Sulfides associated with podiform bodies of chromite at Tsangli, Eretria, Greece

Small bodies of pyrrhotite, chalcopyrite, minor pentlandite, and magnetite occur at the peripheries of podiform bodies of chromite in ultramafic ophiolitic rocks at Tsangli, Eretria, central Greece. Banding of magnetite and sulfide within the bodies is reminiscent of magmatic banding. A magmatic origin has been proposed for similar sulfide masses in the Troodos ophiolite (Panayiotou, 1980). The compositions of the host rocks, chromite, and of the sulfides have been investigated. On average, the sulfide mineralization, recalculated to metal content in 100% sulfide, contains 0.55% Ni, 5.15% Cu, 0.29% Co, 9 ppb Pd, 179 ppb Pt, 16 ppb Rh, 112 ppb Ru, 31 ppb Ir, 58 ppb Os, and 212 ppb Au. These metal contents, particularly the high Cu/(Cu+Ni) ratio of 0.78 and the Pt/(Pd+Pt) ratio of 0.95, are inconsistent with the sulfides having reached equilibrium with their Ni rich host rocks at magmatic temperatures and accordingly it is concluded that they are not of magmatic origin. The average δ 34S value of the sulfide bodies is +2 while that of a sample of pyrite from country-rock schist is -15.6. These values are inconclusive as to the origin of the sulfur. It is suggested that the sulfides have been precipitated by hydrothermal fluids, possibly those responsible for the serpentinization of the host rocks. The source of the metals may have been the host rocks themselves. © 1984 Springer-Verlag

The origin of the fractionation of platinum-group elements in terrestrial magmas

The platinum-group elements (PGE's), when chondrite normalized, have been found to be fractionated in order of descending melting point (Os, Ir, Ru, Rh, Pt, Pd and Au). Mantle-derived material (garnet lherzolite and spinel lherzolite xenoliths and alphine peridotites) have essentially unfractionated PGE patterns. Periotitic komatiites have mildly fractionated patterns (PdIr = 10), pyroxenitic komatiites are slightly more fractionated (PdIr = 30). Both continental and ocean-floor basalts are highly fractionated (PdIr = 100). Data from intrusive rocks show a large range in PGE fractionation from Pd-depleted chromities of ophiolites (PdIr = 0.1) to the extreme Pd enrichment in the JM Reef of the Stillwater Complex (PdIr = 865). Some possible mechanisms for the origin of this fractionation are: alteration, partial melting and crystal fractionation. Carbonate alteration affects Au and Pt and hydrothermal alteration mobilizes Pd. Solid substitution of Ir (and associated Os and Ru) into olivine and chromite, during crystal fractionation or partial melting is rejected as a mechanism of fractionating the PGE's. It is suggested; that the major factor in PGE fractionation is the differences in solubility of the PGE's in a silicate magma, that Pd, Pt and Rh are more soluble than Os and Ir, which form an alloy and Ru which forms laurite. These differences in PGE solubility could fractionate the PGE's during partial melting or crystal fractionation. During crystal fractionation prior to Fe--Ni--Cu sulphur saturation the low solubility of Os, Ir and Ru leads to the formation of Os-Ir alloys and RuS 2 in the magma. These may then be settled out of the magma by whatever phase is crystallizing and the remaining magma becomes fractionated in PGE's

A comparative study of olivine and clinopyroxene spinifex flows from Alexo, Abitibi greenstone belt, Ontario, Canada

A petrological and geochemical study of an olivine and of a clinipyroxene spinifex textured flow, from Alexo, indicates that the initial liquid in both flows probably came from the same mantle melting event and that the source was incompatible element depleted. The starting liquid of the clinopyro~ene flow had experienced more olivine fractionation (10%) prior to its emplacement at Alexo, than the initial liquid of the olivine spinifex flow. The development of each of the textural and compositional zones in the flows can be modelled by means of crystal fractionation. In the case of the clinopyroxene flow the B-zone is formed by the fractionation of olivine, low-Ca pyroxene and chromite. An unusual feature of the Alexo clinopyroxene flow is presence of a peridotitic komatiite above the pyroxene cumulate layer, where a basaltic komatiite would usually be present. The presence of the peridotitic komatiite suggests an influx of new magma and hence a dynamic model for the flow. The composition of the clinopyroxene spinifex zone represents a mixture of clinopyroxene plus liquid, rather than simply a frozen liquid. This could happen if the clinopyroxene needles grew stalactitelike from the chilled upper surface of the flow into a flowing basaltic liquid. In the olivine spinifex flow the zones can be modelled as frozen liquids in the A2-zone, as initial liquid which has fractionated 30% olivine in the A3-zone and as liquid plus 50% olivine in the B-zone. But, if the clinopyroxene spinifex developed by stalactite growth of clinopyroxene needles into the a flowing liquid, the possibility that the olivine spinifex represent fractionated liquid plus stalactite olivines arises

Ore deposits of the platinum-group elements

The formation of ore deposits of the platinum-group elements (PGE) requires that their concentrations be raised about four orders of magnitude above typical continental crustal abundances. Such extreme enrichment relies principally on the extraction capacity of sulfide liquid, which sequesters the PGE from silicate magmas. Specific aspects of PGE ore formation are still highly controversial, however, including the role of hydrothermal fluids. The majority of the world's PGE reserves are held in a handful of deposits, most of which occur within the unique Bushveld Complex of South Africa