IGNEOUS REFINEMENT OF QUARTZ RAW-MATERIALS

Quartz raw-materials are often regarded as an “excessive” commodity that easily may be exploited. This may be correct for low- to medium-purity quartz commodities (>300 ppm impurities). However, intermediary- to high-purity qualities (2-300) are much more challenging to find. This fact agrees with the 100–500 fold value increase from low to high purity quartz (HPQ), i.e., HPQ obtain better profits than many base metal raw materials. Still, the factors controlling the distribution and speciation of trace element impurities in quartz are poorly defined. This study is a first approach to understand the formation of igneous HPQ. Studies of 130 igneous bodies imply that Li, B, Be, Na, Al, P, K, Ti, Fe, Ge are the most common trace elements in granitic quartz. Al, Ti and Li are most abundant, however, depending on T and the melt composition, K, Na, Fe and P may reach considerable concentrations. The temperature (T) is the most important factor. For example, in high T charnokitic melts, several hundred to >1000 ppm of trace elements is common whereas, in low T granitic pegmatites, concentrations rarely exceeds 150 ppm. Ti, K, Fe and Be are enriched in early formed quartz, i.e. in high T quartz. P and Al is complex whereas Ge, Li and Na are enriched in late formed low T quartz. Peraluminous melts generate quartz with relatively high Al-Li-K concentrations, whereas metaluminous melts produce quartz with relatively high P concentrations. In a simplistic way, the trace element speciation is buffered by the melt composition and the total concentration is buffered by T. Interaction with aqueous saline solutions may considerably alter primary igneous quartz. The Li and B concentration is strongly reduced whereas K and Na are enriched. Ti and P is generally not remobilised although rare exceptions are documented. HPQ qualities may particularly form in strongly differentiated granitic pegmatites that are derived from metaluminous melts. Subsolidus aqueous alteration may further enhance the purity of quartz. Our study also imply that quartz-analysis of only 10-20 pegmatite bodies in a pegmatite field suffice to decide if further prospecting is feasible.