Mass balance analysis and geochronology of exotic ore forming processes at the El Salvador porphyry copper deposit district, Chile

At the El Salvador porphyry copper deposit lateral transport and re-distribution of copper into surrounding paleodrainage networks is studied by applying mass balance techniques to identify source, transport pathways and sinks of copper and by constraining the timing of copper transport by applying recent techniques in micro-beam laser Ar dating, thereby enabling us to track copper in a spatial and temporal framework.; Following the analytical strategy for assessing district-scale mass balance budgets developed by Brimhall et al. (1985), a comprehensive district-scale copper mass balance study at the El Salvador porphyry copper deposit is executed to study the lateral transport of copper forming ‘exotic’ deposits. The mass balance results delineated source zones, transport pathways and sinks of transported copper. In the source zones copper was not fixed as secondary chalcocite but instead escaped from the system due to a combination of an increase in permeability, the lack of reducing/neutralizing rock assemblages and the relative lack of Cu sulfides as replacement sites. The model results identify source zones of negative copper flux with unknown sinks and lead to the eventual discovery of exotic mineralization in Quebrada Turquesa.; A suite of cryptomelane [K_1-2(Mn3+Mn 4+)₈O₁₆·xH₂O] and birnessite [K_0.33Mn3.9+₇O₁₄ ·7H₂O] samples from exotic copper ore within the El Salvador district were characterized mineralogically and dated by 40Ar/39Ar laser step heating geochronology. Alunite [KAl₃(SO₄)₂(OH)₆] derived from supergene fluids, found in paleospring feeder systems leading from the source zones of copper out to the exotic mineralization, was dated to independently constrain the exotic ore formation age. The 40Ar/39 Ar analytical results show that Ar and/or K losses, excess 40Ar, and 39Ar recoil do not pose significant problems in dating copper wad in the cryptomelane and birnessite forms. 40Ar/ 39Ar dating of exotic mineralization at El Salvador indicates that supergene and exotic mineralization processes were active within 5 Ma years from the emplacement of hydrothermal mineralization, at ∼35 Ma and continued episodically until the mid-Miocene. The majority of exotic mineralization pulses were shown to extend from the Oligocene-Miocene (24 Ma) boundary through the middle Miocene (11 Ma) emanating in multiple directions from the source of copper in Indio Muerto, the mountainous region above the exotic deposits.; The distinct occurrence of a laminated jarosite [KFe₃(SO ₄)₂(OH)₆] sample within a paleospring feeder system between the supergene and exotic ore bodies system lead us to believe that a majority of the entire enrichment history is recorded in this one sample. The in-situ UV-laser ablation 40Ar/ 39Ar dating technique applied to this sample characterizes the age, duration and episodic nature of weathering there. The results of 39 40Ar/39Ar dates show that weathering processes at the El Salvador porphyry copper deposit were episodic from the early Oligocene (∼32 Ma) to the early Pliocene (∼4Ma) spanning a period of 28 m.y. and were comprised of at least nine discrete mineralization events.