Abstract

The Oxec copper deposit of Upper Cretaceous age lies in the serpentinite body that makes up the Sierra de Santa Cruz, Guatemala. Recognition of the ore host rocks as chloritized, silicified, and sheared pillow basalts, identification of a nearly complete ophiolite assemblage, and an elucidation of the structure indicate that this massive Fe-Cu sulfide deposit is of the ophiolite copper type.In the mine vicinity, the ultramafic-mafic assemblage consists of serpentinized periodotite, a sheeted dike complex, a hydrothermally altered lower pillow lava sequence (ore horizon), and an upper pillow lava sequence. Chemical analyses of the pillow basalts and sheeted dikes reveal an ocean-floor tholeiitic origin. Comparison with ocean-floor tholeiites indicates that the Oxec assemblage is strongly enriched in Na 2 O and H 2 O, strongly depleted in CaO, and slightly depleted in FeO, an indication of ocean-floor metamorphism. The widespread assemblage epidote-albite-chlorite-actinolite-sphene in the altered pillows and dikes indicates that greenschist facies metamorphic conditions were attained. Pillow basalts and dikes intimately associated with the ore are strongly depleted in Na 2 O and CaO and strongly enriched in FeO and H 2 O, an indication that they were subject to local intense hydrothermal alteration.The mineralization in the zone of massive ore consists of pyrite, chalcopyrite, and pyrrhotite. Traces of early hematite and magnetite indicate initial oxidizing conditions at the onset of sulfide deposition. A small, highly silicified breccia zone with pyrite and chalcopyrite may represent the top of a stockwork feeder zone. Late dikes invaded the deposit and disaggregated it into numerous ore pockets. The characteristics of this deposit are consistent with a Cyprus-type genetic model.

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