Deep tectonic processes and super-accumulation of metals

Abstract

Super-accumulation of metals means exceptional large ore deposits with very large ore reserves, which have been called so far by different names as superlarge ore deposits, supergiant ore deposits, gigantic ore deposits, world-class ore deposits, and so on. As they are identical, we propose a unified term of exceptional large ore deposits. From the viewpoint of ore-forming process, the exceptionally large ore deposits are characterized by an explosive anomaly of mineralization. The explosive anomaly usually includes plenty of metal supply, exceptional metallotect convergence and stable physico-chemical conditions needed for super accumulation of metals. This is especially dependent on ore-forming elements, deposit types, metallogenic geochronology, and geological settings. The minimum reserve criteria of exceptional large ore deposits have been proposed for 17 ore-forming commodities, and the exceptional large ore deposits have been selected from the class of world deposits. Their macroscopic distribution generally shows concentration in two gigantic active belts, four stable massifs and on a transitional zone between those two tectonic units. The specific localities could be defined by exceptional metallotect convergence. New attempts have been made to assess the intensity of an explosive anomaly and prove the super-accumulation of metals. These attempts included studies as: (1) ore-forming geochronology by a multidiscipline methods, (2) analysis of thermal events of certain deposit types, (3) analysis of relative abundance of ore reserve and ore-forming period of the deposit. Regarding the genesis of an explosive anomaly, we hereby propose that global thermal event in certain eons and eras in geologic history created them. The global thermal events are tentatively distinguished as an 'excess-oxygen atmospheric event' in the Archean, a 'lack-on-oxygen atmospheric event' in the Proterozoic-Paleozoic, and a major 'tectono-magmatic thermal event' in the Mesozoic-Cenozoic. One example of banded iron formation (BIF), related to an 'excess-oxygen atmospheric event' has been explained. Based on the tectonic settings, geophysical data and metallogenic features mentioned above, we summarize below the deep tectonic processes and super-accumulation of metals as occurred in the Nanling metallogenic province: (1) Continental orogeny is generally divided into 3 types: the marginal continental, intercontinental and intracontinental. The orogenic belt of the Nanling metallogenic province consisted of the Southeastern Coast folded system belonging to the continental marginal orogenic belt in the Mesozoic, and the collision suture belt between the Yangtze and Cathaysian plates and their forelands as the intercontinental orogenic belt in Late Proterozoic to Devonian. After that, a large-scale intracontinental orogenic belt was formed by subduction of the oceanic crust and delamination caused by crust thickening in the Mesozoic, almost completely superimposed on the above two orogenic belts. (2) Eight metallogenic zones and three minerogenic series have been distinguished in the Nanling metallogenic province, based on the tectonic settings and the concept of minerogenic series. One of the mineralization zones is related to continental margin orogeny; another one to intracontinental orogeny; and one is situated in the transitional belt between the two orogenic belts.