scholarly journals Contrasting Textural and Chemical Signatures of Chromitites in the Mesoarchaean Ulamertoq Peridotite Body, Southern West Greenland

Geosciences ◽  
2018 ◽  
Vol 8 (9) ◽  
pp. 328 ◽  
Author(s):  
Juan Guotana ◽  
Tomoaki Morishita ◽  
Ryoko Yamaguchi ◽  
Ikuya Nishio ◽  
Akihiro Tamura ◽  
...  
Keyword(s):  
Mineral Chemistry ◽  
Ultramafic Rocks ◽  
West Greenland ◽  
Temperature Conditions ◽  
Chemical Signatures ◽  
Fe Content ◽  
Compositional Zonation

Peridotites occur as lensoid bodies within the Mesoarchaean orthogneiss in the Akia terrane of Southern West Greenland. The Ulamertoq peridotite body is the largest of these peridotites hosted within the regional orthogneiss. It consists mainly of olivine, orthopyroxene, and amphibole-rich ultramafic rocks exhibiting metamorphic textural and chemical features. Chromitite layers from different localities in Ulamertoq show contrasting characteristics. In one locality, zoned chromites are hosted in orthopyroxene-amphibole peridotites. Compositional zonation in chromites is evident with decreasing Cr and Fe content from core to rim, while Al and Mg increase. Homogeneous chromites from another locality are fairly uniform and Fe-rich. The mineral chemistry of the major and accessory phases shows metamorphic signatures. Inferred temperature conditions suggest that the zoned chromites, homogeneous chromites, and their hosts are equilibrated at different metamorphic conditions. In this paper, various mechanisms during the cumulus to subsolidus stages are explored in order to understand the origin of the two contrasting types of chromites.

scholarly journals Separation of Fe from Mn in the Cryogenian Sedimentary Mn Deposit, South China: Insights from Ore Mineral Chemistry and S Isotopes from the Dawu Deposit

Minerals ◽  
2021 ◽  
Vol 11 (5) ◽  
pp. 446
Author(s):  
Zhiming Xu ◽  
Chengquan Wu ◽  
Zhengwei Zhang ◽  
Jinhong Xu ◽  
Xiyao Li ◽  
...  
Keyword(s):  
South China ◽  
Large Scale ◽  
Early Stage ◽  
Sulfur Isotopes ◽  
Mineral Chemistry ◽  
Manganese Carbonate ◽  
Separation Mechanism ◽  
Geochemical Properties ◽  
Stage Separation ◽  
Fe Content

Manganese and Fe have similar geochemical properties in the supergene environment. Separation of Mn and Fe is an important process for the formation of high-grade sedimentary manganese deposits. Large-scale manganese carbonate deposits (total reserves of approximately 700 Mt) were formed during the interglacial of the Sturtian and Marinoan in South China. The orebodies are hosted in the black rock series at the basal Datangpo Formation of the Cryogenian period. The Fe contents in ores range from 1.15 to 7.18 wt.%, with an average of 2.80 wt.%, and the average Mn/Fe ratio is 8.9, indicating a complete separation of Mn and Fe during the formation of manganese ores. Here, we present element data of manganese carbonates and sulfur isotopes of pyrite from the Dawu deposit, Guizhou, China, aiming to investigate the separation mechanism of Mn and Fe and the ore genesis. The Fe in ores mainly occurs as carbonate (FeCO3) and pyrite (FeS2). The Mn, Ca, Mg and Fe exist in the form of isomorphic substitutions in manganese carbonate. The contents of FeCO3 in manganese carbonates are similar in different deposits, with averages of 2.6–2.8 wt.%. The whole-rock Fe and S contents have an obvious positive correlation (R = 0.69), indicating that the difference of whole-rock Fe content mainly comes from the pyrite content. The δ34SV-CDT of pyrite varies from 40.0 to 48.3‰, indicating that the pyrite formed in a restricted basin where sulfate supply was insufficient and the sulfate concentrations were extremely low. Additionally, the whole-rock Fe content is negatively correlated with the δ34S values of the whole-rock and pyrite, with correlation coefficients of −0.78 and −0.83, respectively. Two stages of separations of Mn and Fe might have occurred during the mineralization processes. The reduced seawater became oxidized gradually after the Sturtian glaciation, and Fe2+ was oxidized and precipitated before Mn2+, which resulted in the first-stage separation of Mn and Fe. The residual Mn-rich and Fe-poor seawater flowed into the restricted rift basin. Mn and Fe were then precipitated in sediments as oxyhydroxide as the seawater was oxidized. At the early stage of diagenesis, organic matter was oxidized, and manganese oxyhydroxide was reduced, forming the manganese carbonate. H2S was insufficient in the restricted basin due to the extremely low sulfate concentration. The Fe2+ was re-released due to the lack of H2S, resulting in the second-stage separation of Mn and Fe. Finally, the manganese carbonate deposit with low Fe and very high δ34S was formed in the restricted basin after the Sturtian glaciation.

scholarly journals Investigating the diamond potential of southern West Greenland

2004 ◽  
Vol 4 ◽  
pp. 69-72 ◽  
Author(s):  
Sven Monrad Jensen ◽  
Karsten Secher
Keyword(s):  
Structural Control ◽  
Mineral Chemistry ◽  
Mantle Xenoliths ◽  
Comprehensive Overview ◽  
West Greenland ◽  
Large Samples ◽  
Indicator Mineral ◽  
The Government ◽  
Indicator Minerals ◽  
Diamond Potential

Southern West Greenland hosts a province of ultramafic alkaline rocks, including swarms of dykes traditionally described as kimberlites and lamproites (Larsen 1991; Jensen et al. 2002). Since the mid-1990s, commercial diamond exploration has been focused on the Sarfartoq region and the region south-east of Maniitsoq (Fig. 1), and has resulted in numerous reports of diamond-favourable indicator minerals from till sampling, finds of kimberlitic dykes, and recovery of diamonds from kimberlitic rocks. A new digital compilation of company data released from confidential status (Jensen et al. 2003a) presents a comprehensive overview of exploration activities and results that have emerged since the Survey’s first compilation of occurrences of kimberlitic and related rocks (Larsen 1991). The new compilation in a GIS (geographic information system) environment allows for refined assessment of the distribution, structural control and possible spatial and petrogenetic relationships that characterise the kimberlitic occurrences. In 2003, the Geological Survey of Denmark and Greenland (GEUS) and the Government of Greenland’s Bureau of Minerals and Petroleum (BMP) went further than has been customary in investigating the economic potential of specific sites. Four areas were temporarily closed to application for exploration licences, pending sampling and testing for diamond content of large samples of more than one tonne each from significant kimberlitic occurrences. Additional characterisation and research initiated on these and other occurrences include magnetic mapping, detailed petrography and studies of mantle xenoliths, as well as indicator mineral chemistry. An extensive programme to determine the ages of kimberlitic and related rocks was also initiated in 2003.

Magnetite Redistribution during Multi-stage Serpentinization: Evidence from the Taishir massif, the Khantaishir Ophiolite, Western Mongolia

2021 ◽  
Author(s):  
Otgonbayar Dandar ◽  
Atsushi Okamoto ◽  
Masaoki Uno ◽  
Noriyoshi Tsuchiya
Keyword(s):  
Hydrogen Generation ◽  
Mantle Peridotite ◽  
Oceanic Lithosphere ◽  
Ultramafic Rocks ◽  
Western Mongolia ◽  
Fluid Infiltration ◽  
Multi Stage ◽  
Fe Content ◽  
Coarse Grains ◽  
Modal Abundance

<p>Magnetite commonly forms during serpentinization of mantle peridotite, involving the hydrogen generation within the oceanic lithosphere. Although magnetite is concentrated in veins, the mobility of iron during serpentinization is still poorly understood. The completely serpentinized ultramafic rocks (originally dunite) within the Taishir massif in the Khantaishir ophiolite, western Mongolia, include abundant magnetite + antigorite veins, which manifest novel distribution of magnetite. The serpentinite records the multi-stage serpentinization, in order of (1) Al-rich antigorite + lizardite mixture with hourglass texture (Al<sub>2</sub>O<sub>3</sub> = 0.46-0.69 wt%; Atg+Lz), (2) Al-poor antigorite composed of thick veins and their branches (Atg), and (3) chrysotile that cut all previous textures. The Mg# (= Mg/ (Mg + Fe<sub>total</sub>)) of Atg+Lz (0.94-0.96) is lower than Atg (0.99) and chrysotile (0.98). In the region of Atg+Lz, magnetite occurs as the arrays of fine grains (<50 μm) around the hourglass texture. In the Atg veins replacing Atg+Lz, magnetite disappears and re-precipitated as coarse grains (100-250 μm) in the center of some veins. As the extent of replacement of Atg+Lz by Atg veins increases, both modal abundance of magnetite and the bulk Fe content decrease. These characteristics indicate that hydrogen generation mainly occurred at the stage of Atg+Lz formation, and magnetite distribution was largely modified via dissolution and precipitation in response to later fluid infiltration associated with the Atg veins. This also indicates the high iron mobility within the serpentinized peridotites even after the primary stage of magnetite formation.</p>

The role of magma injection and crystal sorting in the formation of early gabbros at the Coldwell Complex, Ontario, Canada

2019 ◽  
Vol 56 (7) ◽  
pp. 715-737 ◽  
Author(s):  
Yong-hua Cao ◽  
David J. Good ◽  
Robert L. Linnen ◽  
Iain M. Samson
Keyword(s):  
Major Element ◽  
Mineral Chemistry ◽  
Ultramafic Rocks ◽  
Chemical Compositions ◽  
Olivine Gabbro ◽  
Additional Observation ◽  
Midcontinent Rift ◽  
Layered Series ◽  
Incompatible Elements

The Layered Series of the Midcontinent Rift related Coldwell Complex comprises thick sections of gabbro, without any known associated ultramafic rocks. It represents a major early intrusive unit of the Coldwell Complex and consists of thick accumulations of olivine gabbro and oxide augite melatroctolite. This study combines petrography, mineral chemistry, and lithogeochemistry to constrain the magma composition and petrogenesis of the Layered Series. The presence of cumulus orthoclase together with the observation that the Layered Series rocks plot in the alkaline field on a total alkali–silica diagram indicate that the Layered Series magma has an alkaline parentage. The stratigraphy of the Layered Series cannot be fully correlated between different areas using lithogeochemistry and mineral chemistry. This together with observed normal and reverse trends for mineral chemical compositions in different areas suggest that the processes related to magma emplacement and crystallization were different in different locations. The whole-rock concentrations of incompatible elements and the compositions of major minerals of the olivine gabbro and oxide augite melatroctolite units are chemically similar. However, major element lithogeochemistry is variable, dominantly due to differences in the abundances of olivine, clinopyroxene, plagioclase, and magnetite. An additional observation is that olivine and clinopyroxene are not in chemical equilibrium. Together, these observations are interpreted to reflect a combination of multiple injections of magma and crystal sorting in an open system.

Petrochemistry and mineral chemistry studies on metamorphic ultramafic rocks in Yanghou area, Northern Fujian, China

1999 ◽  
Vol 18 (1) ◽  
pp. 30-38
Author(s):  
Ren Shengli ◽  
Zhou Xinghua ◽  
Li Jiliang ◽  
Sun Min
Keyword(s):  
Mineral Chemistry ◽  
Ultramafic Rocks

The Advocate ophiolite mantle, Baie Verte, Newfoundland: regional correlations and evidence for metasomatismGeological Survey of Canada, Earth Sciences Sector, Contribution 20090211.

2010 ◽  
Vol 47 (3) ◽  
pp. 237-253 ◽  
Author(s):  
Jean H. Bédard ◽  
Monica Escayola
Keyword(s):  
Earth Sciences ◽  
Subduction Zone ◽  
Mineral Chemistry ◽  
Major Elements ◽  
Apparent Absence ◽  
Gold Exploration ◽  
Metasomatic Agent ◽  
Exploration Target ◽  
Chemical Signatures

Mantle rocks of the Advocate ophiolite near Flatwater Pond (Baie Verte, Newfoundland) are dominated by harzburgite tectonites, which are extensively converted to listvenite along the Baie Verte Road fault and represent a potential gold exploration target. Most Advocate harzburgites have forsteritic olivine (Fo90.5 to Fo93) and Cr-spinels, with Cr# (= 100Cr/(Cr + Al)) between 52 and 64 and Mg# (= 100Mg/(Mg + Fe2+)) between 56 and 68. These mineral chemical signatures, together with high whole-rock MgO (46%–48%), low Al2O3 (<1%), and TiO2 (<0.003%), imply the Advocate harzburgites are refractory residues after ca. 25%–35% melting. Cr-spinel compositions of Advocate mantle rocks overlap with Cr-spinels from the mantle rocks of the Point Rousse and Betts Cove ophiolites, with Mg# higher than those of Bay of Islands or Thetford Mines mantle Cr-spinels. Although refractory in terms of major elements and mineral chemistry, Advocate harzburgites contain high La–Ce–Pr–Pb–Nd–Sm–Zr contents suggestive of pervasive metasomatism. Similar geochemical signatures occur in all ophiolitic mantle rocks from the Baie Verte Peninsula examined so far. The enrichments are not consistent with supra-subduction zone syn-melting metasomatism as observed in other Appalachian ophiolites. The apparent absence of visible metasomatic channels in most outcrops suggests that metasomatism occurred before obduction by diffuse percolation, but the nature and origin of the metasomatic agent remain speculative. The similarities of mineral and whole-rock geochemistry imply that all mantle rocks from Baie Verte ophiolites are correlative and may represent remnants of a single obducted slab.

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