MINERAL REPLACEMENT DURING PROGRADE AND RETROGRADE EVOLUTION OF THE CERRO JUMIL GOLD SKARN DEPOSIT, MORELOS, MEXICO

Skarn deposits represent an important resource for precious metals such as gold and silver. However, exploration for and within deposits is complicated by the overprinting nature of skarn development. A better understanding of skarn formation, and crucially, the relative timing of mineralization, can be gained through detailed characterization of the mineral replacement history during prograde and retrograde development of skarn deposits.

The Cerro Jumil skarn deposit, ~12 km southwest of Cuernavaca, Mexico, developed within the contact aureole of a feldspar porphyry intrusion into limestone of the Upper Jurassic to Lower Cretaceous Morelos-Guerrero Platform. Metasomatism developed prograde and retrograde mineral assemblages in both endo- and exoskarn contemporaneous with or following recrystallization of limestone to marble. Prograde skarn is characterized by development of wollastonite-bearing assemblages, mostly along bedding-controlled replacement fronts. Garnet (+/- pyroxene) skarn, occurs as massive zones that replace wollastonite and destroy relict bedding. A first generation of pyrite is associated with the garnet skarn, but appears to be largely unrelated to mineralization. Retrograde tremolite extensively replaces wollastonite but not garnet. A second generation of pyrite appears to have formed during the retrograde evolution of the exoskarn and is spatially associated with epidote that occurs as disseminations and along fractures. The secondary pyrite, locally forming ≤25% by mode, has been entirely replaced by supergene limonite. Limonite is locally accompanied by traces of malachite implying that the pyrite-rich exoskarn originally contained traces of chalcopyrite. Elevated gold concentrations in these zones suggest mineralization was associated with this sulfidation event.

The prograde and retrograde skarn assemblages are overprinted by pervasive supergene alteration causing extensive replacement of sulfides by limonite. Supergene alteration seems to have been important by redistributing gold on a local scale, making Cerro Jumil a more attractive bulk-mineable deposit.