The pegmatite’s of Mica Creek occupy a roughly triangular area at the northernmost extremity of a large pluton known as the Sybella Granite . They intrude units of the surrounding Haslingden Group country rocks, in particular amphibolites of the Eastern Creek Volcanics, and are al so locally abundant within the Sybella Granite itself.
Investigations show that the majority of pegmatites were late stage differentiates from the Sybella Granite and themselves followed a complex crystallisation sequence culminating in the development of unusual textures, exotic mineral phases and at least four replacement assemblages. By contrast, a limited number of small pegmatitic bodies within the Sybella Granite are attributed to partial melting phenomena operating in conjunction with regional deformation.
The mineralogical and textural diversity of the Mica Creek pegmatites has enabled them to be subdivided into eight distinct "types ". Al l are essentially granitic in composition, although a small number have been extensively altered by hydrothermal solutions.
Palaeotemperature estimates for the final crystallisation of the Sybella Granite and partial melt pegmatites are less than 400 C , while the crystallisation of the major pegmatite varieties commenced at approximately 660º C and continued through to about 350º C .
Trace element abundances indicate that the Sybella Granite and Mica Creek pegmatites were generated in an intracratonic tectonic setting .