Stockwork-Like Sulfide Mineralization in Young Oceanic Crust: Deep Sea Drilling Project Hole 504B

Abstract

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.

Os isotopic compositions of MORBs from the ultra-slow spreading Southwest Indian Ridge: Constraints on the assimilation and fractional crystallization (AFC) processes

“…Enrichment of both U and Re in LOC from Site 735B suggests that very high Re may result from hydrothermal alteration (Blusztajn et al, 2000). Such enhanced hydrothermal activity would also deposit secondary sulfides in the LOC (e.g., Honnorez et al, 1985), which may have very radiogenic Os compositions of up to 1 (Brügmann et al, 1998;Ravizza et al, 1996). Furthermore, zircon U-Pb ages of LOC from Hole 735B suggest that construction of ultra-slow spreading oceanic crust may take 2.5 Ma (Schwartz et al, 2005).…”

Hydrothermal Alteration of a Section of Upper Oceanic Crust in the Eastern Equatorial Pacific: A Synthesis of Results from Site 504 (DSDP Legs 69, 70, and 83, and ODP Legs 111, 137, 140, and 148)

“…A mineralized stockworklike zone containing abundant veins of quartz and sulfide minerals occurs in a highly fractured and brecciated pillow unit at 910-928 mbsf within the transition zone ( Fig. 2; Honnorez et al, 1985).…”

“…7; Alt et al, 1985aAlt et al, , 1985bAlt et al, , 1986aAlt et al, , 1986bAlt et al, , 1989aAlt et al, , 1989bHonnorez et al, 1985). First, chlorite and actinolite formed in veins and greenschist minerals (chlorite, actinolite, albite-oligoclase, titanite, and local talc) formed in the host rocks during axial hydrothermal alteration by seawater hydrothermal fluids at temperatures of ~200°C to >300°C.…”

Patterns and processes of alteration in the lavas and dykes of the Troodos Ophiolite, Cyprus

“…Holes CY2A and 504B both intersected fossil upwelling zones that did not vent on the seafloor [Honnorez et al, 1985;Alt et al, 1986a;Adamides, 1987]. Metal-emichrnent in some rocks from the upper dyke zone of hole CY1/1A (Figure 9) suggests that the drillcore also intersected a zone of upwelling [Gillis and Robinson, 1991b].…”