Titanium‐in‐biotite thermometry in porphyry copper systems: Challenges to application of the thermometer

Empirical geothermometer dealing with Ti solubility in the Fe‐Mg biotites was originally proposed for biotites in graphitic, peraluminous metapelites containing ilmenite or rutile that equilibrated roughly at 4–6 kbar. Given that biotites are abundant in the porphyry copper systems, this geothermometer has frequently been used for the determination of magmatic–hydrothermal temperatures in the porphyry copper systems. Common associations of porphyry copper deposits (PCDs), that is, low Al content of biotite, biotite chloritization (causes the biotite to become more magnesian and to lose Ti), and biotite formation by amphibole replacement, as well as disequilibrium, local equilibrium, or re‐equilibration of biotites, especially through potassic alteration, may provide significant uncertainty in the temperatures estimated a by Ti‐in‐biotite geothermometer. In addition, besides the calibration range of thermometer for pressure (400–600 MPa), the temperatures of major sulfide precipitation in PCDs (>~400°C) does not fit with the temperature range of thermometer calibration (480–800°C). Worth noting, as confirmed by fluid inclusion data in the Sarkuh PCD, regardless of presence of mineralogical requirements, obtained temperatures of sulfide mineralization using Ti in biotite thermometer could be overestimated. This may be due to the difference between general conditions of sulfide mineralization and calibration range of Ti in the biotite thermometer for pressure and temperature, as well as the metaluminous nature of biotites in PCDs.