Potassic alteration is a common feature in hydrothermal ore systems, and both its occurrence and degree can be important features applicable to ore deposit exploration. Here we report results from optical cathodoluminescence (CL) and chemical composition investigations on K-feldspar of different origins in and around the Early Cretaceous lode gold deposits in the Xiaoqinling area, located along the southern margin of the North China Craton.
We focus on K-feldspars from an Early Cretaceous biotite monzogranite (G-Kfs), a Paleoproterozoic migmatite (M-Kfs), and a hydrothermal alteration zone of Early Cretaceous gold deposits (H-Kfs). The grain size of G-Kfs ranges from 1 to 4 mm, usually exhibits tartan twinning, and occasionally shows evidence of exsolution. Part of the M-Kfs exhibits tartan twinning, and the grain size shows wide variation (from tens of microns to several centimeters), while the H-Kfs shows no twinning, and the grain size is generally less than 0.5 mm.
Optical CL analyses show that the G-Kfs exhibits two emission bands at around 466–472 and 708–713 nm which occur in multiple, alternating dark red and blue zones. The M-Kfs exhibits blue, red, and/or violet-red luminescence resulting from two emission bands at 446–465 and 694–701 nm. The H-Kfs displays a distinct greenish-yellow luminescence resulting from an emission band at 545–550 nm.
Electron probe microanalyses show that among the three K-feldspar types, the G-Kfs contains higher Na2O (average: 0.71 wt.%) and Al2O3 (average: 18.79 wt.%) and lower K2O (average: 15.62 wt.%), whereas the H-Kfs shows higher K2O (average: 16.31 wt.%) and lower Na2O (average: 0.45 wt.%) and Al2O3 (average: 18.61 wt.%). The K2O, Na2O, and Al2O3 contents of the M-Kfs are intermediate between those of the G-Kfs and H-Kfs. In transitioning from the G-Kfs, M-Kfs, to H-Kfs, it is noted that the concentrations of Sr, Ba, and Pb decrease progressively, whereas W, V, Zn, Mn, Sc, Ge, and Ga gradually increase. Concentrations of La, Ce, and Eu in the H-Kfs are lower than in the G-Kfs and M-Kfs, and the Cr and Cu concentrations in the G-Kfs are the lowest. Scatter diagrams constructed with Sr, Ba, Pb, and Rb concentrations show that the three types of K-feldspars fall into different fields. These, as well as the CL and spectral properties of K-feldspar, can clearly distinguish the K-feldspars of magmatic, metamorphic, and hydrothermal origin in the study area.