scholarly journals Zoned Laurite from the Merensky Reef, Bushveld Complex, South Africa: “Hydrothermal” in Origin?

Minerals ◽  
2020 ◽  
Vol 10 (4) ◽  
pp. 373 ◽  
Author(s):  
Federica Zaccarini ◽  
Giorgio Garuti
Keyword(s):  
South Africa ◽  
Hydrothermal Solution ◽  
Layered Intrusion ◽  
Accessory Mineral ◽  
Sulfur Fugacity ◽  
Thermal Range ◽  
Merensky Reef ◽  
Magmatic Stage ◽  
Placer Deposits ◽  
Local Enrichment

Laurite, ideally (Ru,Os)S2, is a common accessory mineral in podiform and stratiform chromitites and, to a lesser extent, it also occurs in placer deposits and is associated with Ni-Cu magmatic sulfides. In this paper, we report on the occurrence of zoned laurite found in the Merensky Reef of the Bushveld layered intrusion, South Africa. The zoned laurite forms relatively large crystals of up to more than 100 µm, and occurs in contact between serpentine and sulfides, such as pyrrhotite, chalcopyrite, and pentlandite, that contain small phases containing Pb and Cl. Some zoned crystals of laurite show a slight enrichment in Os in the rim, as typical of laurite that crystallized at magmatic stage, under decreasing temperature and increasing sulfur fugacity, in a thermal range of about 1300–1000 °C. However, most of the laurite from the Merensky Reef are characterized by an unusual zoning that involves local enrichment of As, Pt, Ir, and Fe. Comparison in terms of Ru-Os-Ir of the Merensky Reef zoned laurite with those found in the layered chromitites of the Bushveld and podiform chromitites reveals that they are enriched in Ir. The Merensky Reef zoned laurite also contain high amount of As (up to 9.72 wt%), Pt (up to 9.72 wt%) and Fe (up to 14.19 wt%). On the basis of its textural position, composition, and zoning, we can suggest that the zoned laurite of the Merensky Reef is “hydrothermal” in origin, having crystallized in the presence of a Cl- and As-rich hydrous solution, at temperatures much lower than those typical of the precipitation of magmatic laurite. Although, it remains to be seen whether the “hydrothermal” laurite precipitated directly from the hydrothermal fluid, or it represents the alteration product of a pre-existing laurite reacting with the hydrothermal solution.

Naldrettite (Pd2Sb): A new find in Brazil and comparison with worldwide occurrences

2021 ◽  
Vol 59 (6) ◽  
pp. 1801-1820
Author(s):  
Giorgio Garuti ◽  
Federica Zaccarini
Keyword(s):  
Regional Metamorphism ◽  
Layered Intrusion ◽  
Hydrothermal Fluids ◽  
Maximum Temperature ◽  
Magma Intrusion ◽  
Type Composition ◽  
Before And After ◽  
Magmatic Stage ◽  
New Finding ◽  
Podiform Chromitites

ABSTRACT Naldrettite (Pd2Sb) is a PGM discovered in 2005 in Mesamax Northwest deposit, Ungava region, Quebec, Canada. Before and after its approval, PGM with the naldrettite type composition have been reported from a number of localities worldwide. Most frequently, naldrettite has been documented in magmatic Ni–Cu–PGE sulfide deposits, hydrothermal veins in porphyry coppers of the Cu–Au type, and PGE deposits of Alaskan-type zoned intrusions. Naldrettite has been occasionally found in metasomatic Sb–As sulfide ore, metamorphic Ni–oxide ore, and podiform chromitites, although these occurrences have not been fully constrained by solid chemical analyses or paragenetic reconstruction. In this paper we report the first discovery of naldrettite in Brazil. This new finding occurs in a chromitite sample collected in the Luanga Complex, a Neo-archaean layered intrusion in the Carajás Mineral Province. Paragenetic association with alteration assemblages (ferrianchromite, Fe-hydroxides, chlorite) suggests precipitation of naldrettite from metamorphic hydrothermal fluids. The average composition of the Luanga sample (Pd1.76Pt0.24)Σ2.00(Sb0.57As0.43)Σ1.00 shows major substitution of Pt and As. These elements were derived from the breakdown of primary sperrylite, and were incorporated in naldrettite deposited by percolating fluids, at temperature below 350 °C (maximum temperature registered by the crystallization of associated chlorite). An overview of documented occurrences indicates that naldrettite can form in a variety of igneous rocks (ultramafic, mafic, felsic), even involving minimal concentrations of Pd and Sb. Crystallization of naldrettite generally occurs in the post-magmatic stage due to the activity of hydrothermal fluids containing volatile species Sb, As, Bi, Te, and Pd due to its higher mobility compared with the other PGE. A major issue concerns the origin of fluids that can be: (1) “residual”, after the main crystallization of the host magma, (2) “metamorphic”, during regional metamorphism or serpentinization, and (3) “metasomatic”, emanating from an exotic magma intrusion. The combination of two or three of these factors is the most likely process observed in the naldrettite-bearing complexes.

Coupled substitutions in PGE-enriched cobaltite: new evidence from the Rio Jacaré layered complex, Bahia state, Brazil

2015 ◽  
Vol 79 (5) ◽  
pp. 1185-1193 ◽  
Author(s):  
Andrei Y. Barkov ◽  
Yana Fedortchouk ◽  
Robert A. Campbell ◽  
Tapio A.A. Halkoaho
Keyword(s):  
Layered Intrusion ◽  
Fluid Phase ◽  
Primary Phase ◽  
Sulfur Fugacity ◽  
Type Molecule ◽  
Substitution Mechanism ◽  
Compositional Variability ◽  
New Evidence ◽  
Layered Complex ◽  
Grain Core

AbstractMicrocrystals of platinum-group element (PGE)-bearing cobaltite occur in the Gulcari A deposit of vanadiferous titanomagnetite in the lower zone of the Rio Jacaré mafic-ultramafic layered intrusion, Brazil. Aggregates of cobaltite and sperrylite are cluster-like and developed generally along the boundary of Fe-Ti oxide grains with deuteric silicates. Our observations of cryptic zoning, compositional variability and interelement correlations are based on 37 analytical points (wavelength-dispersion spectrometry mode) of cobaltite, and indicate that Ir and Rh behave uniformly with Ni and antipathetically with Co which, in turn, correlates directly with S content. Iridium, Rh and Ni apparently substitute for Co in the As-enriched grain core, and the substitution mechanism invokes solid solution with a cattierite-type molecule: (Ni + Ir + Rh) + (AsS) = Co + (S2). The PGE-bearing cobaltite probably crystallized as a primary phase at 500 to 300°C, from microvolumes of a late fluid phase. The observed enrichment in S and decrease in the As:S ratio at the cobaltite grain margins is a reflection of the increase in sulfur fugacity (fS2) with decrease in temperature of crystallization.

scholarly journals Contamination of the Bushveld Complex (South Africa) magmas by basinal brines: Stable isotopes in phlogopite from the UG2 chromitite

Geology ◽  
2021 ◽  
Author(s):  
Haoyang Zhou ◽  
Robert B. Trumbull ◽  
Ilya V. Veksler ◽  
Ilya N. Bindeman ◽  
Johannes Glodny ◽  
...  
Keyword(s):  
South Africa ◽  
Stable Isotopes ◽  
Hydrogen Isotope ◽  
Bushveld Complex ◽  
Isotope Ratios ◽  
Second Step ◽  
Merensky Reef ◽  
Oxygen Isotope Ratios ◽  
Abundant Evidence ◽  
Hydrogen Isotope Ratios

There is abundant evidence for significant H2O in evolved melts from the platinum-rich UG2 chromitite and the Merensky Reef of the Bushveld Complex (South Africa), but there is no consensus about the source of H2O. We report triple-oxygen and hydrogen isotope ratios of interstitial, late-magmatic phlogopite from three localities of the UG2 layer. The phlogopite yielded δD values of –43‰ to –23‰, which is >30‰ higher than previously known from Bushveld rocks and far above the mantle values of ~–75‰. The phlogopite triple-oxygen isotope ratios are the first to be reported for Bushveld rocks, with values of Δ′17O0.5305 (17O excess relative to the reference line 0.5305) from –0.069‰ to –0.044‰ (δ18O 5.2‰–6.2‰). The oxygen data support existing models of as much as 30%–40% contamination of mantlederived magmas in the lower to middle crust. However, the high δ values require a second step of contamination, which we attribute to brines from the marine sediments in the Transvaal Basin at the emplacement level.

Isotope and trace-element patterns below the Merensky Reef, Bushveld Complex, South Africa: evidence for fluids?

1993 ◽  
Vol 106 (1-2) ◽  
pp. 171-186 ◽  
Author(s):  
D.L. Reid ◽  
R.G. Cawthorn ◽  
F.J. Kruger ◽  
M. Tredoux
Keyword(s):  
South Africa ◽  
Trace Element ◽  
Bushveld Complex ◽  
Merensky Reef

scholarly journals Platinum-Group Minerals in the Placer of the Kitoy River, East Sayan, Russia

Minerals ◽  
2021 ◽  
Vol 12 (1) ◽  
pp. 21
Author(s):  
Evgenia V. Airiyants ◽  
Olga N. Kiseleva ◽  
Sergey M. Zhmodik ◽  
Dmitriy K. Belyanin ◽  
Yuriy C. Ochirov
Keyword(s):  
Primary Source ◽  
Source Rocks ◽  
Southeastern Part ◽  
Reducing Conditions ◽  
Platinum Group Minerals ◽  
Magmatic Stage ◽  
Oxide Phases ◽  
Placer Deposits ◽  
Platinum Group ◽  
Eastern Sayan

The platinum-group minerals (PGM) in placer deposits provide important information on the types of their primary source rocks and ores and formation and alteration conditions. The article shows for the first time the results of a study of placer platinum mineralization found in the upper reaches of the Kitoy River (the southeastern part of the Eastern Sayan (SEPES)). Using modern methods of analysis (scanning electron microscopy), the authors studied the microtextural features of platinum-group minerals (PGM), their composition, texture, morphology and composition of microinclusions, rims, and other types of changes. The PGM are Os‑Ir‑Ru alloys with a pronounced ruthenium trend. Many of the Os‑Ir‑Ru grains have porous, fractured, or altered rims that contain secondary PGE sulfides, arsenides, sulfarsenides, Ir-Ni-Fe alloys, and rarer selenides, arsenoselenides, and tellurides of the PGE. The data obtained made it possible to identify the root sources of PGM in the placer and to make assumptions about the stages of transformation of primary igneous Os-Ir-Ru alloys from bedrock to placer. We assume that there are several stages of alteration of high-temperature Os-Ir-Ru alloys. The late magmatic stage is associated with the effect of fluid-saturated residual melt enriched with S, As. The post-magmatic hydrothermal stage (under conditions of changing reducing conditions to oxidative ones) is associated with the formation of telluro-selenides and oxide phases of PGE. The preservation of poorly rounded and unrounded PGM grains in the placer suggests a short transport from their primary source. The source of the platinum-group minerals from the Kitoy River placer is the rocks of the Southern ophiolite branch of SEPES and, in particular, the southern plate of the Ospa-Kitoy ophiolite complex, and primarily chromitites.

scholarly journals An overview of global layered intrusions, with a special focus on the Bushveld Complex, South Africa

2021 ◽  
Author(s):  
W.D. Maier ◽  
◽  
Qun-Ke Xia ◽  
Keyword(s):  
South Africa ◽  
Bushveld Complex ◽  
Ore Deposits ◽  
Reactive Flow ◽  
Special Focus ◽  
Potential Importance ◽  
Layered Intrusions ◽  
Merensky Reef ◽  
Research Questions ◽  
Water Contents

Layered intrusions host some of the world’s most important ore deposits, yet their global distribution, age, and petrogenesis remain poorly known. Here we provide an update on our current understanding and highlight some key outstanding research questions, notably on the potential importance of reactive flow. We present some preliminary data on water contents of Bushveld orthopyroxenes from which we calculated the water contents of the magma. The analysed Bushveld rocks, including the Merensky Reef, crystallised from relatively water-poor magma, similar to many other continental basalts.

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