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Decoupling of Cu and As in Magmatic-hydrothermal systems Evidence from the Pueblo Viejo Au-Ag deposit, Dominican Republic

Deditius, A.P., Utsunomiya, S., Ewing, R.C. and Kesler, S.E. (2008) Decoupling of Cu and As in Magmatic-hydrothermal systems Evidence from the Pueblo Viejo Au-Ag deposit, Dominican Republic. In: Goldschmidt Conference 2008, 13 - 18 July 2008, Vancouver, Canada


Pyrite from the Pueblo Viejo high-sulfidation deposit provides evidence for decoupling of Cu and As in hydrothermal solutions. The pyrite that shows this decoupling is in the late-stage veins that also contain sphalerite and minor enargite. Pyrite in the veins shows growth zoning that varies in composition with depth into the deposit. Deepest veins (>150m below the present surface) contain fine-grained (<211m) pyrite with 0.4% As, 0.5% Pb, 0.2% Cu, 0.1% Ag, 0.09% Te and 0.06% Sb (wt% by EMPA). At depths of 120- 103m below the present surface, pyrite contains alternating growth zones with either Cu (<0.78%) or As (<0.69%), but never both. Farther upward in the deposit concentrations of Cu and As in the two types of pyrite increase and Pb (<1.8%), Sb (<0.33%), Ag (<0.1%) and Te (<0.08%) are also in As-rich zones. At a depth of - 20m, Cu and As reach concentrations of up to 3 wt% in separate, alternating growth zones. EMPA elemental maps of the shallowest pyrites reveal that increased concentrations of As and Cu coincide spatially with decreasing concentrations of Fe and show no relation to S, suggesting that both elements substitute for Fe. Chemical compositions of Cu-pyrite and As-pyrite are: (Fe0.95CUo 06) 101S2 and (Fe09~s0_05) 101 Sb respectively. HRTEM observations on pyrite with highest Cu and As concentrations reveal that the pyrite consists of single crystals that are continuous from Cu-rich to As-rich growth zones. There is no visible (by TEM) grain boundary between Cu-rich and As-rich zones. Cu-rich growth zones contain no Cubearing inclusions, whereas As-rich growth zones contain numerous ordered nano-domains rich in As.

The alternating sequence of Cu-rich and As-rich zones appears to reflect separation of Cu from As during evolution of the hydrothermal fluids. Similar decoupling of As and Cu is seen in analyses of fumaroles and fluid inclusions (both vapor and liquid), which are enriched in As and Cu, respectively. This suggests that the decoupling is related to magmatic processeses, probably involving vapor-liquid transitions.

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