ALBERT

Description: The Jbel Tirremi fluorite-barite ± sulfide deposit in northeastern Morocco is hosted in a Jurassic-aged structurally high carbonate platform known as the Jbel Tirremi dome. The host rocks consist of unmetamorphosed, flat-lying early Jurassic dolomitized limestones, locally intruded by Eocene lamprophyre dikes. The orebodies consist mostly of fluorite and barite, and occur as open-space fillings and partial to massive replacement of the enclosing medium- to coarse-grained dolomitized limestones. The ore mineralogy is dominated by fluorite of different colors and habits, barite, and, to a lesser extent, sulfides. Rare earth element compositions along with fluid inclusion, halogen and isotopic data suggest that the fluorite barite mineralization and the spatially associated Eocene alkaline magmatism are petrogenetically unrelated, pointing instead to the regional circulation of hydrothermal basinal brines mixed to various degrees with meteoric water in a dominantly closed rock-buffered system at progressively higher temperatures and fluid/rock ratios. In this respect, fluid inclusion microthermometric measurements show that the ore-bearing hydrothermal system developed in two separate stages of fluorite-barite mineralization, as also revealed by isotopic data. Both stages precipitated from saline fluids at shallow crustal levels (i.e., 〈5 km), and were related, in varying degrees, to different stages of basin evolution and salt dome growth (salt mobilization and mineralization). During the first stage, the ore fluid was a highly saline aqueous brine with a total salinity up to 44.2 wt % NaCl + KCl equiv, at temperatures ≥82°C and possibly up to 218°C, whereas in the second stage the mineralizing fluid had a similar temperature range, but lower salinities (~20–10 wt % NaCl equiv). The recorded high salinities are interpreted to represent the involvement of a mixture of halite dissolution water and evaporated seawater component. Oxygen ( 18 O = 21.7 to 29.6 V-SMOW) and carbon ( 13 C = –7.9 to 0.2 V-PDB) isotope data along with strontium ( 87 Sr/ 86 Sr = 0.70300–070789) and lead ( 206 Pb/ 204 Pb = 17.961–20.96, 207 Pb/ 204 Pb = 15.511–15.697, 208 Pb/ 204 Pb = 37.784–39.993) isotope ratios suggest the involvement of a mixture of oil-bearing fluids, basinal brines, and meteoric fluids that interacted extensively with the early Jurassic host carbonates, the underlying Triassic salt-bearing diapir, associated siliciclastic rocks, and the highly fractionated and greisenized Hercynian granitic crystalline basement, resulting in the release of fluoride, metals, and other constituents to form the Jbel Tirremi deposit. Petroleum-bearing fluid, released from overpressured portions of the Guercif Basin at lithostatic pressures, and bittern brines dominated the first stage of mineralization. Mixing of saline, oxidized, CaCl 2 - and sulfate-rich bittern brine with oil-bearing fluid resulted in fluorite precipitation of stage I. Conversely, during the second stage of mineralization, the hydrothermal system was open to the influx of oxidized meteoric water as a consequence of the upward migration of the Triassic salt-bearing diapir and associated pressure decrease. The shift from stage I to stage II is associated with the evolution of the system from lithostatic to mostly hydrostatic pressure conditions. Stage I mineralization is thought to have occurred during the Late Miocene in response to rapid sedimentation and high subsidence rates and subsequent hydrocarbon migration associated with the outward migration of the Rif thrust front. Conversely, stage II mineralization occurred coevally with the uplift phase during Tortonian time.

Description: Ground-based and satellite observations are used in conjunction with the RRTM radiative transfer model to assess climatological aerosol loading and the associated cloud-free aerosol direct radiative effect (DRE) over the Red Sea. Aerosol optical depth (AOD) retrievals from the MODIS and SEVIRI instruments are first evaluated via comparison with ship-based observations. Correlations are typically better than 0.9 with very small root-mean-square and bias differences. Calculations of the DRE along the ship cruises using RRTM also show good agreement with co-located estimates from the GERB instrument if the aerosol asymmetry parameter is adjusted to account for the presence of large particles. A monthly climatology of AOD over the Red Sea is then created from 5 yrs of SEVIRI retrievals. This shows enhanced aerosol loading and a distinct north to south gradient across the basin in the summer relative to the winter months. The climatology is used with RRTM to estimate the DRE at the top and bottom of the atmosphere and the atmospheric absorption due to dust aerosol. These climatological estimates indicate that although longwave effects can reach 10s of W m −2 , shortwave cooling typically dominates the net radiative effect over the Sea, being particularly pronounced in the summer, reaching 120 W m −2 at the surface. The spatial gradient in summer-time AOD is reflected in the radiative effect at the surface and in associated differential heating by aerosol within the atmosphere above the Sea. This asymmetric effect is expected to exert a significant influence on the regional atmospheric and oceanic circulation.