How abundant is high purity silica?
Quoting from this paper: Haus R., Prinz S., Priess C. (2012) Assessment of High Purity Quartz Resources. In: Götze J., Möckel R. (eds) Quartz: Deposits, Mineralogy and Analytics. Springer Geology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-22161-3_2

What purity level is required in naturally occurring silica in order to be used for manufacturing semiconductors?

As high as possible.

...high purity quartz with total impurity levels less than 20 ppm...

20 ppm means that the silica has to be 99.998% pure. It is extremely hard to find silica that pure in nature.
More specifically:

Specific requirements as to tolerable limiting values differ from
industry to industry. In the lamp tubing and optics industries
aluminium content in the refined quartz concentrate should not exceed
20 ppm, other metals should be less than 1 ppm, and total impurities
less than 30 ppm. For semiconductor base materials and crucibles
aluminium content should be even lower, specified to less than 10 ppm,
other metals less than 0.1 ppm, and total impurities not to exceed 15
ppm.
Feedstock for solar silicon used in the photovoltaic industry should
generally have boron and phosphorus contents in the sub-ppm range
since these elements are most difficult to remove and negatively
affect the performance of the solar cells (Geerligs et al. 2002).
For microelectronics applications, e.g. in epoxy moulding compounds,
uranium and thorium, both responsible for soft errors by alpha
radiation, should be less than 2 ppb, and in low alpha applications,
even less than 0.5 ppb.