Geology and mineralogy of the pyrophyllite deposits south of Manuels, Avalon Peninsula

The Late Precambrian (Hadrynian) Harbour Main Group of the eastern side of the Central Block of the Avalon Platform of Newfoundland consists predominantly of acidic volcanic rocks which are intruded on the west by quartz monzonite of the Holyrood Plutonic Series. Pyrophyllite deposits, containing mainly pyrophyllite, sericite and quartz, occur in the acidic volcanic rocks close to the Holyrood granite - Harbour Main contact. The deposits are localized by fault and shear zones. They were studied in the field in relation to the host rocks and in the laboratory by means of an X-ray diffraction technique involving the correlation of peak-height ratios with percentage ratios of mineral pairs. The percentages of pyrophyllite, sericite and quartz can be calculated from these ratios. This method can be adapted to a routine industrial analysis. -- The alteration process responsible for the formation of the pyrophyllite deposits was one of metasomatic replacement by late stage hydrothermal solutions related to the Holyrood granite intrusion. The replacement, contemporaneous with deformation, involved the introduction of Si, K, and possibly Al, the movement of K, Na and Si within the alteration zones, and the migration of K, Na, Si and Mg away from the zones. The alteration took place at a total pressure of 1000 bars, a fluid pressure probably somewhat below this, and at a temperature below 380°C, possibly approaching 270°C