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Notes: Abstract New investigations are carried out on the mineralogy and mineral chemistry of sulfide assemblages obtained in samples from one core in the hydrothermally active, southwest basin of the Atlantis II deep, Red Sea. The most abundant sulfide phases are the exsolved intermediate solid solution (ISS) and chalcopyrite. Sphalerite, pyrrhotite, marcasite, mackinawite, and presumably wurtzite are also observed. Two distinct groups of paragenesis were encountered: (a) Intermediate solid solution with sphalerite incrustations and intergrowths, and (b) intermediate solid solution barren of sphalerite intergrowths. The first group is confined to the upper part of the Co zone and the SOAN zone (Bäcker and Richter 1973), and the second is present in the entire core 100-3-7. An optically isotropic chalcopyrite is found for the first time as a natural mineral in Atlantis II, Red Sea. Yet its existence as a novel phase needs x-ray confirmation. It exhibits a lower reflectivity than normal chalcopyrite and is isotropic. Chalcopyrite occurs either as a single phase or in association with tetragonal chalcopyrite. Our investigations indicate that the formation of Atlantis II deposits is a result of complex processes. These processes are characterized by compositional changes in the ore-bearing fluids and the change in sulfur fugacity (especially with depth). The presence of exsolved chalcopyrite lamellae in ISS indicates slow cooling below 450°C. However, it is difficult to understand why the cubic chalcopyrite is not converted to the tetragonal form even though the temperature of transformation lies above 450°C (470° – 500°C, Cabri 1973). The Cu/Fe ratio changes in the exsolved chalcopyrite lamellae from core to rim of the composite grains. The ratio is higher in the rims. This suggests that primary inhomogenous ISS grains formed from solutions with a continuous increase in the Cu/Fe ratio. Slow cooling is also required to account for the exsolution of chalcopyrite lamellae in ISS. The low sulfur content in isotropic chalcopyrite is also suggestive of low fs2. The low S content in the chalcopyrite may be the controlling factor for the sluggish conversion from cubic to tetragonal chalcopyrite. Mackinawite lamellae show the same orientation in ISS and exsolved isotropic chalcopyrite indicating that mackinawite exsolved before the breakdown of ISS. This strongly suggests that mackinawite is stable above 300°C (contrary to experimental results by Zoka et al. 1973). Pyrrhotite was probably formed by the sulfurization of ilvaite. The pyrrhotite grains with several complex successive zones show the sequence of the sulfurization episodes. Metalliferous sediments related to hot brines were discovered in the Red Sea in 1964 (Miller et al. 1966). Since then, several papers have been published on this subject (Degens and Ross 1969, Bäcker and Schoell 1972, Bäcker and Richter 1973, Bignell et al. 1976, Shanks and Bishoff 1977, Weber-Diefenbach 1977, Nöltner 1979, Pottorf 1980, Pottorf and Barnes 1983, Oudin et al. 1984). Complex sulfide phases including intermediate solid solution (ISS), chalcopyrite, and a chalcopyritelike mineral (which exhibits a lower reflectivity than normal chalcopyrite and appears to be isotropic occur in the metalliferous sediments. These phases were found in association with several minerals in different parageneses. In an attempt to understand the origin of the formation of the sulfide-bearing sediments in the Atlantis II deep of the Red Sea, a detailed study of the phase relations of the Cu-Fe sulfide ores of this locality was carried out.