Marble Contact Research Articles

Garnet Geochemistry of Pertek Skarns (Tunceli, Turkey) and U-Pb Age Findings

The Fe skarn and vein-type Cu mineralization types are common in the Eastern Taurus Mountains. This study aims to determine the U-Pb geochronology of garnets of varying sizes within the skarn zone developed at the quartz diorite–marble contact zone in Ayazpınar, Pertek District, Tunceli Province, Turkey. Additionally, this study aims to determine the age of the skarnization and the types of inclusion minerals in the garnets. Faulting and magma emplacement along the thrust plane caused mineralization in the Eastern Taurus Mountains, especially at the marble and quartz diorite contact zone between the cities of Elazığ and Tunceli. The greenish garnets found in the Ayazpınar deposit are characteristic of distal skarns, while red or brown Pertek garnets are observed in the proximal skarns. The garnets typically feature a core–rim texture. The cores of the garnet crystals are large, reddish in color, and have a high REE (Ce, Pr) content. Moreover, the cores have higher Fe and lower Al ratios, alongside higher La, Ce, and Pr contents, than the rims. We propose that the compositional differences between the rims and the cores reflect the transition from oxidized REE- and Fe3+-rich liquids to liquids with lower REE and Fe3+ contents, producing the differences in the garnet colors. Green garnets show lower REE contents than brown garnets with Fe-rich cores. The skarn under study includes the following successively formed zones: diorite → epidote skarn → garnet–magnetite → pyrite–garnet–magnetite → calcite carbonate. Diopside, magnetite, and hematite, including small grains, are surrounded or enveloped by garnets. The U-Pb age of the mineralization is 74.1 ± 5 Ma, indicating that the mineralization occurred concurrently with the intrusion settlement.

Fluid Infiltration and Mass Transfer along a Lamprophyre Dyke–Marble Contact: An Example from the South-Western Korean Peninsula

In this contribution, we report the metasomatic characteristics of a lamprophyre dyke–marble contact zone from the Hongseong–Imjingang belt along the western Gyeonggi Massif, South Korea. The lamprophyre dyke intruded into the dolomitic marble, forming a serpentinized contact zone. The zone consists of olivine, serpentine, calcite, dolomite, biotite, spinel, and hematite. Minor F and Cl contents in the serpentine and biotite indicate the composition of the infiltrating H2O-CO2 fluid. SiO2 (12.42 wt %), FeO (1.83 wt %), K2O (0.03 wt %), Sr (89 ppm), U (0.7 ppm), Th (1.44 ppm), and rare earth elements (REEs) are highly mobile, while Zr, Cr, and Ba are moderately mobile in the fluid. Phase equilibria modelling suggests that the olivine, spinel, biotite, and calcite assemblage might be formed by the dissolution of dolomite at ~700 °C, 130 MPa. Such modelling requires stable diopside in the observed conditions in the presence of silica-saturated fluid. The lack of diopside in the metasomatized region is due to the high K activity of the fluid. Our log activity K2O (aK2O)–temperature pseudosection shows that at aK2O~−40, the olivine, spinel, biotite, and calcite assemblage is stable without diopside. Subsequently, at ~450 °C, 130 MPa, serpentine is formed due to the infiltration of H2O during the cooling of the lamprophyre dyke. This suggests that hot H2O-CO2 fluids with dissolved major and trace elements infiltrated through fractures, grain boundaries, and micron-scale porosity, which dissolved dolomite in the marble and precipitated the observed olivine-bearing peak metasomatic assemblage. During cooling, exsolved CO2 could increase the water activity to stabilize the serpentine. Our example implies that dissolution-reprecipitation is an important process, locally and regionally, that could impart important textural and geochemical variations in metasomatized rocks.

Geochemistry of the garnets in the Baiganhu W–Sn orefield, NW China

The Baiganhu W–Sn orefield in the southeastern Xinjiang Uygur Autonomous Region is associated with Caledonian S-type syenogranites and metasediments of the Paleoproterozoic Jinshuikou Group. Four types of garnets have been identified in the orefield using petrographic and major and trace element data. Grt-I garnets are generally present as inclusions within magmatic quartz in the syenogranites, with end-member formulas of Sps45–53Alm46–53Adr0–1Grs0–1Prp0–1 and rare earth element (REE) patterns that are enriched in heavy REE (HREE) and contain strong negative Eu anomalies. Grt-II garnets are associated with tourmaline and quartz and occur in interstices between feldspars within the syenogranites. In general, the Grt-II garnets have end-member formulae (Sps64–70Alm29–34Adr0–1Grs0–2Prp0) and REE patterns that are similar to the Grt-I garnets although they are more spessartine-rich and contain higher concentrations of HREE. Grt-III garnets coexist with clinopyroxenes and Mo-rich scheelites within skarns developed along the syenogranite and marble contact. Their compositions are Adr62–88Grs1–18Sps3–12Alm0–8Pyr0 and they have relatively flat REE patterns with no negative Eu anomalies. Grt-IV garnets are present as massive aggregates that are often cross-cut by Mo-poor scheelite-bearing calcite veins. Their end-member formulas are Adr4–22Grs62–73Sps5–16Alm2–10Pyr0 and they have slightly domed REE patterns without negative Eu anomalies. Both Grt-III and Grt-IV garnets contain lower concentrations of the HREE (2–3 and 4–32ppm, respectively) than Grt-I and Grt-II garnets (682–1352ppm with Y=1558–2159ppm, and 6051–12831ppm with Y=9663–13333ppm, respectively).The occurrences, mineral assemblages, major element compositions, and REE patterns of the Grt-I and Grt-II garnets suggest they have magmatic origin and crystallized at relatively low temperatures and pressures, whereas the Grt-III and Grt-IV garnets have hydrothermal origin. The association of magmatic garnet (Grt-II) with tourmaline suggests that boron in S-type magmas has an important role in the formation of W–Sn mineralization during magmatic and subsequent hydrothermal processes. All four types of garnets contain low concentrations of W (⩽1.64ppm) and Mo (<0.77ppm), but relatively high and variable concentrations of Sn (22.96–8364ppm). A positive correlation between SnO2 and andradite molecule contents confirms the substitution of Sn4+ for Fe3+ within these garnets. These data suggest that Sn-rich andradite–grossular garnets in skarns may be used as an indicator mineral for W–Sn exploration when combined with other geological, geophysical, and geochemical signatures.

Petrology and Geochemistry of Calc-Silicate Schists and Calc-Silicate Reaction Bands in the Jakura Marble, Western Nigeria

The Jakura Marble deposit is located ca. 50 km northwest of Lokoja, southwestern Nigeria. Calc-silicate schists occur as intercalations and lenses within the marble. The schists consist of hornblende, biotite, actinolite, plagioclase, quartz, accessory epidote, sphene, zircon, chlorite and apatite. Calc-silicate reaction bands developed at the contacts between the marble and calc-silicate schists consist of diopside, plagioclase, scapolite, late actinolite and epidote.Chemically the calc-silicate schists are characterized by relatively high CaO, MgO, Cr, Ni, Sr, La, Ce and Nd contents compared with the mica schist regionally associated with the marble as well as the Post-Archean Australian Shale (PAAS). Relative to the ultramafic schist the calc-silicate schists are characterized by higher contents of K2O, Ba, Pb, Rb, La and Ce and lower MgO, CaO, Fe, Na2O, Cr and V. The calc-silicate reaction bands have higher contents of CaO and Sr and lower concentrations of K2O, Rb, Ni, and Ba relative to the calc-silicate schists; and relatively higher SiO2, TiO2, Al2O3, Fe2O3, MgO, Na2O, K2O and P2O5 and lower CaO contents compared to the marble.These features suggest that the protoliths of the calc-silicate schists were admixtures of carbonate material with some ultramafic detritus, and the reaction band was produced by diffusion of elements from the calc-silicat schists into the marble contact zone.Keywords: Jakura, Calc-silicate Schists, Calc-silicate Reaction Bands, Marble, Geochemistry

RELICS OF HIGH-TEMPERATURE CLINOPYROXENE ON THE JOIN Di CaTs WITH UP TO 72 mol.% Ca(Al,Fe3+)AlSiO6 IN THE SKARNS OF CICLOVA AND MAGUREAUA VATEI, CARPATHIANS, ROMANIA

In the western Carpathians of Romania, clinopyroxene with a very high content of the Ca-Tschermak (up to 24 wt.% Al2O3) and esseneite components occurs as veinlets and inclusions, in places associated with kalsilite, in calcite and in wollastonite very close to the marble contacts of two shallow monzodioritic intrusive complexes, Magureaua Vatei, in the Apuseni Mountains, and Oravita-Ciclova, in Banat, both of Upper Cretaceous age. These relics originate from magmatic veinlets, emplaced at an early stage of the magmatic process and later recrystallized at high temperature (> 800 degrees C) by a CO2-dominated fluid (stage 1). Such fluid probably resulted from the reaction of calcite with the main intrusive body at the time of its emplacement, a reaction also witnessed by the development of a symplectitic association of anorthite and aluminous pyroxene at the intrusive margins, and by the (exceptional) occurrence of Na-rich melilite. Part of the intrusive boundaries were later invaded by pegmatite-forming liquids or fluids that, upon reaction with calcite, first produced idiomorphic crystals of (Fe,Mg)-rich wollastonite (stage 2), occasionally hosting minerals of stage 1, followed by the development of a fibrous wollastonite-grossular association (stage W) accompanied by an (Al-poor) diopside that progressively became Al-rich (stage P) through further modification of the fluid by reaction with calcite. The later appearance of vesuvianite initiated the large-scale postmagmatic hydrothermal process.

Formation and evolution processes of the Salanfe W–Au–As-skarns (Aiguilles Rouges Massif, western Swiss Alps)

At Salanfe (Aiguilles Rouges Massif, Swiss Alps) several stratabound W–Au–As-skarn bodies were formed at the contact between pre-Ordovician marble lenses and paragneiss in close proximity to an anatectic leucogranite of Late Palaeozoic age. Anatexis and leucogranite formation occurred at peak metamorphic conditions of P=0.45 GPa and T~650–700 °C. This metamorphic event, dated at 317 Ma in the adjacent Mont Blanc Massif, was related to dextral transpression following continental collision. Structural evidence indicates that the initial stages of skarn formation also occurred under the above P–T conditions. The main stratiform skarn body (Robert) consists of a massive facies close to the marble and of a banded facies on the paragneiss side. The massive facies resulted from the replacement of former dolomite marble, whereas the banded facies resulted mostly from the replacement of thin carbonate layers intercalated in the paragneiss. In the massive facies, the prograde stage skarn is composed by hedenbergite with increasing Fe concentrations through skarn evolutionary stages, whereas in the banded facies it consists of diopside–plagioclase–quartz. The imprint of the retrograde stage is minor and unevenly distributed, with replacement of the pyroxene by magnetite/haematite–calcite–quartz close to the marble and by chlorite–epidote close to the paragneiss. The economic ore minerals are scheelite, arsenopyrite, loellingite and gold and are mainly associated with the prograde skarn stage. Most of the scheelite mineralization occurs during the early prograde stage, probably at temperatures higher than 600 °C, and is associated with pyroxene (hedenbergite 55). Arsenopyrite and loellingite precipitated after scheelite at temperatures between 600 and 350 °C and are associated with hedenbergite>80. Gold occurs both in the final prograde stage and in the retrograde stage. Minor second generation scheelite is associated with epidote and chlorite of the retrograde stage. Lead isotopes indicate that skarn ore minerals were precipitated by a low Th/U fluid like that in equilibrium with the anatectic leucogranite. Oxygen isotopes support the contention that the prograde skarn was formed by a fluid in equilibrium with the leucogranite, mixed with increasing amounts of CO2 approaching the skarn–marble contact. Oxygen and hydrogen isotopes suggest the intervention of a non-magmatic fluid (meteoric, connate?) during the retrograde stage. The Salanfe skarns were formed at deep crustal levels by fluids in equilibrium with an anatectic leucogranite, possibly channelled along permeable paragneiss during regional-scale transpression, and could be the deep expression of a widespread W–Au hydrothermal ore event that occurred along the European Variscides at 310–305 Ma.

Skarn-type ilmenite mineralization of the Tuzbaşi–Tunceli region, eastern Turkey

In the Tuzbaşi–Tunceli region of eastern Turkey, skarn-type ilmenite mineralizations occur within the skarn zones along the contact between marbles of the Keban Metamorphic Formation (Permo-Triassic) and gabbros and diorites of the Elazig Magmatic Complex (Coniacian–Campanian). The Elazig Magmatic Complex has characteristics typical of arc magmatism and is composed of plutonic, volcanic, subvolcanic and pyroclastic rocks. Both formations are overlain by sediments of the Alibonca Formation (Lower Miocene) and continental andesitic lava flows of the Karabakır Formation (Upper Miocene–Pliocene). Ilmenite and skarn formation developed as a consequence of plutons (granite–gabbro–diorite) of the Elazig Magmatic Complex intruding into marbles of the Keban Metamorphic Formation. This event also resulted in different types of magnetite mineralization and skarn formation, with different mineral assemblages, in neighbouring areas. Ilmenite concentrations occur as lensoid bodies in garnet–pyroxene calcic exoskarns along (gabbro–diorite)–marble contacts. Ilmenite bodies form syngenetically with skarns, and their TiO 2 contents reach up to 15.83 wt%. The predominant ore mineral is ilmenite, which is often altered to leucoxene and hematite, and occurs disseminated within skarn minerals (predominantly almandine garnet and pyroxene). The titanium of ilmenites apparently originated from the diorites and gabbros, which are otherwise Ti and Al-rich and Si-poor. The intruded marbles supplied the necessary O 2 for ilmenite mineralization.

Millimetre-scale variation in metamorphic permeability of marbles during transient fluid flow: an example from the Reynolds Range, central Australia

The Upper Calc-Silicate Unit of the Reynolds Range, central Australia, contains marbles with centimetre-wide layers of metamorphosed marls. Wollastonite-bearing marbles record fluid infiltration by water-rich fluids (XCO2=0.1–0.3) at 650–700 °C and 300–400 MPa during cooling from granulite-facies metamorphism at ∼1.6 Ga. The pattern of fluid flow recorded by stable isotope ratios of calcite, the distribution of wollastonite, and calcite luminescence is heterogeneous on a millimetre-scale. Calcite from decimetre-size hand specimens has δ18O values and δ13C values that vary by up to 10 and 12‰, respectively. The lowest δ18O and δ13C values are centred on millimetre-wide grandite-rich skarn zones, whereas higher values characterise relict calcite that co-exists with quartz. The variation in δ18O values together with related δ13C variations and the abundance of wollastonite may be used to determine the distribution of zones of higher and lower permeability. Skarn zones and regions of low δ18O values at marl–marble contacts mark channels of fluid flow. Fluid flow at the contacts was promoted by the rheological contrast and by the juxtaposition of calcite (in the marble) and quartz (in the marl) that could form abundant wollastonite via the reaction: calcite + quartz=wollastonite + CO2. This reaction results in a large negative volume change, and fluid flow may have been concentrated in a small number of channels once they were established, provided that fluid pressures were sufficient to overcome compaction. Some isolated skarn zones and low δ18O regions within the marbles may be located at the intersections of fractures that would have also been zones of high permeability. The formation of large volumes (>15–20%) of wollastonite within the marble layers requires that silica metasomatism via the reaction calcite + SiO2(aq)=wollastonite + CO2 has occurred, and the grandite skarns record metasomatic addition of Fe. These metasomatic reactions have positive volume changes that would have sealed existing fluid flow channels, allowing new channels to develop elsewhere in the rock. Preservation of steep stable isotope gradients suggests that fluid flow occurred over only a few thousand years after which the rocks were fluid absent.

Metamorphic discontinuities in the Pennsylvania – Delaware Piedmont: evidence for early Paleozoic assembly

Interpretation of the early Paleozoic tectonic history of the Appalachian Piedmont is commonly complicated by post-Taconian deformation that has disrupted and obscured early Paleozoic structure. Blocks that lie between zones where later deformation has been concentrated, however, may preserve evidence of Taconian high-grade metamorphism. In these cases, analysis of metamorphism allows an interpretation of the early Paleozoic tectonic history of the block and its surrounding region. A block of this type in the Pennsylvania–Delaware Piedmont includes three tectonic units: (1) the basement-cored massifs of southeastern Pennsylvania with Grenvillian gneiss unconformably overlain by the Setters Formation and Cockeysville Marble; (2) the Wilmington Complex arc terrane; and (3) the Wissahickon Group and associated metaigneous rocks. A metamorphic gradient and inferred temperature inversion within the Wissahickon Group and a metamorphic discontinuity at the Wissahickon Group–Cockeysville Marble contact are used to establish that the Wilmington Complex was first joined with the Wissahickon Group and that the two units were then emplaced together onto the rocks now exposed in the massifs. An observed correlation of amphibolite-facies mineral assemblages in the Cockeysville Marble with their distance from the Wissahickon Group indicates that peak metamorphism of the marble occurred after the metapelites were emplaced. As amphibolite-facies metamorphism of the basement-cored massifs has been previously dated as Taconian, it follows that the assembly of these tectonic units occurred in the early Paleozoic.

Petrogenesis of the copper-bearing skarn at the Mason Valley Mine, Yerington District, Nevada

Skarn formation at the Mason Valley mine occurred at a depth of 2,000 m on the outer fringe of a contact metasomatic aureole related to a Jurassic granodiorite to quartz monzonite batholith. The skarn is located in Upper Triassic limestone at the contact with a stratigraphically lower tuff unit and is systematically zoned relative to this contact. The general zonal sequence toward marble is: garnet, garnet-pyroxene-sulfides, pyroxene-sulfides, tremolite-magnetite-calcite, talc-magnetite-calcite, and dolomite-calcite. The zones migrated outward with time.Electron microprobe data indicate that garnets and pyroxenes in the barren garnet footwall zone have compositions similar to the districtwide early metasomatic hornfelses and are represented by low-iron diopsides and intermediate grandites. Pure andradite, as overgrowths and in cross-cutting veins, becomes more abundant as the hanging-wall skarn is approached. Both garnets and pyroxenes shift abruptly to higher iron contents in the hanging-wall skarn which formed at the contact between the early garnet zone and dolomitized limestone. Garnets maintain a constant lower limit of 55 mole percent andradite and are zoned to pure andradite. Later garnets, contemporaneous with chalcopyrite deposition, consist of pure andradite. The iron content of pyroxene increases gradually and systematically toward the marble contact, from an average value of 36 mole percent hedenbergite in the inner garnet-pyroxene zone to 56 mole percent hedenbergite in pyroxene vein centers on the marble contact, and then drops abruptly to 15 mole percent hedenbergite in vein envelopes. Two generations of amphibole are represented by: (1) early tremolite (0 to 10 mole percent ferrotremolite) associated with magnetite-calcite in outermost vein envelopes in marble; and (2) actinolite which contains the same Fe/Mg ratio as associated pyroxene and is contemporaneous with chalcopyrite deposition in the pyroxene and garnet-pyroxene zones.The initial silication process, as represented by zoned veins at the marble contact, may be attributed to isothermal metasomatic diffusion of Ca, Mg, and Si, with X (sub CO 2 ) decreasing toward the vein centers. The abrupt appearance of new minerals coincides with the attainment of appropriate chemical potential values through metasomatism, rather than the crossing of isobaric univariant T-X (sub CO 2 ) equilibria. Bulk composition gradients are extreme and are reflected in the rapid increase in iron content of tremolite and diopside over a few centimeters from vein envelope to vein center.Within the main skarn zone, which formed at higher temperature and/or lower X (sub CO 2 ) , bulk composition gradients are less extreme, and phase-composition trends are opposite to those that would be predicted by a simple isothermal diffusion model. The gradual inward decrease in the iron content of salite within skarn zones of relatively constant bulk composition may have been controlled in part by continuous Fe-Mg reactions.Comparison with phase-composition data from similar zoned skarns indicates that variation of phase compositions within zones is a characteristic phenomenon, but that zonal composition trends in some cases are opposite to those established here.

Vein Assemblages and Metamorphism in Dutchess County, New York

Mineral assemblages of fissure veins in pelitic rocks are a consistent function of metamorphic grade within a 1,700 km 2 area in and near Dutchess County, New York. The following vein assemblages are cb-served with increasing grade: quartz and quartz-calcite up to just above the staurolite isograd, with limited occurrence of quarrz-albite below the biotite isograd; quartz and quartz-plagioclase (An 20 to An 50 ) from the staurolite isograde up to the sillimanite-orthoclase isograd; and quartz, quartz-plagioclase, and quartz-plagioclase-orthoclase above the sillimanite-orthoclase isograd. Extreme deformation makes relative crosscutting relations of veins difficult to determine; however, late quartz veins are fairly common, and late quartz-calcite veins are seen in high-grade rocks near marble contacts. A tentative conclusion is that quartz and quartz-calcite, quartz-plagioclase, and quartz-plagioclase-orthoclase veins form in pelitic rocks at successively higher metamorphic grades from material derived from the surrounding matrix. This may happen during prograde and retrograde metamorphism and during successive metamorphic events.

Mechanisms of magmatic assimilation at a marble contact, northern Haiti

The unusually large Meme-Casseus and Bassin assimilation zones surround marble blocks in the 66 m.y.-old, epizonal Terre-Neuve quartz monzonite of northern Haiti. The Meme-Casseus zone, which contains syenodiorite, granodiorite, and minor nepheline syenite, occupies more volume than did the marble block. Therefore, dispersal, as well as addition, of material was required to form the contaminated zones. Mechanisms of addition of material to a magma can include congruent or incongruent melting of wall rock at normal or depressed melting points followed by solution of the melted material, or solution of solid rock by the magma. At the Meme-Casseus zone, minor melting may have occurred, but solution of solid marble was the dominant addition mechanism. Dispersal can take place by diffusion, crystal sinking, or convection, and at Meme-Casseus dispersal was caused largely by convection. The relation of addition to dispersal at the Meme-Casseus assimilation zone suggests that the formation of large assimilation zones is controlled by the mechanism of dispersal of material.

Formation of Pegmatitic Carbonatite in a Syenite–Marble Contact

IMMEDIATELY north of the village of Karacayir, vilayet Sivas, Turkey, a small intrusion of rather coarse-grained syenite occurs in a banded marble formation of unknown age. Here I had occasion to study in some detail a peculiar coarse-grained marble of pegmatitic appearance, containing biotite and apatite. The banded marbles seem to be regionally metamorphosed, on which the thermal metamorphism caused by the intrusion is superimposed. The part of the marble which protrudes into the syenite is changed into a carbonatite–pegmatite. Otherwise the marbles retain their regular banding, which even seems to be accentuated when nearing the contact. Within the aureole of thermal metamorphism small crystals of phlogopite and diopside develop in small amounts on the stratification planes as well as throughout the marble. Near the contact small dikes of syenite–aplite and more-or-less crushed syenite–pegmatite cut through the marbles; the syenite itself contains locally muscovite.