Mg-Fe diffusivity patterns in sub-continental and ophiolite mantle chromitites

Mg-Fe cation exchange between olivine and spinel has been studied in the past, as its dependence from T and P allowed the calibration of several geothermometers. These can be used to infer temperatures from high-T conditions down to at least 650 \u25e6C. Moreover, cation exchange produces diffusivity patterns close to the intergranular limits, providing information on the thermal history of ultramafic rocks. In rocks where spinel is an accessory phase, diffusion patterns are fully developed only in olivine as the small size of spinel grains prevents the preservation of primary compositions. Chromitites are the focus of the present work. In these rocks diffusivity patterns are fully developed both in olivine and spinel, due to higher chromite modal content and larger grain size. Therefore, they represent a powerful tool to unravel the thermal history of chromitite-bearing mantle rocks. Subcontinental mantle was studied in Finero mafic-ultramafic massif (Southern Alps, Italy) where fresh chromitite lenses are hosted within the Phlogopite-Peridotite unit. Oceanic mantle was studied in the Puka peridotite massif of the Mirdita ophiolite (Northern Albania) and in the Nea Roda peridotite massif of the Halkidiki ophiolite belt (Northern Greece). Puka massif hosts major chromitite occurrences, as tens of meters long and some meters thick lenses, within fresh dunites, exploited at the Iballe mine, while Nea Roda massif hosts small, little serpentinized, centimeter-size chromitite pods, surrounded by small dunite haloes. In order to avoid scattering of data due to coupled rim-rim analyses, diffusivity patterns were modeled with an exponential function through software OriginLab. In Finero, diffusivity curves show trends ascribable to continuous cooling, with an increase in Mg contents approaching the rim in olivines and the opposite trend in chromites. Cooling rates were qualitatively assessed following the approach of Ozawa (1984), and highlighted an increasing cooling rate with decreasing temperature, due to rapid exhumation. Puka and Nea Roda curves are not as well constrained due to minor serpentinization developing from the intergranular rims. However, it is possible to infer that Puka chromitites underwent continuous cooling but do not show the trend of rapid decrease in cooling rates observed in Finero. Nea Roda diffusivity patterns are the most puzzling. They do not fit a continuous cooling model, and suggest that local conditions affected Mg-Fe exchange

Olivine–Spinel Diffusivity Patterns in Chromitites and Dunites from the Finero Phlogopite-Peridotite (Ivrea-Verbano Zone, Southern Alps): Implications for the Thermal History of the Massif

The study of Mg\u2013Fe2+ subsolidus exchange between olivine and spinel is a powerful tool to unravel the thermal history of ultramafic rocks. We have implemented such a study using olivine\u2013spinel diffusivity patterns in fresh mineralogical samples from the Finero mafic-ultramafic Complex in the Ivrea-Verbano zone of Northern Italy. Our analytical suite includes chromitites and dunites of the Phlogopite-Peridotite Unit from the core of the Complex. Primary and re-equilibrated olivine and spinel compositions were derived from diffusivity curves calculated by fitting data via an exponential function. Resulting XMg (Mg/(Mg + Fe2+) values were then used for geothermometry. Samples are found to demonstrate a maximum temperature of 849 \ub0C and a minimum temperature of 656 \ub0C; these temperatures coincide with the limits of elemental exchange in this mineralogic system. We were unable to identify primary olivine/spinel compositions related to the original formation of the dunite\u2013chromitite suite during Early Permian metasomatic activity. Temperature of 849 \ub0C is ascribed to the Late Triassic re-heating event dated at 208 \ub1 2 Ma. Continuous cooling followed this event at rates of 10 124 and 10 122 \ub0C/yr until cessation of elemental exchange activity at ~656 \ub0C. A rapid cooling rate is associated with the uplift and subsequent decrease of geothermal gradient during the early stages of the opening of the Piemont ocean basi

Mineralogical study of rodingitized microgabbros and associated chromitite seams from the Nain ophiolite, Central Iran

The Nain-Dehshir-Baft Ophiolitic Belt (NDBOB), which crops out along the Nain-Baft fault, around the Central Iranian Microcontinent (CIM), comprises a set of dismembered ultramafic, mafic and sedimentary complexes. The northernmost branch of this ophiolitic belt is known as \u201cNain ophiolitic m\ue9lange\u201d and hosts small chromitite bodies, as pods and lenses, within completely serpentinized peridotites. The focus of the present study is the interaction between a 50 cm thick chromitite lens and a crosscutting rodingite dyke. For this purpose, a full transect across chromitite, rodingite and serpentinite was continuously sampled and studied in reflected and transmitted light microscopy. Mineral chemistry of sulfides, silicates, carbonates and oxydes was determined through EMP analyses. Rodingite shows a calc-silicate assemblage with an association of clinopyroxene, xonotlite, chlorite, garnet, vesuvianite, titanite, hornblende and chromite. Chromitite has 60-80% modal chromite, that sporadically shows a slight Fe-chromitization. Silicate assemblage is dominated by serpentine with relics of olivine and, occasionally, diopside, enstatite, hornblende and phlogopite. Later calcite veins crosscut both rodingite and chromitite, extending within serpentinite too. Rodingite shows a widespread range of copper sulfides, the most common ones being chalcocite, followed by native copper, digenite, geerite, and few spotted grains of possible yarrowite and sponkiopite. As secondary Cu oxydes and hydroxydes tenorite and spertiinite were found. In chromitite, close to the upper contact with rodingite, usual secondary sulfides like heazlewoodite and millerite were found together with shandite. Close to the lower rodingite contact, the presence of pyrrhotite, native iron and pentlandite was detected. Very close to the lower contact, again an unusual sulfide assemblage was found, with bornite and galena. Within rodingite clinopyroxenes show both diopside and augite compositions, with XMg ((Mg/(Mg+Fe2+)) of 0.93-0.96 for the former and 0.82-0.86 for the latter. Garnets are grossular and hydrogrossular in the upper rodingite, to which andradite is added in the lower rodingite. Chlorite shows a wide range of compositions with XMg increasing towards the contact with chomitite from 0.47 to 0.60. Very close to the contact XMg of chlorite ranges between 0.67 and 0.94. Chromite accessory grains have XMg ranging between 0.52 and 0.68 and XCr (Cr/(Cr+Al)) ranging between 0.75 and 0.80. Chromite in chromitite has XMg ranging between 0.65 and 0.71 and XCr ranging between 0.68 and 0.71. Olivine is forsteritic with XMg ranging between 0.95 and 0.97 and orthopyroxene is enstatitic with XMg around 0.94-0.95. Chlorite is very rare and has around 3.5 wt% Cr2O3. Rodingite intrusion postdates serpentinization of mantle assemblage and did not affect the chomite+silicate chromitite assemblage. The effect on sulfide variety and distribution was instead remarkable. At least lead and copper were introduced in the contact zone within chromitite to form shandite and bornite. Reducing conditions during rodingite emplacement are witnessed by the abundance of native copper and the presence of native iron. The effect of interaction in rodingite is mainly recorded by the wide range of chlorite compostions that increases its Mg content towards chromitite

New insights into the mineralogy and geochemistry of sb ores from Greece

Antimony is a common metalloid occurring in the form of Sb-sulfides and sulfosalts, in various base and noble metal deposits. It is also present in corresponding metallurgical products (concentrates) and, although antimony has been considered a penalty element in the past, recently it has gained interest due to its classification as a critical raw material (CRM) by the European Union (EU). In the frame of the present paper, representative ore samples from the main Sb-bearing deposits of Greece (Kilkis prefecture, Chalkidiki prefecture (Kassandra Mines), and Chios Isl.) have been investigated. According to optical microscopy and electron probe microanalysis (EPMA) data, the Greek ores contain stibnite (Sb2 S3), boulangerite (Pb5 Sb4 S11), bournonite (PbCuSbS3), bertherite (FeSbS4), and valentinite (Sb2 O3). Bulk analyses by inductively coupled plasma mass spectrometry (ICP-MS) confirmed, for the first time published, the presence of a significant Hg content in the Kilkis Sb-ore. Furthermore, Kassandra Mines ores are found to contain remarkable amounts of Bi, As, Sn, Tl, and Se (excluding Ag, which is a bonus element). The above findings could contribute to potential future exploration and exploitation of Sb ores in Greece

Chromite Grain Diameter (CGD) from the ore-hosting dunite of the Xerolivado-Skoumtsa chrome mine (Vourinos,Western Macedonia, Greece): implications for chrome ore exploration

The largest chrome-ore bearing dunite of the Vourinos Complex occurs within the Xerolivado-Skoumtsa Mine District. The host dunite body has a surface exposure of 3 km2 and extends at least 400m into the subsurface. The dunite body is hosted by harzburgite tectonite interfolded with the dunite body during ductile phases of deformation. The Xerolivado-Skoumtsa mine is one of the world\u2019s largest ophiolite-hosted chrome deposits with a potential of 6 million tons of ore assaying at 22 wt.% Cr2O3. Even so, chrome ore bodies compose less than 1% of the volume of the dunite body. Exploration consisted chiefly of expensive drilling programs. A well-documented suite of chrome ores from the south sector of the mine were collected during the final years of its operation (1987-1988). Samples include twenty-seven (27) samples of serpentinite altered from primary olivine that originally coexisted with the chromite. Two types of serpentinised dunite were distinguished: Type A samples are from serpentinised dunite 1m from the ore bodies, and Type B samples are silicates infolded with the ores during original high-temperature mantle deformation. Thin-polished sections were studied via optical microscopes (transmitted and reflected light). All chromite grains (2,776 total measurements) were photographed and measured in eight directions at angular resolution of 22.5o, starting from the maximum diameter. The average and median chromite grain diameter (CGD) was calculated for each sample. These geometric analyses indicate that CGD decreases depending on the position (Type A or Type B) of the samples: the average and the median of the CGD were smaller in the serpentinites next to the chromite ore bodies (Type A) than those within consecutive chromite ore bodies (Type B). A decrease of the average and median CGD was found in (today\u2019s) vertical ore dimension: samples from mine level 717m have larger average and median CGD than samples from over-lying mine level 738m and from the underlying mine level 692m. This concurs with the position of the thickest size of the ore bodies, decreasing towards the overlying levels and underlying levels. The grain size of chromite in these dunites is due to a combination processes of original Cr-spinel crystallization and grain break-up and deformation concurrent to original dunite thinning around the ore layers. With the lack of preserved olivine morphology, these structures are the only \u201cfingerprints\u201d of the type of deformation undergone on grain scale. These results could provide a \u201cpetrographic tool\u201d useful for the exploration of schlieren-type chrome ores: host dunite (serpentinite) samples present lower average and median CGD when they are found closer to the ore bodies. Future research on this subject should focus on the verification of the results in other similar ore-hosting dunites and mineralogical and/or metallogenetic interpretation of the numeric results

Olivine–spinel diffusivity patterns in chromitites and dunites from the finero phlogopite-peridotite (Ivrea-Verbano Zone, Southern Alps): Implications for the thermal history of the massif

The study of Mg–Fe 2+ subsolidus exchange between olivine and spinel is a powerful tool to unravel the thermal history of ultramafic rocks. We have implemented such a study using olivine–spinel diffusivity patterns in fresh mineralogical samples from the Finero mafic-ultramafic Complex in the Ivrea-Verbano zone of Northern Italy. Our analytical suite includes chromitites and dunites of the Phlogopite-Peridotite Unit from the core of the Complex. Primary and re-equilibrated olivine and spinel compositions were derived from diffusivity curves calculated by fitting data via an exponential function. Resulting XMg (Mg/(Mg + Fe 2+ ) values were then used for geothermometry. Samples are found to demonstrate a maximum temperature of 849 ◦ C and a minimum temperature of 656 ◦ C; these temperatures coincide with the limits of elemental exchange in this mineralogic system. We were unable to identify primary olivine/spinel compositions related to the original formation of the dunite–chromitite suite during Early Permian metasomatic activity. Temperature of 849 ◦ C is ascribed to the Late Triassic re-heating event dated at 208 ± 2 Ma. Continuous cooling followed this event at rates of 10 −4 and 10 −2◦ C/yr until cessation of elemental exchange activity at ~656 ◦ C. A rapid cooling rate is associated with the uplift and subsequent decrease of geothermal gradient during the early stages of the opening of the Piemont ocean basin. © 2019 by the authors. Licensee MDPI, Basel, Switzerland

Rare and new mineralogical phases in the Ni-Cu-Sb-As system from the Gomati ophiolite, Northern Greece

The Gomati ultramafic body (Chalkidiki peninsula, Northern Greece) is located in the Serbo-Macedonian Massif, one of the geotectonic terranes composing the Hellenides orogenic belt. Ophiolite occurrences in this domain have an unclear origin, and consist of altered peridotites hosting scattered chromitite bodies with massive, schlieren and disseminated textures. These ultramafic bodies are enclosed in the Vertiskos unit, an alternation of Silurian gneisses and schists, and are sometimes in contact with late Cenozoic granites. The present work focuses on several accessory minerals in the Ni-Cu-Sb-As system, found in a chloritized clinopyroxenite in contact with chromitite. The composition of these accessory minerals was determined through electron microprobe analyses. Well known mineralogical phases are represented by orcelite (Ni5As2) and breithauptite (NiSb), while other detected phases have been either not well described or never reported. The chemistry of the Gomati minerals clusters around the following ideal stoichiometries: (Ni,Cu)7(Sb,As)3, (Ni,Cu)2(Sb,As), (Ni,Cu)11(Sb,As)8, Ni3As, Ni5(As,Sb)2 and Ni7(As,Sb)3. As orcelite (Ni5As2) is a nonstoichiometric mineral, (Ni,Cu)7(Sb,As)3, Ni5(As,Sb)2 and Ni7(As,Sb)3 may correspond to Cu and/or Sb-rich terms of this phase. A mineral phase corresponding to the (Ni,Cu)2(Sb,As) stoichiometry was first described in the Alaskan-type Tulameen complex of Canada as unknown phase by Nixon and Cabri (1990). A phase with stoichiometry Ni3As is reported by Tredoux et al. (2016), from Bon Accord oxide body (South Africa), and corresponds to the mineral dienerite, known only from one loose crystal found in Austria in 1921 and recently discredited my IMA. (Ni,Cu)11(Sb,As)8 probably represents a Cu-rich Sb analogue of the mineral maucherite (Ni11As8). Such an anomalous mineral assemblage in the Gomati ophiolite is puzzling. While ultramafic rocks contain Ni and As of magmatic origin, the presence of Sb and Cu could be indicative of a metasomatic enrichment, probably linked to the presence of fluids emanating from the granite body in contact with the Gomati ophiolite

Rare and new compounds in the Ni-Cu-Sb-As system: first occurrence in the Gomati ophiolite, Greece

The Gomati ophiolite (Northern Greece) is located in the Serbo-Macedonian Massif, in the Hellenides orogenic belt. It consists of altered peridotites hosting scattered chromitite bodies. The ultramafics are enclosed in Silurian gneisses and schists, and are partially in contact with late Cenozoic granites. The present work focuses on accessory minerals in the Ni-Cu-Sb-As system, found in a chloritized clinopyroxenite in contact with chromitite. The electron microprobe analyses revealed the presence of known minerals such as orcelite (Ni5-xAs2) and breithauptite (NiSb) and new phases that cluster around the following compositions: Ni3As, Ni5(As,Sb)2, (Ni,Cu)5-x(Sb,As)2, (Ni,Cu)2(Sb,As), and (Ni,Cu)11(Sb,As)8. The compound (Ni,Cu)5-x(Sb,As)2 may correspond to a Cu-rich Sb dominant variant of this phase. A phase corresponding to (Ni,Cu)2(Sb,As) was first described in the Tulameen complex of Canada. A phase with stoichiometry Ni3As was formerly known as the mineral dienerite, later discredited by the IMA and only recently under revalidation. The compound (Ni,Cu)11(Sb,As)8 probably represents a Cu-rich Sbdominant analogue of the mineral maucherite (Ni11As8). The mineral assemblage in the Gomati ophiolite is puzzling. While ultramafic rocks contain Ni and As of magmatic origin, the presence of Sb, Ag, Au and Cu minerals could be indicative of a metasomatic enrichment, probably linked to the presence of fluids emanating from the granite body or a nearby porphyry copper mineralization

Rare and new compounds in the Ni-Cu-Sb-As system: First occurrence in the Gomati ophiolite, Greece

The Gomati ophiolite (Northern Greece) is located in the Serbo-Macedonian Massif, in the Hellenides orogenic belt. It consists of altered peridotites hosting scattered chromitite bodies. The ultramafics are enclosed in Silurian gneisses and schists, and are partially in contact with late Cenozoic granites. The present work focuses on accessory minerals in the Ni-Cu-Sb-As system, found in a chloritized clinopyroxenite in contact with chromitite. The electron microprobe analyses revealed the presence of known minerals such as orcelite (Ni5-xAs2) and breithauptite (NiSb) and new phases that cluster around the following compositions: Ni3As, Ni5(As,Sb)2, (Ni,Cu)5-x(Sb,As)2, (Ni,Cu)2(Sb,As), and (Ni,Cu)11(Sb,As)8. The compound (Ni,Cu)5-x(Sb,As)2 may correspond to a Cu-rich Sb dominant variant of this phase. A phase corresponding to (Ni,Cu)2(Sb,As) was first described in the Tulameen complex of Canada. A phase with stoichiometry Ni3As was formerly known as the mineral dienerite, later discredited by the IMA and only recently under revalidation. The compound (Ni,Cu)11(Sb,As)8 probably represents a Cu-rich Sbdominant analogue of the mineral maucherite (Ni11As8). The mineral assemblage in the Gomati ophiolite is puzzling. While ultramafic rocks contain Ni and As of magmatic origin, the presence of Sb, Ag, Au and Cu minerals could be indicative of a metasomatic enrichment, probably linked to the presence of fluids emanating from the granite body or a nearby porphyry copper mineralization. © 2020 Edizioni Nuova Cultura. All rights reserved