Economic Geology Vol. 78, 1988, pp. 57-72 Mineralogy and Geochemistry of Epigenetic Features in Metalliferous Sediment, Atlantis II Deep, Red Sea R. A. ZIERENBERG AND W. C. SHANKS III University of Wisconsin-Madison, Department of Geology and Geophysics, Madison, Wisconsin 58706 Abstract Metalliferous sediment precipitated in the Atlantis II Deep brine poolis finely laminated and undisturbed except in the area of the southwest basin. In this part of the Deep massive bedding, resedimentation, and brecciation are common. In addition,some sediment cores containvein minerals precipitated in open fissures which crosscut unlithified metalliferous sediments. These veins are the conduits by whichnew hot brine vents into the Deep. Vein mineralogy isdominated by anhydrite, talc,smectite, pyrite, sphalerite, andchalcopyrite. The mineralogy and mineral chemistry of the veins show a vertical zonation due to cooling and reaction of the incoming brineasit rises through the metalliferous sediment. Talc and mag- nesium-rich smectites dominate deep veinlets, and mote iron-rich smectite and anhydrite dominate veins nearer the sediment-water interface. Near the southwest basin-west basin transition, metalliferous sediment has been locally recrystallized to a hematite-magnetite-py- roxene assemblage, probably due to the intrusion of basalt into the metalliferous sediments. Introduction THE Atlantis II Deep is a tectonic depression within the activelyspreading Red Sea rift which is filled with stratified, hot, saline brine. The lowest brine layer has a density of 1.2 g/cm a, a temperature of 61.5øC,and chlorinity of 156 per mil as measured in 1977 (Schoell and Hartmann, 1978; Hartmann, 1980).This is overlain by a cooler (50øC), less dense (1.10g/cm a) brinewith a chlorinity of 82 per mil. Recent hydrographic observations (Schoell, 1976; Schoell and Hartmann, 1978;Hartmann,1980) in- dicate that these brines comprise a dynamic system, withsignificant temporal changes in temperature and total dissolved salts related to active brineventing in the southwest basin of the Deep (Fig. 1). The general lithostratigraphy of the metalliferous sediment (Fig. 2) has beendescribed by B//cker and Richter (1978). Disruption of the sediment, including brecciationand resedimentation, are characteristic of the unique facies which are developed in the south- westbasin(Fig. 2). Examination of 291core logs and25 sediment cores from theAtlantis II Deep has revealed that epigenetic features (Table 1) are common in the metalliferous sediment, especially in the southwest basin. Two im- portant types of epigenetic features have been dis- tinguished: veinsand discontinuous layersof anhy- drite and talc, and metalliferous sediment recrystal- lized to hematite-magnetite-pyroxene. Textural and spatial relations of these features are summarized in Figure $. A detailed study of these features has beenunder- taken (1) to provide a descriptive mineralogic frame- work for comparison to epigenetic features observed in ancient deposits and (2) to determine constraints on chemical and physical properties of the venting, mineral-depositing brine. Methods The materials used in thisstudy are subsamples of piston andkasten cores taken during the R/V Wando Rivet 1969 and the R/V Valdivia 1971 and 1978 cruises. The locations of cores are shown in Figure 1 and a summary of textural types and mineralogy is providedin Table 1. The cores have been stored at 4øC and 100 percent relativehumidity since the time of collection. Samples were washed with deion- ized water to removeinterstitial salt and separated into size fractions of greater and less than74 •. Clay mineralogy was determined on the less than2-t•frac- tion of samples treated with sodiumcitrate-dithio- nate to removefree iron oxides (Jackson, 1979). X- ra• diffractometry wasdone using Cu Ka radiation and a curved crystal monochrometer to discriminate against high levels of iron fluorescence. On selected samples, clay mineralhk spacings were determined with a Debye-Scherrer camera. The greater than 74- • fractionwas studied by petrographic microscope, scanning electron microscope with energy dispersive X-ray spectrometer, and single crystal X-ray diffrac- tion using a Gandolphi camera. Chemical data were obtained using an ARL electron microprobe with en- ergy dispersive X-ray spectrometer, with convential corrections Using the MAGIC program (Colby, 1971). Clay mineral samples were hydraulically pressed into stainless steel mounts to obtain a surface suitable for probe analysis. Pressed powders were analyzed with a 50-• beam, with beam current maintained at 0561-0128/85/118/57-16$2.50 57