Effect of alteration on the geochemistry and mechanical properties of granite from Pingjiang, Hunan Province, China

Minghao Ren,Paul Sargent,Wei Wang,Guangxiang Yuan,Shanggao Li,Huaichang Yu,Qi Wu,Zhiquan Huang

doi:10.1007/s12665-022-10197-z

Abstract

The effect of alteration on the geochemistry and mechanical properties of granite from Pingjiang, Hunan Province, China was investigated. Six weathered and 14 hydrothermally altered samples in 3 adits were collected for mechanical strength tests, mineralogical and geochemical analysis. The types of alteration observed within the samples were chloritization and argillization. Samples taken from a silicified fracture zone were enriched in quartz. Weathering was observed to significantly weaken the granite whereas the effects of hydrothermal alteration on strength were more complex. The porosity increased with the enrichment of the altered minerals, indicating that the formation of altered minerals degrades the strength of physical bonds between minerals within the granite. The granite Na2O, CaO, K2O, MgO and SiO2 contents decreased while Fe2O3T increased due to weathering. Variations of major elements within the hydrothermally altered granite were distinguished from those observed in weathered samples—notably Mg was removed whilst Si and Fe were generally stable during the hydrothermal alteration. Whereas the quartz-enriched samples gained Si with slight depletions in Mg and Fe. Trace and rare earth elements were both removed in hydrothermal alteration. The variable behavior of major element was quantified by the mobility index which indicated that the different geochemical changes were attributed tochloritization and argillization. Furthermore, the mobility index of Mg was used to identify the dominated alteration in granite and evaluate the effects of chloritization and argillization. Generally, chloritization was found to be more dominant than argillization in weakening the granite.

Highlights

The mineralogical, geochemical, and mechanical properties of rocks can be significantly changed by alteration
The results show that the relationship between mechanical strength and the degree of alteration, irrespective of whether this is due to weathering or hydrothermal alteration, can be described by an exponential equation
TiO2, K2O, Fe2O3 content (Fe2O3T), Th, Hf, Co, Ni, and V contents were unaffected by hydrothermal alteration; Na2O, Sr, Nd, Sm, and Pb contents were lowered by hydrothermal alteration; and SiO2, Rb, Cr, U, Zn, Mn, and Cs contents were increased due to reactions with the hydrothermal fluids

Summary

Introduction

The mineralogical, geochemical, and mechanical properties of rocks can be significantly changed by alteration (del Potro and Hürlimann, 2009; Huang et al, 2011; Julia et al, 2014; Moon and Jayawardane, 2004; Pola et al, 2012, 2014; Wang et al, 2015; Wyering et al, 2014). Almost all alteration occurs in two ways: (1) rocks interact with water and the atmosphere, which is called weathering (Fritz and Mohr, 1984; Moon and Jayawardane, 2004; Wang et al, 2013); and (2) hydrothermal fluids in contact with rocks cause water–rock reactions The latter process is called hydrothermal alteration (Browne, 1978; Wyering et al, 2014). The most published data are for weathered volcanic rocks, whereas weathered or hydrothermally altered granites have received little attention (Lan et al, 2003)

Methods

Results

Discussion

Conclusion