5.0 THE METALLOGENIC DATABASE AND GIS
This study resulted in the division of all known mineral deposits in the Georgetown district into 55 metallogenic camps. A summary of the characteristics and classification of each camp is tabulated in the metallogenic database (Appendix 1). This new database is an expansion of the database originally prepared by Tate, 1987. A detailed digital map of the study region that contains geology, structure, dykes, veins, mine and deposit locations, camp boundaries and mineralisation styles was prepared as part of this study and can be accessed from the Queensland Geological Survey website.
One hundred and forty-seven historical mines and deposits were visited during the study. Notes on the nature of the geology, structure, mineralisation and alteration were made and samples collected for determination of quartz textures, age dating and trace element geochemistry. A summary of this data is provided as tables in this report and in the accompanying database.
General Classification Scheme
Overall, we have found the most useful features for classification of the camps is the overall environment of mineralisation, age represented as epoch, the quartz textures for depth of formation, multi-element geochemistry for class and the presence or absence of related intrusions.
- plutonic clan (intrusion-hosted but not intrusion-related) with Au-base metals;
- IRGS (Intrusion-Related Gold Systems) clan with genetic links to intrusions and magmatic-hydrothermal fluids and typical Au- Bi-Te polymetallic (Pb Zn Cu As Sb) geochemistry;
- the epithermal clan (formed near surface with little or no magmatic input), with Au-Ag ± Te, As-Sb, basemetals.
The related intrusion is any intrusive body that is demonstrably genetically related to the mineralisation. The best evidence is where dikes or plugs host mineralisation and alteration and occupy the same hydrothermal system, structure or area as the mineralisation. In addition, radiometric dating demonstrates dikes and alteration/mineralisation are of the same epoch. The best examples are in porphyry systems like the Kidston breccia pipe where dikes are pre, syn- and post mineralisation and are demonstrably the cause of the hydrothermal breccia that is the main host to mineralisation and where multiple radiometric ages are all 335+/-5Ma (Morrison, 2007).
In many examples there are intrusions in the same area and orientation as the mineralised structure, but the exact relations are unclear. In these cases, we use the texture of the quartz and the geochemical signature to evaluate intrusion involvement in the hydrothermal system. Typical examples are in the Percyvale District where a significant Permian dike swarm traverses the area with Early Devonian shear-hosted mineralisation. In most outcrops the dikes are seen to cut the shear and to not be mineralised, but in some cases, there are mineralised dikes and clasts of mineralised dike in the shear and fine comb quartz veins with Ag>Au geochemistry that is typical of the Permian dike mineralisation e.g. Carbon Copy.
In the camps where there are no intrusions that are demonstrably genetically related to the mineralisation, there may still be granitoids that host the mineralisation. In previous interpretations by others this may be taken to mean the mineralisation is magmatic, but in this study it is necessary to demonstrate age, quartz-type or chemical evidence that support a magmatic link. In the Georgetown region, the Meso-Proterozoic granites have related Sn-W-Nb-Ta mineralisation in pegmatites and are the most common host to the Devonian and younger gold mineralisation, but they have no genetically related gold mineralisation. Similarly, the Silurian granitoids host Devonian and younger mineralisation, but there is no demonstrated Silurian mineralisation.
6.0 AGES AND EPOCHS OF MINERALISATION
Twenty-seven new K-Ar ages have been completed on alteration minerals from deposits and combined with recent ages obtained by GSQ and published mineralisation and rock ages to interpret mineralising epochs in the Georgetown region (Table 6.1).
There are four Palaeozoic mineralised epochs interpreted from the radiometric dating in the Georgetown region (Figure 7):
- Early Devonian (EDEV) epoch spans 418-370Ma and includes most of the historic and recent gold producers in the Georgetown – Forsayth –Percyvale-Mt Hogan districts that have been linked to the Silurian granitoids historically but have no direct evidence of causative intrusive bodies.
- Early Carboniferous (ECARB) epoch spans 350-320Ma and includes scattered occurrences on the east side of the Newcastle Range that are demonstrably genetically linked to intrusions that are age equivalent to the early phases of the Newcastle Range and at least in part represent sub-volcanic complexes that may have been part of the same magmatic event (e.g. Lochaber-Bagstowe Complex linked to the Kidston deposit).
- Late Carboniferous (LCARB) epoch spans 320-290Ma and includes scattered occurrences near the Carboniferous-Permian volcanic complexes west of Georgetown and locally around the Newcastle Range. These complexes can contain units that span from early Carboniferous (~350Ma) and into early Permian <290Ma), but there seems to be three separate mineralised epochs here. The late Carboniferous deposits are intrusion-related (e.g., Mt Turner) and epithermal (Log Creek) and are associated with dikes and plugs that are sub-volcanic phases of those complexes.
- Early Permian (EPERM) epoch spans 287-270Ma and includes intrusion-related Ag-Cu (Phyllis May) and epithermal Ag-Au (Agate Creek) deposits associated with sub-volcanic intrusions in and near the volcanic complexes west and north of Georgetown and in a SE trending belt along the Robertson Fault from Greenhills to Gilberton.
A notable feature in the Georgetown region is the lack of mineralisation and spatially and genetically related to the Silurian granitoids (Figure 8) of the White Springs and Dido Supersuites of the Pama Igneous Association (Bultitude, Champion & Hutton, 2013). There has been confusion historically about the age of these supersuites with most historic Rb-Sr and K-Ar ages reporting in the Early Devonian, but more recent U-Pb zircon ages in the Silurian (430-420Ma). This is typified by the Dido Tonalite giving 431Ma by U-Pb zircon, 410 on K-Ar biotite and 415Ma on K-Ar hornblende (Table 4.1). This age difference is a general feature noted throughout the Pama Igneous Association, particularly at Charters Towers and ascribed to an Early Devonian tectonic-hydrothermal event overprinting the Silurian granitoids. A notable feature at Charters Towers is that there is pluton-level porphyry Cu-Mo mineralisation in the Silurian granitoids there but not so far seen at Georgetown (Morrison et al., 2016). This might be explained by the generally more felsic un-fractionated I-type granitoids at Georgetown compared with more mafic and fractionated granitoids at Charters Towers. However, in both provinces there is substantial gold mineralisation of similar character in the Early Devonian tectonic-hydrothermal event even though there are no demonstrably linked intrusions.
