Environment of formation and styles of volcanogenic massive sulfides: The Iberian Pyrite Belt

The massive sulfide deposits of the Iberian Pyrite Belt are grouped in different styles of mineralization, reflecting the formation in contrasting geological settings. The shale-hosted orebodies of the southern part of the belt are interpreted as formed in sub-oxic to anoxic third order basins. Here, upwelling deep sulfur-depleted fluids mixed with modified seawater rich in biogenically reduced sulfur, leading to the precipitation of the massive sulfides on the seafloor. These deposits share features such as the intermediate to high aspect ratio (typically 10 to 20), the usually large tonnages, the stratiform morphology, an absence of major metal refining, the abundance of sedimentary structures, and the absence of sulfates and the common presence of siderite-rich facies. Most of these deposits (Aznalcollar-Los Frailes, Sotiel-Migollas, Valverde, Tharsis and Neves Corvo) formed in a short time span in the uppermost Devonian indicating that there was a restricted epoch of vigorous hydrothertual activity that favored the development of long-lived and stable hydrothermal systems. Some of the Rio Tinto massive sulfides (San Dionisio, Filon Sur) are interpreted as of similar origin, but somewhat younger (Early Carboniferous). Most of the massive sulfides in the northern part of the Belt are hosted by a felsic volcanic sequence with only minor mudstone as is the case of Concepcion, San Platon, Aguas Te (n) over tilde idas Este, San Miguel, La Zarza, Aljustrel, or Filon Norte in Rio Tinto. The orebodies are hosted by massive or volcaniclastic, vitriclast- or pumice-rich, felsic rocks. They share some primary features including sharp transgressive contacts with the host rocks, the presence of a pervasive wrapping hydrothermal alteration, the variable aspect ratio, the major metal refining and the abundance of sulfates. These deposits are interpreted as formed by the stratabound replacement of porous or reactive volcanic rocks. Ore precipitation probably took place by mixing of the deep fluids with modified seawater with variably reduced sulfur. Deposits such as usually have lower tonnages than the exhalative ones. The stable (delta O-18(fluid), 0-8 parts per thousand; delta D-fluid, - 4 parts per thousand to 5 parts per thousand) and radiogenic isotope geochemistry (Sr-87/Sr-86, 0.7071 to 0.7221), as well the composition of the fluid inclusions (avg. 3 to 12 wt.% NaCl equiv.), are compatible with the dominant derivation of the deep fluids and metals from the underlying siliciclastic PQ Group. Diagenetic evolution of the sedimentary sequence triggered with convective flow and focusing of hydrothermal fluids along major faults in a regime of high geothermal gradients can account for the formation of this metallogenic province. (c) 2005 Elsevier B.V. All rights reserved.