Evidence for a subseafloor replacement origin of the Cretaceous Palma volcanogenic massive sulfide deposit, central Peru

Abstract

The Palma volcanogenic massive sulfide deposit is one of the most recent significant base metal discoveries in central Peru. Base-metal mineralization can be followed over a strike length of over 2.5 km, with the currently best defined ore zone having an inferred resource of 5.59 million metric tons grading 7.43 wt % Zn, 1.50 wt % Pb, and 44.1 g/t Ag. The massive sulfides at Palma are hosted by a Cretaceous succession of hemipelagic mudstone that was intruded by basaltic dikes and sills. Contacts between the mafic intrusions and the enclosing carbonaceous mudstone are frequently marked by the occurrence of peperite suggesting that the emplacement of the high-level intrusions took place into the wet and unconsolidated sediments, broadly overlapping with the hydrothermal activity at Palma. Massive sulfide formation occurred ~50 m below the paleoseafloor through subseafloor infiltration and replacement of the mudstone host. Textural evidence suggests that the subseafloor replacement processes involved an early stage of carbonate alteration of the mudstone followed by sulfide replacement at increasingly higher temperatures. The carbonate alteration commonly occurs as bands of carbonate infiltration of the carbonaceous mudstone or as wispy, grey bands that contain mudstone clasts of various sizes that form a jigsaw in the carbonate matrix or have been rotated and abraded. The massive sulfides at Palma show many textural similarities to the carbonate-altered mudstone suggesting wholesale replacement of the carbonate by sulfides. Carbonate alteration associated with the concordant massive sulfides lenses overprinted the carbonaceous mudstone and mafic intrusions in the stratigraphic footwall, but decreases rapidly in intensity in the stratigraphic hangingwall of the ore bodies. The location of the paleoseafloor is marked by a ~20-m-thick package of mudstone that has experienced low-temperature hydrothermal clay alteration and silicification. Recent deep drilling into the stratigraphic footwall established the existence of a stringer zone containing abundant chalcopyrite, which are interpreted to represent a structurally controlled hydrothermal upflow zone. Following terrane accretion, the submarine mafic-siliciclastic rocks have been folded, with the Palma deposit today occurring on the eastern limb of an open, drag- folded anticline. The massive sulfides and their host rocks have been overprinted by contact metamorphism related to the emplacement of the Coastal batholith.