600°C) liquid phase in stage 1. Fluid inclusions in an early garnet homogenized over a similar temperature range (440°C and 459°C) into both liquid and vapor phases. Eutectic temperatures ranging from −61.7°C to −35.0°C that indicate the presence of H2O–NaCl–(±MgCl2±CaCl2)–CO2 solutions; (2) coexisting daughter mineral-bearing high salinity (29.5≥70wt.%NaCl equiv.) and vapor-rich moderate salinity (11.5–16.7wt.%NaCl equiv.) inclusions that homogenized in the liquid phase by the disappearance of the vapor phase at a similar temperature range (308°C to >600°C) in stage 2. Eutectic temperatures range from −67.9°C to −51.8°C that shows the presence of H2O–NaCl–CO2–(±CH4/N2) solutions; (3) low-moderate salinity (12.5–7.6wt.%NaCl equiv.) and temperature (320°C to 215°C) inclusions homogenized by the liquid phase in stage 3. Eutectic temperatures range from −59.5°C to −44.2°C indicating the presence of H2O–NaCl–(±MgCl2±CaCl2)–CO2 solutions; (4) inclusions of low salinity (9.9–0.9wt.%NaCl equiv.) and homogenization temperature (183°C to 101°C) in stage 4.These data show that the Kozbudaklar skarn deposit was formed in a magmatic–hydrothermal system. In this model, carbonaceous fluids may have been exsolved from the plutonic rock during its emplacement and crystallization. Fluid inclusion data indicate that fluid boiling and immiscibility occurred at temperatures between 440°C and 459°C and pressures ranging from 50MPa to 60MPa based on hydrostatic considerations. Early scheelite was precipitated with relatively reduced mineral compositions. As a result of depressurization, Mo-rich scheelite with oxidized minerals formed via high salinity and vapor-rich inclusions. The second scheelite mineralization occurred in a normal hydrothermal system by an infiltration mechanism at pressures between approximately 40 and 1.5MPa. At shallow depths (<1.5MPa) with increasing permeability, sulfide and oxide minerals were deposited in the retrograde stage, greatly assisted by meteoric water. Finally, as a result of the diminishing of ore-forming fluids, post-depositional barren quartz and calcite veins were formed.

Interaction of serpentine and chromite as a possible formation mechanism of subcalcic chromium garnet in the upper mantle: an experimental study

An experimental simulation of serpentine and chromite interaction was conducted at the pressure ( P ) and temperature ( T ) conditions of garnet-peridotite stability in order to clarify the potential role of serpentinite as a source for the crystallization of subcalcic garnet in the depleted subcratonic mantle. The experiments were performed at 4 GPa and 1100 °C and 5.5 GPa and 1200 °C using the high-pressure apparatus BARS. Natural antigorite from ophiolites of the Eastern Sayan (Russia) was used as a starting material. Two groups of chromite grains with different Cr# = 100Cr/(Cr + Al) ratios (from spinel peridotite xenoliths from the Udachnaya kimberlite pipe, Yakutia) were added to the antigorite. Newly formed garnet, spinel, olivine and orthopyroxene were observed as the products in the experiments. Garnet formed only around chromite grains with the lower Cr# value (46.4). Garnet has low CaO contents (<0.05–1.10 wt.%) with chromium contents showing wide intra- and inter-grain variations (Cr# = 0.7–33.5). The Cr content increases from core to rim with the outer zones corresponding most closely to the equilibrium composition of the relevant bulk composition. The garnet total FeO content is in the range 3.4–5.8 wt.%. The experiments demonstrate that serpentinite decomposed at a temperature of 700 °C to olivine + orthopyroxene + water. If mingled mechanically with spinel-bearing mantle-wedge peridotite upon subduction, it could react to form the range of subcalcic garnet compositions found as inclusions in diamonds.

Ancient mantle metasomatism recorded in subcalcic garnet xenocrysts: Temporal links between mantle metasomatism, diamond growth and crustal tectonomagmatism

We have identified carbonatitic melts as the main agent for metasomatism in cpx-free garnet harzburgites from the Kaapvaal craton. Substantial overlap of the Ti/Eu and Zr/Hf ratios of subcalcic garnets from xenoliths and garnet inclusions in diamonds corroborate previous findings that mantle metasomatism and the growth of diamonds are connected processes. The key process involves the interaction of a carbonatitic melt with depleted garnet harzburgite leading to dissolution and regrowth of the constituent minerals and the growth of diamonds by redox reactions. Model calculations show that only small amounts of a carbonatite melt, between 0.3 to 3%, are needed to generate the range of sinusoidal Rare Earth Element patterns in the garnets from harzburgites and the inclusions in diamonds. The εHf values of the xenolith garnets range from extremely positive (∼+1000) to extremely negative values (−65) with accompanying εNd values varying from +38 to −41. It can be shown that the negative εHf and εNd values correspond to the initial ratios of the metasomatizing agent. They can only stem from a very old (early Archean or Hadean) crustal component. Very negative εNd values have been found previously in subcalcic garnets from xenoliths and inclusions in diamonds. They yield early Archean model ages for the growth of diamonds. In these studies, very high, unsupported 87Sr/86Sr ratios were found as well. The combined evidence suggests that the metasomatic agents are derived from an old source with a high Rb/Sr ratio, typical for pelitic crustal rocks. Metasomatism in the mantle, the formation of diamonds and tectonomagmatic events in the crust occurred contemporaneously at least within the time span between the early Archean and the middle Proterozoic as can be seen from the comparison of crustal ages, Lu–Hf isochron ages from subcalcic garnets and Sm–Nd and Re–Os ages of inclusions in diamonds.

The buffering capacity of lithospheric mantle: implications for diamond formation

Current models for the formation of natural diamond involve either oxidation of a methane-bearing fluid by reaction with oxidized mantle, or reduction of a carbonate-bearing fluid (or melt) by reaction with reduced mantle. Implicit in both models is the ability of the mantle with which the fluid equilibrates to act as an oxidizing or reducing agent, or more simply, to act as a source or sink of O2. If only redox reactions involving iron are operating, the ability of mantle peridotite to fulfill this role in diamond formation may not be sufficient for either model to be viable. Using the recent experimental recalibration of olivine–orthopyroxene–garnet oxybarometers of Stagno et al. (2013), we re-evaluated the global database of ~200 garnet peridotite samples for which the requisite Fe3+/Fe2+ data for garnet exist. Relative to the previous calibration of Gudmundsson and Wood (1995), the new calibration yields somewhat more oxidized values of Δlog fO2 (FMQ), with the divergence increasing from 60 mol%), with CH4 being the next most abundant species. To ascertain the capacity for mantle peridotite to act as a source or sink of O2, we developed a new model to calculate the fO2 for a peridotite at a given P, T, and Fe3+/Fe2+. The results from this model predict 50 ppm or less O2 is required to shift a depleted mantle peridotite the observed four log units of fO2. Coupled with the observed distribution of samples at values of fO2 intermediate between the most reduced (metal-saturated) and most oxidized (carbonate-saturated) possible values for diamond stability, these results demonstrate that peridotites are very poor sinks or sources of O2 for possible redox reactions to form diamond. A corollary of the poor redox buffering capacity of cratonic peridotites is that they can be employed as faithful indicators of the redox state of the last metasomatic fluid that passed through them. We propose that diamond formation from CHO fluids is a predictable consequence either of isobaric cooling or of combined cooling and decompression of the fluid as it migrates upward in the lithosphere. This establishes a petrological basis for the observed close connection between subcalcic garnet and diamond: based on high solidus temperatures of harzburgite and dunite effectively precluding dilution of CHO fluids through incipient melts, such highly depleted cratonic peridotites are the preferred locus of diamond formation. Due to a rapid increase in solidus temperature with increasing CH4 content of the fluid, diamond formation related to reduced CHO fluids may also occur in some cratonic lherzolites.

Geochronological and geochemical constraints on the formation and evolution of the mantle underneath the Kaapvaal craton: Lu–Hf and Sm–Nd systematics of subcalcic garnets from highly depleted peridotites

Subcalcic garnets carry the major inventory of most trace elements of their host harzburgites and are thus proxies of the bulk composition. We used single garnet grains from heavy mineral concentrates from the Kaapvaal craton (Roberts Victor and Lace mine) to determine the major and trace elements and the Sm–Nd and Lu–Hf isotope systematics of these highly depleted members of the peridotitic suite. The combination of the results with previous work from the Finsch mine (Lazarov et al., 2009a) allowed us to reconstruct the formation and evolution of the mantle underneath the Kaapvaal craton.Several lines of evidence from major and trace elements suggest that mantle melting was mainly at shallow pressures followed by subduction into the garnet stability field. A 3.22Ga metasomatic event underneath the East block occurred in a previously depleted mantle (εHf=+16) which was sufficiently stabilized by that time to hold a crust with tonalite–trondhjemite–granodiorite (TTG’s) and greenstone belts. Such high εHf values can be produced within a few hundred million years by 25% non-modal fractional melting in the spinel stability field. This is the first prove of a mantle underneath the East block with an age similar to a 3.65Ga crustal age.Before the amalgamation around 2.88Ga, oceanic lithosphere was created between the W- and E-block around 2.95Ga (group RV1 samples from the Roberts Victor) and subducted underneath the W-block. Another mantle portion (group RV2 garnets) from Roberts Victor yielded a seeming age of 3.27±0.15Ga with εHf=+17.6. It actually results from an enrichment process in a highly depleted mantle about 2.8–2.9Ga ago. This may have been the depleted mantle wedge above the subduction and final collision between the West and the East block. The creation of a cratonic nucleus for the West-block is unknown until 3.2Ga when the oldest mantle TRD and oldest crustal zircon ages are reported. Such ages were not directly obtained from the study of the subcalcic garnets but the high positive εHf value (+25) of the 2.62Ga enrichment age from Finsch and the existence of a highly depleted mantle wedge before 2.8–2.9Ga at Roberts Victor suggests that the depletion occurred several hundred million years earlier. Subsequently, the 2.6–2.8Ga old voluminous Ventersdoorp volcanics poured over a major part of the unified Kaapvaal craton, an event which also may have modified portions of the mantle underneath Finsch. The attachment of the Kheis–Magondi belt to the Kaapvaal craton from the West caused further modification around 1.90Ga. The latest stages of craton-scale metasomatism recorded in the subcalcic garnets lie between 0.9 and 1.3Ga as can be seen in the Sm–Nd isotope systematic. This age range coincides with the Namaqua–Natal belt orogeny. The present study and previous work on subcalcic garnets show that they can be excellent recorders of multiple mantle events which can be correlated with the tectonomagmatic evolution of the craton.

Oxygen isotope constraints on the origin of high-Cr garnets from kimberlites

The association between diamonds and high-Cr, sub-calcic garnets from kimberlites suggests that garnets have experienced high pressure (and presumably, high temperature) conditions in the subcontinental lithosphere mantle (SCLM). The oxygen isotope compositions of these garnets are generally reported to be lower than average values of mantle olivines (~ 5.2‰)—opposite the sense of oxygen isotope fractionation expected at high-temperature equilibrium, and to correlate negatively with their Cr-contents. These observations were interpreted to indicate the sub-calcic garnets either are derived from melting Cr-enriched but 18O-depleted source in the SCLM, or have experienced cryptic metasomatism by high-Cr, high-Mg#, but 18O-depleted fluids/melts in the SCLM or during their ascent to the surface. We investigate the possibility that oxygen isotope characteristics of these garnets instead reflect an analytical artifact that reduces the measured δ 18O of garnets in proportion to its Cr content—a so-called ‘Cr-effect’. Mixtures of garnet standards (UWG-2) plus various amounts of pure Cr-metal were measured for δ 18O-values by infrared-laser fluorination technique (ILFT). Our results show that oxygen isotope compositions of sample mixtures vary systematically as a function of Cr-content and O 2-yield. Mass-balance analyses indicate that the correlation between the measured δ 18O-value and Cr-content is an analytical artifact mostly due to isotopic fractionation between the extracted-O 2 and chromium oxyfluorides left as residues in the sample chamber during the ILFT analyses, which amounts to a decrease in the measured δ 18O values of ~ 0.064‰ for the increase of every wt.% Cr 2O 3 present in garnets. After applying our experimentally-calibrated correction factor to garnet samples in garnet peridotite xenoliths from South Africa, our study reveals an equilibrium oxygen isotope fractionation between the Cr-poor garnet and coexisting olivine (i.e., garnet ≥ olivine) with a fractionation factor of ~ 0.28‰. More importantly, high-Cr garnets (Cr 2O 3 > 5%), after correction for the ‘Cr-effect’, are actually enriched, rather than depleted, in 18O compared with low-Cr garnets, suggesting the involvement of 18O-enriched crustal materials during their formation. We hypothesize a fluid-flow model using depth-dependent Cr partitioning. This model simultaneously explains relatively high-δ 18O-values and Cr and Mg enrichment in garnets, implying that the reduced C H O fluids that are responsible for the formation of diamonds in both eclogite and peridotite xenoliths in kimberlites, are also responsible for transporting Cr to oxidized locations where the Cr partition coefficient between garnet and fluids is high.

Structure and evolution of the lithospheric mantle beneath Siberian craton, thermobarometric study

Monomineral thermobarometry (MTB) data derived from EMP analyses for heavy monomineral separates from > 20 Yakutian kimberlite pipes were used to compile a SSE–NNW traverse of the mantle beneath the Siberian craton. Orthopyroxene (Opx) MBT for the mantle section beneath Udachnaya gives three PT paths: a low temperature (LT) conductive branch (Boyd et al., 1997), estimated with thermometers of Krogh (1988) or/and O'Neill and Wood (1979), and two other HT paths. The three paths correspond to different values of Fe# in Ol (0.075, 0.085 and 0.11). They are reproduced by the modified MTB equations for clinopyroxene, garnet, chromite and ilmenites (Ashchepkov et al., 2008a), a mono-version of the O'Neill and Wood (1979) thermometer with corrections to Cr and Ca/Mg ratios which mark conductive geotherm. PT estimates for garnets and pyroxenes reflect mantle layering, whereas those for ilmenite reflect varying conditions of polybaric mantle protokimberlite systems and metasomatism. MTB for xenoliths from the Udachnaya, Mir, Dalnyaya, and Komsomolskaya kimberlites show the colder branches if PT path using heavy mineral analyses them according to PT for xenoliths according to TB (O'Neill and Wood, 1979; Finnerty and Boyd, 1987; McGregor, 1974; Brey and Kohler, 1990) producing smoother geotherms. MTB gives a wider range PT points reflecting heating near magmatic channels. Regularities of mantle sections and layering in the Daldyn field are recognized on the PT and P– X diagrams. The lower part of the mantle sections were heated by protokimberlite melts which created the megacrystalline associations. PT values of sub-calcic garnets correlate with those of picroilmenites. Mantle columns beneath the large pipes reveal stepped 7–12 layering for Udachnaya pipe units correlating with the peaks of Re/Os ages (Griffin et al., 2002b) is marked by periodic increase in Fe# in minerals. Pyroxenites and ‘hot’ eclogites (Pokhilenko et al., 1999) are found in layers at ∼ ∼ 40, 50–65, and ∼ 70 kbar. Sub-continental lithospheric mantle (SCLM) beneath the Alakite field has been subjected to pervasive multistage metasomatism, as indicated by Fe-enriched Cr-diopsides and Ti-rich low-Ca garnets. Ilmenite PT trends were formed by rising protokimberlites that underwent AFC. In the Upper Muna field the mantle is similar in structure to that of the Alakite region. Fe-rich clinopyroxene-bearing rocks (60–55 kbar) are located between the ilmenite-forming systems (70–60 and 55–40 kbar), sub-Ca garnets start from 40 kbar and become more abundant downward. Beneath the Nakyn field, rhythmic layering is found for peridotites in the lower part ( P > 40 kbar), fertilization by Fe–Cpx (40–50 kbar) follow the Ilm-forming system ∼ 55–60 kbar correlating with the occurrence of depleted (low-Ca) peridotites. Beneath the Anabar fields highly depleted mantle at depth > 40 kbar has been subjected to Fe-metasomatism and pervasive metasomatism that accompanied protokimberlite feeders marked by low Cr-ilmenites accompanied by fertilization. In the upper section abundant garnet- and clinopyroxene-rich peridotites are typical. Comparison of mantle sections reconstructed from monomineral PT estimates from Paleozoic and Mesozoic kimberlites show differences in entrainment levels which were elevated after the Permian–Triassic superplume to > 55–40 kbar without delamination.