Diamond and Other Exotic Mineral-Bearing Ophiolites on the Globe: A Key to Understand the Discovery of New Minerals and Formation of Ophiolitic Podiform Chromitite

Ophiolite-hosted diamond from peridotites and podiform chromitites significantly differs from those of kimberlitic diamond and ultra-high pressure (UHP) metamorphic diamond in terms of occurrence, mineral inclusion, as well as carbon and nitrogen isotopic composition. In this review, we briefly summarize the global distribution of twenty-five diamond-bearing ophiolites in different suture zones and outline the bulk-rock compositions, mineral and particular Re-Os isotopic systematics of these ophiolitic chromitites and host peridotites. These data indicate that the subcontinental lithospheric mantle is likely involved in the formation of podiform chromitite. We also provide an overview of the UHP textures and unusual mineral assemblages, including diamonds, other UHP minerals (e.g., moissanite, coesite) and crustal minerals, which robustly offer evidence of crustal recycling in the deep mantle along the suprasubduction zone (SSZ) and then being transported to shallow mantle depths by asthenospheric mantle upwelling in mid-ocean-ridge and SSZ settings. A systematic comparison between four main genetic models provides insights into our understanding of the origin of ophiolite-hosted diamond and the formation of podiform chromitite. Diamond-bearing peridotites and chromitites in ophiolites are important objects to discover new minerals from the deep earth and provide clues on the chemical composition and the physical condition of the deep mantle.

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

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.

New podiform chromitites Occurrence from the Masirah Ophiolite, Oman

<p>The Masirah ophiolite is one of the few true ocean ridge ophiolites that have been preserved (Rollinson, 2017) and lacks any indication that it formed in a subduction environment. The Masirah ophiolite in south-eastern Oman is a different and older ophiolite from the more famous northern Oman ophiolite. Chromite and copper ores comprise large deposits in the Samail ophiolite, northern Oman. In comparison, chromite and copper deposits have not been described in previous reports or previous exploration in Masirah ophiolite. Rollinson (2017) has proposed that the apparent absence of chromitites in the mantle section of Masirah ophiolite is an important discriminant between subduction related and ocean ridge ophiolites.  However, during recent studies on the Batain ophiolite mélange, and Masirah ophiolite, several chromitite pods have been discovered. The chromitites occur as separated small concordant, lenticular pods (3–10 m in thickness), which have been extensively altered and deformed, with the host pyroxenite serpentinites serpentinized harzburgites and dunites. The largest chromitite pods found within the pyroxenite and dunite of Masirah are up to 10 m across.  Unusual minerals and mineral inclusions (orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the chromitites from the  Masirah ophiolites.  The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. Both  phlogopite and hornblende were possibly formed from alkali-rich hydrous fluids/melts trapped within the chromite during the chromitite formation. High-T green hornblende and phlogopite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands. The chromitites parental magmas are rich in K, Na, LREE, B, Cs, Pb, Sr, Li, Rb and U relative to HREE, reflecting the alkalic fluids/melts that prevailed during the chromitites genesis.</p><p>The mineral inclusions  in association with host peridotites may have been brought by the uprising asthenosphere at mid-oceanic ridges due to the mantle convection. It appears that this chromite has been formed through reaction between amid-ocean-ridge basalt-melt with depleted harzburgite in the uppermost mantle.  The chromitite deposits have similar cr# (55-62% Al-chromitites), mg# Al2O3 and TiO2 contents to spinels found in MORB, and have been interpreted as having formed in amid-ocean ridge setting.  This suggests that this chromitites is residual from lower degree, partial melting of peridotite, which produced low-Cr# chromitites at the Moho transition zone, possibly in a mid-ocean-ridge setting. The chemistry of both mineral inclusions and chromite   suggests MORB-related tectonic setting for the chromitites that were crystallized at 1000 °C–1300 °C under pressures <3 GPa . The host peridotites were generated during the proto-Indian Ocean MORB extension and emplaced as a result of the obduction of the ophiolite over the Oman Continental margin during Late Cretaceous-Early Paleocene.</p><p>Rollinson, H., 2017. Geoscience Frontiers, 8: 1253–1262.</p>

Genetic Link between Podiform Chromitites in the Mantle and Stratiform Chromitites in the Crust: A Hypothesis

No genetic link between the two main types of chromitite, stratiform and podiform chromitites, has ever been discussed. These two types of chromitite have very different geological contexts; the stratiform one is a member of layered intrusions, representing fossil magma chambers, in the crust, and the podiform one forms pod-like bodies, representing fossil magma conduits, in the upper mantle. Chromite grains contain peculiar polymineralic inclusions derived from Na-bearing hydrous melts, whose features are so similar between the two types that they may form in a similar fashion. The origin of the chromite-hosted inclusions in chromitites has been controversial but left unclear. The chromite-hosted inclusions also characterize the products of the peridotite–melt reaction or melt-assisted partial melting, such as dunites, troctolites and even mantle harzburgites. I propose a common origin for the inclusion-bearing chromites, i.e., a reaction between the mantle peridotite and magma. Some of the chromite grains in the stratiform chromitite originally formed in the mantle through the peridotite–magma reaction, possibly as loose-packed young podiform chromitites, and were subsequently disintegrated and transported to a crustal magma chamber as suspended grains. It is noted, however, that the podiform chromitites left in the mantle beneath the layered intrusions are different from most of the podiform chromitites now exposed in the ophiolites.