Archean evaporites of the North China Craton:evidence on major and trace elements and S-isotope

There are some Archean evaporites in North China Craton. Up to now Chinese geolo gists have never paid attention to Archean evaporites. We have found some evapor ites in Inner Mongolia and some trace of Archean evaporites at eastern Hebei Pro vince. It is iron ore and related rocks in Qianlishan of Inner Mongolia. That wo uld be resulted from Archean evaporites. The Qianlishan Iron Mine lies in wester n Inner Mongolia, China, tectonically in the north of the west Eldos fault belt, North China Craton. There are four dome anticlines in the mine area, which are composed of Archean metamorphic rocks called the Qianlishan Group. Archean rocks are widespread, whereas the cover rocks are scattered only somewhere. From the bottom to the top Qianlishan Group may be divided into four formations, namely, Siquanshan, Chganguole, Qianligou and Habuqigai. The Qianlishan granite intruded into the cores of the anticline domes and directly contacted with the Siquansha n Formation, which mainly contains amphibole plagiogneiss. There are also inclus ions of the Siquanshan Formation in the granite. As an introduction we only give some sets of evidence to show the Qianlishan Iron Mine and related rocks belong ing to evaporite. (1) Evidence on petrology and mineralogy. The Chaganguole Formation is an ore-bearing rock series, in which there are nin e layers of iron ore; two of them are rich. The ore-bearing rock series consist s mainly of diopsidite, diopside-magnetitite, and calcipyre. The calcipyre cont ains various accessory minerals in different layers, such as olivine, serpentine , diopside, chondrodite and phlogopite. In the ore-bearing series scapolite dio psidite is especially well developed. These rock assemblages are very similar to those of the South Yakutia Mine, Aldan Shield, Russia, which were studied and d efined as evaporite by Russian geologists. Even the overlying rocks of the ore- bearing series are similar to Aldan’s, too. They are gneisses and schists and h ave some thin layers and lenses of marble also. In these gneisses some cordierit e-bearing meta-pelite is well developed.A kind of rare rock hyalophane-bearing gneiss was found in the ore-bearing ser ies. (2) Evidence on petrochemistry, mineral chemistry, major and trace elements. The protolith of the ore-bearing rocks are determined using major elements by m any methods. They were resulted from chemical sediments. The metapelites were de termined as semi-saline water sediments by ploting trace elements Ba and Sr. An d studies of petrochemistry and mineral chemistry indicate that the Qianlishan G roup has many characters very similar to those of the south Yakutia Mine, Aldan Shield, Russia. It is well known that SO 2- 4, CO 2- 3 and Cl - ani on cluster in scapolite should and could be from evaporite. The assemblages of d iopside + scapolite ± magnetite show an evaporite origin for their protolith. T he high content of Ba and the occurrence of hyalophane indicate that there was B aSO 4 in the protolith rocks. Comparing the scapolite diopsidite of Inner Mongo lia with those of Aldan shield and Madagascar - Australia shield, and Canadian s hield shows that the scapolite-bearing rocks of Qianlishan Mine area should be resulted from evaporites, too. Diopsidite chemical composition and scapolite com position are similar to those of the Aldan’s and south Australia’s too. Of cou rse there are some differences among these minerals and rocks of different areas .(3) Evidence on S-isotope. S-isotope fractionation in Archean is very small, δ 34 S close to 0. T he S-isotope of Qianlishan Group conforms to this feature. The δ 34 S values of Qianlishan show a narrow range, -1.3‰ to 5.7‰ the mean value is 1.4‰ for pyrite and an average value is 2.2 ‰ for scapolite in Qianlishan Grou p. These data are very similar to those of other Archean evaporite in the world ( e.g. North Pole and Barberton). This similarity has been taken as an eviden ce for the fact that Qianlishan Group should also be Archean evaporite. All the above evid

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