Timing of gold and crustal evolution of the Palaeozoic south central Andes, NW Argentina—implications for the endowment of orogenic belts

Major and trace element concentrations and Nd isotope compositions of Ordovician volcano-intrusive rocks in the Puna region of northwestern Argentina suggest formation in a pericratonic setting above thickened and evolved continental crust. Input from juvenile sources was limited and there is no evidence for oceanic crust or the presence of a terrane-bounding suture in the study region. Geochronological constraints from 40Ar/39Ar (sericite-rich whole-rock) and Re–Os (arsenopyrite–pyrite) dating indicate initial deposition of gold in the turbidite-dominated Sierra de Rinconada during orogenic events in the Late Ordovician. This period corresponds to an epoch of major continental growth that includes development of abundant gold deposits along the long-lived and extensive accretionary margins of Gondwana. The ca. 440 Ma mineralisation ages represent the oldest recorded timing of gold emplacement in the central Andes. These ages also coincide with ca. 440 Ma ages obtained for the timing of mineralisation in each of the four largest orogenic gold deposits in the western Lachlan Orogen of eastern Australia. In contrast to analogous terranes elsewhere in the Central Andes (e.g., Peru and Bolivia), however, the Palaeozoic succession in northwest Argentina hosts relatively minor gold mineralisation. Possible explanations for the lack of a more substantial gold endowment include the (i) specific tectonic evolution of this region, (ii) absence of hydrated oceanic substrate in the northern Puna, (iii) lack of first-order, trans-crustal conduits, (iv) restriction of possible asthenospheric upwelling to a short period in the Early Ordovician, and (v) relatively limited extent of crustal shortening and thickening during Ordovician and subsequent orogenic events, when compared to analogous Palaeozoic metamorphic terrains elsewhere that are characterised by substantial orogenic gold endowment.