48 
PROCEEDINGS OF THE ROYAL SOCIETY OF QUEENSLAND. 
Showing Occurrence of Best Mica in Basic Gneiss. 
Fig. 5. 
Dykes in fine-grained schists, and in medium and coarse mica- 
schists in Central Australia, however rich in muscovite they may be, very 
rarely contain any book-mica of sufficient size and quality to warrant 
mining. 
Coarse-grained wall-rocks, like coarse gneisses and amphibolites, 
favour the production of a coarse-grained pegmatite. Where a steeply 
inclined pegmatite dyke invades such rocks there is a tendency for large 
mica books to form alongside either wall, as well as along certain cross 
flaws in the body of the dyke. 
The dykes also have a considerable influence on any basic country- 
rock which they traverse. There is a great development of garnet and 
of calcite veins. These are produced by the pneumatolytic solutions 
permeating the country-rock. Garnetiferous gneiss so often lines the 
mica-rich dykes that garnet can almost be considered as indication. 
Highly potassic dykes which were charged with pneumatolytic solu- 
tions when injected, often metamorphose a selvage of the country-rock 
traversed into an all-mica rock, resembling a muscovite-biotite schist, the 
schistosity of which is parallel to the dyke. This selvage maj^ range from 
a few inches to several feet in width. It can be seen lining the walls 
of many of the best mica-mines in the Territory, including the Spotted 
Tiger, and can be said to be indication of the dyke ’s favourable chemical 
composition for mica-production. Probably the dyke intrusion rendered 
the original gneiss composing this selvage plastic. Potassic solutions 
from the dyke entered into combination with the plastic wall-rock and a 
complete rearrangement of minerals took place, the hornblende of 
amphibolite and biotite gneiss being changed to biotite and the felspar 
to muscovite, with the principal axis of the crystals at right angles to 
the dyke. 
Where mica-bearing pegmatites are not characterised by intermittent 
quartz blows along the outcrop there is usually a band of massive quartz 
near the hanging-wall side, and sometimes parallel bands within the dyke. 
This quartz deposition takes place as the vapours and solutions work 
their way into the country rock, escape of C0 2 being accompanied by 
deposition of Si0 2 . The various quartz bands may mark progressive 
stages in consolidation. 
