安徽峙门口矿床黄铁矿的Pb同位素测定和成矿物质来源探讨

PB ISOTOPE DETERMINATION AND THE METALLOGENETIC MATERIAL SOURCE OF THE SHIMENKOU PYRITE DEPOSIT, ANHUI PROVINCE

安徽峙门口矿床中黄铁矿的Pb同位素组成为:206Pb/204Pb=18.06～18.202,07Pb/204Pb=15.55～15.58,208Pb/204Pb=38.19～38.30。其变化范围很小,落在侵入岩Pb的变化范围内,而与安徽沿江地区沉积岩Pb的变化范围不同,说明峙门口矿床不是沉积成因的。前人的Pb和S同位素以及微量元素分析支持这一结论。前人的Re-Os年龄(303 Ma左右)表明该矿床的形成很可能与石炭纪海底喷流有关,但大的年龄误差指示中生代岩浆活动的改造。钟鸣地区叶山石炭系黄龙组底部砾岩中黄铁矿的Pb同位素组成为206Pb/204Pb=18.11～18.142,07Pb/204Pb=15.57～15.59,208Pb/204Pb=38.23～38.31,与峙门口黄铁矿的一致,变化范围也很小,两者可能有相同成因。但是长江中下游的大部分铜、铁、金、硫矿床的成矿与晚中生代岩浆活动同时,石炭纪海底喷流成因这一结论不能推广到这些矿床。同样,现有的同位素和年龄证据不能判别这些矿床的成矿物质来自石炭纪喷流形成的层状黄铁矿还是来自晚中生代岩浆岩。

Pb isotopic compositions of pyrites from the Shimenkou deposit,Anhui province have been determined.206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb ratios vary from 18.06 to 18.20,15.55 to 15.58 and 38.19 to 38.30,respectively.Pb isotopic ratios of the Shimenkou pyrite are significantly different from those for the sedimentary rocks in the region along the Yangtze River,Anhui province,but fall within the field of feldspars of intrusive bodies of the region,suggesting that Pb(and other ore-forming metals) for Shimenkou deposit might not be derived from a sedimentary source.Previous Pb and S isotope and trace element studies of the Shimenkou deposit support this conclusion.However,303 Ma Re-Os age given by previous study suggests that the Shimenkou pyrite deposit may be related to Carboniferous sea floor exhalation,but great uncertainty in age suggests the reworking by the Yanshanian magmatism.Disseminated pyrites in the basal conglomerate of the Carbonaceous Huanglong Formation from Yeshan,Zhongming region give corresponding Pb isotopic ratios ranging from 18.11 to 18.14,15.57 to 15.59 and 38.23 to 38.31,respectively,similar to those for the Shimenkou deposit,indicating a magmatic origin similar to that of the Shimenkou deposit.However,available isotope and geochronological evidences can neither support nor deny the conclusion that disseminated pyrites in the basal conglomerate of the Huanglong Formation are the source of the Mesozoic deposits in the region.