(Table 1, page 605) Chemistry of concretions from Grand Lake (GL) and Ship Harbour Lake (SH), Nova Scotia and Mosque Lake (ML), Ontario

Studies by optical microscopy, x-ray diffraction, and electron probe techniques of ferromanganese concretions from three Canadian lakes reveal chemical banding of amorphous hydrated iron and manganese oxides. The average ratio of iron to manganese in concretions from these lakes varies from 0.43 to 2.56. The concentrations of cobalt, nickel, copper, and lead are one to two orders of magnitude below those reported for oceanic ferromanganese concretions.

Chemical composition of the chalcophanite rich surface of an encrusted rock ashore Lake Macquarie, Australia

Mineralogical interest in the nature of manganese oxide particulates in natural marine water (Suess, 1979), natural lake water (Klaveness, 1977), and simulated lake water (Giovanoli, 1980), prompted a search for such particulates in a large New South Wales coastal lake. The investigated waters did show the existence of manganese oxide replacement phenomena in fragmentary sedimentary rocks near the south margin of Lake Macquarie. The black crusts of manganese oxide discovered on rocks close to the waterline have revealed a three layers structure. Layer A (0-35 micron), adjacent to the rock, is composed essentially of kaolinite of weathering origin, together with low levels of manganese oxide without detectable Zn. Layer B (35-80 micron) follows as a manganese oxide layer containing admixed kaolinite and low amounts of Zn. Layer C (80-130 micron) is the closest to the surface and is made of Chalcophanite containing 10-15% of ZnO.

(Table 1, page 168) Iron, manganese, and phosphorus concentrations in Fe-Mn-concretions from the Gulf of Bothnia, northern Baltic Sea

Fe-Mn-concretions of a spheroidal type were found according to electron probe determinations to consist of alternating iron- and manganese-rich layers. This pattern was ascribed to seasonal variations in the physico-chemical conditions governing the precipitation of the hydrous oxides of iron and manganese. Calculations based on the rhythmic growth of the concretions investigated gave a mean accumulation rate of 0.15-0.20 mm/yr. The rather high phosphorus content (average 3.5 % P2O5) of the concretions was found to be concentrated in the iron-rich layers, probably as a result of the scavenging effect of ferric hydroxide.

Geochemical composition of sulfide and oxide minerals from ODP Sites 193-1188 and 193-1189, PACMANUS field

Ocean Drilling Program (ODP) Leg 193 recovered core from the active PACMANUS hydrothermal field (eastern Manus Basin, Papua New Guinea) that provided an excellent opportunity to study mineralization related to a seafloor hydrothermal system hosted by felsic volcanic rocks. The purpose of this work is to provide a data set of mineral chemistry of the sulfide-oxide mineralization and associated gold occurrence in samples drilled at Sites 1188 and 1189. PACMANUS consists of five active vent sites, namely Rogers Ruins, Roman Ruins, Satanic Mills, Tsukushi, and Snowcap. In this work two sites were studied: Snowcap and Roman Ruins. Snowcap is situated in a water depth of 1670 meters below sea level [mbsl], covers a knoll of dacite-rhyodacite lava, and is characterized by low-temperature diffuse venting. Roman Ruin lies in a water depth of 1693-1710 mbsl, is 150 m across, and contains numerous large, active and inactive, columnar chimneys. Sulfide mineralogy at the Roman Ruins site is dominated by pyrite with lesser amounts of chalcopyrite, sphalerite, pyrrhotite, marcasite, and galena. Sulfide minerals are relatively rare at Snow Cap. These are dominated by pyrite with minor chalcopyrite and sphalerite and traces of pyrrhotite. Native gold has been found in a single sample from Hole 1189B (Roman Ruins). Oxide minerals are represented by Ti magnetite, magnetite, ilmenite, hercynite (Fe spinel), and less abundant Al-Mg rich chromite (average = 10.6 wt% Al2O3 and 5.8 wt% MgO), Fe-Ti oxides, and a single occurrence of pyrophanite (Mn Ti O3). Oxide mineralization is more developed at Snowcap, whereas sulfide minerals are more extensive and show better development at Roman Ruins. The mineralogy was obtained mainly by a detailed optical microscopy study. Oxide mineral identifications were confirmed by X-ray diffraction, and mineral chemistry was determined by electron probe microanalyses.