Characteristics,origin and role of the carbonate minerals in gold mineralization of Dayingezhuang gold deposit,Jiaodong area
The genetic studies of gold deposits in Jiaodong area have always been hot topics,among which the source of gold and its re-activation and precipitation mechanism are the most concerned issues.Dayingezhuang gold deposit is located in the middle section of Zhaoping fault zone in northwestern Jiaodong,which is a typical disseminated and stockwork-style gold deposit in the region.It is distinguished from other gold deposits in Jiaodong area by the presence of a large number of carbonate veins or veinlets which play an important role for gold mineralization in this deposit,and thus it is a good target to track the material source and mineralization mechanism of Au.Through detailed petrographic observations,this study found the carbonate minerals in Dayingezhuang gold deposit mainly include siderite and calcite,which exist in the pyrite sericite ore in veined and disseminated forms.Some siderites have obvious symbiotic relationships with metal sulfides such as pyrite,while others are associated with quartz.Calcites occur in disseminated and veined forms,and can be divided into two types according to their mineral associations.Both of the siderite and calcite are hydrothermal carbonate minerals.Scanning electron microscopy studies show that gold minerals exist in both siderite and calcite.Combined with Au content,it is indicated that the siderite and calcite are also the important gold-bearing minerals in Dayingezhuang gold deposit,they are formed in the main stage of mineralization.The rare earth elements data obtained by in situ analysis(LA-ICP-MS)of carbonate minerals also show that they have characteristics of hydrothermal origin,and the changes of Eu abnormalities indicate that calcite may be more strongly affected by terrigenous materials than siderite.Isotopic geochemical data show that the carbonate minerals in Dayingezhuang gold deposit are obviously different from those in the Precambrian metamorphic basement in Jiaodong area.The C-O isotopic analysis shows that the δ13 CPDB and δ18 OSMOW of siderites in Dayingezhuang gold deposit are within the ranges of-5.12‰~-4.82‰ and 12.13‰~12.80‰,respectively,showing mantle source characteristics.However,the 813 CPDB and 818 OSMOW of calcites are-0.64‰ and 6.21 ‰,which means that calcites in the deposit are affected by country rock to some extent.Siderite in the gold deposit relatively shows loss of light rare earth elements and enrichment of heavy rare earth elements,with a slight negative Eu anomaly,and its(87Sr/86Sr)i and εNd(t)values are 0.709842~0.710087 and-18.5~-17.1,respectively.The measured(87Sr/86Sr)i and εNd(t)of calcite are 0.710861 and-12.7,indicating that the carbonate minerals might be originated from a basic magma of mantle source.Combined with the Mesozoic geological evolution of Jiaodong area and the whole North China Craton,we suggest that giant gold in Jiaodong area might be sourced from the enriched lithospheric mantle,and CO2 fluid played a promoting role during the process for the ore-forming materials migrated upward along the weak surface such as faults in the crust with basic magma which eventually migrated to the shallow crust with gold-bearing fluid to produce gold mineralization.