A genetic linkage between subduction- and collision-related porphyry Cu deposits in continental collision zones

Abstract

The genesis of continental collision-related porphyry Cu deposits (PCDs) remains controversial. The most common hypothesis links their genesis with magmas derived from subduction-modified arc lithosphere. However, it is unclear whether a genetic linkage exists between collision- and subduction-related PCDs. Here, we studied Jurassic subduction-related Cu-Au and Miocene collision-related Cu-Mo porphyry deposits in south Tibet. The Jurassic PCDs occur only in the western segment of the Jurassic arc, which has depleted mantle-like isotopic compositions [e.g., ( 87 Sr/ 86 Sr) i = 0.7041–0.7048; e Nd(t) as high as 7.5, and e Hf(t) as high as 18]. By contrast, no Jurassic PCDs have been found in the eastern arc segment, which is isotopically less juvenile [e.g., ( 87 Sr/ 86 Sr) i = 0.7041–0.7063, e Nd(t) < 4.5, and e Hf(t) ≤ 12]. These results imply that incorporation of crustal components during underplating of Jurassic magma induced copper accumulation as sulfides at the base of the eastern Jurassic arc, inhibiting PCD formation at this time. Miocene PCDs are spatially confined to the Jurassic arc, and the giant Miocene PCDs cluster in its eastern segment where no Jurassic PCDs occur. This suggests that the arc segment barren for subduction-related PCDs could be fertile for collision-related PCDs. Miocene ore-forming porphyries have young Hf model ages and Sr-Nd-Hf isotopic compositions overlapping with those of the Jurassic rocks in the eastern segment, whereas contemporaneous barren porphyries outside the Jurassic arc have abundant zircon inheritance and crustlike Sr-Nd-Hf isotopic compositions. These data suggest that remelting of the lower crustal sulfide-bearing Cu-rich Jurassic cumulates, triggered by Cenozoic crustal thickening and/or subsequent slab break-off, led to the formation of giant Miocene PCDs. The spatial overlap and complementary metal endowment between subduction- and collision-related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts.