Braidwood Geological Mapping Project
Project status
The first edition geological mapping of the Braidwood 1:100 000 map sheet was undertaken by the Geological Survey of NSW (GSNSW) in the 1970s. Geological mapping is nearing completion for the second edition Braidwood 1:100 000 map sheet which will be the GSNSW flagship project for production of a 3D geological model. Approximately 10 east-west oriented, balanced cross-sections have been compiled to aid 3D geological model construction. Cross-section validity will be tested via geophysical inversions prior to integrating the map and cross-sections into a 3D framework. The aim of the project is to run geophysical inversions on the cross-sections during the course of the geological mapping to help resolve geological and geophysical validation issues. The 3D model will undergo final validation and 3D geophysical inversion using the software package 3D GeoModeller.
Mapping during the 2007-08 financial year was focused on the completion of the northern half of the map sheet and the construction of five east-west cross-sections. At present, mapping is nearing completion for the southern half of the sheet and the validation and construction of a 3D model for the northern half of the map sheet has commenced. It is expected that 3D model construction will begin early 2009.
Geology
The Braidwood 1:100 000 map sheet area lies in the Southern Highlands of New South Wales which, geologically, forms part of the eastern Lachlan Orogen. The area is dominated by Palaeozoic age formations (Ordovician to Late Devonian) overlain in part by Cainozoic age basalt and sediment.
Ordovician basement
Early to Middle Ordovician turbidite sequences of the Adaminaby Group and to a lesser extent the thinly bedded turbidites and black shale of the Late Ordovician Bendoc Group constitute the oldest known rocks in Braidwood map sheet area.
In the Windellama-Oallen areas (north eastern corner of the map sheet), the exposed Adaminaby Group consists of the Lower to Middle Ordovician quartzose turbidites of the Abercrombie Formation which is particularly chert-rich. In this area, the Abercrombie Formation and its Lower to Middle Ordovician chert members are affected by regional scale thrust imbrication. A similar structural style is evident further south, around Mongarlowe, although here an Early Ordovician feldspathic-lithic sandstone package (Willigam Sandstone Member) is exposed at a stratigraphic level below the oldest chert horizons. Late Ordovician black shales and thin bedded turbidites of the Bendoc Group are exposed in the Windellama area (northern central area) and are fault bounded to the west against the Abercrombie Formation.
to west over east thrust
in the Abercrombie Formation
Siluro-Devonian volcanosedimentary sequences
The extensive Ordovician sequences are unconformably overlain by, and faulted against, middle to Late Silurian sequences in the map sheet area. A complex sequence of Silurian volcanic and clastic strata occurs along the western margin and central northern areas of the map sheet. The units were deposited within, and on the margin of a marine basin which rapidly opened in the Late Silurian and hosted deep water environments before shallowing significantly in the Early Devonian. Volcanic and sedimentary rocks deposited in the basin opening stage are host to VHMS mineralisation.
In the Woodlawn area (north western corner of the map sheet), the Late Silurian Woodlawn Volcanics can be divided into a lower sequence of crystal-rich mass-flow rhyolitic volcaniclastic rocks with discontinuous siltstone lenses, and an upper sequence of mostly flow banded rhyolite lavas. The rhyolitic volcanics are overlain by the Currawang Basalt and quartzose turbidites of the regionally extensive Covan Creek Formation. Dolerite intrusions occur throughout the area. The lower volcaniclastic sequence of the Woodlawn Volcanics is up to about 1 km thick and is host to the Woodlawn Pb-Zn-Cu-Ag-Au orebody. It has discontinuous limestone bodies at the base where it unconformably overlies Middle Ordovician Abercrombie Formation quartzose turbidites, the Late Ordovician Warbisco Shale and a package of siltstone with prominent sandstone layers which, lithologically, resembles the latest Ordovician Mundoonen Sandstone near Yass. The Ordovician sequence is extensively intruded by granitic sills and dykes related to the Ellenden Granite further to the south. The area is affected by numerous north to northwest trending shear zones which typically obscure stratigraphic relationships and apparently structurally thicken the lower sequence of the Woodlawn Volcanics in the vicinity of the Woodlawn Mine.
The Woodlawn Volcanics sequence grades into the De Drack Formation to the south and east of the mine area with a thinning of the crystal-rich volcaniclastics, decrease in extent of the rhyolite lavas, and increase in the proportion of siltstone and fine-grained quartzose to tuffaceous material. This relationship suggests that the Woodlawn area is proximal to a Late Silurian volcanic centre.
In the Bombay area (south western corner of the map sheet), the Early Devonian Long Flat Volcanics is divided into a sequence of porphyritic rhyolitic ignimbrite (Toggannoggra Rhyolite Member) that is fault bounded against crystal-rich dacitic pyroclastics (Kadoona Dacite Member) and locally intruded by dacitic to rhyolitic porphyry (Kain Porphyry Member). The volcanic sequences are locally overlain by conglomerate and quartzose sandstones that bear a strong resemblance the Late Devonian Minuma Range Group, although no definitive age constraints have been gained as yet. A series of regional scale, north-south oriented faults including the Shoalhaven and Mulwaree faults often obscure the stratigraphic relationships in this area.
In the northern central area, Silurian volcanic and clastic sequences unconfomably overlie the Abercrombie Formation as well as parts of the late Ordovician Bendoc Group. Preliminary mapping has revealed that the Silurian sequences consist of quartzose sandstones with occasional pods of limestone that are locally altered adjacent to intrusive bodies. The volcanic sequences exposed slightly further to the east include Early Devonian dacitic to rhyolitic ignimbrites of the Long Flat Volcanics.
Intrusive rocks
Ordovician, Silurian and Early Devonian sequences of the Braidwood area are variably intruded by a series of Late Silurian to Early Devonian granite to granodiorite plutons including the Ellenden Granite, Boro Granite and the Braidwood Granodiorite. A number of granite plutons within the eastern half of the map sheet area have previously been assigned to the Early Devonian Boro Granite, although significant variation is observed between plutons and further subdivision is imminent.
The Braidwood Granodiorite dominates outcrop in the central southern map area and displays intrusive contact relationships with Early Devonian volcanic sequences to its west and Ordovician sequences to its east. The Braidwood Granodiorite is divided into an eastern equigranular phase and a western hornblende phyric phase. The western phase of the Braidwood Granodiorite is host to several vein and disseminated gold and sulphide deposits south of the Braidwood map sheet. Isolated granodioirte plutons to the north display geophysical continuity with the main body of the Braidwood Granodiorite and are related to localised skarn mineralisation in the central north of the map sheet area.
A series of broadly east-west oriented brittle structures crosscut the Braidwood Granodiorite and provided a focus for the accumulation of alluvial gold deposits.
Locally, the Ordovician and Silurian sequences are unconformably overlain and faulted against the Early Devonian Tarago Conglomerate in the northeast of the map sheet area. The Tarago Conglomerate forms the base of a package of shallow marine rocks which includes the Bongalaby Formation and the Lake Bathurst Limestone Member.
in the shallow marine
Bongalaby Formation
Late Devonian fluviatile sequences
Late Devonian fluvial to shallow marine strata of the Minuma Range Group locally overlie the Early Devonian volcanic sequences in the southwest of the map sheet area. Late Devonian Merrimbula Group sequences which form the rugged Budawang Range, are faulted against Ordovician Adaminaby Group in the southeast.
Cainozoic strata
The youngest sequences exposed on the Braidwood map sheet area include alluvial gravel and alkali basalt that were deposited along Cainozoic river channels.
Mineralisation
A number of major mineral systems are located within or adjacent to the Braidwood project area. These include the Woodlawn volcanic-hosted, base-metal deposit and the Dargues Reef-Majors Creek intrusion-related gold system.
Preliminary studies by the GSNSW Minerals Systems Group suggest that several base-metal deposits in the Braidwood 1:100 00 map sheet area, including Mulloon and Boro, are orogenic in style rather than being intrusion-related or syngenetic. The majority of these deposits are associated with steeply-dipping, northerly-trending structures that cross-cut the regional foliations.
The gold endowment of intrusion-related mineralisation associated with the Braidwood Granodiorite, which hosts a number of zones including the Dargues Reef-Majors Creek system, exceeds 50t Au. Initial results from the Dargues Reef-Majors Creek suggest that there may have been a greater fluid flux and higher fluid temperatures at Dargues when compared with to Majors Creek.
with interstitial sulfide
(Fe-rich sphalerite)
from the Limekilns prospect
A number of base-metal rich skarn deposits are also present in the central northern part of the sheet around Mayfield and Limekilns, Preliminary sulfur isotope studies suggest that these skarns have similar isotopic signatures with the majority of sulfur being sourced from magmatic reservoirs, possibly equivalent units to the Braidwood Granodiorite.
The Mongarlowe gold field produced about 1.43 t Au with the majority of production being between the 1870s and 1910 from mainly alluvial sources. Metasedimentary rocks of the Adaminaby Group host auriferous low-sulfide quartz veins. These mineralised veins are short, discontinuous and include extension veins that trend ~105°, orthogonal to the dominant regional foliation. Mineralised veins may have been focused by the competency contrast between curvilinear mafic dykes of unknown age and the host sedimentary rocks. These deposits are interpreted to be orogenic in style.
Future work
Future work involves:
- Completion of geological mapping in the south of the map sheet area:
- Construction of remaining cross-sections
- Continued compilation of explanatory notes.
- Validation and Integration of map and cross-sections for the northern half of the sheet into the 3D environment.
- Integration of deposit scale cross-sections with located skarn deposits.
Additional products
For more information on the mapping project please contact the Geological Mapping Team.
(Last updated on 24/02/2009)