The determination of alteration extent using minero-petrographical, geochemical, and geomechanical properties in granitic rocks from the Tavşanlı Zone (NW Turkey)

The effects of alteration on granites outcropping around Sivrihisar area within the Tavşanlı Zone (NW Turkey) were investigated considering minero-petrographical, geochemical, and geomechanical properties. For this aim, polarizing light microscope, scanning electron microscope, X-ray diffraction, chemical analyses, and geomechanical tests were conducted on the granitic samples. The new petrographic index, referred as feldspar alteration index, was developed from the quantitative ratios of total and altered constituents in feldspar minerals. Based on solution-precipitation processes, plagioclase and alkali feldspar were altered to smectite, sericite, and kaolinite along microfractures, cleavage, twinning, and zoning structure. Anorthite-rich core is more altered than the rim of zoned plagioclase due to higher solubility in water and selective alteration of calcic plagioclase. EDS studies indicate that Na, Si, and Ca decrease and Mg, Al, K, and Fe progressively increase. The normalized values of whole-rock chemical analyses of the granites show depletion of Si, Al, Na, and Ca reflecting the dissolution of plagioclase. Enrichment of Si, Al, Na, Ca, Mg, K, and Fe reveal precipitation of smectite, sericite, and kaolinite minerals during alteration. Minero-petrographical and geochemical changes led to an increase in water absorption and apparent porosity and a decrease in apparent density, P-wave velocity, Schmidt hammer rebound, and compressive strength. Significant relationships between FAI and geomechanical properties (R2 = 0.7116–0.8676), FAI and P-wave velocity index (Ivp) (R2 = 0.8676), and FAI and strength ratio (Rs) (R2 = 0.7614) demonstrate that FAI can be used to determine the alteration degree of granites.