Geologcal-Geochemcal Features and Genesis of Dugou Gold Deposit of Li County, Gansu Province

Controlled by plutons and faults, the Dugou gold deposit is located in the western portion of the Hercynian fold belt in the Qinling Mts., and occurs in the Liba group of the outer contact zone of the Zhongchuan pluton. The natural type of the ore deposit belongs to the structure-alteration rock type. The deposit has already reaches largesize. The paper studies geological characteristics of the deposit from orebearing strata, ore-control structures, relationship of vein rock and mineralization, ore features and alteration of country rocks. Geochemistry of the Dugou gold deposit has been studied in terms of stableiso-topes, ore-forming fluids and REE. The isotopic geochemistry indicates that 34S changes in a small range and shows strong homo geneity, which is related to the emplacement of the Zhongchuan pluton. Hand O isotopes fall in between magmatic water and atmosphere water in the δD-δ¹⁸O diagram, reflecting that the ore-forming fluid is mixed ther malf luid. Lead isotopes are anom alous lead derived from mixture between U and Pb. This could be genetically in relation with the Zhongchuan pluton rich in U. From the Pb-Sriso-topic analy tical results, the main metallogenic period is inferred to be in the early Yanshan orogeny (171.6 Ma～173.4 Ma) close to that of the Zhongchuan pluton (181.5 Ma～219 Ma). The middle Devonian Liba group has Auinitial enrichment duringits deposit and diagenesis. Faults basically controlled spatial distribution and location of gold ore, while magma is anim portant thermal dynamic source and a major factor necessary for the deposit. Finally it formed the ore deposit by thermal brine rich in ore-forming materials along the faults. The genetic type for the deposit is the lixiviation composit type of post-magmatic hydrothermal and underground thermal brine. Tectono-dynamics superposed ore layers in 1～5 km range around the plut on are targets for prospecting.