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Magmatic Precursors of Hydrothermal Fluids at the Río Blanco Cu-Mo Deposit, Chile: Links to Silicate Magmas and Metal Transport

Paul Davidson, Vadim Kamenetsky, David R. Cooke and Peter Frikken

Center for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania, Australia

Peter Hollings

Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1

Chris Ryan and Esme Van Achterbergh

CSIRO, P.O. Box 136, North Ryde, NSW, Australia

Terry Mernagh

Geoscience Australia, GPO Box 378, Canberra, ACT, Australia

Jorge Skarmeta

CODELCO-Chile, Gerencia Exploraciones, Santiago, Chile

Luis Serrano and Ricardo Vargas

CODELCO-Chile, Division Andina, Saladillo, Chile

Corresponding author: e-mail, paul_d{at}postoffice.utas.edu.au

In most porphyry systems the obscuring effects of hydrothermal processes and subsequent alteration, along with limited exposure of source rocks, preclude a detailed understanding of how and where metals and volatiles were derived. However, in this study we examine melt inclusions which have escaped alteration and that sampled all of the phases coexisting in the magma, in late- and postmineral rhyolitic units from the Río Blanco Cu-Mo deposit, Chile. These inclusions demonstrate the existence of a volatile-rich melt, the exsolution from it of an aqueous volatile-rich phase, initially as melt + vapor bubble emulsions, and the disruption of these emulsions into melt and primary magmatic fluids. Trapping of these emulsions may explain the occurrence of melt inclusions containing widely varying proportions of melt and aqueous fluid found at Río Blanco. We demonstrate the sequestering of metals into the exsolved volatile phases and the derivation from these of possible ore-forming hydrothermal fluids, with particular reference to the implications for metal transport. Melt inclusions show differences between adjacent comagmatic intrusions that may be directly related to the extent of mineralization of the respective bodies. In one of the Río Blanco postmineral rhyolite bodies melt inclusions show exsolution of the volatile-rich phase but only minor evidence of trapping of a metal-rich vapor. In contrast, inclusions from an adjacent late mineral rhyolite body show similar volatile phase exsolution but also provide evidence of ponding of metal-rich hydrothermal fluids during the final stages of cooling.

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