SULFIDE AND OXIDE PETROLOGY

Mineralization in seafloor sulfide deposits can occur as massive sulfides, semimassive sulfides, sulfide minerals in stockworks, stringers, or veins, sulfide-filled breccias, disseminated sulfides, Fe-Mn-Si oxide deposits, and metalliferous (sulfide and oxide) sediments. The sulfide or sulfide-bearing rocks are referred to in this report as simply "sulfides" or "sulfide rocks." Any rock containing 5% sulfides was regarded to be a sulfide rock. The manner in which they are curated, sampled, described, and analyzed overlaps procedures established for both igneous and hydrothermally altered rock (see "Hydrothermal Alteration," "Igneous Petrology," and "Geochemistry"). In addition, for rocks in which the sulfide content was <5%, microscope observations with reflected light of sulfides (and oxides) in polished thin section are described as appropriate.

Core pieces were cut with a rock saw. Cores were designated using leg, site, hole, core, section, piece, and core type as discussed in "Introduction". Each piece was examined for volcanic or relict volcanic features, structural orientation, sulfide and oxide mineralogy, mineral textures, vein distribution, and hydrothermal alteration.

The archive half of each hard-rock sulfide core was described on a cruise-specific form (see "Sulfide Logs" in the "Core Descriptions" contents list) and was photographed before storage. Photographs (macroscopic, binocular, and microscopic) were taken to illustrate representative structures and textures. During the core description process, samples were selected from the working half for analysis of physical and chemical properties.

We tried to retard the oxidation of sulfide-bearing rocks by sealing them as quickly as possible in nitrogen purged bags, but tattletale oxygen indicators sealed with the cores indicated the storage bags were permeable.

Sulfide-rich samples for shore-based spectroscopic surface analysis were taken before the core was split to avoid oxidation. This sampling was done in conjunction with the microbiological sampling (see "Microbiology" for further details). Large whole-round pieces were removed from the core and transferred into an anaerobic chamber. The whole-round pieces were split and pristine (expected to be unoxidized) center samples were obtained. The rest of the sample (all outside parts) was returned to the original core for curation.

Sulfide rocks and their alteration phenomena were described using a variant of the standard VCD form for igneous rocks, to which was added a set of symbols for mineralization. All visual descriptions of sulfide rocks were entered into a computerized database (see the "Sulfide Logs" or the "Alteration Logs," both in the "Core Descriptions" contents list, for rocks containing <5% sulfides). These forms were completed for each lithologic unit and are described below (see "Sulfide Description Spreadsheet"). More detail was obtained in some instances from examination of polished thin sections as described below (see "Polished Thin-Section Description").

A classification system was established based on the macroscopic description of the samples. Each rock type is defined by its content of principal minerals, as determined by hand-sample examination and its most prominent textures. The mineral content and textural characteristics of representative samples of each type were confirmed using polished thin-section data. The principal minerals are generally distinguishable macroscopically. Common opaque minerals are pyrite, chalcopyrite, and sphalerite (includes wurtzite). Common nonopaque minerals, referred to as "gangue minerals" by sulfide petrologists, are quartz, silica (which includes cristobalite and amorphous silica), anhydrite, and undifferentiated clay minerals. The opaque mineral magnetite was included with the gangue minerals as part of the alteration assemblage unless it was clearly of igneous origin. Gangue minerals typically fill veins, fractures, pore spaces, and cavities in host volcanic rocks; they also are present as discrete grains disseminated throughout the sulfides or intimately intergrown with sulfides. By convention, major constituents are 10 vol%, minor constituents are 2 and <10 vol%, and trace constituents are <2 vol%.

Determination of sulfide textures is more subjective. Various terms used conventionally by sulfide petrologists are defined in Table T5. The determination of "grain size" refers primarily to the size of discrete minerals or aggregates of intergrown minerals and not to individual grains or crystals within an aggregate. For the purposes of Leg 193 sulfide descriptions, "extremely fine grained" is <0.001 mm in diameter, "very fine grained" is between 0.001 and 0.5 mm, "fine-grained" is >0.5 mm to 1 mm, "medium-grained" is >1 to 2 mm, and "coarse-grained" is >2 mm (see note on Table T6). Individual crystals, when visible, are described as anhedral, subhedral, or euhedral. Special mention is made of textural relationships between sulfide minerals such as overgrowths, inclusions, and replacements that are the basis for establishing a detailed paragenetic sequence. The term "paragenesis" is taken to mean an assemblage of minerals, whereas "paragenetic sequence" is the order of precipitation of minerals within a given assemblage (Table T7).

The various sulfide types are determined without any particular reference to stratigraphic relationships, and the major characteristics of the different types are summarized in Table T7. This classification system is meant to account only for the sulfide-bearing rocks obtained during Leg 193. Mineralogical and textural features of sulfides found in other sites, including sediment-starved mid-ocean ridges (Leg 158; see Humphris, Herzig, Miller, et al., 1995) and sedimented ridges (Legs 139 and 169; see Davis, Mottl, Fisher, et al., 1992; Fouquet, Zierenberg, Miller, et al., 1998), are mostly different from those observed during Leg 193. Mineral abundances listed in Table T7 are approximate and meant to serve only as a guide for identification of different sulfide rock types. Mineral abundances within individual sulfide types may vary widely.

Sulfide Description Spreadsheet

Descriptions of mineralized sections containing 5% sulfides were summarized on the VCD sheets (see "Hydrothermal Alteration" and "Geochemistry"). Detailed descriptions of these mineralized intervals were also entered on "Sulfide Log" spreadsheets (see the "Core Descriptions" contents list).

As for the other description recording tables used during Leg 193, identifier information is presented for each piece of core described and each line of the spreadsheet represents a separate piece of core or a bay in the core tray. The heading "Mineralization" refers to the proportion of each logged interval that is "mineralized" (i.e., composed of sulfide minerals and associated gangue). Pervasive alteration associated with disseminated mineralization is not considered to be gangue. The "Style" heading refers to the sulfide mineralization styles that are listed in Table T5. Space is then provided to record texture, grain size, and proportion of total mineralized material for individual mineral species, both sulfide and gangue. The textural terms that were used are also listed in Table T5. The percentage of individual minerals was estimated visually from hand specimen, except where additional information was available from polished thin sections or XRD. By convention, the sum of the percent of sulfide and gangue for each interval totals 100%. The "Comments" column was used to record additional information on each sample, including a denotation of intervals sampled for geochemistry or from which polished thin sections were cut.

Most of the cores recovered during Leg 193 contained sulfide minerals and a few were even semimassive sulfides. Polished thin sections of these sulfides were examined to refine visual observations and to document textural patterns and mineral relationships in greater detail. Information about sulfides was recorded on a common microscope description form with igneous petrology and alteration. The following information specific to sulfide minerals was recorded: mineral name, mineral percentages, size, morphology, and general comments. The percentages of individual phases were estimated visually. To avoid subjectivity in determining styles of mineralization, sulfide textures, and grain size, conventional terms were used as defined in Tables T5 and T6. These tables also provide abbreviations for these terms. The terminology for styles and textures of sulfide mineralization is consistent with that used for macroscopic descriptions. The polished thin-section descriptions are included in the "Core Descriptions" contents list.