Rb‐Sr systematics of the Coolgarra Batholith, North Queensland

Abstract

The Coolgarra Batholith in north‐east Queensland is composed of the Go Sam and Nettle granite suites which were emplaced approximately 314 Ma ago. Initial ⁸⁷Sr/⁸⁶Sr ratios of the Nettle Suite appear to be marginally above 0.710, a value characteristic of a wide range of Late Palaeozoic felsic igneous rocks in the region. This suite appears to have been derived from an extensive isotopically homogeneous crustal source distinct from rocks which crop out in the Palaeozoic Hodgkinson Basin or Precambrian Georgetown Inlier. Higher and more variable initial ⁸⁷Sr/⁸⁶Sr ratios (0.727–0.744) of the Go Sam Suite are attributed to a combination of magmatic and hydrothermal processes. Isotopic data, thus, cannot be used to constrain the nature of the Go Sam source rocks. Tungsten (with or without tin) mineralization is associated with the Nettle Suite plutons, with which it shares a common age, whereas the Go Sam Suite is characterized by prominent tin mineralization. One tin occurrence is the same age as, or only slightly younger than, the Nettle Suite tungsten (with or without tin) mineralization. Another tin deposit yields an age of 301 Ma, suggesting the presence of a significantly younger granite at depth.

Key words:

• tin granites
• Rb/Sr systematics
• isotopic ages
• initial ratios
• Herberton‐Mt Garnet tinfield
• carboniferous granites
• North Queensland