Stockwork-like metal sulfide mineralizations were found at 910-928 m below seafloor (BSF) in the pillow/dike transition zone of Hole 504B. This is the same interval where most physical properties of the 5.9-m.y.-old crust of the Costa Rica Rift change from those characteristic of Layer 2B to those of Layer 2C. The pillow lavas, breccias, and veins of the stockwork-like zone were studied by transmitted and reflected light microscopy, X-ray diffraction, and electron microprobe analysis. Bulk rock oxygen isotopic analyses as well as isolated mineral oxygen and sulfur isotopic analyses and fluid inclusion measurements were carried out. A complex alteration history was reconstructed that includes three generations of fissures, each followed by precipitation of characteristic hydrothermal mineral parageneses: (1) Minor and local deposition of quartz occurred on fissure walls; adjacent wall rocks were silicified, followed by formation of chlorite and minor pyrite I in the veins, whereas albite, sphene, chlorite and chlorite-expandable clay mixtures, actinolite, and pyrite replaced igneous phases in the host rocks. The hydrothermal fluids responsible for this first stage were probably partially reacted seawater, and their temperatures were at least 200-250° C. (2) Fissures filled during the first stage were reopened and new cracks formed. They were filled with quartz, minor chlorite and chlorite-expandable clay mixtures, traces of epidote, common pyrite, sphalerite, chalcopyrite, and minor galena. During the second stage, hydrothermal fluids were relatively evolved metal-and Si-rich solutions whose temperatures ranged from 230 to 340° C. The fluctuating chemical composition and temperature of the solutions produced a complex depositional sequence of sulfides in the veins: chalcopyrite I, ± Fe-rich sphalerite, chalcopyrite II ("disease"), Fe-poor sphalerite, chalcopyrite III, galena, and pyrite II. (3) During the last stage, zeolites and Mg-poor calcite filled up the remaining spaces and newly formed cracks and replaced the host rock plagioclase. Analcite and stilbite were first to form in veins, possibly at temperatures below 200°C; analcite and earlier quartz were replaced by laumontite at 250°C, whereas calcite formation temperature ranged from 135 to 220°C. The last stage hydrothermal fluids were depleted in Mg and enriched in Ca and 18 O compared to seawater and contained a mantle carbon component.This complex alteration history paralleling a complex mineral paragenesis can be interpreted as the result of a relatively long-term evolution of a hydrothermal system with superimposed shorter term fluctuations in solution temperature and composition. Hydrothermal activity probably began close to the axis of the Costa Rica Rift with the overall cooling of the system and multiple fracturing stages due to movement of the crust away from the axis and/or cooling of a magmatic heat source.

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Mineralogy, chemistry, and some magnetic properties of altered basaltic rocks from DSDP Site 396B are rather simply related to one another but differ dramatically with the nature of the igneous material and with the sharp alteration zonation within individual basalt fragments. Low‐temperature alteration of the major igneous minerals appears to have been primarily a function of oxidation, which affects titano‐magnetite pervasively (gray interiors) and olivine locally (brown zones). Destruction of olivine in the pillow basalts released Mg++, SiO2, Co, and Ni to the circulating solution and resulted in a calculated 9–10% decrease in density of the most altered (brown) zones in the rock. Seawater phosphate was incorporated in secondary ferric oxides replacing the olivine. In the massive basalts, all of the SiO2 and much of the Mg from olivine recombined as smectite, and the inferred rock density is almost unchanged. In almost all of the crystalline rocks, primary Ca, Na, Fe, Ti, and Al were immobile, owing to the metastability of plagioclase and clinopyroxene. The filling of cracks by smectite, Fe‐Mn oxides, phillipsite, and calcite consumed elements from various sources and was probably more important than basalt alteration for some chemical fluxes. Values of δ18O for the altered rocks are roughly 1–3‰ higher than their fresh equivalents, owing to the addition of 18O‐rich secondary oxides, smectites, and lesser carbonates and phillipsite. The highest values are associated with the smectite‐rich samples. However δ18O is not a good indicator of the degree of alteration because of strong leaching in the most altered zones of the pillow basalts. Pervasive oxidation of titanomagnetite to titanomaghemite has caused all samples to have high Curie temperatures, indicating up to about 70% oxidation. With the destruction of olivine in the brown zones, strong oxidation has caused breakdown of titanomaghemite to more stable nonmagnetic oxides, hence a drop in saturation intensity. These reactions have proceeded further in the pillow basalts than in the massive basalts. Freshly created oceanic crust consisting of various basaltic materials with abundant void space and (or) interstitial sediments is clearly heterogeneous; this primary heterogeneity is accentuated by alteration.

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The Snake Pit active hydrothermal field was discovered at 23°22'N on the Mid-Atlantic Ridge during ODP Leg 106. Among the ten holes drilled in the mound at the foot of an active chimney, only three (649B, 649F, and 649G) had substantial recovery, and produced cores of unconsolidated hydrothermal deposit made up of porous sulfide fragments with minor talc pellets and biological debris, and a few pieces of brassy massive sulfides. Eight representative samples from the 6.5-m-long core from Hole 649B were analyzed for bulk chemistry, both by XRF (major elements) and NAA (trace elements). Major elements average compositions show high Fe (36 wt%), S (37 wt%), and Cu (12 wt%) contents, and minor Zn (6.7 wt%), reflecting a mostly high-temperature deposit. Trace elements are characterized by a high Au content (600 ppb) which could express the maturity of the mound. Mineralogical assemblages show evidence of sequential precipitation, and absence of equilibrium. Major sulfide phases are pyrrhotite, pyrite, Fe, Cu sulfides, marcasite, and sphalerite. Three types of samples are distinguished on the basis of textures and mineral assemblages: type 1, rich in pyrrhotite, with approximately equivalent amounts of Cu, Fe sulfides, and sphalerite and minor pyrite; type 2, rich in Cu, Fe sulfides, which are cubic cubanite with exsolutions and rims of chalcopyrite; and type 3, essentially made up of sphalerite. Type 2 samples likely represent fragments of the inner chimney wall. The presence of talc intergrown with cubic cubanite/chalcopyrite in one big piece from Hole 649G is probably related to mixing of the hydrothermal fluid with seawater.