Petrology and geochemistry of Silurian-Triassic sedimentary rocks in the Tongling area:Constraints on the genesis of stratabound skarn deposits

Stratabound skarn deposits are widespread across the globe.These deposits are exploited for a wide variety of economic commodities,and the skarns,intrusives,altered rocks,and ores within skarn deposit have been the focus of a significant amount of researches.However,there is currently no model for the role of marine sedimentary rocks in contact metasomatism and mineralization associated with skarn formation.Paleozoic to Mesozoic marine clastic and carbonate sediments are widespread throughout the Tongling area of Anhui province,East China,and are associated with a number of skarn deposits,making this area an excellent natural laboratory for studying interactions between marine sediments and skarn mineralization.Here,we present the results of the petrology and geochemistry of Silurian to Triassic sedimentary rocks in this area,focusing at the distribution of different types of skarn deposit within different sedimentary units.Petrology shows that there are three types of abrupt lithologic change in sedimentary rocks of Silurian to Triassic age in Tongling area:dolomite-siltstone,dolomite-siliceous sediments,and dolomite-limestone.The first two suddenly changed associations are the main hosts for skarn orebodies in the study area.Geochemical analysis indicates that the Clarke concentration(i.e.,the concentration of an element in a rock compared with its average concentration in the earth's crust)of ore forming elements within sedimentary rocks in the Tongling area is much lower than the concentration coefficient(i.e.,the amount by which an element must be increased above its normal crustal abundance to make it commercially viable to mine),meaning that the skarn mineral deposits in this area are unlikely to have formed without involvement of external material.However,the relatively high contents of Fe within Carboniferous dolomites in parts of the study area mean that these dolomites may represent potential sources for metals within the skarn deposits.We show that the formation of the stratabound skarn deposit in this area was mainly controlled by the tectono-lithological association,source rocks,and the presence of organic matter and evaporites.Among these factors,the tectono-lithological association is the most important,with the source rocks(the protore)for pre-concentrating ore-forming elements,the gypsum-evaporite providing the sulfur source for part of the ore deposits and organic matter enabling the adsorption of ore-forming elements.The organic matter and evaporites contribute to the reduction of sulfate to sulfide in some,but not all,of the skarn deposits in this area.