Formation of Mn-skarn Ores at Thapsana Mines, Paros Island, Attico-Cycladic Metallogenetic Massif, Greece

Mn-skarn ore deposits are relatively infrequent worldwide. A typical example of a Mn-skarn in the Attico-Cycladic Metallogenetic Massif is located at the mining area of Thapsana, Paros Island. The skarn occurs adjacent to the Thapsana, highly sericitized, biotite-garnet-tourmaline-lepidolite leucogranite apophyses of the Paros granitoid and related pegmatites and aplites The Mn-skarns orebodies occur as lenses and NE-trending veins hosted in the Cyclades Blueschist Unit (CBU) marbles and intercalated calcic schists. They comprise two discontinuous paragenetic zones (with widths of ≤ 10 m): A zone that contains vesuvianite, Mn-enriched salite to johannsenite-diopside and spessartine with cores enriched in grossular component (Sps ~75 Grs ~15 ), placed close to the Thapsana leucogranite, and a zone of grossular (Sps ~85 Grs ~5 ), actinolite to Mn-cummingtonite (with Mn ~ 0.6 apfu) and phlogopite more distal from the leucogranite. The Mn-skarns are crosscut by later WNW-to W-trending veins filled with Ca-K-Mg-bearing pyrolusite, manganite, rhodonite and rhodochrosite, carbonates, hydroxylapatite and johnbaumite. The Mn-ores

An illustrative example of a Mn-skarn is placed in the Attico-Cycladic Metallogenetic Massif, Greece, and exposed at the mining area of Thapsana, Paros Island.The Thapsana Mnskarn is unusual as occurs adjusted to the homonymous spessartine-tourmaline-biotitelepidolite leucogranite apophyses of the Paros granitoid and related pegmatites and aplites.
Herein, we report the results of a comprehensive study of the geological, mineralogical, paragenetic and thermodynamic estimations of the Mn-skarns and stable isotopic compositions of the ore minerals (e.g., δ 18 Ο, δD, δ 44 Ca, δ 26 Mg).We have used this information to constrain the physicochemical conditions of the Thapsana skarn and ore formation.

Geological setting of Cyclades and Paros Island
The Attico-Cycladic Metallogenetic Massif (ACM) has a complex geological, magmatic and tectono-metamorphic history that documents the closure of a Neotethyan ocean basin and associated subduction and collision during the Cenozoic.The latter resulted from the convergence between the Apulian microplate and the Eurasian continent from Jurassic until Miocene [8].Later, an extensional tectonic setting associated with southward retreat of the Hellenic subduction zone and westward extrusion of the Anatolian plate contributed to the final exhumation and exposure of the ACM rocks at the surface [9].
Above the CBU and below the Upper Cycladic unit (UCU), there is a very low-grade unit of possibly Permian age that consists of marbles, phyllites and metabasic rocks (formerly the Dryos nappe of [10]).A low angle detachment separates the MCC footwall block from the hanging-wall UCU (formerly the Marmara nappe of [10]).The structurally higher UCU is a poorly preserved heterogeneous sequence that locally overlies the CBU.It mainly comprises Permian to Mesozoic marbles, dismembered ophiolites and Late Cretaceous greenschist-toamphibolite facies rocks that show no evidence of high pressure metamorphic conditions [17].
The tectonic juxtaposition of UCU with the underlying CBU was possibly accomplished in early Miocene [18].The UCU sediments on Paros are mainly exposed on the eastern part of the island and consist of a gently folded, incomplete marine sequence overlain by a polymictic conglomerate interbedded with sandstones of variable grain size and bed thickness.These lower clastic sedimentary rocks are topped by flat-lying limestone and travertine deposits of possible Pliocene age [19].

The Paros and Thapsana leucogranites and contact aureole
In the mid-to late Miocene, the magmatic arc migrated to the south producing syn-to postextensional granitoids that cut the regional foliation and in some cases are deformed by the extensional detachments [20].However, in few cases granitoids ascent and emplace under a transitional setting from transpression to extension [21].The Miocene granitic intrusions of Paros occur as small ellipsoid laccoliths, pipes and apophyses in the areas of Kolymbithres, Kamares, Paroikia, Taxiarxes, Logovathra (at the northern part of Paros), Thapsana and Trypiti (at the central and southern parts of Paros) (Fig. 1).These intrusive bodies are classified as monzogranites-to-granites and leucogranites, appear highly porphyritic and intense sericitized and propylitized and contain dioritic enclaves [22,23].They have been dated at 11.5-12.4± 0.2 Ma (K-Ar biotite and muscovite [11]) and 17.1 Ma (K-Ar and Rb-Sr biotite and muscovite [11]), respectively.The Kolymbithres leucogranite was dated by [24] at 15.5 ± 0.3 to 15.7 ± 0.2 Ma ( 238 U/ 206 Pb on zircon).
The intrusion of Paros leucogranites has caused an extensive contact aureole (≤ 500m [10]) over the adjacent metamorphic rocks of BU and CBU.The hornfelses proximal to the leucogranitic intrusions with metabauxite protoliths comprise the assemblage sillimanitekyanite-staurolite-corundum whereas those with metabasite protoliths scapolite-diopsidegarnet.More distal from the intrusions hornblende and biotite hornfelses are formed over the metapelites of the CBU [10].

Local geology
At the abandoned mines of Thapsana, with reserves of ~ 1Mt, mining activity took place during the period of 1950-1965 by Giannoukos Co, with an annual production of ≈ 2500 t.The skarns outcorp at the migmatized gneisses of the BU and cataclastic marbles that intercalate with the calcic schists, amphibolites and hornfelses of the CBU.The Thapsana biotitemuscovite leucogranite which occurs as an apophysis, mostly intrudes in the CBU marbles and amphibolites along an N-to NNE-trending zone that is also crosscut by pegmatite and aplite dikes.In this zone the skarns are more frequent.The skarn orebodies occur as NE-trending veins, lenses and layers that follow the foliation of their host CBU rocks.
The Mn-ores occur as massive aggregates or even disseminated, enclosed in the skarns.

Petrography
In the Thapsana area, microscopically, three metasomatic zones were recognized based on spatial and textural relations of Mn-spinels, -pyroxenoids, -pyroxenes, -garnets, andamphiboles.The first zone (with widths of ≤ 10 m) is placed close to the Thapsana leucogranite and contains subhedral johannsenite and euhedral spessartine with crystals of ≤ 2 cm (Jhn-Sps zone).This assemblage that occurs as massive aggregates or disseminated, relates to stage I of the Mn-mineralization, i.e., jacobsite, hausmannite, magnetite and braunite.The oxides hausmannite and jacobsite occur as ≤ 2 cm, rounded crystals in intergrowths with each other and with clinopyroxenes and garnets or sometimes replacing the silicate minerals.
More distal from the Thapsana leucogranite, the second metasomatic zone (≤ 20 m wide) comprises fine-grained spessartine, hornblende, cummingtonite and phlogopite replacing the minerals of the johannsenite and spessartine zone (Sps-Cum zone).Phlogopite replaces the amphiboles and garnets with distinctive kink-bands texture.This zone relates to the secondary Mn-mineralization, i.e., stage II that occurs as massive aggregates of hollandite, pyrolusite, manganite, cryptomelane, manjiroite, vernadite and supergene Fe-oxides.Stage II Mn-ore minerals mainly deposit as stockwork veins with open space filling and boxwork textures.The third metasomatic zone (of ≤ 5m in width) is barren, occurs adjusted to the marbles and calcic schists and composes of rhodonite and subordinate vesuvianite.

Analytical methods
The mineralogical and textural characteristics of the samples were studied in polished-thin sections in polarizing optical and scanning electron microscopes (SEM).Mineral microanalyses were performed using a JEOL JSM-6300 SEM equipped with energy dispersive and wavelength spectrometers (EDS and WDS) and INCA software at the Laboratory of Electron Microscopy and Microanalysis, University of Patras, Greece.Operating conditions were accelerating voltage 25 kV and beam current 3.3 nA, with a 4-m beam diameter.The total counting time was 60 s and dead-time 40%.Synthetic oxides and natural minerals were utilized as analytical standards.Detection limits are ~ 0.01% and accuracy better ≥ 5% was obtained.
Materials of 500-mg splits for stable isotope studies were obtained from the coarse-grained Mn-oxides, i.e., jacobsite, hausmannite, magnetite and braunite from the johannsenitespessartine zone.All minerals selected were handpicked and checked under a binocular microscope to ensure a purity of > 95%.Isotopic compositions of oxygen and hydrogen analyzed using a MAT-253 stable isotope ratio mass spectrometer.All of our isotopic analyses were performed at the Beijing Research Institute of Uranium Geology, China National Nuclear Corporation (CNNC), the Modern Analysis Center, Nanjing University, Nanjing, China as well as, the Chinese Academy of Geological Sciences (CAGS), Beijing, China.Oxygen and hydrogen were released using the BrF5 extraction technique of [26].Hydrogen was released from fluid inclusions in the Mn-oxide grains (with a weight of approximate 2 g) by thermal decrepitation.The samples were pre-heated up to 200 °C, and then re-heated ≥ 500 °C, and reacted with zinc powder at ~ 400 °C to generate hydrogen, and by this treatment we have largely eliminated the effects of the secondary fluid inclusions to the hydrogen isotope composition as the majority of them were decrepitated.Analytical precision was better than ± 0.2 ‰ for δ 18 Ο and ± 2 ‰ for δD.
Ca and Mg isotope analyses were performed in the Mn-oxides from the johannsenitespessartine skarn zone, using a Thermo-Fisher Triton multicollector thermal ionization mass spectrometer, following the method of [27].Measurements were made yielding a 44 Ca beam intensity that was typically between 3.5 and 4.0 V. Mass 43.5 was measured, which records doubly charged 87 Sr, to correct for Sr interference.Every sample is bracketed by measurements of an in-house high-purity ICP Ca standard (HPS).Typical uncertainties 44 Ca/ 42 Ca, 44 Ca/ 43 Ca and 43 Ca/ 42 Ca ratios range between 0.01 and 0.02 per mil (all errors are reported at ± 1σ), ( 40 Ca/ 44 Ca = 47.162 and 42 Ca/ 44 Ca = 0.31221).Similar to Ca, precision of Mg isotope measurements is assessed also by synthetic dolomite standard.For both the Ca and Mg isotopes the analytical accuracy was better than ± 0.01 per mil.Reproducibility for δ 44/42 Ca was 0.09 per mil (2σ) and δ 44/40 Ca values are reported relative to Bulk Silicate Earth (BSE), and for δ 26 Mg was 0.1 per mil (2σ) relative to DSM-3 (Deep Sea Magnesium).
Garnet formulae have been calculated normalized on 12 oxygens and 8 cations, based on the formula X3Y2Z3O12 (X = Ca 2+ , Mn 2+ , Fe 2+ , Mg 2+ , Na 1+ , Y = Al IV , Fe 3+ , Ti and Cr 3+ , and Z = Si and Al VI , where X > Y, with Fe 2+ /Fe 3+ calculated assuming full site occupancy, and for high Cr and Mn contents MnO corrected for Cr interference).For our calculations we have used the excel sheet GNTCALC provided by GabbroSoft.Garnets from the Jhn-Sps and Sps-Cum zones are isotropic and homogeneous from cores to rims, and appear in equilibrium with johannsenite and cummingtonite.They contain Si = 3.1, Mn ranging from 2.1 to 2.3 and have low Fe 3+ (≤ 0.2) and Ca (≤ 0.5) values (all in apfu).Their composition is almost pure spessartine (Sps~80 to 85), with minor andradite component (Adr ~ ≤10) for the Jhn-Sps zone, and almost pure spessartine (Sps~75 to 80), with minor grossular component (Grs~ ≤15) for the Sps-Cum zone.

Clinopyroxene and pyroxenoids
Clinopyroxenes from the Jhn-Sps zone are coarse-grained and euhedral with characteristic greenish pleochroism.Usually crystals are highly altered and replaced, particularly around their peripheries and along cleavage planes by hornblende.Pyroxene formulae have been calculated normalized on 6 oxygens, based on the formula M2M1Z2O6 (M2 = Na, Ca, Fe 2+ , Mn and Mg, M1 = Mn, Fe 2+ , Fe 3+ , Mg, Cr, Al IV , and Ti and Z = Si and Al VI , where M2>M1).These clinopyroxenes are zoned and their cores and rims contain Si ranging from 1.8 to 2.0 (all in apfu).Their cores comprise Mn ranging from 0.91 to 0.95 and low Mg (≤ 0.07) and Ca (≤ 0.09), whereas their rims compose of ranging from 0.86 to 0.90, higher Mg (≤ 0.13 to 0.17

Amphiboles
The amphiboles from the Sps-Cum zone form hypidiomorphic crystals showing intensive pleochroism in shades of green.Their formulae have been calculated normalized on 23 oxygens, based on the formula A0-1X2Y5Z8O22(OH)2 (A and X = Na, Ca, and K, Y = Al VI , Ti, Cr, Mn, Fe 2+ , Fe 3+ , and Mg, and Z = Si and Al IV ).The analyzed amphiboles related to Thapsana Mn-skarn are classified as magnesio-mangani-hornblende passing towards cummingtonite (as they have Si-Ca+Li=15, Si-Mg+Li=13 and Si-Na=15).Their Sitotal and Mg# values range from 6.9 to 7.6 and 3.6 to 4.3 respectively.Additionally their Altotal values range from 0.84 to 1.24.

Geo-thermo-barometry
For the estimation of pressure of the Thapsana Mn-skarns we have employed the Al-inhornblende geobarometer of [28].In the magmatic hornblendes the Al tot content correlates linearly with crystallization pressure.The Al-in-hornblende geobarometer was re-evaluated by [29,30].The analyzed amphiboles have Altotal values that range from 0.84 to 1.24.We have used the [29] equation as it suits better for skarns (for our calculations we have used the excel sheet Pmeter-Al-in-HbBt).Based on the Al-in-hornblende geobarometer the estimated pressure during the formation of Sps-Cum zone ranged from 0.11 to 0.12 GPa.
Temperature estimates were made using the garnet-clinopyroxene geothermometer of [31] which fits best in skarns.The calcium, magnesium and manganese distribution between clinopyroxene and garnet has been calibrated by [32].In the present calibration, ferric iron in the garnet has been calculated on a stoichiometric basis assuming Z = Si = 3.0, and recalculating ΣXY = 5.0, with ΣX = (Ca + Mn + Fe 2+ + Mg) = 3.0 and ΣY = (Fe 3+ + Al + Cr) = 2.0.For the co-existing clinopyroxene, which all are Na-and Al-poor, Fe 2+ /Fe 3+ has been estimated by assuming no jadeite component.The main limitations of the pyroxene-garnet geothermometer are that the clinopyroxene and garnet crystals are in equilibrium and minimum compositional uncertainties must be considerably greater than 0.01Mg# values of analytical errors.
The Grt Ca X , Grt Mn X and Grt Mg# X values of spessartine and johannsenite in equilibrium range from 0.51 to 0.52, 0.79 to 0.80, and 0.21 to 0.23 respectively and lnKD values are 100.Application on the clinopyroxene-garnet geothermometer of Pattison and Newton (1989) re-calibrated by [32] using an average calculated pressure of 0.2 GPa gave temperatures ranging from ~ 440º to ~ 510ºC for the Jhn-Sps zone (average of 501ºC and st.d. of 18.3ºC, for our calculations we have used the GrtCpxThermoV5 excel sheet utilizing the equation of [32]).

Stable isotopes
We have analyzed ten crystals of braunite and hausmanite from the Jhn-Sps zone, for their stable isotopic compositions, i.e., δ 18 Ο, δD, δ 44 Ca, and δ 26 Mg.The isotopic compositions (n = 10) obtained from the Mn-ores are almost constant with an average of δ 44 CaBSE and δ 26 MgDSM-3 of 0.5 ± 0.05 and -0.6 ± 0.1.The δ 18 O and δD values of these Mn-oxides are 7.2 ± 0.5 and -92 ± 2 per mil.These values suggest a magmatic source for the metasomatic ore fluids related to the Thapsana leucogranite.The mineralizing fluids have also interacted and isotopically equilibrated with the host CBU marbles, schists and amphibolites.

Conclusions
Our concluding remarks based on the synthesis and analysis of geological, mineralogical, and isotopic data in Thapsana Mn-skarns and ores can be summarized as follows: • Three metasomatic zones were recognized, a Jhn-Sps zone proximal to the leucogranite, a Sps-Cum zone, more distal and a rhodonite and subordinate vesuvianite adjusted to the CBU marbles.
• The minimum estimated pressure during the Mn-skarn formation at Thapsana was from 0.11 to 0.12 GPa, at temperatures ranging from ~ 440º to ~ 510ºC.
• Stable isotopic compositions suggest a magmatic source for the metasomatic ore fluids related to the Thapsana leucogranite.The mineralizing fluids have interacted and isotopically equilibrated with the host CBU marbles, schists and amphibolites.Table 2. Electron microprobe analyses of I and II stages (wt.%) from Mn-skarn zones, Thapsana, Paros.