Studies On The Genesis Of Dashui Superlarge Hematite Silicalite-Type Gold Ore Deposit In Gansu Province

Dashui gold ore deposit, being of a superlarge deposit newly discovered in western Qinling orogenicbelt, has very unique ore-forming characteristics. A systematic studies including ore-generatinggeologic settings, ore-controlling geological factors, metallogenic mechanism and metallogenicregularities have been done in this paper for the first time.The gold mineralization in Dashui deposit mainly occurred in the Triassic limestones and dolomites.Other gold-hosted rocks include Permian limestones, Jurassic conglomerates and intermediate-basicvein-like magmatic rocks formed during Yanshannian intracontinental orogenic period. The orebodies were produced in major forms of vein-like, sack-like and funnels. The ore-controllingstructures are NWW-EW strike, nearly SN strike and NEE-NE strike faults. The convergent partsof different strike faults and the contact zones between vein-like magmatic rocks and sedimentaryroeks are the best ore-hosted structures. The ore mineral assemblages are golds+hematites+chalc-edonieso The gold mineralization was strongly related to silifications and hematitizations. Thefabrics of gold ores mainly consist of metasomatic relict texture, crypto-micro crystal texture,colloform texture and oolitic-pisolitic texture; laminated-banded structure, brecciated structure,veinlet-stockwork structure, porous or vuggy structure and massive structure etc. The ores can beclassified into three genetic types: hydrothermal metasomatic, hydrothermal infilling andhydrothermal sedimentary mmeralizationo They are the serial products of the one hydrothermal systemat different geological environments. The hydrothermal sediments or ores include hvdrothermalsiliceous rocks, hydrothermal explosive breccias, hydrothermal sedimentary, oolitic-pisoliticconglomerates and hydrothermal carbonate rocks. There is a very good regularity for the time-spatialdistribution of all the hydrothermal products: In space, from the center toward peripheral parts of thehydrothermal active systems, the hydrothermal sediments are produced in the order of hydrothermalexplosive breceias+hydrothermal siliceous rocks+hydrothermai carbonate veins, hydrothermalmetasomatic jasperoids+hydrothermal metasomatic granodiorites, hydrothermal metasomaticlimestones. From the lower to upper parts, the hydrothermal products are respectively hydrothermalfilling veinlet-stockwork siliceous rocks+carbonate veins, hydrothermal sedimentary laminated-banded siliceous rocks+oolitic-pisolitic conglomerates+horizontally occurred carbonate crusts. Intime, hydrothermal metasomatic mineralizations are a bit earlier than hydrothermal infilling andsedimentary mineralizations, and the compositions of the hydrothermal systems evolved from ferro-silico rich in the earlier stages towards carbonate rich in the late periods. The red or brown silicalitetype ores were formed due to the directly infilling or sedimentation of the ferri-rich siliceoushydrothermals.Evidences from the geological and geochemical characteristics of the deposit, the Au abundances ofgold-hosted geological bodies, the stable isotope elements (C, Si, O, H) and rare elements etc.indicate that the ore-forming materials mainly came from the deep sources which are closely related toYanshannian tectono-magmatism. The shallow tectonic activities, the magmatic intrusion orvolcanism, the hydrothermal eruptions and the late active hot-springs are all the serial evolutionaryproducts of the regional crust due to the deep upwelling and mantle-crust interactions. Thus, they accompanied and associated each other in time and space, and the active intensity and extensivenessof these serial products became much weaker and narrower with the time going on.Au was transported probably in the complex form of AuH₃SiO₄⁰. When the ore-containinghydrothermal solutions rapidly got to the surface being of full-open and oxidative environment from thedeep sources, the concentration of silica would be changed into supersaturated relative to quartzbecause of the abruptly descendment of the temperature and pressure of the hydrothermal systems.Therefore, the gold transporting in the complex of AuH₃SiO₄⁰ would be wholly depositedaccompanied with the depositing of quantities of chalcedonies. The ore-forming temperature andpressure obtained from the fluid inclusions show that the whole mineralization suffered a quite shortprocess with a wide range of temperature variation, and the metallogenic depth might be very shallowor almost near the surface of the crust. The compositions of the fluid inclusions demonstrate thehydrothermals might be rich in alkaline components and the concentration of K⁺ far more than that ofNa⁺. This implies the sources of the ore-forming fluid be closely related to the Yanshannian alkalinemagmatic rocks. The gas reduction parameters (R=(CO+H₂+CH₄)/CO₂) of fluid inclusions arevery low, showing a high oxidation metallogenic environment. The metallogenic epoch belongs tothe earlier stage of Yanshannian according to the geologic evidences.In a word, Dashui-type gold ore deposits should be classified into the magmatic hydrothermalshallow-near surface metasomatism-infilling-sedimentation ore deposits from whatever viewpoints ofthe mineralization.