Queensland Geology 11

Geology and mineralisation of the Texas region, south-east Queensland

This report describes the geology, mining history, and mineralisation of rock units in the Queensland portion  of Allora (and western part of Warwick), Inglewood, Texas (and north-eastern part of Ashford), and Stanthorpe (and western part of Drake) 1:100 000 map sheets which cover the bulk of the Texas region in south-eastern Queensland. The area forms part of the Woolomin and Silverwood Provinces within the central part of the New England Orogen.

The Woolomin Province comprises Palaeozoic rocks of the Texas and Silver Spur Subprovinces, as well as igneous rocks of the Stanthorpe Batholith. The Silverwood Province contains the constituent formations of the Silverwood Group, a ?Cambrian to Devonian package of island arc volcanic and sedimentary rocks. These rocks together with the accretionary wedge rocks of the Texas beds (belonging to the Texas Subprovince of the Woolomin Province) were accreted to the craton margin during Late Devonian to Late Carboniferous Andean-style subduction at the eastern margin of the Australian continents. The Texas beds sediments are rich in felsic volcanic detritus derived from the active magmatic arc to the west.

During the Late Carboniferous, the tectonic setting changed from a purely convergent plate margin to a combination of transform faulting and subduction. The site of subduction moved to the east. The rocks of the now inactive accretionary wedge were folded into a large double oroclinal structure, the western portion of which forms the Texas Megafold. Back-arc extension and also possibly orocline-related transtension produced a number of Early Permian basins of the Silver Spur Subprovince, developed on the older deformed accretionary wedge. Immature diamictite-rich volcaniclastic sediments and minor felsic volcanics were deposited in these basins. During this time a felsic pluton the (Bullaganang Granite) was intruded into the core of the Texas Megafold, followed later in the Early Permian by intrusion of the Greymare Granodiorite.

After the period of Early Permian sedimentation, the region was affected by uplift, faulting and local folding related to the first phase of the episodic Hunter-Bowen Orogeny. This uplift was succeeded by post-orogenic Late Permian mainly felsic volcanism, and sedimentation (concentrated mainly across the border in New South Wales) produced a range of rock packages assigned to the Wandsworth Province. The volcanism was accompanied by the emplacement of numerous co-magmatic granitic plutons ranging from small stocks to large composite batholiths. The latter continued to be intruded until the Early Triassic as dominantly hornblende-bearing I-type plutons assigned to the New England Batholith.

The most extensive bodies of granite occur in the Stanthorpe area, where they are mapped as the Ruby Creek Granite, the Ballandean Granite, and variants of the Stanthorpe Granite. With the exception of the Ballandean Granite these granites are post-orogenic leucogranites. The Ruby Creek Granite is considered to be the source of most of the tin-tungsten-molybdenum mineralisation in the area. The extent of this pluton is difficult to map throughout the area.

Large volumes of quartz-rich sand were derived from the uplifted and unroofed granitic rocks in the Latest Triassic to Middle Jurassic and were deposited by an extensive stream network to form the basal units of the Clarence-Moreton and Surat basin. Tertiary deep weathering and concentration of heavy minerals in stream channel alluvium resulted in the formation of rich alluvial cassiterite deposits.

Mineralisation in the Texas region in Queensland includes tin, tungsten, arsenic, molybdenum, bismuth, gold, silver, copper, lead, zinc and manganese. Limestone, marble, building stone, and gemstone deposits also occur in the area. Tin was the main commodity mined in the Stanthorpe area. Alluvial cassiterite deposits in the Stanthorpe area and minor lode deposits at Sundown, Mineral Hill and Sugarloaf have produced 56 537t of cassiterite concentrates since 1872. The entire arsenic production of Queensland (2290.5t of arsenic oxide and 2150t of arsenic concentrates) came from the State Arsenic Mine at Jibbinbar and from mines in the Sundown area.

The first gold deposits in Queensland were discovered in the south-east of Warwick in 1852 and further productive reef and alluvial fields were established west of the town. The main period of production was from the late 1800s to early 1900s, but sporadic small-scale activity continued on these fields until 1998.
Some notable base metal deposits were also developed in the region, the most important of which was the Silver Spur Mine, discovered east of Texas in 1890. The mine was worked most intensively from 1892-1914, with some sporadic production until 1976. Total production was 68t of silver, 1950t of lead, 690t of zinc, and 140kg of gold.

Sporadic mining of manganiferous jasperoidal bodies in the Texas beds has occurred since the late 1800s. Most of the major manganese mines (War Effort, Mount Fuller and Mount Devine) are located on Inglewood, although some small deposits (Mount Gammie, The Glen) also occur to the east on Allora.
Mineral deposit types in the study area include: epithermal to mesothermal reef gold, strata-bound manganese and late stage alteration-related magnetite accumulations in the Texas beds; tin, tungsten, molybdenum, arsenic and base metal deposits related to the Ruby Creek Granite; strata-bound and shear-hosted base metal sulphide deposits in Early Permian units; and alluvial/ eluvial placer deposits.

Mineral exploration since 1966 has been mainly for large tonnage, low-grade tin deposits, alluvial tin deposits, and precious metal-base metal sulphide deposits and industrial minerals such as building stone. Significant, but subeconomic, discoveries include sheeted Sn vein systems in the Sundown and Sugarloaf areas.

Sheeted vein systems offer the best potential for hard rock tin deposit discoveries. Palaeogravels of probable Tertiary age have potential as sources of alluvial tin. Permian sedimentary and volcanic units are targets for precious and base metals.

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