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The recent recognition that metallic mineral deposits are concentrated by hot aqueous (hydrothermal) solutions at sites along the earth-encircling oceanic ridge-rift system of spreading centers constitutes a scientific breakthrough. The sites where metal-rich hot springs discharge at sea floor spreading centers are natural laboratories to investigate ore-forming processes of economically useful deposits such as massive sulfides in volcanogenic rocks on land. The occurrence of hydrothermal mineral deposits at sea-floor spreading centers enhances the metallic mineral potential of oceanic crust that covers two-thirds of the earth both beneath ocean basins and exposed on land in ophiolite belts.
The occurrence of the series of hydrothermal mineral deposits, including polymetallic sulfides (Cu, Fe, Zn, associated Ag, Au) precipitated from high-temperature primary hydrothermal solutions and metallic oxides (Mn, Fe, associated metals) precipitated from low-temperature dilute solutions, is considered in terms of spatial and temporal frames of reference. Spatial frames of reference comprise structural features along the spreading axis (linear segments that are the loci of sea floor spreading alternating with transform faults) and perpendicular to the axis (axial zone of volcanic extrusion and marginal zones of active extension) common to all sea floor spreading centers. Regional tectonic setting is determined by stage (early, advanced) and rate (slow, fast) of opening of an ocean asin around a sea floor spreading center, and by local tectonic subsetting that incorporates anomalous structural and thermal conditions conducive to mineral concentration (thermal gradient, permeability, system geometry). Temporal frames of reference comprise the relation between mineral concentration and timing of regional plutonic, volcanic, and tectonic cycles, and of episode local physical and chemical events (transient stress, fluctuating heat transfer, intrusion-extrusion, sealing, hydraulic fracturing, pH and Eh changes, etc).
At this early stage in exploration of the sea floor, most of the hydrothermal mineral deposits so far discovered at sea floor spreading centers may be characterized as mineralized showings rather than as commercial prospects. The most remarkable observation about these mineral occurrences is that they are in situ, in contrast to ancient deposits on land that were displaced from their place of formation some time in the geologic past. A dialog is developing between marine geologists and land geologists, directed to evaluating the information being acquired from the sea floor hydrothermal mineral deposits in terms of analogous deposits presently on land. The dialog is gradually translating the understanding of ore-forming processes gained from the sea floor into exploration guidelines t identify ancient geologic settings on land where former conditions favored the concentration of large and accessible hydrothermal mineral deposits.
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