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Gympie
Queensland, Qld, Australia
Main commodities: Au


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The Gympie gold field is located within the town of Gympie, 177 km north of Brisbane in south-east Queensland, Australia (#Location: 26° 12' 53"S, 152° 41' 16"E).

Alluvial gold was discovered at Gympie in 1867, with subsequent, mainly hard rock, mining activity being continuous for 60 years to 1927 and by 1930 most of the mines of the field had closed. The gold field is some 10 km long by 3 km wide and between 1867 and 1930 produced 105 tonnes of gold, predominantly from lode deposits. In 1969, the fragmented mining tenements in the field were amalgamated into the Gympie Eldorado gold mine. Modern exploration began between 1980 and 1995, first by Freeport, then by BHP in joint venture with Gympie Eldorado Gold Mines (GEGM). In 1995 GEGM started a small-scale mining operation with the objective of providing the basic mining infrastructure and cash flow to fully explore the goldfield. This involved dewatering and reopening some of the old shafts and underground workings. However, by 2004, the mine was sold after GEGM went into receivership. The new owner, Gympie Eldorado Mines Ltd wound down the operation soon after and put it into care and maintenance.

The hard rock mineralisation of the gold field comprises of an extensive, 'mesothermal' quartz vein system hosted within the Late Permo mafic to intermediate island arc volcanic and sedimentary rocks of the Gympie Group within the New England Fold Belt. The Gympie Group sits on a basement of deformed Devonian deep marine, basalt, chert and sediments called the Amamoor Beds. These have been intruded by mid- to late-Triassic granite and diorite, although the nearest intrusion to the Goldfield lies 6 km to the south east. The field is also intruded by dolerite dykes of probable Triassic age.

At Gympie, the host Late Permian Gympie Group, comprises, from the base:
• Highbury Volcanics - >350 m of agglomerate, amygdaloidal basalt and basaltic andesite;
• Rammutt Formation - 700 to 1000 m of volcanic conglomerate, andesitic and basaltic lava, tuff, agglomerate, feldspathic arenite and carbonaceous shale;
• South Curra Limestone - up to 140 m of bioclastic calcarenite with carbonaceous mudstone, mudstone and siltstone; and
• Tamaree Formation - at least 600 m of shale interbedded with fine lithic arenite.

All of the quartz lodes are structurally controlled and hosted by the upper 400 m of the Rammutt Formation. No gold has been recovered from the Highbury Volcanics or the lower Rammutt Formation or South Curra Limestone. Most of the high-grade mineralisation occurs where the quartz veins are in contact with carbon-rich sediments, historically described as 'The Productive Beds'. The Productive Beds, which are developed over a stratigraphic thickness of up to 60 m, lie above the volcanic units and dip from 10 to 45°E.

The gold field occupies the strongly faulted core of a partly dismembered domal structure.   The volcanics and the Rammutt Formation were deposited near a volcanic centre possibly controlled by one of the long lived fault structures of the gold field.

The mineralisation at Gympie occurs as low sulphide, quartz-carbonate veins that are often associated with carbonate-altered dolerite dykes. The gold occurs as free grains, which can be very coarse. Isotopic dating of the mineralisation gives a Triassic age (Cunneen and Levy, 2001).

There are four distinct styles of quartz vein at Gympie (Cunneen and Levy, 2001), namely:
i). Gympie vein quartz lodes which produced high grade shoots developed preferentially where the lodes cut carbonaceous units of the upper Rammutt Formation. These are narrow, 0.2 to 2.0 m thick tension veins that strike north-south and dip steeply to the west, perpendicular to bedding. These reefs can be 1 to 2 km long. In the carbonaceous zones, the veins carry coarse free gold and often contain minor galena, sphalerite and chalcopyrite. Historically they averaged 34 g/t Au when in contact with the carbonaceous sediments and contained bonanza pods with several percent gold. Gympie Veins often come in groups of several veins <100 m apart. Individual veins have produced over 3 tonnes of gold.
ii). Break-style lodes, which are bedding-parallel shears filled by quartz and enriched in graphitic carbon, often found at the top and bottom of the carbonaceous shales of the 'Productive Beds'. The quartz was injected into these flat shears where another more vertical vein came into contact with the break. The steep Gympie vein quartz appears to be diverted from its usual steep path into the shallower 'break' (carbonaceous shear), forming a jog, before resuming its normal steep orientation. Break-style quartz may run along the intersection of the vein with the carbon-rich shear as a narrow ribbon for several hundred metres. These break-zones of quartz have produced spectacular specimens, but limited tonnage, although, where they occur in conjunction with other vein styles, they improve the grade. In the renovated Monkland Mine, break-style ore represents only 15% of tonnes mined but produces over 20% of the gold.
iii). Inglewood lodes which are northwest striking, stacked, sub-vertical structures that are 1.8 to 6.0 m thick with gold grades that can locally be >30 g/t Au, but averages 6 to 10 g/t Au. They occur as laminated quartz vein lodes hosted in a quartz-dolerite dyke, and are composed of vein quartz, usually containing brecciated volcanics of the Rammutt Formation and dolerite (from dykes) within the NW trending, near vertical Inglewood Fault. They contain free fine gold (with minor amounts of copper and arsenic). The total thickness of the quartz and dykes within the structure is from 10 to 20 m and extends over a strike-length of 2 km with a vertical extent of >800 m. Continuous ore grade shoots within the Inglewood structure may have strike lengths and vertical extents of 100 to 800 m and are usually terminated by late-stage dykes that offset the lode channel by 2 to 20 m across strike. The mining widths of the lode channels range from 1 to 7 m averaging ~2.5 m. The free gold is fine-grained and generally not visible to the naked eye. The Inglewood structure does not require the carbonaceous stratigraphy to produce ore grades. Two of the largest mines, the Scottish Gympie and No. 2 South Great Eastern mined Inglewood lodes.
iv). 'Stockworks', where numerous thin, sub-parallel veinlets filling tensional zones with the same orientation as the Gympie style veins are sufficiently densely developed to permit bulk mining. These occur over widths of up to 100 m and lengths of up to 500 m, with bulk grades of 8.0 to 12.0 g/t Au. Like the Gympie Veins they require the carbonaceous stratigraphy to maintain grade and therefore have a limited vertical thickness of up to 60 m. Individually the thin stockwork veins are less than 10 cm wide but have much higher grades than the larger Gympie Veins.

Alteration associated with either vein style is only weak, comprising patchy chlorite, hematite, and epidote and has not been conclusively connected with the gold mineralisation.

In addition to the 105 t of gold mined from 1867 to 1930 (Kitch and Murphy, 1990), ~15 t of gold was extracted between 1995 and 2004 (Aurum Resources website, viewed 2023). In 1990 there was potential for an additional 31 t of Au over a strike length of 1300 m between the depths of 600 and 1000 m with lodes averaging 2 m in thickness and carrying 12 to 16 g/t Au, based in part on historic production and grades (Kitch and Murphy, 1990).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this decription was dated: 2001.     Record last updated: 28/6/2023
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Gympie

  References & Additional Information
   Selected References:
Cunneen, R. and Levy, I.,  2001 - The Gympie Gold Story: Exploring and Reopening A Historic Goldfield In Todays Business Reality.: in    SMEDG - AIG SYMPOSIUM 2001 with ASEG, Smith Auditorium, Shore, Blue Street, North Sydney, Friday, 27th April, 2001.   https://smedg.org.au/old_site/Sym01RC.htm, 3p.
Kitch R B, Murphy R W  1990 - Gympie Gold field: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v2 pp 1515-1518


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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