zones suggesting that the metal-rich crust and 

 overlying sediments emanate from the rift and 

 move toward the active trenches [Figure 18]. 

 Preliminary isotope studies suggest that ore 

 bodies in igneous rocks above subduction 

 zones result from partial melting of the 

 subducted crust — the melting of which in con- 

 junction with upward transport through vol- 

 canism can be considered the second stage 

 of a two-stage geochemical enrichment pro- 

 cess. Since such ore bodies are known only on 

 land, the study of metallogenesis on the mar- 

 gin edges will be a partial contribution to the 

 understanding of a much larger problem.* 



Nazca Plate 



The Nazca Plate (Figure 19] has been recog- 

 nized as suitable for a detailed investigation 

 of the complete cycle from crustal formation 

 along the East Pacific Rise to its consumption 

 in the Peru-Chile Trench. The presence of 

 major ore deposits in the Andes overlying the 

 zones of subduction supports the thesis that 



* Guidelines for metallogenesis proposals are avail- 

 able from the IDOE Office. 



they originated through this process. The 

 Nazca Plate is small enough to be studied as 

 a single geologic entity yet large enough to be 

 representative of the great lithospheric plates 

 which make up the surface of the earth. Since 

 the spreading rate along the East Pacific Rise 

 is among the most rapid yet measured, the 

 volcanic processes producing metalliferous 

 crust and sediments must be quite intense. 

 Furthermore, because the Plate itself receives 

 little sediment from the land, the dilution 

 process must be minimal. 



The Hawaii Institute of Geophysics, Ore- 

 gon State University and the Pacific Oceano- 

 graphic Laboratory of NOAA are conducting 

 a study of the plate margins using comple- 

 mentary geophysical, geochemical and geo- 

 logical techniques. Scientists from Colombia, 

 Ecuador, Peru, and Chile are all actively 

 participating in the cruises and data analysis. 

 Simultaneously, a large-scale geophysical 

 study of the subduction zone under the Andes, 

 as it extends from Colombia south through 

 Chile, is being carried out. Although this 

 study goes beyond the scope of IDOE, the 

 data on the subduction zone has obvious im- 

 plications for the Nazca Plate metallogenesis 

 study and vice versa. 



THE GENESIS OF COPPER DEPOSITS 



OCEANIC SEDIMENTS (LAYER) WITH METAL 

 RICH HORIZON AT THEIR BASE 



COPPER CONCENTRATIONS 

 IN OCEANIC CRUST 



OCEAN RISE 

 (SITE OF METAL-RICH 

 EXHALATIONS) 



PORPHYRY COPPER DEPOSITS 



VOLCANIC CHAIN 



50 KM 



Figure 18 



700 KM 



32 



