Study of Short-Wave Infrared spectroscopy (SWIR) on hydrothermal chlorite in the Tongshan porphyry Cu-Mo deposit, Heilongjiang

Abstract

Short-Wave Infrared spectroscopy (SWIR) is widely used in the study of the mica-type minerals such as muscovite, phengite, phlogopite, and paragonite for mineral exploration. However, for dark-colored mineral chlorite, which is also a common hydrothermal mineral in most magmatic-hydrothermal systems, there are fewer key indicators available. This study performed a systematic SWIR analysis on four ∼2000 boreholes, along the newly discovered Ore Body V in the Tongshan porphyry Cu-Mo deposit. Systematic petrographic observations were conducted and the four boreholes here are primarily composed of andesitic tuff and porphyritic andesite of the Duobaoshan Formation. No distinct alteration mineral zoning was observed, with chloritization is widespread in the shallow part, accompanied by minor epidotization and carbonation (i.e. chlorite with small amount of sericite-epidote-calcite), while the ore body in the deeper part of the cores exhibits chloritization, silicification, chalcopyrite alteration, and pyritization (i.e. chlorite-sercite-chalcopyrite-pyrite). No porphyritic intrusions or large-scale potassic alteration related to copper mineralization were found in Ore Body V. SWIR analysis revealed that at the “Tongshan” fault, the absorption peak of Fe-OH in chlorite decreases from ∼2255 nm to ∼2253 nm. After evaluating the interference of epidote, the “SWIR-IC” of chlorite exhibits a linear decreasing trend with borehole depth both before and after the fault, dropping from approximately 1.4 to below 1.0, and found it may indicate the formation temperature. This trend of chlorite undergoes significant changes near the “Tongshan” fault, shifting from approximately 0.8 to 1.2 before and after the fault, further confirming the presence of the “Tongshan” fault. Data coupling and correlation analysis conducted through programming showed the “SWIR-IC” is negatively correlated with copper grade, further supports the genetic link with temperature. Based on the linear variation characteristics, combined with the engineering data of the boreholes, this study infers that the slip distance of the “Tongshan” fault on Ore Body V is ∼570 m. Our study shows that chlorite SWIR spectral analyses have the great potential to provide new insights into porphyry copper deposits, and can provide indication information such as temperature and copper grade for exploration.