Formation of volcanic-exhalative nickel-sulfide deposits at a Late Proterozoic spreading ridge in the proto-Arabian Shield

Abstract

Field and petrologic studies indicate that the strata-bound nickeliferous sulfide deposits of Jabal Mardah were formed contemporaneously with sedimentation of an active spreading ridge. Mineralization is distributed discontinuously over a strike length of 10 km within a 20–250 m thick sequence of basaltic turbidites of submarine-canyon to inner-fan-channel facies. These rocks are stratigraphically underlain and overlain by pillow basalts of the Late Proterozoic Darb Zubaydah ophiolite. Individual deposits (as large as 1.5 Mt at 0.8% Ni) exhibit characteristics that indicate formation by volcanic exhalation, such as (i) soft-sediment deformation, (ii) graben-controlled geometries, and (iii) underlying zones of stockwork mineralization. The assemblage pyrite + polydymite (Ni3S4) + vaesite (NiS2) filled veins and replaced wall rock in the stockwork zone; pyrite + millerite (NiS) + polydymite partially replaced unconsolidated sedimentary particles and directly precipitated in pore spaces. Cu and Zn sulfides are scarce. Hydrothermal alteration that was overprinted by lower greenschist-facies metamorphism is principally albite + chlorite + epidote in deeper portions of the turbidite sequence and albite + chlorite + calcite or quartz + chlorite + calcite in shallower portions. Epiclastic basaltic sediments were deposited intermittently during periods of mineralization. Graben structures controlled the distribution of sediments and the discharge of mineralizing fluids. Thick sedimentary piles acted as chemical sinks for upwelling fluids and prevented significant precipitation of sulfides on the sea floor. Contemporaneous mineralization, sedimentation, and erosion (indicated by basal scour zones) produced a complex pattern of sulfide distribution near points of fluid discharge. These nickeliferous deposits are similar in most physical aspects to ophiolite-related Cu–Zn deposits of Phanerozoic age. The divergence in mineralogy at Jabal Mardah principally resulted from the discharge of lower temperature (less than 200°) fluids, lower [Formula: see text], and probably higher [Formula: see text].