The Heemskirk granite massif, Western Tasmania : a study of chemical variability within plutonic rocks

The Heemskirk granite massif was intruded as a diapir about 354 m.y. ago into Precambrian and Palaeozoic sediments, it fractionated into two types the red and white granites, which are distinguished by their colour and mineral and chemical composition. It is suggested that they also differed in volatile content and oxygen fugacity. The characteristic mineral assemblage of the red granite (hematite - magnetite - allanite) indicates a higher oxygen fugacity than the white type which is characterised by tourmaline. Both rock types are biotite or two mica granites sensu stricto (Hietanen, 1963). Differences between the granite types have developed during and after emplacement of the intrusion. The granite body has a generally stratiform structure with subhorizontal layering and the upper part of the intrusion is characterized by a very flat dome. Its internal structure consists of several layers of granites with slightly different mineralogical, textural and chemical properties. Within each layer cryptic layering can be defined by the behaviour of the trace elements. The red granite occupies the topmost part of the intrusion and forms a comparatively thin layer in the eastern part of the body; it acts as country rock, having been consolidated earlier than most of the white granites. Towards the west the red granite is laterally re-placed by the white granite, which probably represents the major pert of the Heemskirk granite at depth. The roof zone of the white granite (contact zone with the red granite) is enriched by tourmaline nodules, which are interpreted as originally volatile segregations in a crystallizing magma. The white granite shows a higher degree of geochemical and mineralogical homogeneity. Interpretation of the granite composition in the light of experimental data suggests that the Heemskirk massif was emplaced at shallow depths in the crust. A temperature of 650°C is estimated for the crystallization of K-feldspars from the red granite. The surface of the body reflects on a subdued scale the anticlinorial structure of the original sedimentary cover. The existence of the ghost structure of the country rocks in the roof of the intrusion is also indicated by the distribution of the contaminates and positive anomalies of some trace elements (particularly within the red granite). The Heemskirk granite massif is characterized by very regular fracture systems. Extensive greisenisation and hydrothermal alteration follow the dominant cross joints. The filling of the fissures is quartz, tourmaline and commonly cassiterite in economic concentrations, and rare columbite. The majority of the tin deposits are found on the white/red granite contact. The abundance of Sn concentration at the present surface in the eastern part of the body is related to exposure of this contact and the depth of red granite cover. Where the red granite exceeds 600 feet in thickness tin mineralization is unlikely to be economic.