Ninth Annual V. M. Goldschmidt Conference 7120.pdf

GEOCHEMISTRY AND METALLIZATION OF MYLONITE-HOSTED GOLD DEPOSIT IN SOUTHERN CHINA. H. N. Wang, J. Chen, J. F. Ji, and C. Y. Sun, Nanjing University, China ([email protected]).

Introduction: Many large gold deposits have a close cal characteristics of REE and trace elements to the genetic relationship to ductile shear zones. These country rock, which may have provided ore-forming gold deposits can be divided into three type: materials for the first stage of mineralization. The 1)mynolite-hosted; 2)Crush belt-hosted; 3) Quartz chemical and isotopic characteristics of the fluid in- vien; In recent years, the mynolite-Hosted gold de- clusions from the auriferous quartz veins indicate posit has been found to be one of the largest gold de- that the ore-forming fluid responsible for the second posits in South China. stage of mineralization came from meteoric water or 1. Ore Deposit Geology: gold deposit is contained magmatic hydrothermal. The sulfur and lead iso- within a ductile shear zone. The ores occur as veins topes of ores reflect the fact that the ore-forming or as lenticular bodies within altered mylonite and materials originated mainly from a wall-rock source. ultramylonite host rocks. Native gold is dissemi- 4. Gold Enrichment by Ductile Shearing nated in silicified, pyritized, and sericitized phyllo- The geological characteristics, orebody occurrences, nites. The auriferous shear zone has undergone two and gold existence forms all indicate that ductile stages of evolution. The early stage was character- shearing leads to gold enrichment in mylonite belts. ized by ductile deformation duing which auriferous This conclusion is supported by the following obser- mylonite containing n x 106 Au was formed. The vations:1) The orebodies occur in the mylonite belts later stage was developed in brittle-ductile and brit- and are assocated with phyllonite and ultramylonite. tle environments in which magmatic hydrothermal The occurrence of orebodies correlates directly with mineralization was superimposed, resulting in local the occurrence of the mylonite belts. The scale of gold enrichments or auriferous quartz vein. orebodies is positively proportional to the scale of 2. The Features of Au in Dislocation Wall the mylonite belts and the strength of shearing Based on the study of TEM, dense dislocations can strain. The dip and angle of orebodies are basically be seen under bright field transmission. The Au- consistent with those of the shear zone; 2) Native concentrated bright spots are regularly arranged in gold is disseminated in alternated mylonite ores. In the dislocation wall. polished section, native gold grains are orented Gold deposits in ductile shear zone have an obvious along phyllonite cleavages. 3) A large quartz sub- cause of structural formation. During ductile shear- grain with some bright Au-concentrated area regu- ing, large quantities of quartz which exist in a shear larly arranged in the dislocation wall. 4) regional zone form the dynamic dislocation walls which are geochemical sections show that the metamorphosed the important structures for Au to migrate to the rocks and migmatite that underwent ductile shear- shear zone and for increasing the total gold content ing have a preliminary gold enrichment. of the shear zone. 3. Trace Element and Isotopic Geochemistry: The auriferous silicified mylonite is similar in geochemi-