The Mechanisms Forming the Five–Floor Zonation of Quartz Veins: A Case Study in the Piaotang Tungsten–Tin Deposit, Southern China

Xiangchong Liu,Dehui Zhang,Wenlei Wang

doi:10.3390/min11080883

Abstract

It is common among many vein–type tungsten deposits in southern China that the thickness of ore veins increases from <1 cm to >1 m with increasing depth. A five–floor zonation model for the vertical trend of vein morphology was proposed in the 1960s and has been widely applied for predicting ore bodies at deeper levels, but the causative mechanisms for such a zonation remain poorly understood. The Piaotang tungsten–tin deposit, one of the birthplaces of the five–floor zonation model, is chosen as a case study for deciphering the mechanisms forming its morphological zonation of quartz veins. The vertical trend of vein morphology and its link to the W–Sn mineralization in Piaotang was quantified by statistical distributions (Weibull distribution and power law distribution) of vein thickness and ore grade data (WO3 and Sn) from the levels of 676 m to 328 m. Then, the micro–scale growth history of quartz veins was reconstructed by scanning electron microscope–cathodoluminescence (SEM–CL) imaging and in situ trace element analysis. The Weibull modulus α of vein thickness increases with increasing depth, and the fractal dimensions of both vein thickness and ore grade data (WO3 and Sn) decrease with increasing depth. Their vertical changes indicate that the fractures that bear the thick veins were well connected, facilitating fluid focusing and mineralization in mechanically stronger host rocks. Three generations (Q1–Q3) of quartz were identified from CL images, and the CL intensity of quartz is possibly controlled by the concentrations of Al and temperature. From the relative abundance of the Q1–Q3 quartz at different levels, the vertical trend of vein morphology in Piaotang was initially produced during the hydrothermal event represented by Q1 and altered by later hydrothermal events represented by Q2 and Q3. Statistical distributions of vein thickness combined with SEM–CL imaging of quartz could be combined to evaluate the mineralization potential at deeper levels.

Highlights

A possible reason for their differences is that a major part of quartz veins at the 676 m level have been denudated by later geological processes, leaving the remaining quartz veins deviated from their original statistical distribution
Our study suggests that the growth histories of those thin veins can be traced by scanning electron microscope– cathodoluminescence (SEM–CL) imaging of quartz, which, together with the five–floor zonation model, could be used as a guide for evaluating the mineralization potential at deeper levels
Statistical distributions of vein thickness and ore grade and scanning electron microscope (SEM)–CL imaging of quartz at different levels were combined to decipher how the vertical zonation of vein morphology in the Piaotang tungsten–tin deposit was produced, and our results provide the following implications: (1) Fractal dimensions of both vein thickness and ore metal grade (WO3 and Sn) decrease with increasing depth, while the Weibull modulus α of vein thickness increases with increasing depth

Summary

Introduction

The Nanling Range is a world–class tungsten province located in southern China [1]. Most tungsten deposits in this region have a close genetic relationship to the granitic magmatism during the Late Mesozoic [2,3]. Vertical zonation in vein morphology is common among many tungsten deposits in the Nanling Range [4]. Based on the vein morphology, field geologists have identified five zones since the 1960s and proposed a five–floor zonation model for deep exploration [5,6]

Results

Discussion

Conclusion