Copper-rich tin deposits

Vein, stockwork, skarn, and carbonate-replacement Sn deposits commonly contain little or no Cu, but examples rich in Cu are also well known in many Sn belts worldwide. The origin of Sn-Cu deposits in association with fractionated and reduced, ilmenite-series granites of mostly metasedimentary crustal parentage is enigmatic because Cu, in contrast to Sn, normally accompanies oxidized, magnetite-series intrusions ultimately of mantle origin. Decompression mantle melting in back-arc or post-collisional settings generates mafic magmas, which are commonly invoked as the triggers for partial melting of thick overlying metasedimentary crust to form the fractionated, peraluminous, ilmenite-series magmas required to generate any associated Sn mineralization. Therefore, it is reasonable to suppose that oxidized fluids exsolved from these mafic magmas can contribute Cu to the synchronous and coexisting silicic chambers, thereby adding Cu to reduced Sn-bearing fluids and any resultant Sn deposits. Copper could also be released from Cu-bearing immiscible magmatic sulfides, formed during early crystallization of the reduced silicic magmas, as a result of either the introduction of these oxidized fluids and/or magma degassing. These putative mechanisms are restricted to thick metasedimentary crust, whereas in dominantly metaigneous crust the resultant magnetite-series magmas generate Cu deposits. This fundamental crustal influence on Sn versus Cu metallogeny of back-arc or post-collisional settings is responsible for both along- and across-strike transitions from the Central Andean Sn belt to regions displaying partly coeval Cu ± Au mineralization.