REE GEOCHEMICAL FEATURES OF THE QIFENGCHA-DETIANGOU GOLD FIELD,HUAIROU,BEIJING

The Qifengcha-Detiangou gold field in Huairou county,Beijing,is one of those that constitute the huge E-W-trending gold mineralization belt at the north margin of the North China Block and is located in the southwestern flank of the Miyun-Huairou uplift.The orefield is composed of the Qifengcha and the Detiangou gold deposits,and a group of discrete gold occurrences and Au-mineralized spots.The gold bodies and Au-mineralized bodies are mainly in veins or in lenses and are controlled mainly by faults,fractures and ductile-shear zones,associated with hydrothermal alterations such as silicification, chloritization, sericitization, pyritization and carbornatization.They mainly occur as native gold,and tellurides (calaverite and petzite),or as very fine grains and colloids within the crystal lattice of pyrite,pyrrhotite,chalcopyrite,galena and sphalerite.Quartz vein type and rock alteration type are two types of gold mineralization in this orefield,indicating a hydrothermal mineralization process.
In this paper,REE composition of sulfides,gold-bearing quartz veins,altered rocks,Mesozoic intrusive rocks and Archean metamorphic rocks in and around the orefield is studied.The main results are as follows:
(1) Fractionation of light and heavy REE of relatively earlier and larger acid or alkali acid magmatic bodies is usually higher than that of the later and smaller veins.
(2) The fact that the ΣCe/ΣY,La/Sm,(La/Sm)_N,Gd/Yb,(Gd/Yb)_N,La/Yb,(La/Yb)_N and Sm/Nd ratios of sulfides are higher than those of Au-bearing quartz veins,indicates a higher light REE and higher LREE/HREE fractionation in sulfides than in-quartz veins.
(3) The syenite porphyry veins and syenite in this orefield are products of magmatism of the same source,with the former evolved from the latter at a late stage.REE geochemistry of syenite porphyry veins,such as ΣREE,ΣCe/ΣY,La/Sm,(La/Sm)_N and Sm/Nd is more similar to that of the Au-bearing veins compared with the syenite masses.
(4) Acid intrusive rocks especially the beschtauite and granite porphyry veins are similar to syenite and syenite porphyry veins in REE geochemical features in many aspects such as the WEu,ΣCe/ΣY,La/Sm and (La/Sm)_N ratios.The beschtauite veins are more similar to the Au-bearing quartz veins compared with the other acid rocks.
(5) Relationship between the Archean metamorphic rocks (such as plagioclase amphibolite,biotite granulite,biotite hornblende plagioclase gneiss and migmatite) and the Au-bearing quartz veins in REE composition is not very clearly recognized.Rocks derived from hydrothermal alteration of metamorphic rocks are similar to the acid or alkali acid magmatic veins in their Chondrite-normalized REE patterns.
We come to the conclusion that the present gold deposits are of hydrothermal origin associated with the Yanshanian alkali and acid magmatism.This knowledge will be helpful for a better understanding of the spatial distribution of the gold deposits and the ore bodies in the Qifengcha-Detiangou gold field,and for the further prognosis of gold deposits in this area.It may also provide an example for the study of gold deposits of similar origin occurring in areas of old metamorphic rocks using REE geochemistry.