滇西北铜厂沟夕卡岩-斑岩型Mo-Cu矿床成矿岩体的高氧逸度特征及区内斑岩矿床成矿元素组合差异控制因素分析

Highly oxidized characteristics of Tongchanggou porphyry associated with skarn-porphyry Mo-Cu mineralization and key factors controlling the elemental association of porphyry deposits in northwestern Yunnan Province

滇西北铜厂沟夕卡岩-斑岩型Mo-Cu矿床位于特提斯构造域的东部,成矿岩体为晚白垩世花岗闪长斑岩,矿化和岩体紧密共生,主要为夕卡岩型和斑岩型矿化。分析了花岗闪长斑岩中锆石Ce^(4+)/Ce^(3+)、Eu_N/Eu_N~*比值、岩体氧化物共生组合及岩体Sr-Nd同位素组成。铜厂沟花岗闪长斑岩中锆石具高的Ce^(4+)/Ce^(3+)比值(250～1415)和高的Eu_N/Eu_N~*比值(0.61～0.75),与智利北部以及中国的玉龙、德兴斑岩型矿床的基本一致,显示高氧化性特征;岩体发育赤铁矿-磁铁矿组合及据锆石微量元素组成计算岩浆lgfO_2值在-2.8～-20.0之间,也表明岩浆具高氧化特征,岩浆中的硫主要为氧化硫。岩浆中氧化硫主要通过与岩浆中的Fe^(2+)反应,形成磁铁矿,使氧化硫还原,为硫化物沉淀析出提供还原硫。岩体全岩初始(^(87)Sr/^(86)Sr)_i比值在0.706881～0.706970之间,ε_(Nd)(t)值在-5.0～-5.5之间,两阶段模式年龄t_(DM2)范围在1297～1337 Ma之间,表明其主要为古老下地壳部分熔融形成的,并混有少量幔源组分。滇西北地区发育斑岩Cu-Mo(Mo-Cu)、Cu-Au-Mo及Cu-Au矿床,不同元素组合斑岩矿床成矿岩体岩浆都具高氧化特征,但矿床成矿元素从Cu-Mo→Cu-Au-Mo→Cu-Au,矿化岩体的(^(87)Sr/^(86)Sr)i范围从0.7076～0.7101逐渐减少到0.7053～0.7059,ε_(Nd)(t)值范围从-5.9～-6.1逐渐增加到-2.2～-3.8,显示幔源物质增加或地壳物质减少。这表明区内斑岩-夕卡岩型矿床成矿元素组合受源区控制,斑岩铜金矿床岩浆源区含较多幔源物质,斑岩铜钼矿床岩浆源区含较多壳源物质。

The Tongchanggou skarn-porphyry Mo-Cu deposit occurs in the eastern Tethyan tectonic domain in the northwestern Yunnan province. Mineralization associated with the granodiorite porphyry can be classified as veinlet-to-disseminated mineralization hosted in porphyritic and skarn mineralization. Skarn mineralization consists of the primary orebodies. Our work analyzed zircon trace element composition, iron oxide mineral assemblage, and whole rock Sr-Nd isotopic composition to show highly oxidized features and sources of melts as well as factors controlling the development of porphyry deposits of different elementals associations. The Tongchanggou porphyry has high Ce^4+/Ce^3+(250–1415) and EuN/EuN^* values (0.61–0.75) similar to those of the Chile giant porphyry deposit and the Yulong and Dexing porphyry deposits, suggesting that the melts of Tongchanggou porphyry are characterized by high oxygen fugacity. The iron oxide assemblage of magnetite and hematite found in the porphyry and calculated log (fo2) values of the melts ranges from -2.8 to -20.0, in the area between FMQ + 2 and HM, providing more strong evidence that the melts of the Tongchanggou porphyry are highly oxidized. Sulfur in the melts exists mainly as sulfate. Given that magnetite is commonly found in the porphyry, it is suggested that the formation of magnetite could have resulted in the reduction of sulfate to sulfur, providing sufficient sulfur to form sulfides for the porphyritic ore-forming system. The Tongchanggou porphyry has initial 87Sr/86Sr values of 0.706881–0.706970 and εNd(t) values of -5.0 – -5.5 with two model ages of 1297–1337 Ma, suggesting that it originated from partial melting of Proterozoic basement with minor contribution from mantle sources. Porphyry-Cu-Au, Cu-Au-Mo, and Cu-Mo or Mo-Cu deposits are found in northwestern Yunnan province. Initial (87Sr/86Sr)i ratios increase from 0.7053–0.7059 to 0.7076–0.7101 from porphyry-Cu-Au to Cu-Au-Mo and to Cu-Mo or Mo-Cu deposits, while the εNd(t) values decrease from -2.2– -3.8 to -5.9– -6.1 accordingly, suggesting their mantle source material decreases from porphyry-Cu-Au to Cu-Au-Mo and to Cu-Mo or Mo-Cu deposits. Porphyry-Cu-Au deposits were derived from sources with more mantle contribution than those of the porphyry-Cu-Mo or Mo-Cu deposits. Source differences are the key factors resulting in porphyry deposits with different element associations in the area.