Geoscience Australia, in collaboration with the relevant State Geological Surveys and Phil Blevin of Petrochem Consultants, started a new project in 2003 focussed on the Tasmanides of eastern Australia. The project will provide a synthesis of publicly available data sets to assess the metallogenic potential of major Australian Phanerozoic granite suites and their associated host rocks. These datasets are seen as key ingredients for better definition and targeting of potential granite related gold (Au) base mineralisation in brown and greenfield areas of eastern Australia. The data also will be important for explorers investigating new deposit styles, in particular the intrusion-related gold model.
Figure 1. Geological coverage of Stages 1 (North Qld) and 2 (Southern Qld). Select image for a larger version [210KB]
A significant proportion of the mineral endowment of eastern Australia is related to Phanerozoic granites and comagmatic volcanic rocks. Accordingly, considerable scientific research and data gathering has been focussed on these magmatic systems over the past 15 years. A major outcome¹ has been the recognition that the nature and style of mineralising systems associated with these felsic rocks, including porphyry copper-gold (Cu-Au) and copper-molybdenum (Cu-Mo), vein style tungsten (W), molybdenum (Mo), tin (Sn), and gold (Au) varies with both the mineralogy and chemistry of the related magmatism.
However, it is recognised increasingly that the development of mineralisation is controlled by many factors other than just the chemistry and intensive parameters intrinsic to the magmatic systems. Wall rock controls, such as oxidation state, competency, porosity and structural preparation are considered critical for a fertile system. The mineralogical composition of the host rocks also appears important and certainly plays a significant role in metal precipitation, e.g., many deposits are hosted by rocks rich in reductants such as magnetite, graphite and/or sulphide. Interestingly, empirical data also indicate that deposit types may be host rock specific, e.g., Haynes² suggests iron oxide Cu-Au and porphyry Cu deposits are found in settings where host rocks contain little or no reduced carbon minerals (e.g., graphite). Conversely, Wyborn³ suggests Au only deposits occur where host rocks are carbonaceous.
The potential for host rocks to play a significant role in the mineralising environment is clear, even more so when it is considered that mineralisation may occur up to three kilometres from the granite body. Unfortunately, previous studies of magmatism and related metallogenesis in eastern Australia have largely ignored country rocks and their role beyond simple criteria such as level of emplacement and level of exhumation.
To address this shortcoming Geoscience Australia, in collaboration with the relevant State Geological Surveys and Phil Blevin of Petrochem Consultants, is undertaking a comprehensive data collation and synthesis program on intermediate to felsic intrusive rocks and their associated host rocks. The goals of this project are:
The documentation of the spatial and temporal evolution in magmatism over time will provide an improved framework for understanding the known distribution of mineralisation. This, coupled with similar studies of host rock data, will assist in the prediction and better targeting of mineralised sites for a range of commodities and newly recognised classes of deposits, in particular intrusion related gold deposits8,9.
Methodology for this project is based on previous Geoscience Australia regional studies4,5 and industry sponsored projects (AMIRA P147B, P425, P426, P515). The igneous unit dataset (GRANITE) will include a wide range of mineralogical, geophysical, geochronological, chemical, metallogenic association and classification (suite, supersuite) data considered necessary to characterise each unit and its metallogenic potential (Table 1, left). Data being collated for the country rock dataset (HOSTROCK) include lithological, mineralogical, geophysical, geochronological and metamorphic parameters (Table 2, right).
All units, igneous and host rock, are uniquely identified by stratigraphic unit number (STRATNO), which relates to Geoscience Australia's Australian Stratigraphic Names Database, the primary national standard for formal and informal geological names. Datasets from the project will be linked to any digital dataset which contains STRATNO, independent of scale.
Table 1, left. Parameters for the GRANITE data table. Select image for a larger version [103k]
Table 2, right. Parameters for the HOSTROCK data table. Select image for a larger version [64k]
The 2003/2004 program concentrated on Proterozoic and Phanerozoic granites and associated rocks from the north Queensland region (Fig. 1). This region was chosen for a number of reasons. Firstly it capitalises on the extensive previous work undertaken in this area (e.g., the various AMIRA projects and studies by Geoscience Australia and Geological Survey of Queensland), and on the recent comprehensive synthesis of geoscience information for this region, encapsulated in the North Queensland Geology volume6. Secondly, the region hosts significant mineralisation of various styles, including those interpreted to be intrusion-related gold deposits, e.g., Kidston7. Outputs from this first phase will include GRANITE and HOSTROCK data sets.
In conjunction with Mineral Resources Tasmania and Phil Blevin from Petrochem Consultants, a similar project is being undertaken for granites and host rocks of Tasmania. Initial outputs will be via Minerla Resources Tasmania. All data will be incorporated into the present project as they become available.
A similar Phanerozoic granites data collation project is being undertaken in New South Wales by the Geological Survey of New South Wales in collaboration with Phil Blevin.
The major focus of the 2004/2005 program was the Phanerozoic granites and associated rocks from southern Queensland (Fig. 1). This phase of the project built on the recent new regional geological mapping of this part of Queensland undertaken by the Geological Survey of Queensland and the concomitant increase in geological understanding. This will result in completion of the coverage for Queensland.
Enhanced understanding of Australia's mineral potential and metallogeny for the promotion of exploration and land use decisions. Increased exploration activity in, and promotion of, the Tasmanides for mineral exploration.
Geoscience Australia Record 2001/12 was the result of 'The Metallogeny of Australian Proterozoic Granites' project undertaken in 1996/1997 by the then Australian Geological Survey Organisation in collaboration with 20 minerals exploration companies.
The original project was a data-driven exercise which asked:
'Where hydrothermal Au, Cu, Zn, Pb, Sn, W, and Mo mineralisation occurs within five kilometres of the boundaries of Proterozoic granites, are there any specific characteristics of either the granites and/or their host rocks?'
The project compiled data on the mineralogy, geochemistry (~7,500 analyses), and age of Proterozoic granites and felsic volcanics as well as the age and mineralogical composition of sediments within five kilometres of Proterozoic granite boundaries for 20 provinces. The project investigated a spatial association only and was not considered of major importance if the metals came from the granite or had been leached from the country rock by processes related to granite emplacement.