Abstract

A tectonic model developed for the northeast Pacific spreading centers explains many of the diverse morphological and structural attributes of medium-spreading ridges and may provide a basis for explaining the distribution and composition of their associated base metal massive sulfide deposits. Stage 1 of the tectonic model is a period of excessive extrusive axial volcanism that builds the axial crestal ridge; an elongate summit depression (axial valley) is narrow or absent during this stage and inflated pillow lavas are dominant (e.g., southern Explorer Ridge). Sulfide deposition during stage 1 occurs immediately adjacent to the principal bounding faults of the elongate summit depression. The deposits are large in comparison with those of the East Pacific Rise and are composed of pyritic mounds containing copper and zinc. During stage 2, volcanism is rare to absent and the elongate summit depression develops and widens by collapse of the summit of the crestal ridge, as a result of tectonic stretching (e.g., southern Endeavour Ridge). Sulfide deposits forming during stage 2 are also the bounding faults and the deposits are similar in size and composition of those of stage 1. Renewed volcanism along axial fissures occurs during stage 3; sheet and lobate flow forms dominate within the elongate summit depression (e.g., southern Juan de Fuca Ridge). Sulfide deposition during stage 3 is in axial fissures in the floor of the summit depression. Deposits are very small relative to those in stages 1 and 2 and are very zinc rich, and copper and iron poor. Deposits in both stages 1 and 2 more closely resemble deposits preserved in ophiolitic assemblages than do those in stage 3.Axial Seamount, which straddles Juan de Fuca Ridge and is the western termination of the Cobb-Eickelberg seamount chain, has small zinc-rich deposits within its caldera. The caldera floor is composed of disrupted and fissured sheet lava flows and the deposits are in fissures and along fractures near the caldera wall. They are similar in composition and setting to those in stage 3 areas. Deposits in heavily sedimented failed rifts (e.g., Middle Valley of Endeavour Ridge) are the largest (tens of millions of metric tons) and are formed of pyrrhotitic, zinc-rich mounds. The heat source for the latter deposits is similar to that for their volcanic-hosted counterparts, but heat is more effectively conserved in the high-temperature reaction zone because the impermeable sediments inhibit cooling of the heat source by inflow of cold seawater.The largest volcanic-hosted deposits thus occur in those segments that formed closest in time to a period of major constructional volcanism. Upwelling hydrothermal fluid is focused along the principal marginal faults of the elongate summit depression and rises from the reservoir without significant cooling or mixing as a result of the relative impermeability of the cap rocks (i.e., inflated pillow lava flows, stages 1 and 2); the major volcanic edifices in the latter areas also provide some insulation from the overlying seawater, thus inhibiting cooling and promoting the growth of a relatively large high-temperature reservoir. In the stage 3 areas, and at Axial Seamount, the exceptionally high permeability in the sheet lava flows promotes the cooling and mixing of hydrothermal fluid immediately prior to venting. This, and the evident shallow position of the hydrothermal reaction zone (at the southern Juan de Fuca site), inhibits the formation of a large, stable reservoir. Also, some sulfide precipitation probably occurs immediately beneath the vent area, further contributing to the size of the deposits. Deposits are much larger in heavily sedimented rifts than in volcanic-associated areas of ridge crest that have relatively little sediment cover because of the broad insulating and protective nature of the sediments.

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