Plate Tectonics and Metallogenesis Studies 



The concept of plate tectonics, which has provided a 

 unifying theory to the study of the earth, is also providing a 

 new framework within which the origin and genesis of the 

 substantial economic mineral deposits associated with subduc- 

 tion zones can be studied. This framework is embodied in the 

 "geostill" concept which in essence states that these economic 

 minerals (1) have their initial source at active spreading cen- 

 ters; (2) are to some extent mobilized, altered, and concen- 

 trated during their emplacement at and transport away from 

 spreading centers; (3) are further mobilized to a significant 

 extent in subduction zones; and (4) are transported and con- 

 centrated to form economic mineral deposits such as porphyry 

 coppers, massive sulfides, Ag, Pb, and Zn ores in the near 

 surface rocks above the subducting oceanic lithospheric plate 

 (fig. 31). 



A comprehensive research program designed to test and 

 refine this geostill concept would clearly need to address all of 

 the numerous questions that can be derived from the compo- 

 nent parts of the concept as listed above. Many of these ques- 

 tions can be most reasonably answered by studies of the ore 

 deposits themselves and of their geological, geochemical, and 

 geophysical setting on the continents. Many, however, can only 

 or best be addressed by marine studies of the lithospheric 

 plates themselves. 



As part of its program in Seabed Assessment, IDOE 

 supports basic research studies that contribute to our under- 

 standing of the genesis of metallic ore formations. The IDOE 

 interest is based on hypotheses on the origin of ores that 

 developed from the concept of plate tectonics. Since many 

 metallic ore deposits were formed under marine conditions, 

 better understanding of these conditions will provide more 

 powerful exploration tools in the search for new deposits of 

 strategic metals. Conversely, it is recognized that the occur- 

 rence of the metals in themselves provides evidence of the 

 conditions under which the earth's crust is formed. 



Since the scope of the subject is so broad, IDOE must 

 limit its support to the study of those processes that operate on 

 the sea floor. Those processes are summarized as follows: 



1. Active spreading centers are sites of new crustal forma- 

 tion. Knowledge of the processes active along the ridge crests 

 should help determine metal sources and their initial mobiliz- 

 ing forces. Geophysical methods, heat flow, and seismicity will 

 help localize the more active zones. Chemical measurements 

 will help understand: (1) the processes by which metals are 

 transferred by circulating solutions; (2) chemical interactions 

 among fresh oceanic crust, sea water, and sediments; (3) rates 

 of chemical change. Determination of the sources should in- 

 clude investigation of the upper mantle, crust, surface sedi- 

 ments, and the overlying seawater. 



2. The metals may be mobilized during transport from 

 the spreading center towards the subduction zone. Chemical 

 processes in the stable plate during transport include exchange 

 of elements among crust, sediments, and seawater as a function 

 of time. Changes will be progressive with increasing distance 

 from the spreading center. Processes of crustal formation may 

 also be active within the stable plate. Occurrence of metallifer- 

 ous sediments in ponds or basins within the stable plate may 

 be evidence of fissures along which new crustal material is 

 beine added. 



3. Study of trench sediments and rocks will indicate 

 what metals are being carried down into the subduction zones. 

 In order to follow the pathways of the metals from source to 

 deposition, comparison of materials at ridges and landward ore 

 deposits will help elucidate the processes in the geostill, which 

 is predicted to be operating in the subduction zone at a depth 

 of approximately 100 kilometers. At the present state of tech- 

 nology, the concept of the geostill can be studied only from the 

 input at the subduction zone and the output on the landward side. 

 The program, however, remains open to proposals that could 

 provide insights into the workings of the geostill at depth. 



The Nazca Plate was selected for a detailed investigation 

 of the complete tectonic cycle from crustal formation along the 

 East Pacific Rise to its consumption along the Peru-Chile 

 Trench. Occurrences of metalliferous sediments along the East 

 Pacific Rise parallel a major trend of ore deposits in the Andes 

 from southern Chile to northern Peru, suggesting that these 



T 1 1 r 



T 1 r 



D 



SITE SURVEYS OF MAJOR BASINS 



115°W 

 _1 I l____l I l_ 



LA PAZ 



no» 



Figure 30. — Gulf of California Study tracklines 

 of Scripps Institution of Oceonagraphy. 



OCEANIC SEDIMENTS (LAYER) WITH METAL 

 RICH HORIZON AT THEIR BASE 



COPPER CONCENTRATIONS 

 IN OCEANIC CRUST 



OCEAN RISE 

 (SITE OF METAL-RICH 

 EXHALATIONS) 



H^.'.J'^.V.'Tjg 



PORPHYRY COPPER DEPOSITS 



VOLCANIC CHAIN 



Figure 31. Geostill concept of genesis of 



copper deposits. 



V 



700 KM 



31 



