Multiple centers of mineralization in the Indio Muerto district, El Salvador, Chile

The porphyries that formed copper deposits in the Indio Muerto district inc lude both 44 Ma rhyolitic sub-volcanic intrusions and 42 to 41 Ma granodior itic stocks and dikes. All are aligned for 4 km along a north-northeast tre nds, which is the apparent hinge line of a 58 Ma trap-door caldera. Three s eparate centers of mineralization were associated with the earlier rhyolite s, Cerro Pelado, Old Camp, and probably M Gulch-Copper Hill. Four separate centers were associated with the later granodiorites, M Gulch-Copper Hill, O Nose, Turquoise Gulch, and Sector Granito. Patterns of lithology, structu re, mineralization, and alteration in the various centers, which have been documented by systematic mapping partially supported by studies of mineralo gy and geochemistry, are presented and compared to the previously described Turquoise Gulch center. Cerro Pelado is a 750-m diameter rhyolitic volcanic neck, intruding host an desite at the northeast end of the district. Very weak Cu but much stronger Mo mineralization, with analogies to Climax-type deposits, is associated w ith this intrusion. Surficial sericitic alteration with weak pyrite-(chalco pyrite-bornite)-specularite-gold mineralization shows decreasing sericitc, pyrite, Cu, and Au contents downward. This strong vertical zonation grades downward to alkali feldspar-biotite alteration with increasing molybdenite and magnetite content below an abrupt top of quartz veining. The Old Camp i s a 250-m diameter, roughly cylindrical body of disseminated chalcopyrite-b ornite and quartz veining with K feldspar-biotite alteration, centered with in a rhyolitic quartz porphyry dike. This dike was emplaced and mineralized within 1 m.y. of Cerro Pelado. Mineralization zones outward through chalco pyrite-pyrite to pyrite with sericite-chlorite and then chlorite-epidote-al bite alteration. A supergene sulfide enrichment blanket has been largely ox idized. M Gulch-Copper Hill was first mineralized in and around quartz porp hyry dikes with quartz veining, disseminated chalcopyrite-bornite, and K fe ldspar-biotite alteration. Roughly 1 m.y. later, intrusion of a series of feldspar porphyries and igne ous breccia displaced the center of tile mineralization at hi Gulch-Copper Hill. Late-stage hydrothermal activity reworked earlier mineralization and superimposed pyrite-chalcopyrite assemblages with sericite. Aside from some rich D veins containing pyrite-bornite-digenite, economic grad in the M Gu lch-Copper Hill pit is mostly due to supergene enrichment. Ore at O Nose wa s largely supergene chalcocite-covellite on chalocypyrite-pyrite, centered on early QG feldspar porphyry and brecciated and biotized andesite. Subsequ ent intrusion on intramineral L feldspar porphyry and late hydrothermal act ivity apparently reworked earlier mineralization. At the contact, this prod uced a fringing chalcopyrite-pyrite zone in andesite, which is sharply zone d upward to pyrite-bornite-chalcopyrite with higher grade. This is apparent ly the same L porphyry that intruded the center of the main Turquoise Gulch orebody and controlled transitional and late patients of alteration and mi neralization described previously. The supergene enrichment blanket which m akes ore in Sector Granite, to the southwest of Turquoise Gulch, is connect ed to the main orebody but primary mineralization is spatially. separate an d related to a different set of intrusions. Although less well known than o ther centers, a chalcopyrite-bornite core is apparently related to unusuall y intense, texture-destructive K feldspar alteration in early equigranular X porphyry and later G feldspar porphyry. This core mineralization grades o utward to pyrite-sericite and abruptly upward to pyrite-bornite-chalcopyrit e with sericite-andalusite-prophyllite-diaspore. Extending the trend to the southwest is the barren Granite Gulch feldspar porphyry, emplaced between the two mineralizing periods. Ar-40/Ar-39 dating demonstrates that two episodes of mineralization associa ted with the early rhyolitic and later granodioritic intrusions each lasted approximately 1 to 1.4 m.y., from 44.5 to 43.5 and 42.3 to 40.9 Ma, respec tively. Tile barren Granite Gulch porphyry and perhaps the early and equigr anular X and O porphyries were intruded between 43 and 42 Ma. Magmatic acti vity, therefore, occurred episodically over at least 3 m.y. The individual centers are best explained as expressions of different cupolas on a magma c hamber, which evolved from granitic to granodiorite to quartz diorite with progressive injection of new magma. Despite our best efforts to date 16 car efully selected samples, it was not possible to define the sequence and dur ation of tile partially overlapping centers within each of the larger miner alization periods. The different centers exhibit a wide diversity of charac teristics, which mirror variations on the general theme of porphyry Cu-Mo d evelopment. Changing hydrothermal input from an evolving and growing magma chamber and major remobilization of early mineralization by both intrusion and hydrothermal activity are major factors in this diversity.