Newcrest Mining Limited, Level 2, 349 Coronation Drive, Milton, QLD 4064.
Key words: gold, epithermal, Klondyke, Royal Shoot, Cracow
The recent discovery of high-grade epithermal gold mineralisation at Cracow in south-east Queensland has resulted from targetted area selection, persistence in exploration and continued management support. The area selection process involved the identification of a large hydrothermal gold system which had the potential for economic gold mineralisation beyond that exploited by earlier intermittent mining since 1932. Over the four years leading up to this discovery, a stable project management team planned and implemented the exploration programs, which, together with key drillcore observations made by site geologists, convinced management to persist with deep exploration drilling.
The Klondyke gold mineralisation is situated within the Cracow Goldfield, approximately 350km north-west of Brisbane. Gold was first found in the Cracow district in 1875, however payable gold was not discovered until 1931. In 1932, the Cracow Goldfield was proclaimed and gold production from underground and open pit operations proceeded intermittently until 1992. A total of 850,000 ounces of gold was produced from the Cracow Goldfield with the majority (800,000oz) from the Golden Plateau mine. The recent gold discovery at the Klondyke Prospect occurred 2km west of the Golden Plateau mine beneath the historical Royal Standard and Klondyke workings (Figure 1). The gold mineralisation occurs in steeply dipping quartz-adularia-carbonate epithermal veins, hosted in shallow dipping, early Permian andesitic lavas and epiclastics of the Camboon Volcanics.
In the mid 1990’s, Newcrest Mining Limited (NML) approached area selection for gold exploration in Queensland by focussing on known gold hydrothermal systems which offered potential for deeper economic gold mineralisation. This exploration philosophy was influenced by NML’s recent exploration success at Cadia and Telfer where deeper drilling had shown that the hydrothermal systems had considerable depth potential. Several gold hydrothermal systems in Queensland were targetted for data review including the Cracow area. In early 1995, NML approached Sedimentary Holdings Limited (SHL), who at that time held exploration tenements over the Cracow area. After a review of the past exploration data, a joint venture was formed to explore for gold deposits in the Cracow Goldfield. The Cracow Joint Venture (CJV) comprises 70% NML and 30% SHL, with NML being the manager of the CJV.
Exploration between January 1995 and October 1997 in the Cracow district
targetted large tonnage gold deposits.
Geological mapping and surface geochemical sampling conducted over the Cracow Goldfield indicated a broad area (20sq km) of sericite-quartz-pyrite alteration to the immediate east and south of the Golden Plateau mine. This area had been identified by previous explorers but had not been drill tested or systematically sampled. A program of helimagnetics and surface geochemical sampling showed the quartz-sericite-pyrite alteration to be anomalous in Mo-Bi-Au and spatially related to intrusives which outcrop to the south-east of the Cracow Goldfield. The alteration was interpreted to be of a higher temperature porphyry style. A program of widespaced, shallow, reverse circulation drilling was undertaken, aimed at identifying near surface, large tonnage gold mineralisation. The results from this program were discouraging with no broad intervals of gold mineralisation encountered in the drilling.
The second exploration phase between October 1997 and June 1998 targetted small tonnage, high grade, epithermal gold deposits.
In 1997, information on the Vera-Nancy discovery in North Queensland was disseminating throughout the exploration industry. A key observation was that high grade epithermal gold mineralisation was being discovered at deeper levels on quartz vein structures which had previously been drill tested to shallow levels. The similarities between the Cracow Goldfield and the Vera-Nancy/Pajingo area suggested potential for deeper, high grade gold mineralisation at Cracow, and the previous drilling data from the known epithermal quartz vein structures in the Cracow Goldfield were re-evaluated. Five quartz vein structures were targetted for deep drill testing, with the order of prospectivity being Golden Plateau, White Hope, Golden Mile, Roses Pride and Klondyke. The depth extent to which these quartz vein structures had previously been drilled ranged from 70m below surface at Klondyke to 300 metres below surface at Golden Plateau. At that time, the Golden Plateau area was the most promising exploration target, as previous drilling and underground sampling data from the deepest levels in the mine (250 metres below surface) indicated the presence of epithermal quartz veining with sporadic high grade gold values. An audio-frequency magneto-tellurics geophysical survey was undertaken over the Golden Plateau mine area and this showed a large resistivity anomaly at 600m to 1km depth beneath the Golden Plateau mine. In January 1998, an 800m hole was drilled to test this target and this hole encountered a broad zone of quartz-sulphide hydrothermal brecciation with anomalous silver and tellurium. Gold values, however, were low. Drilling was also conducted along the White Hope (6 drillholes), Golden Mile (3 drillholes), Roses Pride (1 drillhole) and Klondyke (2 drillholes) quartz vein structures. In this program an encouraging drill intercept of 12m @ 7.4g/t Au was intersected in quartz-adularia-carbonate veining at the southern end of the Klondyke quartz vein structure, 170m beneath the Royal Standard workings. In April 1998, a further five drillholes (KDD013 to KDD017) were drilled at the Klondyke Prospect (Figure 2). All five drillholes intersected the epithermal quartz vein structure, however assay results from these holes only returned low grade gold values. A key observation noted in the deeper drillcore intercepts of KDD015 and KDD017 was that the quartz vein structure showed at least two phases of brecciation with clasts of banded crustiform and colloform-textured epithermal quartz within the breccia. This was interpreted as indicating a deeper and/or lateral source for these clasts. It was also noted that the epithermal quartz vein structure was broader at depth and was gold mineralised to at least 500m below surface. It was primarily these drillcore observations which led the site geologists to persist in searching for high grade gold mineralisation within the Klondyke quartz vein structure.
The third exploration phase commenced in January 1999 and led to the discovery of the Royal Shoot.
In February 1999, hole KDD021, drilled at the Klondyke prospect, returned 16m @ 38g/t Au. This intercept accelerated the drilling program, and eventually led to the estimation of an Inferred Resource of 1.1Mt @ 11g/t Au and 9g/t Ag within a high grade gold zone known as the Royal Shoot (Figure 2).
The objective of the forward exploration program at Cracow is to discover additional high grade gold mineralisation, with a focus on the prospective quartz vein structures in the district.
There is potential for blind, high grade epithermal gold mineralisation along the strike extensions of the Klondyke and Roses Pride quartz vein structures where they are covered by the younger, post-gold mineralisation, late Permian shallow marine sediments of the Back Creek Group. The Roses Pride epithermal quartz vein structure has a similar geochemical signature and quartz vein textures to those of the Klondyke quartz vein structure and this had been an observation noted by previous work done in the goldfield (Summons, 1995).
Geological, geochemical and geophysical data collected from surface exploration and drilling of the Royal Shoot have been used in the search for additional high grade epithermal gold mineralisation. Ground magnetics conducted over the Klondyke structure shows a strong correlation between the quartz vein structure and a north-west trending linear magnetic low. The Royal Shoot high grade gold mineralisation trends northerly (350o) and correlates with the intersection of the Klondyke trend (315o) and a north-south cross structure (Figure 3). Structural data collected during drill core logging confirm this relationship with the southerly plunge of the Royal Shoot being a product of the intersection of these two structures.
Paragenetic and geochemical studies (Braund, 2000) indicate at least two distinct gold phases. One is a high grade Au-Ag phase with a strong gold bias, where Au:Ag is typically 3:1 and the other is a Au-Ag-Te phase with a silver-telluride bias, and a Au:Ag:Te ratio of typically 1:3:3.
Exploration tools that have been used successfully in the Cracow district, and will continue to guide future exploration, include ground magnetics, geochemistry and textures of the auriferous quartz-adularia-carbonate veins and an understanding of the structural controls on known mineralisation.
The authors would like to thank the Cracow Joint Venture for permission to publish this paper, and would also like to acknowledge the contribution and commitment of all of the geologists who have been involved in the Cracow Project over the past six years.
Braund, K., 2000, Geology, Geochemistry and Paragenesis of the Royal Shoot low sulphidation epithermal quartz vein structure, Cracow, South-east Queensland: unpublished University of Queensland paper.
Summons, T., 1995, Cracow Gold Project, Information Memorandum, unpublished internal report, Sedimentary Holdings Ltd.
Figure 1. Regional Geology of the Cracow Goldfield