HIGH-PRECISION GEOCHRONOLOGY CONSTRAINS ON MIOCENE MAGMATIC AND MINERALISING EVENTS IN THE PB-ZN-AG-CU DOMO DE YAULI DISTRICT, PERU

The Domo de Yauli is located 100 km east of Lima in the Western Cordillera of Peru. It is one of the major Zn-Pb±Cu±Ag mining districts of Peru and a classical ore district with high temperature, carbonate-hosted base metal deposits. Outcrops of magmatic rocks are more abundant in the northern part of the Domo de Yauli , where numerous small stocks of monzogranite intrude the large Anticona diorite. In the south there are multiple small apophyses of the very altered Chumpe intrusion. The northern monzogranite stocks are related to the formation of four different ore deposit types: Cu-porphyries, Zn-Pb skarns, Zn-Pb±Ag carbonate replacement deposits (mantos), and veins. In the south, only polymetallic mantos and veins are associated to the Chumpe intrusion.

This study is aimed at understanding the relationship between Miocene magmatic activity, tectonic setting and ore formation in the Domo de Yauli area. Predominant input of magmatic lead in each ore deposit is indicated by similar Pb isotopic composition between sulphides and whole rock of Miocene intrusions. U-Pb, Re-Os and ⁴⁰ Ar/³⁹Ar dating allow the definition of a sequence of magmatic, tectonic and mineralizing events and the duration of mineralisation in the Domo de Yauli area.

Preliminary U-Pb dating of zircon from the northern intrusions gives concordant ages of 14.11 ± 0.04 Ma for the Anticona diorite and close to 9 Ma for different monzogranite stocks related to Cu-porphyry style and skarn deposits. Late veins of the Cu-porphyry deposit have been dated at 7.9 ± 0.1 Ma by Re-Os on molybdenite and phlogopite from a Zn-Pb skarn give a ⁴⁰ Ar/³⁹Ar plateau age of 7.2 ± 0.2 Ma. U-Pb analyses of zircon from the southern Chumpe intrusion result in discordant points defining a lower intercept age about 6.6 Ma, in agreement with ⁴⁰ Ar/³⁹Ar ages of 4.90 ± 0.15 Ma and 4.78 ± 0.16 Ma obtained on sericite from wall rock alteration selvages. Age determinations reveal the existence of three distinct magmatic events at 14.1, 9.1 and 6.6 Ma, with the two latter ones related to a phase of mineralisation. We therefore conclude that the particularly large abundance of economic ore bodies at the Domo de Yauli is the result of successive hydrothermal systems. Ages and structural data indicate that they are associated to transtensional stress following the deformation pulses.