西藏谢通门县雄村铜金矿主要地质体的40Ar/39Ar年龄及地质意义

雄村铜金矿Ⅱ号矿体在2007-2008年取得了重大的找矿突破,详细的地质编录成果表明,矿体同样受含眼球状石英斑晶的角闪石英闪长玢岩和角闪石英闪长玢岩控制.含眼球状石英斑晶的角闪石英闪长玢岩和角闪石英闪长玢岩的锆石U-Pb年龄已经确定(164～177 Ma),因此,雄村铜金矿不同地质体的云母类、长石类矿物Ar-Ar同位素年龄的测定显得尤为关键.文章通过对穿切I号矿体的黑云母花岗闪长岩、云煌岩脉的黑云母Ar-Ar同位素测年,结合其他研究者的成果,得到一组十分重要的年龄数据.穿插矿体的黑云母花岗闪长岩中的黑云母(样号6187-335)坪年龄(46.96±0.42)Ma,穿插矿体的无矿化的云煌岩(5053-324.4)(Cu含量0.0551%,Au含量0.034 g/t,Ag0.6 g/t)中的黑云母给出了一个较好的似坪年龄,加权平均年龄为(49.59±0.58)Ma.结合其他研究者测定的中侏罗世侵位的角闪石英闪长玢岩(不含矿)锆石U-Pb年龄为(177.1±2.0)Ma]中黑云母的At-At同位素年龄为(48.57±0.31)Ma;含矿凝灰岩围岩锆石U-Pb年龄为(176±5)Ma,MS3VD=0.63}的蚀变绢云母Ar-Ar年龄为(47.07±0.30)Ma;似伟晶岩中长石的At-At年龄为(47.62±0.7)Ma,认为不同形成时代、不同产出空间、不同矿化程度的地质体的云母类、长石类矿物的Ar-Ar同位素年龄的一致性,反映了后期岩浆热事件对中侏罗世早期形成的地质体和矿体的黑云母氩同位素体系产生了较强的扰动或置换.谢通门大岩基黑云母花岗闪长岩的侵位致使各地质体发生显著的退变质,形成典型的角岩化带,这种退变质的时限在46-48 Ma之间的始新世lutetian期,进而认为各地质体中云母类矿物的~(40)Ar/~(39)Ar同位素年龄不能作为成矿年龄.

~(40)Ar/~(39)Ar isotope ages of main geological bodies in Xiongcun copper-gold deposit, Xietongmen County, Tibet, and their geological significance

The exploration of No. II ore body in the Xiongcun copper-gold deposit made significant breakthrough dur ing 2007-2008. Detailed core logging shows that the deposit is controlled by augen quartz hornblende diorite porphyry and hornblende diorite porphyry. The zircon U-Pb ages of augen quartz hornblende diorite porphyry and hornblende diorite porphyry are 164-177 Ma, indicating that the Ar-Ar ages of mica and feldspar minerals within different rock types are especially important. In this paper, the authors got a group of age data based on biotite Ar-Ar isotopic dating of biotite granodiorite and lamprophyre in combination with the results obtained by other researchers. The plateau age of biotite (Sample No. 6187-335) within biotite granodiorite is (46.96 ± 0.42) Ma. The surface ages of six consecutive heating stages are the same within the error range. Released ~(39)Ar accounts for 94.6 % of total ~(39)Ar. The surface ages of the first two heating stages are lower than the plateau age. The content of radiogenic argon accounts for a small percentage of released radioactivity argon at these two stages. This means that the biotite surface was subjected to a very slight loss of argon. At the same time, a small amount of atmospheric argon was adsorbed on the mineral surface or mixed with the outer crystal lattice of the mineral. The samples don't contain excessive argon and have not been polluted by post-magmatic minerals, as evidenced by an analysis of the surface age. The formation of biotite was not subjected to significant thermal event interference, suggesting that there did not exist significant heating-magma events after the formation of bi otite granodiorite. Argon isotopes are in a closed state. The ages of anti-isochron are (46.72 + 0.81) Ma and (46.84 + 0.67) Ma respectively, the same as the plateau age in the same error range. This proves that the bi otite cooling age of 46-47 Ma is reliable. According to the intercept from anti-isochron to the ~(36)Ar/~(40)Ar axis, the initial ratio of ~(36)Ar/~(40)Ar is 298 + 13, similar to the normal ratio of atmospheric argon. This implies that there was no superposition of late thermal events. Biotite from unmineralized lamprophyre (5053-324.4) (Cu 0.0551%, Au 0.034 g/t, Ag 0.6 g/t) gives a good plateau age. Released ~(39)Ar from four consecutive heating stages accounts for 63.28% of total ~(39)Ar. The surface ages are the same in the same error range, and the weighted average age is (49.59 ±0.58)Ma. Released ~(39)Ar at the stage 2-3 and stage 4-5 of the sample heat ing stages accounts for 36.45% of total ~(39)Ar. The surface ages are the same within the same error range. The ages of these two stages show that late thermal events made strong disturbance on argon isotope of biotite from the lamprophyre, and this led to partial loss of radioactive isotope of argon. The Ar-Ar age of biotite from Juras sic unmineralized hornblende quartz diorite porphyry is (48.57 ± 0.31) Ma, with zircon U-Pb age being (177.1 + 2.0) Ma; the Ar-Ar age of altered sericite from tuff rock is (47.07 ± 0.30) Ma, with zircon U-Pb age being (176 + 5)Ma; and the Ar-Ar age of feldspar from pegmatite is (47.62 + 0.7)Ma. According to the above data, the Ar-Ar ages of micas and feldspar-type minerals within different periods, different spatial positions and diffe rent mineralized rock types are consistent with each other. This implies that the late thermal event made strong disturbance on argon isotope of biotite and led to partial loss of radioactive isotope of argon. Ar-Ar dating results indicate that biotite granodiorite emplacement resulted in significant retrograde metamorphism. The time limits for retrograde metamorphism were 46-48 Ma in Eocene, and this means that the Ar-Ar ages of mica minerals can not be regarded as the ore-forming age.