Submersible investigation of an extinct hydrothermal system on the Galapagos Ridge; sulfide mounds, stockwork zone, and differentiated lavas

Fifteen dives along the Galapagos Ridge in the region between 85 degrees 49 minutes and 85 degrees 55 minutes W were made to examine detailed relationships among tectonics, hydrothermal activity and lava compositions. Extensive tectonic activity and physical weathering have exposed the inner parts of large Cu-Zn sulphide mounds and the uppermost part of the underlying stockwork zone. The mineralization occurs at the top and southern base of a horst block, 40-80 m high, that separates the present neovolcanic zone to the north from an older rift valley to the south. The lavas in the neovolcanic zone are homogeneous MORB pillows; those on the horst block and within the southern valley are evolved MORB to andesite pillow and sheet flows. The alteration zone exposed beneath the sulphide mounds comprises a network of fracture- controlled pipe- and sheet-like bodies of highly altered material which changes outward into relatively fresh but similarly closely fractured rocks. The hydrothermal upflow zone is extensively brecciated on a centimetre scale and encloses a stockwork of veinlets now filled largely by silica, clays and sulphides. The most highly altered rocks are strongly depleted in Ca, Na, K and Mn, and are enriched in S, Fe, Cu and Zn relative to their fresh analogues. Si and Mg are variable, the latter showing local depletions and enrichments according to the proportion and distribution of chlorite. Depletions in (super 18) O with increasing (super 87) Sr / (super 86) Sr suggest extensive sea-water-rock interaction at T < or =350 degrees C. Deep-tow and ALVIN-based magnetic profiles have a relative magnetization low centred over the southern valley and the horst block that could reflect more extensive hydrothermal alteration zones associated with older sea- floor. The Galapagos stockwork is most analogous to the alteration zones associated with massive sulphide deposits in the ophiolites of Cyprus and Oman.