Amphibole Geochemistry of the Yanacocha Volcanics, Peru: Evidence for Diverse Sources of Magmatic Volatiles Related to Gold Ores

The Yanacocha mining district is located in the Andes of northern Peru in an area of relatively thick continental crust (∼35 km) and long-lived Cenozoic subduction-related volcanism. Volcanic activity in the district began at ∼20 Ma, and gold deposits (total resource of ∼1500 tonnes of gold) are spatially and temporally associated with eruption of the ∼80 km
³
Miocene Yanacocha Volcanics from 14·5 to 8·4 Ma. The Yanacocha Volcanics consist of five successive eruptive groups: the Atazaico Andesite lavas, the Colorado Pyroclastics (andesite–dacite), the Azufre Andesite (and dacite) lavas, the San Jose Ignimbrite and related domes (dacite), and small volumes of Coriwachay Dacite dikes, domes and rhyolite ignimbrite. Most dacite magmas probably did not erupt, but rather are inferred to have episodically crystallized to granite at depth to produce ore fluids. Two distinct populations of amphiboles, distinguished by their aluminum content, are found in the dacites. On the basis of phase equilibrium, the low-aluminum (low-Al) amphiboles were formed at 750–840°C and 110–240 MPa, whereas the high aluminum (high-Al) amphiboles are estimated to have formed at 900–950°C and P
H2O
> 250 MPa. The trace element contents of amphibole and whole-rocks are consistent with crystallization of the high-Al amphibole at near-liquidus temperatures from a basaltic-andesite to andesite magma, whereas the low-Al amphibole crystallized at lower temperatures in equilibrium with a rhyolitic melt derived from a crystal-rich dacite magma. Hydrogen isotopic compositions of both high- and low-Al amphibole exhibit a large range from –40 to –120‰ for the 12·5–11·0 Ma andesite–dacite and have a restricted range from –100 to –112‰ for the younger Coriwachay Dacite (10·8–8·4 Ma). The high δD values of some high-Al amphiboles (–41‰) probably represent subduction-derived water dissolved in a water- and fluorine-rich, chlorine-poor, and sulfate-saturated basaltic andesite magma. This magma was injected into an upper crustal, chlorine-rich, silicic magma chamber characterized by low δD of low-Al amphibole (–60‰ or lower). Short residence times (<1 year) of high-Al amphibole in the upper crustal chamber are estimated from dehydration rims and hydrogen diffusion lengths. Following the eruption of the lower San Jose Ignimbrite at 11·5 Ma, a new shallow dacite magma chamber was established and minor amounts of mafic magma input continued, as shown by the high-Al amphiboles present in the San Jose domes and in the middle and upper San Jose Ignimbrite. These high-Al amphiboles (δD
Amph
= –81 to –102‰) had sufficiently long residence time (>1 year) in the shallow chambers prior to eruption to equilibrate isotopically with the predominant low-δD dacite. The young Coriwachay Dacite magmas probably assimilated meteoric–hydrothermally altered low-δD rocks to generate the low δD of the low-Al amphibole (–100 to –120‰). These dacites are related to the main gold ore stages, and contain low-Al amphibole that is zinc- and chlorine-rich, but copper- and fluorine-poor, compared with the associated high-Al amphibole. These results imply that deep mafic magmas may have supplied much of the sulfur, fluorine, copper, and by inference gold, whereas upper crustal recycling may have supplied a significant proportion of the water and chlorine to the late dacite magmas and ore fluids.