Li, Xuanxuan, Zhou, Taofa, Fan, Yu, Chen, Jing, White, Noel C., Zhang, Lejun, Tang, Mingying, Lin, Xin, Du, Yao, and Zhang, Yuang
The Qianpu area, located in the center of Luzong basin, develops advanced argillic alterations and well-occurring alunite. Three formation environments of alunite, magmatic-hydrothermal, magmatic-steam, steam-heated, indicate that there are multiphase hydrothermal activities. The mineral compositions including quartz, alunite (and APS), pyrite, zunyite, dickite, pyrophyllite, kaolinite, and minor illite are consistent with the characteristic of typical lithocap. Alteration zones in Qianpu lithocap develops sequentially from the center to the periphery quartz-alunite-pyrite alteration, alunite-dickite alteration, dickite-pyrophyllite alteration, alunite-kaolinite-illite alteration, and alunite veins superimposed on quartz-alunite-pyrite alteration and alunite-dickite alteration. The alunite geochemistry of different origin indicates the formation temperatures are more than 200°C and the fluid gradually decreases in temperature and acidity. Zunyite is a mineral with the specific environmental significance of high temperature and close to the fluid channel. In combination with the potential of Luzong basin, the presence of zunyite in Qianpu lithocap, as well as the characteristics and genetics of Huangzhuyuan silver polymetallic deposit near Qianpu lithocap, it is inferenced that the Qianpu-Huangzhuyuan area has the prospecting potential of porphyry-epithermal deposits. [Display omitted] • There are multiphase hydrothermal activities forming different alunite in Qianpu: disseminated I-alunite in magmatic-hydrothermal environment, II-alunite veins in magmatic-steam environment, powdery III-alunite in steam-heated environment. • Qianpu area is a typical lithocap, developing alteration zones from center to outward following quartz-alunite-pyrite, alunite-dickite, dickite-pyrophyllite, and alunite-kaolinite (illite) alteration. • The Qianpu-Huangzhuyuan area has good prospects for exploring porphyry-epithermal Cu-Au deposit. The lithocap is a large-scale alteration including silicification, advanced argillic and argillic that developed near the surface by hydrothermal activities. It is helpful for exploring high-sulfidation epithermal deposits and porphyry deposits in its root and deep parts, respectively, and is used as a new prospecting indicator. There are many minerals in the lithocap and it is difficult to identify them; alunite is the most representative mineral and one of the new minerals used to guide exploration; it plays an important role in assessing the metallogenic potential and guiding ore prospecting. The Luzong basin is dominated by iron oxide apatite deposits (IOAs). In recent years, many researchers have been searching for porphyry copper–gold deposits and epithermal deposits as studies on alunite and lithocaps in the basin have gradually been undertaken. In field explorations and investigations of the Luzong basin, the author found that in Qianpu area of the central basin, where kaolinite deposits were mainly explored in the past, Well-occurring alunite minerals and advanced argillic alterations have developed. A typical lithocap developed in Qianpu area after a series work of geology and mineralogy, such as alteration mineral species, alteration types, and alteration zoning, which were formed by the hydrothermal alteration of the volcanic rocks of the Zhuanqiao Formation. The minerals in this lithocap are mainly quartz, alunite (including aluminum-phosphate-sulfate, APS), pyrite, zunyite, dickite, pyrophyllite, kaolinite, and a small amount of illite, which are present sequentially from the center to the peripheral quartz-alunite-pyrite alteration, alunite-dickite alteration, dickite-pyrophyllite alteration, alunite-kaolinite-illite alteration, and alunite veins superimposed on quartz-alunite-pyrite alteration and alunite-dickite alteration. Three types of alunite were produced in large quantities and were formed in three kinds of environments. The I-type alunite, which was formed by hydrothermal metasomatism alteration in the early stage and is densely disseminated distribution, coexisting with quartz and pyrite, is a magmatic hydrothermal alunite. Relatively pure alunite veins (II-alunite) of varying thicknesses formed in the open space in the mid-hydrothermal period in a magmatic steam environment. The fine-grained III-alunite that coexists with kaolinite formed in the late hydrothermal period and in a steam-heated environment. The alunite produced by various origins represents multiphase hydrothermal activity in the Yanshanian period in the Qianpu area; the Na content decreased, the K and Pb content increased, and the K/Na molar ratios ranged from 2.82 to 6.58 (EPMA) and 1.59 to 3.93 (LA-ICP MS) from the early to middle and late hydrothermal stages, indicating that the formation temperatures were more than 200 °C and gradually decreased with hydrothermal evolution. There is a high-sulfidation epithermal system in Qianpu area. In combination with the specific environmental significance of zunyite indicating a high temperature and closeness to the fluid channel, as well as the characteristics and genesis of the Huangzhuyuan silver polymetallic deposit near Qianpu, it is inferred that the Qianpu-Huangzhuyuan area has prospecting potential for porphyry-epithermal deposits. [ABSTRACT FROM AUTHOR]