短波红外光谱技术在浅剥蚀斑岩铜矿区勘查中的应用——以西藏念村矿区为例

作为世界上最重要的一种矿床类型,斑岩铜矿一直是工业界勘查的首选.对此类矿床的勘查,在中-深剥蚀程度的矿区相对简单,而在浅剥蚀的矿区则变得较为困难,这是因为在浅剥蚀的矿区,矿床热液/矿化中心很难快速有效定位.近年来,在矿产勘查领域逐渐得到广泛应用的短波红外光谱技术,可通过特定蚀变矿物反射光谱特征参数系统变化的规律来厘定热液/矿化中心,在块状硫化物及浅成低温矿床中显示出良好的应用效果,这为浅剥蚀斑岩铜矿热液/矿化中心的快速、有效厘定提供了一种途径.为此,文章选择了位于冈底斯斑岩铜矿带东段、剥蚀较弱的念村(即夏玛日)矿区,拟通过矿区样品短波红外光谱的系统测量,寻找出蚀变矿物反射光谱特征参数系统变化的规律,进而约束矿床热液/矿化中心.本次研究在念村矿区共识别出7种蚀变矿物,按出现频率由多至少依次为伊利石、绿泥石、蛋白石、叶蜡石、高岭石、绿帘石及多硅白云母；而且,矿区外围以伊利石-绿泥石±绿帘石蚀变矿物组合为主,向内逐渐过渡为伊利石±蛋白石、伊利石-叶蜡石±高岭石组合.通过对伊利石反射光谱特征参数的计算发现,伊利石结晶度及Al-OH吸收峰位,这些通常被认为与伊利石形成温度有关的光谱学参数,在该矿区呈现出系统的变化规律:在矿区东北部,伊利石结晶度较大(＞1.6),Al-OH吸收峰位较小(＜2 203 nm),而该区域的外围,伊利石结晶度变小,Al-OH吸收峰位变大.这表明矿区东北部伊利石的形成温度更高,暗示该区域可能为矿床热液/矿化中心.因此,建议在本次研究所圈定的热液/矿化中心范围内,在适当开展物探工作的基础上,尽快布置勘查工程进行验证,以实现矿床的尽快查找和突破.

Application of short wavelength infrared (SWIR) technique in exploration of poorly eroded porphyry Cu district: A case study of Niancun ore district, Tibet

As the most important type of ore deposit, porphyry Cu deposit has long been taken as the primary exploration target. The exploration of such type of ore deposits is relatively easy in strongly eroded areas, but is relatively difficult in poorly eroded area where mineralization/hydrothermal (M/H) centers cannot be easily located by traditional alteration mineral mapping. Short Wavelength Infra-Red (SWIR), a newly-developed technique in the field of mineral exploration, was tentatively used in this study to locate M/H centers of poorly eroded porphyry Cu deposits at Niancun, Tibet based on the fact that it has shown powerful function in locating M/H centers for VMS-type and epithermal deposits. About 300 surficial samples and 30 drill hole samples were measured by TerraSpec in this study. As a result, seven types of alteration minerals were recognized. They are illite, chlorite, opal, pyrophillite, kaolinite, epidote, and phengite according to frequency of occurrences from high to low. Spatially, the alteration minerals show clear zoning comprising an outer zone of illite-chlorite ± epidote assemblage to an inner zone of illite-pyrophillite ± kaolinite. Numerical values for reflectance spectral features of illite were also extracted, and some of them (e.g., wavelength position of Al-OH absorption and illite crystallinity) show systematic changes. The wavelength position of the illite Al-OH absorption varies between ~2 192 nm and 2 220 nm with the mode at 1 904 nm to 1 914 nm, and it is obviously lower (<~2 203 nm) in the eastern portion of the district where illite and pyrophillite constitute dominant alteration minerals. Illite crystallinity (IC) at Niancun typically changes from 0.6 to 3.1, with the mode at 1.0 to 1.8. In contrast, it is relatively higher (>1.6) in the eastern portion of the district. For illite, higher IC value and lower wavelength position of Al-OH absorption generally indicate that their formation occurred at higher temperature. So the eastern portion of the district with higher IC value, lower wavelength position of Al-OH absorption and alteration of illite-pyrophillite may be the M/H center of the Niancun deposit.