The mineral chemistry data of sulfide-oxide mineralization and associated gold occurrences for Sites 1188 and 1189 were collected from different samples at different depths. Table T2 summarizes the minerals analyzed with the electron microprobe with respect to their origin site and depth.
Pyrite is the dominant sulfide mineral throughout the drilled cores. Pyrite composition is nearly stoichiometric. No cobalt or nickel were detected. Trace amounts of arsenic and, more rarely, copper and/or zinc were detected in a few analyses. Tables T3, T4, T5, and T6 present the data collected from each studied sample.
We attempted, without success, to analyze pyrrhotite in a single sample. The mineral is extremely fine grained (<2 µm) and occurs as inclusions in pyrite.
Chalcopyrite is more common in samples from Site 1189 (Roman Ruins). Chalcopyrite commonly occurs in association with pyrite, sphalerite, and quartz. Chalcopyrite is also present as isolated anhedral grains within strongly chloritized zones and/or in the groundmass. Tables T7, T8, and T9 present the chemical composition of the analyzed chalcopyrite that occurs, respectively, at Site 1188 and in Holes 1189A and 1189B.
Sphalerite is present both in veins and vesicle linings. Sphalerite was seen associated with chalcopyrite and, in some cases, on the edge of pyrite crystals. Chemical compositions of sphalerite were obtained in samples from Roman Ruins only (Hole 1189B). These analyses are presented in Table T10.
Galena was described in a single sample from Site 1189 (Roman Ruins), at 147.4 meters below seafloor (mbsf) in Hole 1189B. Galena occurs in close association with sphalerite, pyrite, and lesser amounts of chalcopyrite. It occurs as irregular bodies in the groundmass, in sphalerite, or in pyrite next to sphalerite. The galena grains are usually very small with poor polish quality, which makes them very hard to identify. A few larger grains were found both associated with sphalerite and in the groundmass. Electron microprobe analyses (EPMA) included arsenic, selenium, and silver in addition to lead and sulfur elements. Silver contents are low (0.10-0.25 wt%), and no arsenic or selenium were detected. Table T11 presents the chemical compositions.
Subsurface gold mineralization occurs in the Roman Ruins site at 118 mbsf, as micrometric grains of silver-poor (0.4-2.5 wt% Ag) native gold grains. It occurs as fine inclusions in three different minerals: (1) on the edge of sphalerite grains associated with hydrothermal silica vein, (2) filling voids and/or lining vesicles in quartz, and (3) as inclusions in pyrite. All these grains contain silver (0.4-2.5 wt%) as well as trace amounts of copper. No mercury was detected. Silver content varies with the gold grain mineral association. Those gold grains associated with sphalerite show the minimum silver content (0.36 wt%). Gold in pyrite contains on average 1.23 wt% silver, and gold in quartz shows the highest silver composition (2.48 wt%). Gold grain compositions are presented in Tables T12, T13, and T14. Low totals are a result of the difficulty in analyzing the very small grains of gold, 90% of which are <5 µm across. Table T15 summarizes the gold data obtained for each occurrence type.
Oxide minerals are represented by Ti magnetite, magnetite, ilmenite, hercynite (Fe spinel), hematite, and less abundant chromite (average = 10.6 wt% Al2O3 and 5.8 wt% MgO), Fe-Ti oxides, and a single occurrence of pyrophanite (Mn Ti O3). Electron probe microanalyses were performed on samples from Holes 1188A and 1188F. No samples from Site 1189 were used in this analytical investigation. Hematite was not analyzed because of its grain size and shape. It occurs as platy inclusions in quartz <3-5 µm across.
Magnetite is a trace component of the rocks but is the dominant iron oxide mineral throughout the drilled cores. Ti magnetite is, in some cases, closely associated with magnetite. Magnetite is present within veins of intergrown quartz, brown clay, and pyrite. Detailed microscopic observations reveal a few examples of Ti magnetite-ilmenite exsolution. Magnetite also is present as remnants in leucoxene within the groundmass (Binns, Barriga, Miller, et al., 2002; Pinto et al., 2003). Tables T16 and T17 present the chemical compositions of the analyzed magnetite and Ti magnetite in samples from Holes 1188A and 1188F.
Chromite was found only once, occurring as a big relict crystal in Sample 193-1188A-21R-1 (Piece 3, 29-34 cm) at 183.4 mbsf. Table T18 provides the chemical composition data set obtained by the electron microprobe for this mineral.
Ilmenite is present at Snowcap, Hole 1188F, between 336 and 346 mbsf. Ilmenite is associated with hercynite (Fe spinel) and magnetite. Less commonly, coarser ilmenite is intergrown with magnetite. Table T19 shows the composition of ilmenite from Hole 1188F.
A transparent to translucent spinel occurs enclosed within quartz and coarser grains of magnetite and ilmenite. The color of the mineral varies in plane-polarized transmitted light from bright apple green to a dark greenish brown. The spinel has been identified as a hercynite (Fe spinel), contains tiny inclusions of magnetite, and is rimmed by a thin film of magnetite. Hercynite has been observed only in samples from Hole 1188F at depths ranging from 336 to 346 mbsf. Table T20 presents EPMA results.
A single sample from Snowcap (193-1188F-34Z-1 [Piece 9A, 45-47 cm]) revealed a few crystals of Fe-Ti oxides (average = 0.56 wt% Al2O3 and 4.49 wt% FeO with traces of V2O3). EPMA results are presented in Table T21.
Another rare occurrence is pyrophanite (Mn Ti O3) in Sample 193-1188F-34Z-1 (Piece 9B, 46-49 cm) from Hole 1188F. The single chemical analysis obtained is shown in Table T22.