| 其他摘要 | The Yangshan gold deposit, Wenxian county, Gansu province, containing 308 t Au with average grade of 4.74g/t, is now ranked as the China’s largest gold deposit.. Locating in western Qinling Orogen, central China, it is a syn-collisionally formed Carlin-like gold deposit. Its orebodies are controlled by an E-trending shear zone and hosted in the Devonian carbonaceous carbonate-phyllite-slate sequence or the granite-porphyry dikes intruding into the Devonian strata. Its hydrothermal ore-forming process includes several stages: (1) the early stage forming quartz-sericite-pyrite assemblage; (2) the main stage forming quartz-pyrite-arsenide-galena etc. polymetallic mineral assemblages; and (3) the late stage forming carbonate-quartz-stibnite veinlets.
The granite-porphyry dykes in the Yangshan gold belt, Gansu Province, are rich in LILE and LREE, and depleted in Ba, Sr, Nb, Ta, P and Ti, ΣREE＝54.35～124.01 μg/g, (La/Yb)N = 9.72 ~ 27.80, and δEu = 0.70 ~ 0.89, the granitic magmas were originated from partial-melting with little of plagioclase as residual phase. The porphyries have ISr ratios of 0.70806 ~ 0.71756 with average of 0.71107, εNd(t) values of -2.9 ~ -5.0 with average of -3.4, and Nd-model ages (T2DM) of 1.24 ~ 1.41 Ga with average of 1.34 Ga, which suggests that the magmas must be sourced from lowly-matured Mesoproterozoic basement. The averages of (206Pb/204Pb)220Ma, (207Pb/204Pb)220Ma and (208Pb/204Pb)220Ma ratios are 17.875, 15.604 and 38.296, respectively, similar to those of both the Bikou Group of the Bikou terrane and the Mesoproterozoic basement of the Qinling micro-continent. Considering that the Bikou Group has been dated to have developed during 1.235 ~ 1.367 Ga in previous studies, and that the Qinling micro-continent, which hosts the Yangshan gold belt at its southern margin, southwardly overthrusted onto the Bikou terrane along the Mian-Lue suture, we suggest that the interested granite-porphyries were generated by metamorphic devolatilization-related partial melting of the underthrust slab mainly consisting of the Bikou Group. Hence the granite-porphyries in the Yangshan gold belt are typical syn-collision granitoids formed during the Mesozoic collision between the Yangtze and North China continents.
New S ,Sr and Pb isotopic data, presented in this paper show, 34S values for sulfides reveal large isotopic variations (-15.5‰~6.59‰), and presents no tower style distribution, suggest a contribution from sedimentary strata. 34S values for Pyrite from porphyry-granite shows a narrow range(-1.47‰～2.12‰) that can not supply such larger 34S isotopic variations for ore deposit of Yangshan. The extremely variable initial 87 Sr/ 86 Sr ratios 0.70877～0.71697 are reveal the characteristic of several different kinds of Sr source. Considering the porpgry-granite formed before the minaralizion, it may supply the source of pyrite and other metal minerals with low 87 Sr/ 86 Sr ratios, so basement does as well. Lead in sulfides of the Yangshan Au deposit is more radiogenic (µ = 9.46~10.06,ω=36.96～42.21) than average crustal, which suggests that the metallogenic material must be sourced from hightly-matured Devonian strata which comprise of carbonaceous phyllite, slate, chert and carbonate. The Pb isotope ratios from the sulfides are also variable , 206Pb/204Pb=17.552～18.853, with average of 18.260；207Pb/204Pb=15.574～15.928, with average of 15.685；208Pb/204Pb=37.894～39.293, with average of 38.680, the Pb-isotope ratios range within the range of those of the Devonian strata. Part of sulfides with low Sr and Pb isotope may be derived from granite or/ and basement rocks.
Hydrogen, oxygen and carbon isotope systematics of the Yangshan deposit from a previous work show the ore-forming fluid-system varies from early, deep metamorphic to late, shallow meteoric, and the early stage fluids were derived from metamorphism and/or reworking from the Devonian strata or/and similar lithologies which comprise of carbonaceous phyllite, slate, chert and carbonate, which are consistent with conclusions drawn from our studies in this paper.
The comparison of the hydrogen-oxygen-carbon-sulfur-lead -strontium isotope systematics between the Yangshan Au deposit and the main lithologies in the Bikou Terrian, shows that a likely source of ore fluids and metallogenic materials for the Yangshan Au deposit was country-rocks of Devanian strata.
The Yangshan Au deposit was hosted by Devonian strata and formed at about 190Ma, followed the syn-collsion-type granite-porphyry formed. During the Mesozoic collision events, a north-dipping A-type subduction zone was active along the Mian-Lue suture. The slab south of the Mian-Lue suture, mainly consisting of the North Yangtze Terrane (including Bikou Groups), was subducted beneath the Qinling micro-continent and subsequently metamorphosed and dehydrated. This provided fluids to the overriding slab (i.e. Qinling micro-continent) and was ultimately responsible for the development of the Yangshan Au systerm. The main metallogenic stage happened during the collisional compression-to-extension transition, this evolution of ore-forming tallies well with the CFM model.
The Yangshan gold deposit could be a typical representative of a large class of gold deposits occur in continental collision orogens in the world, especially representing those found in the western Qinling gold province. Hence, we introduce a concept of “Qinling-type” or “Yangshan-type” to cover and represent a group of gold deposits whose geological and geochemical characteristics are similar to the Yangshan gold deposit. |
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