Effects of chemical alteration on fracture mechanical properties in hydrothermal systems

Abstract:

Fault and fracture networks often control the distribution of fluids and heat in hydrothermal and epithermal systems, and in related geothermal and mineral resources. Additional chemical influences on conduit evolution are well documented, with dissolution and precipitation of mineral species potentially changing the permeability of fault-facture networks. Less well understood are the impacts of chemical alteration on the mechanical properties governing fracture growth and fracture network geometry.

We use double-torsion (DT) load relaxation tests under ambient air conditions to measure the mode-I fracture toughness (K_IC) and subcritical fracture growth index (SCI) of variably altered rock samples obtained from outcrop in Dixie Valley, NV. Samples from southern Dixie Valley include 1) weakly altered granite, characterized by minor sericite in plagioclase, albitization and vacuolization of feldspars, and incomplete replacement of biotite with chlorite, and 2) granite from an area of locally intense propylitic alteration with chlorite-calcite-hematite-epidote assemblages. We also evaluated samples of completely silicified gabbro obtained from the Dixie Comstock epithermal gold deposit.

In the weakly altered granite K_IC and SCI are 1.3 ±0.2 MPam^1/2 (n=8) and 59 ±25 (n=29), respectively. In the propylitic assemblage K_IC is reduced to 0.6 ±0.1 MPam^1/2 (n=11), and the SCI increased to 75 ±36 (n = 33). In both cases, the altered materials have lower fracture toughness and higher SCI than is reported for common geomechanical standards such as Westerly Granite (K_IC ~1.7 MPam^1/2; SCI ~48). Preliminary analysis of the silicified gabbro shows a significant increase in fracture toughness, 3.6 ±0.4 MPam^1/2 (n=2), and SCI, 102 ±45 (n=19), compared to published values for gabbro (2.9 MPam^1/2 and SCI = 32).

These results suggest that mineralogical and textural changes associated with different alteration assemblages may result in spatially variable rates of fracture initiation and growth in different parts of hydrothermal systems. Contrasting fracture mechanical properties between alteration assemblages may constitute a new mechanism of chemical-mechanical feedback that contributes to the localization of conduits in hydrothermal systems.