Dredged basalts from the western Nazca plate and the evolution of the East Pacific Rise

Ocean-floor basalts and glasses were recovered from three stations along the western Nazca plate, from a sublinear topographic feature believed to represent the proto-East Pacific Rise (EPR), and include abyssal tholeiites, FeTi-basalts and glasses, as well as transitional and little fractionated compositions. When compared with their coexisting fresh glasses, the FeTi-basalts have higher total alkalies, TiO₂ and MgO, and lower FeO^*, suggesting that they have also been affected by non-oxidative post-magmatic alteration processes. The FeTi-glasses form a remarkably uniform compositional group through space and time. A little fractionated composition having anMg-number= 73, similar to those reported from the Mathematician Ridge, has higher Na₂O and TiO₂, and slightly lower CaO than similar compositions from the slowly accreting Mid-Atlantic Ridge. The basalts and glasses reported here exhibit the compositional diversity expected for propagating rifts and probably represent more than one volcanic episode.Both geochemical and geophysical interpretations support the inference that the EPR grew from Miocene times by the progressive growth and propagation of mantle perturbations, leaving a remnant sublinear zone of rough topography characteristic of slower accretion as the trace of the proto-EPR. Continuing translations and rotations of axial segments are occurring along the EPR, probably in response to self-reorganizations of mantle flow patterns arising from rapid melting and depletion of the source regions. The data allow the inference that the youthful rift systems of the eastern Pacific are far from thermodynamic equilibrium as might be expected if such systems were to drive fundamental life processes.     

Mass-transport complexes (MTCs) document subsidence patterns in a northern Gulf of Mexico salt minibasin

Mass-transport complexes (MTCs) dominate the stratigraphic record of many salt-influenced sedimentary basins. Commonly in such settings, halokinesis is invoked as a primary trigger for MTC emplacement, although the link between specific phases of salt movement, and related minibasin dynamics, remains unclear. Here, we use high-quality 3D seismic reflection and well data to constrain the composition, geometry and distribution (in time and space) of six MTCs preserved in a salt-confined, supra-canopy minibasin in the northern Gulf of Mexico, and to assess how their emplacement relate to regional and local controls. We define three main tectono-sedimentary phases in the development of the minibasin: (a) initial minibasin subsidence and passive diapirism, during which time deposition was dominated by relatively large-volume MTCs (c. 25 km³) derived from the shelf-edge or upper slope; (b) minibasin margin uplift and steepening, during which time small-volume MTCs (c. 20 km³) derived from the shelf-edge or upper slope were emplaced; and (c) active diapirism, during which time very small volume MTCs (c. 1 km³) were emplaced, locally derived from the diapir flanks or roofs. We present a generic model that emphasizes the dynamic nature of minibasin evolution, and how MTC emplacement relates to halokinetic sequence development. Although based on a single data-rich case study, our model may be applicable to other MTC-rich, salt-influenced sedimentary basins.     

Constraints on the composition of ore fluids and implications for mineralising events at the Cleo gold deposit,Eastern Goldfields Province,Western Australia

The Cleo gold deposit, 55 km south of Laverton in the Eastern Goldfields Province of Western Australia, is characterised by banded iron‐formation (BIF)‐hosted ore zones in the gently dipping Sunrise Shear Zone and high‐grade vein‐hosted ore in the Western Lodes.There is evidence that gold mineralisation in the Western Lodes (which occurred at ca 2655 Ma) post‐dates the majority of displacement along the Sunrise Shear Zone, but it remains uncertain if the ore in both structures formed simultaneously or separately. Overall, the Pb, Nd, Sr, C, O and S isotopic compositions of ore‐related minerals from both the Western Lodes and ore zones in the Sunrise Shear Zone are similar. Early low‐salinity aqueous‐carbonic fluids and late high‐salinity fluids with similar characteristics are trapped in inclusions in quartz veins from both the Sunrise Shear Zone and the Western Lodes. The early CO₂, CO₂–H₂O, and H₂O‐dominant inclusions are interpreted as being related to ore formation, and to have formed from a single low‐salinity aqueous‐carbonic fluid as a result of intermittent fluid immiscibility. Homogenisation temperatures indicate that these inclusions were trapped at approximately 280°C and at approximately 4 km depth, in the deeper epizonal range. Differences between the ore zones are detected in the trace‐element composition of gold samples, with gold from the Sunrise Shear Zone enriched in Ni, Pb, Sn, Te and Zn, and depleted in As, Bi, Cd, Cu and Sb, relative to gold from the Western Lodes. Although there are differences in gold composition between the Sunrise Shear Zone and Western Lodes, and hence the metal content of ore fluids may have varied slightly between the different ore zones, no other systematic fluid or solute differences are detected between the ore zones. Given the fact that the ore fluids in each zone have very similar bulk properties, the considerable differences in gold grade, sulfide mineral abundance, and ore textures between the two ore zones most likely result from different gold‐deposition mechanisms. The association of ore zones in the Sunrise Shear Zone with pyrite‐replaced BIF suggests that wall‐rock sulfidation was the most significant mechanism of gold precipitation, through the destabilisation of gold‐bisulfide complexes. The Western Lodes, however, do not exhibit any host‐rock preference and multistage veins commonly contain coarse‐grained gold. Fluid‐inclusion characteristics and breccia textures in veins in the Western Lodes suggest that rapid pressure changes, brought about by intermittent release of overpressured fluids and concomitant phase separation, are likely to have caused the destabilisation of gold‐thiocomplexes, leading to formation of higher‐grade gold ore zones.