The Navidad polymetallic (Ag+Pb±Cu±Zn) epithermal deposit, Cañadón Asfalto basin, Patagonia Argentina: Sources of metals and volcanic rocks

FRANCHINI MARTA; BOUHIER VERONICA; LAURA MAYDAGÁN; DAVID R. LENTZ

Cape Town

Congreso; 35th International Geological Congress; 2016

International Union of Geological Sciences

Navidad (42°4?S-68° 8?W) is a Ag+Pb± (Cu,Zn) world-class deposit located in the North Patagonian Massif, Patagonia Argentina. The polymetallic mineralization has epithermal characteristics and is hosted by intermediate to basic volcanic and sedimentary rocks of Jurassic age deposited in the Cañadón Asfalto basin. LA-ICPMS studies yielded U-Pb ages of zircon grains of 173.9±1.9 Ma and 170.8±3.0 Ma (Middle Jurassic) for the middle and upper volcanic units that host the mineralization at Navidad. Other 206Pb/238U ages of 281.3±8.7 Ma, and 218.1, 246.5, and 201.3 Ma are recognized in zircon crystals from the lower and middle volcanic units are consistent with xenocrysts derived from the Permian basement and Middle-Upper Triassic rocks, respectively. The presence of these xenocrysts in the volcanic units is interpreted as evidence of assimilation of crustal rocks in the basaltic-andesite magmas.Basalts from the lower volcanic unit have the highest isotopic 87Sr/86Sr (0.7112 -0.7117) and lowest ξNd (-6,9 to -6) ratios. Andesites from the upper and middle volcanic units have isotopic 87Sr/86Sr (0.706-0.708) compositions uniform and less radiogenic and more dispersed values of ξNd (-3.2 to -6.1). Thus, the lower volcanic unit is more radiogenic than the younger units (middle and upper volcanic units), suggesting a higher contribution of middle-upper crustal components in its petrogenesis. Pb isotopic composition of the volcanic rocks shows ratios of 206Pb/204Pb of 18.28 to 18.37; 207Pb/204Pb of 15.61 to 15.62, and 208Pb/204Pb of 38.26 to 38.43. Lead isotopes of sulfides (galena and pyrite) are slightly lower: 206Pb/204Pb from 18.22 to 18.26, 207Pb/204Pb from 15.58 to 15.61, and 208Pb/204Pb from 38.17 to 38.26. Navidad sulfides have Pb ratios similar to sulfides from other epithermal deposits located in the North Patagonian Massif and hosted by Early Jurassic volcanic rocks (188-178 Ma, V1, Pankhurst et. al. 2000) and close to the mantle curve of Zartman and Doe (1981)[2], suggesting a mantle-dominant Pb reservoir. A narrow range in all these sulfides indicate that the hydrothermal fluids were slightly modified by the host rocks. In Patagonia, the Pb isotopic composition of sulfides increases progressively from NE to SW, reflecting variations in Pb source while volcanism was migrating during the Jurassic. Increased crustal compression and subduction component during the Jurassic may explain the increased contribution of crust observed in the Pb ratios of ores and host rocks (from V1 to V3 episodic volcanic events, Pankhurst et al. 2000 [1]). V1 is considered related to a predominantly extensional period, whereas V2 (172-162 Ma), and V3 (157-153 Ma) reflect the increasing involvement of subduction activity along the Pacific margin of Gondwana.