Constraining geological conditions for high-purity quartz formation
|Location||International Geological Congress,oslo 2008|
|Author||Ihlen, Peter M.|
|Holding Date||29 September 2008|
This presentation summarises some important results from a nationwide survey of high-purity quartz (HPQ) resources in Norway based on laser ablation ICP-MS analysis of lattice-bound trace elements coupled with SEM-CL studies. Quartz containing <25 µg•g-1 Al and <10 µg•g-1 Ti and up to a few µg g-1 of P, B or Fe is regarded by NGU as HPQ. In contrast to other impurities lattice-bound trace elements substituting Si4+ or occupying interstitial space in the quartz lattice are very difficult to remove by mineral processing. Thus, ’quartz quality’ can conveniently be presented as the total sum of substituting elements.
As per today, NGU has assessed about 500 occurrences of various types of quartz-rich rocks. Weakly deformed syn- to late-kinematic granitic pegmatites in the Caledonides of northern Norway and in the Sveconorwegian domain (~0.9-1.1 Ga) of southern Norway contain predominantly primary igneous quartz with high concentrations of lattice-bound trace elements. Quartzites of medium metamorphic grade mainly consist of medium- and low-purity quartz with a large spread in composition. Finally, all types of quartz-rich rocks affected by high-grade metamorphism (e.g. in the Sveconorwegian of southern Norway) lack HPQ potential, including hydrothermal quartz veins.
However, hydrothermal quartz deposits formed at low to medium metamorphic grade are frequently composed of HPQ. The importance of hydrothermal alteration of quartz to form HPQ deposits is demonstrated by the Sveconorwegian pegmatites of the Haugalandet area in southwest Norway. Towards the Caledonian Hardangerfjord fault zone representing a major fluid conduit, the impure primary pegmatitic quartz is progressively replaced by HPQ along micro-fractures and grain boundaries. Close to the fault and second order shear zones with most extensive fluid-rock interaction, HPQ pegmatites occur together with large HPQ veins and breccias. In contrast to pegmatites, meta-sedimentary quartzites generally appear less susceptible to fluid-induced purification. Until present, no large volumes of HPQ raw material have yet been identified in such rocks.
Kyanite quartzite deposits occurring locally along the Palaeoproterozoic Transscandinavian Igneous Belt do contain a substantial volume of HPQ. The quartzites supposedly represent metamorphosed argillitic alteration (quartz-kaolinite-pyrophyllite) of felsic precursors, related to shallow hydrothermal systems and hot springs along former volcanic arcs. However, the commonly fine grain size of quartz in deposits developed at shallow depth poses challenges in mineral processing.
In conclusion, the most promising targets for HPQ raw material are hydrothermal quartz deposits. Quartz-rich pegmatitic rocks in medium-grade metamorphic terranes of poly-stage orogenic belts are another potential source, as long as they represent intrusions emplaced prior to major episodes of tectonic and hydrothermal activity.