Cu-Ni-PGE Magmatic Sulfide Ores and their Host Layered Gabbros in the Haymiliyah Fossil Magma Chamber (Haylayn Block, Semail Ophiolite Nappe, Oman)

Abstract

A disseminated sulfide-rich zone has been discovered in the gabbroic magma chamber of the Wadi Haymiliyah (Haylyn Block), located in the upper part of the main coarse-grained layered gabbro unit, immediately below a thick, laminated and fine-grained noritic unit. In this 150 m thick zone, thin, fine-grained, concordant layers of noritic gabbro first appear in the layered gabbros, and contain most of the disseminated sulfides. The cumulates are weakly affected by hydrothermal alteration and hence, magmatic textures are well preserved. At the base of the section, the cumulates comprise olivine gabbros, gabbros and two-pyroxene gabbros. In the uppermost two-thirds of the sulfide-rich zone, two-pyroxene gabbros become predominant and grade locally into noritic gabbros and writes. This zone constitutes a transitional unit between an open-system fractionation represented by thé main layered gabbro unit, and a closed-system fractionation corresponding to the planar-laminated noritic gabbros.

At the thin section scale, the sulfide content ranges from <1% to 30%. From the bottom to the top, the relative proportion of sulfides increases in parallel with the abundance of orthopyroxene. When fine-grained twopyroxene gabbros and coarse-grained gabbros are in contact, the sulfides are preferentially concentrated in the two-pyroxene gabbros. Sulfides are observed as spherical inclusions in plagioclase, clinopyroxene and orthopyroxene crystals, as well as intercumulus grains. “Net-textured” sulfides are observed in the layers richest in sulfides. Textural criteria evoke the segregation of an immiscible sulfide liquid which separated from a basaltic melt, prior to the first appearance of orthopyroxene as a main cumulus phase.

Three sulfide associations were observed: 1) Pyrrhotite with pentlandite and chalcopyrite characterize the base of the sulfide-rich zone. 2) Massive pyrite may be associated with chalcopyrite and pyrrhotite, accounting for more than 60% of the sulfide paragenesis. 3) In hydrothermally altered samples, pyrrhotite is pseudomorphosed into marcassite ± pyrite (“bird eyes” textures). This transformation may be related to low-temperature off-axis hydrothermal circulations. Pentlandite and chalcopyrite occur in two habits, as blocky grains adjacent to pyrrhotite, or as exsolution bodies within pyrrhotite. PGE analyses in one of the layers richest in sulfides yield (in ppb): Os < 8, Ir < 3, Ru 32, Rh 4, Pt 37, Pd 130 and Au 150. The chondrite normalized PGE pattern has a positive slope, with a Pd/Ir ratio > 30 and Au/Ir ≈ 160 (normalized to cliondrites) typical of magmatic concentrations. Textural and mineralogical criteria are in agreement, supporting a magmatic origin for these sulfide concentrations. These sulfides are contemporaneous to the fractionation of orthopyroxene, and were precipitated as immiscible liquid droplets from an already evolved basaltic magma. The absence of sulfides in coarse-grained gabbros suggests that the MORB-type magma which has periodically replenished the Haymiliyah magma chamber was sulfur-undersaturated. Sulfur saturation is due to an increase of volatile content in the already evolved magma.