Metallogeny and the New Global Tectonics

— The economic implications of the new global tectonics are reviewed according to plate anatomy. The uprises, stable platforms, downflows, and median zones exhibit particular features in ore genesis. Intercontinental metallogenic provinces, and intersection of transform fault trajectories with local structures as ore controls are discussed. Possible applications to Turkey are considered. HISTORICAL INTRODUCTION Thomas Crook in presenting a history of the theory of ore deposits (1933) argued for a dominant role of exogenic processes (from without) in contrast with endogenic processes (from within). The argument is reminiscent of the current controversy over the meteoritic impact versus volcanic origin of the craters of the moon. Crook found that so many theories of origin of ores had been put forward that it was almost impossible to advance any new general theory. American, and to a large extent worldwide, thinking on the subject of ore genesis is still strongly influenced by Waldemar Lindgren's classical genetic approach—the classification of ores according to the geological processes by which they were formed. In this classification the quantitative importance of magmatic and hydrothermal ore deposits is recognized. The pendulum is now swinging farther to the endogenic side and global or plate tectonics provides a mechanism which was hitherto lacking. This does not mean that the time-proven concepts of magmatic segregation and injection, pneumatolysis, hydrothermal and meteoric solutions in veins and replacements, weathering, sedimentation and metamorphism, etc. in ore concentration are being thrown overboard; rather, a new unifying casual principle is emerging in the philosophy of ore search. The new rationale appears at a time when the well-recognized spatial association of metallic ores with intrusives, as in the Western United States, is being explained as structural rather than genetic (Noble, 1970) and with the metals probably coming from the mantle, in the upper part of which there was a primitive heterogeneous distribution of metals. That the subject of plate tectonics and ore deposits is a lively one is evidenced by the topic discussed at the March, 1971 meeting of the American Institute of Mining, Metallurgical, and Petroleum Engineers: «Regional Tectonics— How Has it Helped in the Search for Ore?» METALLOGENY AND THE NEW GLOBAL TECTONICS 131 We shall continue to employ conventional ore guides and targets—physio graphic, mineralogic, stratigraphic-lithologic, and structural, etc.—for indispensable local clues, but large-scale mineral resource investigators such as United Nations, governmental Geological Surveys, and other exploration organization may be ex pected to apply to some degree the synoptic approach which plate tectonics pro vides. Whether the new thinking will prove more satisfying to the mind than pro fitable to the purse remains to be determined. Potentially the concept may prove as useful to the economic geologist as it has to the seismologist in explaining the geometry of the phenomena with which each is concerned. THE NEW GLOBAL TECTONICS The crust of the earth (lithosphere) is believed to be made up of a dozen or more plates that are growing at ocean ridges by addition of new material from the mantle, moving independently, colliding, and descending into the mantle where they are remelted. The plates are as much as 150 km thick. They comprise both continents and oceanic crust and they serve as conveyor belts for both. (Dewey & Bird, 1970; Walker, 1970; and Guild, 1971). They are the surface ex pression of convection cells within the earth. How mineral deposits fit into this picture is tentatively summarized by Guild in Table 1. The Alpine orogenic period which began about 200 million years ago serves as a model. It includes events from the Triassic onward and is now in terminal cordilleran form. The Alpine drifts eradicated much of the preceding platforms and uprises but events similar to those of the Alpine period may have character ized all ten orogenic periods since 3500 million years ago (Walker, 1971). The different environments of Alpine mantle cells to which mineralization can be related are as follows (Walker, 1971) (Fig. 1) :