Magnesian Skarn Research Articles

The Carlés copper–gold–molybdenum skarn (Asturias, Spain): geometry, mineral associations and metasomatic evolution

This paper presents the petrographical, mineralogical and geochemical characteristics of the Carlés Cu–Mo–Au ore deposit, located in the Rio Narcea Gold Belt (Cantabrian zone of the Iberian Massif). It is related to a small postkinematic calc-alkaline monzogranite, which intrudes as a cedar-tree laccolith into the upper siliciclastic Furada Formation (late Silurian age) and the Nieva carbonates (early Devonian age). The Carlés deposit consists mainly of a well-developed exoskarn. The exoskarn is mostly calcic skarn made up of early garnet and pyroxene, and later amphibole, magnetite and sulfides. The presence of magnesian skarn has been recorded on the north side of the intrusion (roof of granitoid). Magnesian skarn consists of olivine, which is partially replaced by diopside and phlogopite and spinel. Close to the igneous rock, skarns are overprinted by strong potassic alteration. The ore is related to the skarn retrogradation and post-skarn veining and faulting. The skarn-related ore consists of earlier, uneconomic magnetite and Fe–As sulfide assemblages and economic Cu–Au–Ag (Bi–Te) assemblages on the eastern and western sides of the contact aureole, and uneconomic Mo and subeconomic Fe–As–Cu–Au–Ag on the northern side of the contact. Later subeconomic Fe–As–Sb–(Zn–Sn–Cu–Au–Ag) assemblages crosscut the granitoid, skarn, marbles and mineral associations developed previously, and are related to younger episodes of fracturing and faulting. Fluid inclusions in the first hydrothermal stage consist of an aqueous solution with significant contents of CO2, which reach unmixing conditions as a result of a decrease in P–T conditions. This led to two types of solutions, aqueous solutions of moderate to high salinity and hydrocarbon solutions of low salinity. This unmixing phenomenon controlled the first stage of gold precipitation. During the late hydrothermal activity, primary low-salinity-aqueous-carbonic inclusions with contrasting densities are found. They homogenize into vapor, critical or liquid phase. Homogenization temperatures are practically the same in all inclusions, indicating a boiling phenomenon that could control a new precipitation of gold.

300 Million years of episodic hydrothermal activity: stable isotope evidence from hydrothermal rocks of the Eastern Iberian Central System

The Eastern Iberian Central System has abundant ore showings hosted by a wide variety of hydrothermal rocks; they include Sn-W, Fe and Zn-(W) calcic and magnesian skarns, shear zone- and episyenite-hosted Cu-Zn-Sn-W orebodies, Cu-W-Sn greisens and W-(Sn), base metal and fluorite-barite veins. Systematic dating and fluid inclusion studies show that they can be grouped into several hydrothermal episodes related with the waning Variscan orogeny. The first event was at about 295 Ma followed by younger pulses associated with Early Alpine rifting and extension and dated near 277, 150 and 100 to 20 Ma, respectively (events II–IV). The δ18O-δD and δ34S studies of hydrothermal rocks have elucidated the hydrological evolution of these systems. The event I fluids are of mixed origin. They are metamorphic fluids (H2O-CO2-CH4-NaCl; δ18O=4.7 to 9.3‰; δD ab.−34‰) related to W-(Sn) veins and modified meteoric waters in the deep magnesian Sn-W skarns (H2O-NaCl, 4.5–6.4 wt% NaCl eq.; δ18O=7.3–7.8‰; δD=−77 to −74‰) and epizonal shallow calcic Zn-(W) and Fe skarns (H2O-NaCl, 14 wt% NaCl eq.) from evaporites hosted in the post-Variscan sequence. The δD values are very similar to most of those recorded by Kelly and Rye in Panasqueira and confirm that the Upper Paleozoic meteoric waters in central Iberia had very negative δD values (≤−52‰) whereas those of Early Mesozoic age ranged between −65 and −36‰.

The influence of Al on the physical and crystallographic properties of ludwigite in three Romanian occurrences

Ludwigite samples from three boron-bearing magnesian skarns in Romania (Ocna de Fier, Masca-Baisoara and Pietroasa) were used to determine the influence of Al on the main physical and crystallographic constants of this mineral. The selected samples form a minor solid solution towards vonsenite (0.74 to 23.87 mol.%) and have X Al = Al/(Al + Fe 3+ ) ratios between 0.004 and 0.300. Other substitutions, such as those involving Mn, Ni, Co, Ti, Sn, Sb or V are of minor importance. Generally speaking, it is proven that ludwigite shows a negative correlation between X Al and the density, indices of refraction, cell volume and c cell parameter. It is not possible to quantify the variations in the physical and crystallographic parameters by advancing the Al-for-Fe 3+ substitution because both Fe 2+ -for-Mg and Al-for-Fe 3+ substitutions are involved. However, it is clear that Al plays a major role in decreasing the cell volume, c cell parameter, density, absorption coefficient, bireflectance and refraction indices of ludwigite. Because of the influence of Al, natural compositional members of the ludwigite-vonsenite series cannot be unambiguously distinguished, neither by their cell dimensions nor by their physical parameters as previously suggested ( e.g., Aleksandrov, 1968; 1982).

Geology of gold-bearing skarn deposits in the middle and lower Yangtze River Valley and adjacent regions

The middle and lower Yangtze River Valley and adjacent regions are the most important metallogenic belt of gold (and copper)-bearing skarn deposits in China. The total gold reserves in this belt have been estimated at more than 600 t. The gold-bearing skarns are mainly distributed in the southeastern Hubei, Tongling and northern Anhui regions. Favorable tectonic settings are depressions and fold zones of the platforms, i.e., mobile belts. These skarns are hosted by platformal limestone, dolomitic limestone and dolomite of the Triassic, Carboniferous-Permian and Middle to Lower Cambrian formations. The related intrusions are Yenshanian (180 to 113 Ma) calc-alkaline quartz monzodiorite, granodiorite, quartz monzonite, monzogabbro, and their hybabyssal facies. The intrusions have high Fe 2O 3/FeO (>0.5) and intermediate initial 87 Sr/ 86 Sr ratios (0.7046 to 0.7087). Their REE distribution patterns are LREE-enriched and exhibit smooth, right-dipping curves. These suggest that the source materials mainly came from upper mantle, with contamination by sialic crustal components. The auriferous skarns are both calcic and magnesian, but calcic skarns are most common. The constituent minerals of the calcic skarns are diopside, garnet, wollastonite, vesuvianite and scapolite, whereas magnesian skarns are dominated by forsterite, spinel, diopside, phlogopite, chondrodite and clinohumite, with abundant superimposed serpentine, clinochlore and brucite. The compositions of coexisting pyroxenes and garnets are diopside and andradite, indicating the high oxygen fugacity and low acidity conditions. Gold is closely associated with Cu (Pb, Zn) sulfides and exists mainly in the form of native gold and electrum. Arsenides, tellurides, bismuthides and selenides are present in many ore deposits. Therefore, Cu, As, Bi, Te, Ag, Pb, Zn, Se and Co are the major metals present in the deposits and are important geochemical ore-searching indicators. In some Au (Fe, Cu) magnesian skarns, magnesiomagnetite, magnesioferrite and ludwigite are locally abundant. The metasomatic zoning in many gold skarn deposits is very distinct consisting of an outward sequence of: Fe (Cu)→Cu (Mo)→Cu (Au)→Au (Cu)→Au (Pb, Zn). The geologic characteristics of Au (Cu) skarn deposits that formed in the mobile platformal setting of China have distinct differences compared to Au skarns formed in orogenic belts at convergent plate margins in British Columbia and the western USA.

The Discovery of Magnesioferrite from Au (Fe, Cu) Magnesian Skarn Deposits and Study of the Magnesioferrite‐Magnesiomagnetite Series

Abstract: Magnesioferrite, a rare metasomatic mineral, was discovered for the first time in China from the Qinlou Au (Fe, Cu) magnesian skarn deposit, Sanpu, Huaibei, Auhui Province, and the Mulonggou Fe (Mo, Cu) magnesian skarn deposit, Luonan County, Shaanxi Province. In this paper, the geological setting, mineral associations, chemical composition, some physical properties, X‐ray powder diffraction data and infrared spectroscopy of magnesioferrite and magnesiomagnetite are discussed. Magnesioferrite contains 17.66%–13.48% of MgO. Its main associated minerals are clinohumite, chondrodite, serpentine, calcite and magnesiomagnetite. The density of magnesioferrite is 4.537–4.720, reflectances in percent are: 17.8–18.1, hardness is 838–900 kg/mm2, and the cell parameter ao = 8.371–8.379 Å. A systematic study of the magnesioferrite‐magnesiomagnetite‐magnetite series suggests that along with the increase of magnesioferrite molecules in the mineral, the density, reflectances and cell parameters decrease correspondingly, the hardness heightens, and the infrared absorption spectral band becomes wider. The authors consider that magnesioferrite is a product of contact metasomatism between hypabyssal intermediate‐acid intrusions and dolomitic marble. It was formed in shallow exocontact zones under relatively oxidized conditions.

Geochemistry and mineralization age of magnesian skarn-type iron deposits of the Janggun mine, Republic of Korea

The Janggun iron deposits, Republic of␣Korea, occur as lens-shaped magnesian skarn, magnetite and base-metal sulfide orebodies developed in the Cambrian Janggun Limestone Formation. Mineralization stage of the deposits can be divided into two separate events. The skarn stage (107 Ma) consists of magnetite, pyrrhotite, base-metal sulfides, carbonates and magnesian skarn minerals. The hydrothermal stage (70 Ma) consists of base-metal sulfides, native bismuth, bismuthinite, tetrahedrite, boulangerite, bournonite and stannite. Mineral assemblages, chemical compositions and thermodynamic considerations indicate that formation temperatures, −log fs2 and −log fo2 values of ore fluids from the skarn stage were 433 to 345 °C, 8.1 to 9.7 bar and 29.4 to 31.6 bar, and the hydrothermal stage was 245 to 315 °C, 10.4 to 13.2 bar and 33.6 to 35.4 bar, respectively. Thermochemical considerations indicate that the XCO2 during magnesian skarnization ranged from 0.06 to 0.09, and the activity of H+ presumably decreased when the fluids equilibrated with host dolomitic limestone which resulted in a pH change from about 6.1 to 7.8, and decreases in fo2 and fs2. The δ34S values of ore sulfides have a wide range from 3.2 to 11.6 ‰ (CDT). Calculated 34SH2 S values of ore fluids are 2.9 to 5.4 ‰ (skarn stage) and 8.7 to 13.5 ‰ (hydrothermal stage). These are interpreted to represent an initial deep-seated, igneous source of sulfur which gave way to influence of oxidized sedimentary sulfur to hydrothermal stage. The δ13C values of carbonates in ores range from −4.6 to −2.5 ‰ (PDB). It is likely that carbon in the ore fluids was a mixture of deep-seated magmatic carbon and dissolved carbon of dolomitic limestone. The δ18OH2 O and δD values (SMOW) of water in the ore fluids were 14.7 to 1.8 and −85 to −73 ‰ during the skarn stage and 11.1 to −0.2 and −87 to −80 ‰ in the hydrothermal stage.

Chemical and stable isotopic compositions of Proterozoic metamorphosed evaporites and associated tourmalines from the Houxianyu borate deposit, eastern Liaoning, China

An integrated major element, trace and rare earth element, and stable isotope (B, Si, O and S) study has been carried out on the Houxianyu borate deposit, eastern Liaoning, China. The present-day mineralogy of the deposit is closely similar to that of magnesian skarn deposits and the borates are underlain by granite; however, the geochemical and isotopic evidence does not favour a skarn-type origin. In particular, the boron isotopic compositions of the borate minerals and associated tourmalines preclude their origin from fluids derived from the granite. Rather, the data are compatible with the borate deposit being a metamorphosed evaporite. The mineralogy and chemistry of the deposit are most compatible with a non-marineorigin, although the high δ 34S- and δ 11B-values suggest that there may have been some marine components to the evaporitic brines. Overall, the premetamorphic origin of the Houxianyu borate deposit is similar to that of the Tertiary non-marine evaporite borate deposits of western Turkey. There are also similarities with other Proterozoic early rifting environments (e.g., the Upper Proterozoic Damaran Orogen of South Africa) that have implications for the structural evolution and mineral potential of the region.

Geochemical characteristics of halogen-bearing hastingsite, scapolite and phlogopite from the Sangan iron skarn deposits, northeastern Iran.

Mesozoic shale, siltstone, sandstone and carbonate rocks and Eocene volcanic, volcaniclastic and pyroclastic rocks are intruded by late Eocene granitic batholith and related acidic dikes. As a result of hydrothermal activity associated with the granitoid, two distinct skarn zones of calcic skarn zone and magnesian skarn zone were formed in the carbonate rocks. Cl-bearing amphiboles of ferroactinolite to potassium rich ferrohastingsite composition are restricted to the calcic skarn zone and associating with salite, grandite, magnetite, calcite and sometimes with scapolite. The Cl content of amphiboles shows positive relationships with Fe2+/(Fe2++Mg) and K/(K+Na) and a negative relation with Si/(Si+AlIV). In contrast, F bearing phiogopite is a major mineral in the magnesian skarn. F content exhibits positive relationship with wt% of SiO2 and MgO. The estimated log(fH2O/fHF) equilibrating with the phlogopite ranges from 4.6 to 5.5 at 400°C. Cl rich scapolite is widespread in both calcic and magnesian skarn zones. The scapolite has a compositional range of 23 to 40 mole% meionite in marialite-meionite solid solution series. XC1 of the scapolite ranges from 0.44 to 0.98 (mostly more than 0.75). Scapolite of XC1=0.98 from the hornfels is the most Cl-rich scapolite ever reported. Cl content of the scapolite positively correlate with Na/(Na+Ca+K). Calculated XNaCl values for fluid in equilibrium with scapolite from the Sangan deposits reach up to more than 55 wt% equivalent NaCl. The Cl-rich hastingsite, Cl-rich scapolite, F-rich phiogopite, and fluid inclusions with high salinity (up to 70 wt% eq. NaCl) in granitic rocks and in skarn zones suggest that hydrothermal fluids were derived from the granitoid magma. The ore elements such as Fe and Cu could be transported by solutions as metal-chloride complexes. Large amounts of cations such as Al, Si, and Ca incorporated in the fluids should be derived from the country rocks and from the breakdown products of early skarn minerals.

Dynamics of O re‐Forming Processes of the Stratabound Skarn Copper Deposits of Tongling, Anhui Province

Abstract The skarn and ore bodies of the stratabound skarn copper deposits of Tongling, Anhui Province, are both controlled by definite stratigraphic horizons, and they are concordant with the strata. They occur as layers and layer‐like bodies in permeable carbonate rocks of the Middle‐Upper Carboniferous Huanglong and Chuanshan Formations which are underlain by impermeable shale or siliceous rocks of the Upper Devonian Wutong Formation. The authors study the dynamics of ore‐forming processes of the ore deposits with the dynamic model of coupled transport and reaction, and the following results are obtained: The salinity gradient and flow rate of the ore‐forming fluids can both promote the mixing and reaction of juvenile water and formation water, and the permeable strata are favourable sites for the intense transport‐reaction of mixing and the formation of deposits. (2) As isothermal transport‐reaction took place along the bedding of strata, the moving transport‐reaction front formed at the contact between the ore‐forming fluids and the rocks advanced slowly along the permeable strata, and then stratiform skarn and ore bodies concordant with the strata were formed. (3) The gradient transport‐reaction taking place across the isotherms in the cross‐bedding direction caused the mineralogical composition to alter gradually from magnesian skarn to sulphide ore bodies.

The Palaeoproterozoic boron deposits in eastern Liaoning, China: a metamorphosed evaporite

We have carried out a field study of the Palaeoproterozoic borate deposits of eastern Liaoning Province, China. The borates are hosted within Mg-silicate and Mg-carbonate rocks and have previously been interpreted as either skarns or metamorphosed volcanogenic exhalative deposits. The deposits contain textures that are compatible with a sedimentary origin of the borates and there is a lack of the associated mineralization that is characteristic of magnesian skarns. In addition, preliminary boron isotope data indicate that the boron within the borates was not derived from the footwall rocks that have been proposed as the source of the skarn fluids. A purely exhalative model can be dismissed on the basis of geochemical arguments and comparison with modern environments, which show that borates are too soluble to precipitate directly from hydrothermal fluids. The geology and field relationships show a number of similarities with recent non-marine evaporite settings and young continental rifts in arid environments. We conclude, therefore, that the Liaoning deposits are metamorphosed evaporites, in which the boron was originally present as primary, evaporite borate minerals. This model places the Liaoning borates in the same category as the majority of other economic borate deposits in the world.

Skarn CuAu orebodies of the Gunung Bijih (Ertsberg) district, Irian Jaya, Indonesia

The major CuAu skarn deposits of the Gunung Bijih (Ertsberg) district in central Irian Jaya are products of hydrothermal systems that developed in association with Pliocene magma emplacement in an active continental margin. The CuAu skarn orebodies occur within a Cretaceous to Tertiary sedimentary sequence that was deformed as the northern Australian continental margin entered a north-dipping subduction zone at ∼ 12 Ma. The intermediate-composition intrusions consist of fine-grained porphyritic stocks, dikes, and sills that have K-Ar ages ranging from 2.7 to 4.4 Ma. Most intrusions are slightly potassic, but these data could be affected by alteration. The skarn orebodies in the Ertsberg district are hosted in deformed lower Tertiary New Guinea Group carbonate strata along the periphery of the Pliocene Ertsberg intrusion. Major skarn orebodies include the Ertsberg (GB), the Ertsberg East (GBT) complex, including the GBT, the Intermediate Ore Zone (IOZ) and the Deep Ore Zone (DOZ), and the Dom. Chalcopyrite is the dominant ore mineral in the GB and Dom orebodies, whereas bornite dominates in the GBT complex. Native Au occurs within bornite and chalcopyrite in GB and GBT ores. The district calc-silicate alteration assemblages are characterized by high-temperature skarn minerals, including forsterite, monticellite, and minor melilite. Diopsidic clinopyroxene is common, particularly in GBT. Anhydrite and phlogopite are abundant in the GBT complex, and the anhydrite:calcite ratio increases with depth from GBT to DOZ where anhydrite is ubiquitous and calcite rare. At least three types of garnets have been identified at the Dom and show a progressive increase in ferric iron content. Garnet decreases with depth in the GBT complex. Talc, serpentine, tremolite-actinolite, and chlorite are common retrograde minerals. Copper sulfide mineralization is texturally associated with early retrograde alteration. Differences among the skarn orebodies are related in part to variable protolith composition that affected skarn development within different stratigraphic positions. Distinctive fossil replacement textures preserved within skarn indicate that the Oligocene-Miocene Ainod Formation is the most likely protolith for the GB and Dom orebodies. The GBT and upper IOZ orebodies probably are hosted by the Eocene Faumai Formation. The DOZ and lower IOZ orebodies, dominated by magnesian skarn alteration, appear to be developed in a dolomitic unit within the lower New Guinea Limestone Group, which probably is equivalent to the Paleocene Waripi Formation.

Boviksgruvan, a Au-Bi-bearing sulphide deposit in the Bergslagen Province, south central Sweden

Abstract The Boviksgruvan deposit, situated 18 km northeast of Falun, is a gold-bearing magnesian skarn deposit which was mined for iron and zinc during the 18th and 19th centuries. Magnetite, sphalerite, galena, chalcopyrite, pyrite, and pyrrhotite dominate the ore mineral assemblage. In addition, gold, native bismuth, galenobismutitc, cosalite, joseite A, and joseite B are specific constituents recognized in the Boviksgruvan deposit. The ore can be divided into two mineralogical types. Ore type 1 consists of a biotite-magnetite assemblage with disseminated galena and Pb-Bi-sulphosalts. Ore type 2 consists mainly of iron sulphides, base metal sulphides, and anthophyllite. Gold is often intergrown with native bismuth, as inclusions in galena or as cavity fillings in magnetite. According to the cross-cutting relations recognized between magnetite and Pb-Bi-sulphosalts in ore type 1, these sulphosalts, as well as gold, are considered to represent a younger ore forming epoch in relation to the magnetite. Fur...

The Savage Lode magnesian skarn in the Marvel Loch gold–silver mine, Southern Cross greenstone belt, Western Australia. Part 1: Structural setting, petrography, and geochemistry

The Marvel Loch mine in the Archean Yilgarn Block. Western Australia, has produced 9.70 t gold and 1.92 t silver from 3.28 × 106 t of oxide–sulphide ore. The deposit is located in the Southern Cross greenstone belt, and occurs within the medium-grade metamorphic aureole of the Ghooli Dome granitoid batholith, about 1.5 km from the granitoid–greenstone contact, The deposit is controlled by a broad ductile shear zone and spatially associated with late syn- to post-mineralization pegmatite dykes. The major orebodies, represented by the Savage Lode, are hosted by a uniform sequence of metakomatiites, and form zoned replacement bodies oriented subparallel to the steeply dipping foliation of the shear zone. Lens-shaped domains of pillowed metakomatiiles (hornblende + cummingtonite + chlorite) are locally preserved between the orebodies.The gangue of the Savage Lode is laterally zoned, and shows distinct similarities to Phanerozoic magnesian skarns. Calcite–olivine rock and calcite–phlogopite–chlorite schist occur in the centre, and are enveloped by three types of calcite-poor ore, namely banded diopside–amphibole rock, quartz–diopside veins, and tremolite–phlogopite schist. The latter grades laterally into subeconomic, outer tremolite–chlorite schists. Local retrogression of prograde alteration minerals is evident in the partial replacement of olivine by iddingsite and serpentine, and in the occurrence of late muscovite, clinozoisite, and prehnite. The gangue in the lode reflects strong carbon dioxide, calcium, and potassium metasomatism.Hydrothermal oxides and sulphides (2–5 vol.%) occur disseminated throughout the Savage Lode. The oxide assemblage includes hercynite–spinel, magnetite, ilmenite, and scheelite, whereas the sulphide assemblage is dominated by pyrrhotite, loellingite, and arsenopyrite. Native gold occurs as discrete grains (0.001–3 mm) intergrown with sulphides or enclosed in gangue minerals. The average magnesian ore skarn is characterized by a MgO–FeO ratio of 2.0:1, a Au–Ag ratio of 1.7:1, and low base metal (< 500 ppm), anomalous tungsten (20–40 ppm), and high arsenic (2900 ppm) contents. The Savage and other lodes in the Marvel Loch mine may be classified as gold or gold–silver skarns by economic metal content. The magnesian rather than calcic nature of the Marvel Loch skarns is related to the high magnesium content (21 wt.% MgO) of the precursor metakomatiite rocks.

The Savage Lode magnesian skarn in the Marvel Loch gold–silver mine, Southern Cross greenstone belt, Western Australia. Part 2: Pressure–temperature estimates and constraints on fluid sources

The Savage Lode gold skarn orebody in the Marvel Loch mine, Southern Cross greenstone belt, Western Australia, replaces foliated metakomatiites in a subvertical ductile shear zone, and is located within the broad metamorphic aureole of the Ghooli Dome granodiorite–granite batholith. Pressure estimates based on metamorphic as well as alteration mineral parageneses indicate that skarn formation took place at P = 4 ± 1 kbar (1 kbar = 100 MPa), corresponding to a burial depth of about 13 km. Metamorphism reached temperatures of 550–630 °C in the mine area, and preceded skarn formation.Pressure–temperature calculations constrain the maximum temperature of the hydrothermal fluid in the Savage Lode to 640 ± 20 °C during the formation of early olivine–calcite and diopside–amphibole rocks, and to 500–600 °C during the formation of phlogopite–chlorite–calcite schists and quartz–diopside veins. Lower fluid temperatures (500 to ~ 400 °C) are recorded by retrograde serpentine (after olivine), talc (after tremolite), and petrographically late aggregates of muscovite + clinozoisite + prehnite. The oxygen fugacity of the fluid is estimated at 10−20–10−24 bar (1 bar = 100 kPa), based on the assemblage magnetite + ilmenite + olivine in calcite-rich prograde skarn. The sulphur fugacity of the fluid is estimated at 10−7–10−9 bar, based on the assemblages pyrrhotite + loellingite, pyrrhotite + arsenopyrite and pyrite + marcasite, which were deposited in a retrograde régime, when fluid temperatures fell from 550 to < 400 °C.The isotopic composition of the fluid (206Pb/204Pb = 13.77, 87Sr/86Sr = 0.7024, δ13C = −4.8‰), as inferred from hydrothermal galena, scheelite, and calcite, indicates equilibration at high temperatures with rocks of granodioritic or granitic composition. Late syn- to post-mineralization pegmatite dykes, exposed in the present mine workings, provide evidence for magmatic activity at depth. The Marvel Loch deposit represents the first well-described example of an Archean gold skarn system.

Prediction-estimate and prospecting criteria for ore deposits in magnesian and calc skarns

Prediction-estimate and prospecting criteria for ore deposits in magnesian and calc skarns

Inclusions of mineral-forming media in principal minerals of magnesian skarns in the Katalakh and Tsvetkovo phlogopite deposits

(1981). Inclusions of mineral-forming media in principal minerals of magnesian skarns in the Katalakh and Tsvetkovo phlogopite deposits. International Geology Review: Vol. 23, No. 8, pp. 927-930.

The temperature regime of crystallization of post-phlogopite mineral associations in magnesian skarns of Central Aldan region

The temperature regime of crystallization of post-phlogopite mineral associations in magnesian skarns of Central Aldan region

Chemical features of spinels from a magnesian skarn formation

(1980). Chemical features of spinels from a magnesian skarn formation. International Geology Review: Vol. 22, No. 12, pp. 1411-1420.

The högbomite polytypes

SummarySingle crystal X-ray examination has shown that högbomite forms a series of polytypes, designated nH or nR, with hexagonal or rhombohedral lattices and hexagonal unit-cell dimensions a 5·72 Å, c 4·6 × n Å. The polytypes arise by variation, in a manner as yet undetermined, of the stacking sequence of approximately close-packed oxygen layers with interstitial cations on fourfold and on sixfold sites; the composition of 1/nth of a unit-cell may be represented as R2+1.0_1.6T4+0.2-0.4R3+3.7-4.3O2-7.6-8.0(OH)-0-0.4,, where R2+ = Zn, Fe, Mg, and R2+ = Fe, Al. The polytypes so far observed are 4H, 5H, 6H, 15H, 15R, and 18R. Minerals structurally related to högbomite are nigerite (3H) and taaffeite (4H). A new occurrence of högbomite, polytype 5H, with composition Ti1·7Fe1·6Mg6·3Al18·8Si0·2O40, is described from a spinel-free paragenesis in a magnesian skarn at Mautia Hill, Tanganyika. Another new occurrence in an aluminous xenolith in the Cashel gabbro in Co. Galway, Ireland, is recorded. X-ray powder data are given for two of the polytypes.