New insights into the structure and subdivision of the Ravenswood Batholith ? a geophysical perspective

Abstract

A significantly improved geological and structural map of the Ravenswood Batholith is being developed by the integration of high-resolution magnetic and radiometric data with detailed geological information acquired from 1986 to 1991. Some new geological models being proposed are a direct result of geophysical input. The Ravenswood Batholith evolved during three main periods of intrusion spanning the Middle Ordovician to Early Permian. Most of the plutons of the first two intrusive periods are relatively shallow, sheet-like bodies, probably emplaced through a buoyancy-controlled mechanism. However, some plutons in the southwest of the batholith tend to be deep-rooted diapiric intrusives. The third main period of plutonism (Late Carboniferous-Early Devonian) comprises felsic stocks and ring complexes, and mafic to felsic dyke-like complexes which were intruded into the cooled Ordovician-Devonian granitoids. Almost all of the granitoids are magnetite-bearing, and aeromagnetic anomalies associated with the plutons vary considerably owing to the effects of remanent magnetisation as well as changes in magnetic susceptibilities. A general relationship between pluton ages and their associated aeromagnetic anomalies is established within the Ravenswood 1:100 000 sheet. Potassium, thorium and uranium variations within and between individual plutons are clearly illustrated by high-resolution radiometric data. Predominant trend directions of geophysical lineaments in the Ravenswood Batholith are northeast, northwest and less commonly north-south. Some of the major aeromagnetic lineaments appear to have controlled the emplacement of plutons within the batholith. A set of parallel northeasterly trending magnetic lineaments extends for up to 200 km and reflects major, probably pre-existing structures. Mineralisation within the Ravenswood Batholith is almost completely represented by gold ? the Charters Towers and Ravenswood Gold Fields, between them, having produced in excess of 200 t of gold ? with seven mines recently active. The source of the gold mineralisation in the eastern portion of the batholith is unknown but some of the major deposits can be spatially and temporally related to major structural features and to rocks emplaced during the final Late Carboniferous-Permian intrusive phase of the Ravenswood Batholith.