摘要
青藏高原东段三个斑岩型矿床（玉龙、马厂箐和西范坪）产出于大陆碰撞环境，与喜马拉雅期埃达克质斑岩有关，并为新生代大规模走滑断裂所控制。在印度-亚洲碰撞带东部3个斑岩Cu-Mo—Au矿床已识别出3个明显的成矿幕次：①玉龙矿区，石英绢云母蚀变带中的硫化物石英脉辉钼矿Re—Os等时线年龄为40．1±1．8Ma，与赋矿围岩二长花岗岩的锆石SHRIMP年龄40．9±0．1Ma一致，表明Cu—Mo矿化发生在斑岩岩浆作用的晚期阶段（约40Ma），但热液系统至少延长到约36Ma，热液系统持续时间大于4Ma，其间，构造控制的高级泥化蚀变叠加于早期斑岩型矿体中高硫化物矿化之上形成富矿体。②马厂箐矿区，辉钼矿Re—Os等时线年龄为35．8±1．6Ma，与容矿花岗岩的锆石SHRIMP和全岩Rb—Sr年龄（35～36Ma）一致，但早于含金石英正长斑岩的全岩K—Ar年龄（31～32Ma），表明马厂箐斑岩热液系统的寿命为约4Ma，其间，约36Ma有钾硅酸盐蚀变和Cu-Mo矿化，而同Au矿化密切的高级泥化蚀变发生在晚期（31～32Ma）。③西范坪矿区，钾硅酸盐蚀变带内辉钼矿等时线年龄32．1±1．6Ma最年轻，晚于热液蚀变黑云母和角闪石的K—Ar年龄33．5～34．6Ma，很可能反映了斑岩热液系统在约32Ma终止，如此短时的热液系统正是导致西范坪绢云母化蚀变微弱和高级泥化蚀变的缺失的原因。斑岩热液系统的寿命与矿床金属吨位（规模）的正相关，本区巨量玉龙斑岩铜矿可能与其热液活动时期延长有关。而热液系统的延长又与多期次的岩浆侵入有关。因此，从走滑挤压场（55～40Ma）到走滑拉张场（24～17Ma）的构造应力转换期内，幕式的应力松弛引起多期岩浆侵入是导致印度亚洲碰撞带内热液系统的延长和叠加成矿作用发生的关键。
New Re-Os molybdenite dates are nere presented from three porphyre deposits（Yulong,Machangqing and Xifanping）, associated with the Himalayan adakitic porphyritic intrusives that occurred in a continental collision environment and were controlled by large-scale Cenozoic strike-slip faults in eastern margins of the Qinghai-Xizang （Tibet） plateau. The Re-Os isotopes in molybdenites have been used to determine mineralization time and constrain on duration of the porphyry hydrothermal systems. Three distinct mineralization episodes have been recognized for the porphyry Cu-Mo-Au deposits in the eastern Indo- Asian collision zone. At Yulong, Re-Os analyses of four molybdenite samples from sulfide-quartz veins in the quartz-sericite alteration zone yield an isochron with an age of 40. 1±1. 8Ma （2σ）, coincident to zircon SHRIMP age of 40. 9±0.1 Ma for the host monzogranite. The molybdenite Re-Os datings, together with K- At, Rb-Sr, U-Pb and 4^40Ar/^39Ar datings on the pre- and intra-mineral porphyries suggest that Cu-Mo mineralization took place at the late stage （-40Ma） of lifetime of porphyry magmatism, but hydrothermal system prolonged at least to-36Ma. During the longevity of 〉4Ma, structurally controlled advanced argillic alteration and the overprinting of high-sulfidation mineralization over porphyry-type orebody were developed at Yulong. At the Machangqing deposit, molybdenite Re-Os data yield an eight-point isochron with an age of 35. 8＋1.6Ma （20）, which is identical to the zircon SHRIMP and bulk-rock Rb-Sr ages （35±36Ma） of the host granites, but lower than bulk-rock K-Ar ages （31±32 Ma） for associated Au-bearing altered quartz syenite. This suggests a porphyry hydrothermal system longevity of -4Ma, during which Cu-Mo introduction associated with the K silicate alteration at -36Ma, while Au mineralization associated with advanced argillic alteration at the last period （31-32Ma） of the Machangqing hydrothermal system. At Xifanping, five molybdenite samples from the K silicate alteration zone yield the youngest isochron age of 32.14-1.6Ma （20） in the area. The Re-Os molybdenite ages are younger than K-Ar ages （33.5~34.6） of hydrothermal biotite and amphibole, which most likely reflects the cessation of the porphyry hydrothermal system at -32 Ma. Such short duration of hydrothermal system probably explains the weakly sericitization and absence of the advanced argillic alteration in the district. Episodically stress relaxation during tectonically transforming from transpressional （55-40Ma） to transtensional （24-17Ma） regimes probably caused multiple magmatic intrusions, which most likely result in the protraction of the hydrothermal system and superimprosed mineralization in the eastern Indo-Asian collision zone.
出处
《地质论评》
CAS
CSCD
北大核心
2006年第1期72-84,共13页
Geological Review
基金
科技部国家基础研究项目(编号2002CB41260)
国土资源大调查项目(编号200310200001-4)
国家自然基金项目(编号40272046)的成果
