The Mineralization Of Changkeng-Fuwan Gold-Silver Deposits, Guangdong Province, South China

Changkeng-Fuwan large-scale gold and super-large silver deposits have been explored in Sanshui Mesozoic-Cenozoic sedimentary basin (about 50km southwest Guangzhou, Guangdong Province) in 1990s. Gold and silver orebodies independently occurred in the same field and yielded in one brecciated zone with alteration. The inter-formational tectonic belt directly controlled the orebodies, which was located between the strata of Zimenqiao_Fm. (C1z) of Lower Carboniferous and Xiaoping Fm. (T3x) of Upper Triassic. The tectonic belt was also the unconformable contact boundary of Lower Carboniferous and Upper Triassic. The orebodies of gold and silver approximate are bedded or big lens striking trends to NEE and dipping towards SSE with 30 to 50 degrees but 15 to 30 degrees in lower part. The boundary between orebodies and host rocks is quite clear which is characterized with several ten meters of mylonites. The silicification, clavization and sulfidization were the main alteration for the host rocks. The silicification formed several stages of quartz and chalcedony, calvization formed illite, dickite and minor kaolinite. Sulfidization formed pyrite, stibnite, realgar, orpiment, sphalerite and galena. The gold is mainly micro or sub-micro desseminated gold and distribute in the fissures of pyrite and quartz or their edges or surrounding illite. The silver occurred with sulfosalt and single sulfide such as argentite and acanthite and minor native silver. Most of the silver-bearing minerals include pyrargyrite, andorite, miargyrite, freibergite, argentite, and minor owyheeite, proustite, and polybasite.The gold orebody is apparently characterized with all the features of micro-desseminated Carlin-type gold deposit in Nevada of the U.S, but it occurs in the same ore field and one inter-bed tectonic brecciated zone with silver orebody related to the quartz veins rich in sphalerite and galena obviously, which makes it a little different from the real Carlin-type deposit. Recent 10 years, quite many Chinese economic geologists and geochemists researched on this special deposit in different aspects such as stratigraphy, petrology, mineralogy, and geochemistry. About the genesis of the ore deposits and the metallogenic model, several different ideals have been initiated to be tectonic alteration type (Du JE, Ma CH, Zhang GH, 1993), hot-water sedimentary deposit type (Xia P, Zhang H, Wang XZ, Cheng JP, 1996), hot brine lixiviation (Rao JG, Liu HQ, Yang SY, et al., 1997) and hydrothermal replacement (Zhang S, Li TJ, Wang LK, 1997a, 1997b). But no any mentioned about the separating reasons of the gold and silver if they were one deposit and formed by the same metallization, and some different isotopic ages have been dated and obtained as well.Focusing on the trace element geochemistry for the gold and silver orebodies and its hostrocks, also the background values of Au and Ag in regional stratigraphies, the paper concludes that the sources of Au are quite different from the Ag. Probably Au came from the stratum of Zimenqiao Fm., Some materials of silver ore was supplied by the regional Mesoproterozoic or Paleoproterozoic stratigraphies such as Yunkai Complex. According to the REE contents of orebodies and host rocks and its chondrite normalized distribution charts, the paper predicts that the mineralization or silicification was resulted from the hydrothermal replacement not related to the hydrothermal sedimentary deposition or the sedimentary exhalation.According to the analyzing results of sulfur isotope compositions of gold and silver deposits and a quantity of data of other geochemists, this paper focuses on the characters of 6 34S. The diagram of 8 34S shows that the 6 34S of the minerals (pyrite , stibnite) of gold deposit distributes as a wavy shape, and that of the minerals (sphalerite, galena) in silver deposit distributes as tower form. They indicate that the sources of sulfur are complicated for gold but simple for silver, respectively. By using the method which was promoted by D.M.Pinckney and T.A.Rafter (1972), the...