PETROGENETIC SIGNIFICANCE OF TRACE ELEMENTS IN IGNEOUS QUARTZ

The trace element evolution of igneous quartz was studied in granites that formed at 900 to 480 oC and 6 to 1 kb in charnokites, hornblende and biotite granites and granitic pegmatite.

Several hundred samples of quartz were in situ analysed with the LA-HR-ICP-MS technique.

The most important trace elements are (in order of concentration by weight) Al, Ti, P, Li, Na, K, Fe, Ge, B and Be and the total trace element contents vary from hundreds of ppm in high and medium T granitic systems to generally less than hundred ppm in granitic pegmatite.

Only Ti and Ge follow a consistent evolutionary path throughout all types of granite quartz. The concentration of Ti is falling and Ge rising. Accordingly, the Ge/Ti ratio is a strong index of igneous evolution much like the Rb/Sr and Mg/Fe ratios. We are currently calibrating the Ge/Ti ratio to a universal geothermometer for granitic rocks.

Al, P, Li, Na, K, Fe, B and Be is directly buffered by the composition of the granitic melts. Accordingly, the distribution and concentration of these elements monitor the contemporary petrogenetic processes in the magma chamber. E.g. Al in quartz is increasing in peraluminous melts at progressively lower temperatures. The concentration of P in quartz may also increase however, if apatite begins forming, P in quartz is abruptly falling. Li in quartz will typically increase with falling T, however, if an aqueous phase is forming, Li, together with the other alkalies, may partition in to the aqueous phase, and Li in quartz will fall.

Deuteric alteration has profound effects upon earlier formed quartz. At medium to low salinities, Ti, Ge, P and Be are immobile whereas the alkalies, B, Fe and Al may either increase or decrease. If the aqueous fluids are strongly saline Ti, Ge and P is mobilised and, together with all other trace elements, are strongly depleted under the formation of economically attractive high purity quartz.

Quartz is one of the last igneous minerals that entirely recrystallized and where alteration has obliterated all primary igneous phases, quartz may be partially intact preserving the igneous signature of the granite.