Chondrite-normalized Platinum-group Elements Patterns Research Articles

Podiform Chromitites in the Luobusa Ophiolite (Southern Tibet): Implications for Melt-Rock Interaction and Chromite Segregation in the Upper Mantle

The Luobusa ophiolite in the Indus—Yarlung Zangbo suture of southern Tibet hosts the largest known chromite deposit in China. The podiform chromitites occur in a well-preserved mantle sequence consisting of harzburgite with abundant lenses of dunite. The harzburgites have relatively uniform bulk-rock compositions with mg-numbers [100 Mg/(Mg + Fe)] ranging from 89 to 91 and show flat, unfractionated, chondrite-normalized platinum group element (PGE) patterns. Their accessory chromite varies widely in cr-number [100Cr/(Cr + Al)] (18–66). These rocks are essentially residua left after extraction of mid-ocean ridge basalt (MORB)-type magmas. The podiform chromitites display nodular, massive, disseminated and banded textures and typically have dunite envelopes that grade into the surrounding harzburgite and diopsidic harzburgite with increasing pyroxene contents. They consist of relatively uniform chromite with high cr-numbers (74–82), have strongly fractionated, chondrite-normalized PGE patterns with enrichment in Os, Ir and Ru relative to Rh, Pt and Pt, and are believed to have formed from a boninitic magma produced by a second stage of melting. Dunites contain accessory chromite intermediate in composition between those of harzburgite and chromitite and are believed to be the products of reaction between new boninitic magmas and old MORB-type peridotites. The melt-rock reaction removed pyroxene from the peridotites and precipitated oli-vine, forming dunite envelopes around the chromitite pods. The melts thus became more boninitic in composition and chromite saturated, leading to precipitation of chromite alone. The interplay of melt-rock interaction, chromite fractionation and magma mixing should lead to many fluctuations in melt composition, producing both massive and disseminated chromitites and phase layering within individual podiform bodies observed in the Luobusa ophiolite.

PGE distribution in 2.7-Ga layered komatiite flows from the Belingwe greenstone belt, Zimbabwe

The well-preserved Anderson's and Onias's komatiite flows from the late Archaean Belingwe greenstone belt (∼ 2.7 Ga), Zimbabwe, were sampled in the SASKMAR drill hole. Anderson's Flow possesses a 1.1-m-thick B1 subzone and a clinopyroxene spinifex subzone, whereas Onias's Flow has spinifex and cumulate layers typical of komatiite flows. Platinum-group element (PGE) abundances throughout these two flows provide constraints on the formation of layered komatiite flows.All samples from both flows have similar chondrite-normalized PGE patterns that are enriched in Rh, Pt and Pd relative to Os and Ir, which are attributed to large degrees (>30%) of partial melting of the upper mantle. Their Pd/Ir ratios are highly variable (2.0–31 in Onias's Flow and 2.3–61 in Anderson's Flow). PGE variations within both flows, except the spinifex A zone and B1 subzone of Anderson's Flow, together with those of major and other trace elements, are consistent with a model involving olivine fractionation. The spinifex A zone of Anderson's Flow has low Pd/Ir ratios (6–8) which occur in an interval having negative Eu anomalies caused by hydrothermal alteration. The Pt fractionation from Pd and Rh is documented from their relationship to MgO contents: behaviours of Pd and Rh are similar and more incompatible than Pt at the early stage of relatively slow cooling but less incompatible at the late stage during the formation of komatiite flows. PGE in the B1 subzone of Anderson's Flow do not follow an olivine fractionation trend. Pd/Ir ratios in this subzone increase from 28 at the top to 61 at the base. These unusually high values can be accounted for by late-stage development of the subzone and strong fractionation.

Platinum-group element abundance and distribution in chromite deposits of the Acoje Block, Zambales Ophiolite Complex, Philippines

Platinum-group elements (PGE) occur in ore-grade concentration in some of the chromite deposits related to the ultramafic section of the Acoje Block of the Zambales Ophiolite Complex. The deposits are of three types: Type 1 - associated with cumulate peridotites at the base of the crust; Type 2 - in dunite pods from the top 1 km of mantle harzburgite; and Type 3 - like Type 2, but in deeper levels of the harzburgite. Most of the deposites have chromite compositions that are high in Cr with Cr/(Cr + Al) (expressed as chromium index, Cr#) > 0.6; high-Al (Cr# < 0.6) chromite deposits are relatively rare, occurring as interspersed orebodies in regions of Types 2 and 3. Sulfide-bearing Type 1 high-Cr deposits are PGE-enriched; the high-Al deposits have lowest Ir and Pd. Among the PGEs, Ir ranges from <20 to 550 ppb, Rh from <100 to 1100 ppb, Rh from 0.7 to 760 ppb, Pt from <1.0 to 5960 ppb and Pd from <0.5 to 8350 ppb. Chondrite-normalized (CN) PGE patterns are diverse, and include those with both negative slope (Ir > Pd, thought to be characteristic of PGE-barren deposits) and positive slope (Ir < Pd, characteristic of PGE-rich deposits). Iridium, Ru and Os commonly occur as micron-size laurite (sulfide) inclusions in unfractured chromite. Laurite and native Os are also found as inclusions in interstitial sulfides. Platinum and Pd occur as alloy inclusions (and possibly as solid solution) in interstitial Ni-Cu sulfides and as tellurobismuthides in serpentine and altered sulfides. Variability of PGE distribution may be explained by alteration, crystal fractionation or partial melting processes. Alteration and metamorphism were ruled out, because PGE contents do not correlate with degree of serpentinization or the abundance and type (hydroxyl versus non-hydroxyl) of silicate inclusions in chromite. Preliminary Os isotopic data do not support crustal contamination as a source of the PGEs in the Acoje deposits. The anomalous PGE concentrations in Type 1 high-Cr chromite deposits are attributed to two stages of enrichment: an early enrichment of their mantle source from previous melting events and a later stage of sulfide segregation accompanying chromite crystallization. High-Al chromite deposits which crystallized from basalts derived from relatively low degrees of melting owe their low PGE content to partitioning of PGEs in sulfides and alloys that remain in the mantle. High-Cr deposits crystallized from melts that were previously enriched with PGEs during early melting events of their mantle source; Pt and Pd ore concentrations (ppm levels) are attained by segregation of magmatic sulfides. The Acoje deposits indicate that ophiolites are a potential economic source of the PGEs.

Platinum-group elements and gold in komatiitic rocks from the Agrilia Formation, Othrys ophiolite complex, Greece

Highly magnesian lavas (32 wt.% MgO) from the Agrilia Formation, Othrys ophiolite complex, central Greece, were analyzed for major and trace elements. Average platinum-group elements (PGE) concentrations (all in ppb) are: Os 3.6, Ir 0.68, Ru 4, Pt 1.2, Pd 8.5, Rh < 0.5 and Au 19. Although the studied lavas from the Agrilia Formation have unusually high Mg, Cr and Ni contents, they differ from boninites in respect to the dominance of olivine phenocrysts, the lack of orthopyroxene and primary hornblende and the preferential association of chromite with groundmass. The higher Pd than Pt concentrations and the lack of detectable Rh appear to be characteristic of the Agrilia ultramafic lavas. The PGE concentrations, the relatively low values of Pd/Ir ratios (∼ 12), the chondrite-normalized PGE patterns — which are almost flat, with exception of a Pt anomaly — and the mineral chemistry of the studied lavas are comparable to typical komatiites, suggesting that these lavas may represent primitive magmas. The derived parental magma for the studied lavas (in equilibrium with Fo 90.5) contains ∼ 17 wt.% MgO and is similar to those which gave rise to modern analogues of the Archean komatiites from Gorgona island, Colombia. However, the variation in Ti/V and Ti/Sc ratios in Agrilia lavas and the enrichment in some incompatible elements (Sr, Rb, Ba, La) may indicate a modification of the magma composition in the mantle source region by a subduction component. The occurrence of the Triassic komatiitic (and boninitic) lavas in combination with tholeiitic lavas with mid-ocean ridge basalt (MORB) affinities of the Othrys complex, may suggest changes in the geotectonic setting during the development of the Othrys complex from a fore-arc (formation of the highly magnesian lavas) to back-arc tectonic setting (formation of the main complex).

Geochemistry of the platinum-group elements in various types of spinels from the Vourinos ophiolitic complex, Greece

Samples of spinel from three different environments in the Vourinos ophiolitic complex were analysed for the platinum-group elements (PGE) Os, Ir, Ru, Pd, Pt and Au. Chondrite-normalized PGE patterns of both spinels from dunites and massive chromites are characterized by high Os, Ir and Ru normalized values as compared to the low Pd and Pt values, whereas spinels from harzburgite have nearly chondritic patterns. The bulk chemistry of Os-Ir-Ru of spinels from dunites and chromitites reflects the composition of the platinum-group minerals (PGM) trapped in the spinels. The analysed PGE, mostly Pd and Pt, in spinels from harzburgites are considered to be trapped at grain boundaries as sulfides. The geochemical behaviour of the PGE during partial melting and crystallization is used in order to define the conditions of formation of chromitites and/or PGE concentrations.

Platinum-group elements in rocks from the voikar-syninsky ophiolite complex, Polar Urals, U.S.S.R.

Analyses of platinum-group elements (PGE) in rocks collected from the Voikar-Syninsky ophiolite in the Polar Urals suggest that the distribution and geochemistry of PGE in this Paleozoic ophiolite are similar to those in Mesozoic ophiolites from elsewhere. Chondrite-normalized PGE patterns for chromitite, the tectonite unit, and ultramafic and mafic cumulate unit have negative slopes. These results are similar to those found for chromitites from other ophiolites; stratiform chromities show positive slopes. If the magmas that form both types of chromitite originate from similar mantle source material with respect to PGE content, the processes involved must be quite different. However, the distinct chondrite-normalized PGE patterns may reflect differing source materials.