THE CONTROLS OF THE DEPOSITIONAL SYSTEMS TRACTS ON ORE DEPOSITS: EXAMPLES FROM SOME OF THE DEVONIAN SEDIMENTARY AND STRATABOUND DEPOSITS IN SOUTHERN CHINA

The study is designed to examine the relationship between the depositional systems tracts and ore deposits with the aid of the concepts and methods of sequence stratigraphy. The Devonian strata in southern China may be divided into four depositional sequences, of which Sequences 1, 2 and 4 are controlled by relative sea-level changes, while Sequence 3 by a combination of relative sea-level changes and tectonic activity. In fact, the effective basin spaces are often controlled by the sea-level changes and tectonic activity which, in turn, control the characteristics and geometry of the sediments. depositional styles, sedimentary environments and geochemical field of the sea area. Therefore, some of the depositional systems tracts may be in itself the ore-bearing beds. For example, the Ningxiang-type iron deposits consist of the sand bodies of the transgressive systems tract, the coastal onlap sand bodies formed synchronously in response to the sea-level rises, whereas the low-velocity sedimentary manganese deposits are often associated with condensed sections. Since the condensed sections are in the oxygen-deficient conditions.they may serve as the infilled spaces of the ferruginous sulfides, as exemplified by the Chengbu pyrite deposit. The ore bodies of the Baiyunpu lead-zinc deposit and Xikuangshan antimony deposit are all hosted in the palaeokarst cavities of carbonate rocks. which belong to the highstand systems tract where subacrial corrosion by fresh water took place during the sea-level fall. The palaeokarst surface was then onlapped by micritic limestones formed during the sea-level rise.
It follows that some of the depositional systems tracts not only consist of the ore-bearing beds, but also provide the infilled spaces, favourable sedimentary environments and ore-forming material for mineralization.