Geological Features And Origin Of The Piedmont Potassic Zone In Hadamengou Gold Ore District, Inner Mongolia, China

Hadamengou gold ore deposit is located in middle position the northern margin ofthe North China Craton. The tectonic position is belonging to southern edge ofYinshan uplift belt and southern part of Wulashan anticlinorium. The main host rockis the new Archean Wulashan Group metamorphic rock. Magmatism activitydeveloped in the region, which includes Dahuabei granite and Houyanghai graniteformed in Hercynian, Shadegai granite and Xishadegai granite formed in theIndosinian. The geochemical characteristics of these granite showed high silicon,alkali-rich and weakly peraluminous, similar to the A-type granites. These intrusionsmay be some contact on the material sources and evolution course. The Hadamengougold deposit was formed by a multi-stage mineralization, where the mainmineralization age is early Hercynian with hydrothermal activity superimposed later.The piedmont potassic zone is located on the south side of gold deposit. About8kmlong, a few meters to tens of meters wide. It is EW extension, and parallel with veins.It is secondary faults of Piedmont deep faults and a broken belt which constructed bythe K-feldspar altered rock. Its lithological composition is simplicity, with potassicrocks of differences in the degree of alteration. The feldspar in piedmont potassicfeldspar zone and neighboring13thgold veins have the same characteristics, they aremainly feldspar. Elemental analysis characteristics of samples from piedmont potassiczone are also similar to the gold veins, HREE and LREE fractionation, Eu positiveanomalies. The characteristics of zircons in piedmont potassic zone show they areinherited zircon. The zircons U-Pb isotopic age of the test found that they wereformed in between1946~1825Ma ago, and the age data recorded the information ofzircons from Wulashan Group was transformation and change by late hydrothermalalteration and become younger. This evidence description piedmont potassic zone isnot an independent alkaline rock, but by the old Archean rock formations evolvedthrough hydrothermal alteration. And the age data of323~119Ma from other scholarsdating on alteration minerals or whole-rock show that since the outbreak of Hercynianmagmatic activity in the region, piedmont potassic zone being superimposed bymulti-stage hydrothermal alteration. The potassic altered rocks whole rock Rb-Sr, Sm-Nd isotopic data from Piedmont potassic zone, show that εNd (t) values rangedfrom-8.2~-8.6, which imply its diagenetic material from the earth’s crust. And thewhole rock test of samples with data of high（87Sr/86Sr）ⅰand εSr(t) value, this featurealso reflects the diagenetic material source of piedmont potassic zone rocks is uppercrust. Piedmont potassic zone is formed by hydrothermal metasomatism and alterationwith host rock during the slightly alkaline fluid rising along the deep faults, andthrough tectonic fracture zone and into host structures. The frequent late magmatichydrothermal brings multi-stage transformation and superposition of its overlay.Piedmont potassic zone played a role of major ore conducting structure during theprocess of gold mineralization in the Hadamengou gold deposits.