Chronology of exotic mineralization at El Salvador, Chile, by Ar-40/Ar-39 dating of copper wad and supergene alunite

Exotic copper mineralization is a complex hydrochemical process linking sup ergene enrichment, lateral copper transport, and precipitation of copper ox ide minerals in the drainage network of a porphyry copper deposit. At the E l Salvador porphyry copper deposit in northern Chile the majority of the ex otic ore comprised a mixture of copper-bearing manganese oxyhydrates termed "copper wad." X-ray diffraction, scanning electron microscopy, and electro n probe microanalysis show that the copper wad is composed of copper-bearin g cryptomelane [K1-2(Mn3+Mn4+)(8)O-16 . xH(2)O] and birnessite [K0.33Mn73.9 +O14 . 7H(2)O] structures. These natural occurrences within the exotic ore provide the opportunity to directly date the formation of these deposits us ing recent advances in Ar-40/Ar-39 geochronology of supergene K-Mn oxides f ormed by weathering. A suite of copper-bearing cryptomelane and birnessite samples from exotic d eposits within the El Salvador district were characterized and dated by Ar- 40/Ar-39 laser step heating. Supergene alunite [KAl3(SO4)(2)(OH)(6)] found in paleospring feeder systems leading from tile source zones of copper outw ard to the exotic mineralization was dated to independently constrain the a ge of exotic ore formation. Although the Ar retentivity of the layered birnessite structure has been qu estioned by others, the AR retentivity of these samples is thought to be a function of their natural preservation and limited postcrystallization grou nd-water interaction in the hyperarid Atacama desert. The Ar-40/Ar-39 analy tical results show that Ar and/or K losses after crystallization, excess Ar -40, and Ar-39 recoil do not pose significant problems; therefore, in the c ontest of exotic copper deposits within hyperarid environments this dating method is applicable to both cryptomelane and birnessite within copper wad. The Ar-40/Ar-39 dating of exotic mineralization at El Salvador indicates th at supergene and exotic mineralization processes were active at similar to 35 Ma, about 5 m.y. after the emplacement of hydrothermal mineralization, a nd continued until the middle Miocene. The majority of exotic mineralizatio n extends from the Oligocene-Miocene (24 Ma) boundary through the middle Mi ocene (11 Ma) and relates to supergene fluid emanating in multiple directio ns from the source of copper in Indio Muerto, which represents a topographi c high above the exotic deposits. One reconnaissance date of exotic mineralization at the Chuquicamata porphy ry copper deposit yielded an age of 17.03 +/- 0.03 Ma coincident with the k nown supergene alunite dates for this deposit. Reconnaissance dating at Exo tica-Mina Sur and El Abra proved problematic due to the presence of contami nant silicate minerals within the copper wad. Ages derived here for exotic mineralization are similar to the known superg ene ages throughout northern Chile. Tile dates indicate that supergene exot ic mineralization began at the Eocene-Oligocene boundary and continued thro ugh the Oligocene, eventually ceasing in the middle Miocene due to desiccat ion of tile Atacama desert. A series of 10-cm-thick supergene alunite veins were microsampled across th eir widths and dated to address the kinetics of alunite vein growth. Appare nt growth rates of the veins vary from 71 to 100 mm/m.y. in the horizontal direction to 24 mm/m.y, in the vertical, presumably caused by differences i n pressure regimes due to their orientation