GEOLOGIC, PETROLOGIC AND GEOCHEMICAL RELATIONSHIPS
BETWEEN MAGMATISM AND MASSIVE SULFIDE MINERALIZATION
ALONG THE EASTERN GALAPAGOS SPREADING CENTER
Michael R. Perfit1, W. Ian Ridley2 and Ian Jonasson3
1. Department of Geological Sciences, University of Florida, Gainesville, FL 32611
Email: mperfit@ufl.edu
2. US Geological Survey, Mineral Resources Program, Denver Federal Center, MS 973,
Denver, CO 80225
3. Geological Survey of Canada, 601 Booth Street, Ottawa Canada
Petrologic and geochemical analyses following detailed seafloor observations and
sampling of fresh and altered lavas and sulfides from the eastern Galapagos Spreading
Center (EGSC) at ~ 86°W in the Pacific Ocean provide a unique opportunity to
investigate the relationships between magmatism, hydrothermal activity and massive
sulfide mineralization on the seafloor. The extinct hydrothermal system (Galapagos
Fossil Hydrothermal Field), includes sulfide mounds, chimneys, and an underlying
stockwork zone, that are intimately associated with fresh and altered lavas. Although
comparisons have been made with ancient volcanogenic massive sulfide (VMS) ore
bodies, some associated with ophiolites, there are few places on the modern seafloor
where the “stockwork” zone has been exposed or studied in as much detail.
Eruptions of highly evolved magmas, including ferrobasalts, FeTi basalts and
andesites, in the environs of the GFHF are unusual in a MOR tectonic setting, but appear
to be a common phenomenon along the eastern Galapagos Rift as well as at other ridge
transform intersections (e.g. S. Juan de Fuca). Major elements, trace elements, Sr, Pb and
O isotopes demonstrate the cogenetic nature of the diverse assemblage of oceanic lavas.
Geochemical models document the dominant role of low-pressure fractional
crystallization, together with some crustal assimilation and minor magma mixing in
determining the evolution of the suite. Small magma reservoirs within 2 km of the
seafloor facilitate these processes if the supply of melt from below was limited allowing
substantial cooling and fractional crystallization. These magmatic processes were
accompanied by a minor amount of contamination from the surrounding oceanic crust,
the thermal energy being supplied by heat released during crystallization. Using this
framework, the behavior of the trace base metals and sulfur during magmatic evolution
has been evaluated. The physical and chemical relations between magmatism and
hydrothermal mineralization indicate that there is a synergy between these geologic
processes, which may be of general application.
The hydrothermal system, of duration < 10,000 years, developed in response to
injection of magma into the crust. Fluid outflow, at temperatures up to ~350°C, along
tectonically-controlled lineaments at the sea floor, produced a black and white smoker
field. Cu-rich sulfides were precipitated on the sea floor and in the shallow sub-surface,
producing a highly altered zone of stockwork mineralization. The waning stages of
hydrothermal activity produced ferroan-sphalerite, and a number of low-temperature
precipitates, including silica gel that coated earlier sulfides. These unusual magmatic
conditions were required in order to produce hydrothermal fluids with high Cu/Zn ratios;
an atypical characteristic of hydrothermal fluids in the MOR setting. The presence of Cu-
rich magmatic sulfides within the crust is the only reasonable source of additional Cu to
hydrothermal fluids, and such accumulations require extensive fractional crystallization.
The unusual bulk composition of the crust, being dominated by FeO-rich volcanics,
buffered fluids to very low oxygen fugacities. These fluids possessed a high oxygenbuffering capacity during mixing with seawater, and the sulfide trace element
geochemistry is consequently unusual. Highly evolved volcanics are not unique to the
GFHF and its local environs, because volcanics with closely similar geochemical
characteristics have been observed and collected elsewhere along the Galapagos Rift,
Juan de Fuca and EPR ridges. The inter-relations between local, specialized magmatism,
tectonism, and fluid flow, which resulted in exhalative mineralization, are likely to be
repeated elsewhere along the MOR system and in some back-arcs. Thus, the presence of
other sulfide fields similar to the GFHF is predicted.