Summary:
The gold deposits at Meekatharra are consistent with the greenstone-hosted quartzcarbonate vein (mesothermal) gold deposit model. Exploration for extensions of these deposits and new deposits are therefore based on these models as described below:
• Banded Iron Formation sulphide replacement (Prohibition).
• Shear-related (Bluebird, Caledonian, South Emu – Triton, Vivian – Consols).
• Fault-related (Vivian’s thrust lodes).
• Stockwork vein and alteration-related (Fatts, Hendrix, Euro).
• Secondary deposits (Gidgee Laterite in Boomerang – Kurara group).
Meekatharra Gold Operations can be subdivided into five major geological domains: Meekatharra North, Paddy’s Flat, Yaloginda, Nannine, Reedy’s.
Meekatharra North
The Meekatharra North area is located to the north of the Haveluck open pit, extending to the northern limit of the Westgold tenement package, approximately 15 km to the northeast. The area includes the eastern contact of the Chunderloo shear zone, running along the western side of the tenement group. To the east is a sequence of chlorite schists, interflow sediments, chlorite-talc schists, with small granitic intrusions and felsic porphyry dykes. Within and at the contact of the Chunderloo shear zone, rocks have undergone amphibolite facies metamorphism, but elsewhere are mid-to upper-greenschist facies.
Three existing deposits are known at Meekatharra North and have seen open pit mining by Westgold; Five Mile Well, Maid Marion and Sabbath. Five Mile Well is a quartz veined fault / shear zone that trends 20 degrees east of the regional foliation trend. Maid Marion is related to near-surface enrichment and silicification of a possible sulphide facies BIF, with intersections at depth within banded silica-chloritepyrite rock of unknown origin.
Paddy’s Flat
The geology of the Paddy’s Flat area is a simple sediment - mafic, ultramafic and intermediate volcanic succession. The mafic volcanic - sedimentary succession is present in the western parts of Paddy’s Flat and consists of tholeiitic basalt flows with thin bands of interflow sediment. A thick (>50 m) package of volcaniclastic sediment and banded iron-formation (BIF) is present near the top of the sequence. Tholeiitic basalt is variably deformed and contains abundant vesicles that are now filled with chlorite and chalcedony. Rare channel-like structures, possibly related to de-gassing of the lava and the presence of rare pillow structures suggest a submarine environment. Drill core shows that the basal contacts with sediments are often diffuse and suggest minor melting of the underlying sediment. In contrast, the upper contacts of flows are well defined and show sediment infilling of surface features. The volcaniclastic sediments are intermediate in composition and grain size ranges from fine ash to lapilli and graded bedding is evident in fresh exposures. The fine nature of the bedding laminations and the small-scale graded bedding suggest deposition in a water column. The BIF varies from an iron carbonate +/- magnetite BIF, to a chert - magnetite BIF. Individual BIF units range from less than 2 m to 40 m in width and are generally strongly magnetic.
The mineralisation at Paddy’s Flat can be classified into three groups which, in part, relate to the host lithology and style of veining. The three styles of mineralisation are summarised as:
- Sulphide replacement BIF hosted gold.
- Quartz vein-hosted shear-related gold.
- Quartz-carbonate-sulphide stockwork vein and alteration related gold.
The three styles of mineralisation represent a general progression from west to east across the Paddy’s Flat area.
Yaloginda
The Yaloginda area is a gold-bearing Archaean greenstone belt situated 15 km south of Meekatharra and encompasses the Bluebird Mill adjacent to the Great Northern Highway. The deposits in the area are hosted in a strained and metamorphosed volcanic sequence that consists primarily of ultramafic and high-magnesium basalt with minor komatiite, peridotite, gabbro, tholeiitic basalt and interflow sediments. The sequence was intruded by a variety of felsic porphyry and intermediate sills and dykes.
Gold mineralisation is not limited to a particular rock type at Yaloginda. Instead, the location of mineralisation is structurally and rheologically controlled. Mineralisation styles fit into two main categories, shear zone and vein related style. In shear zone style mineralisation, pervasive zones of metasomatism and associated low-grade mineralisation have resulted from gold-bearing fluid that has exploited the vertically connective fault and shear systems and high-strain domains that developed late during north-northeast to north-northwest trending folding. Alteration assemblages proximal to gold typically include quartz, iron carbonate, pyrite, +/- fuchsite, +/- chlorite +/- sericite, with distal halos of weak iron-carbonate +/- mica alteration.
Vein related gold is associated with zones of intense, variably orientated quartz +/- carbonate +/- chlorite veining, commonly with sulphides within veins or their selvedges. Veins tend to overprint rocks with coarse textures at structurally complex sites, such as at the contact of rheologically contrasting units, or the intersection of stronger rocks and fault or shear zone structures. Favourable vein orientations for gold mineralisation include moderate to shallow dipping east-west striking veins, horizontal veins, and arrays of sigmoidal (tension gash) veins. Tension gash kinematics are generally top-to-the west, consistent with the reverse dextral kinematics on the fault-shear zone systems. Gold is locally enriched in the vicinity of brittle to semi-brittle cross structures that include late steep northeast-southwest to east-west trending faults that displace mineralisation.
Nannine
In the Nannine area, the Meekatharra-Mount Magnet Archaean greenstone belt is dominated by a sequence of intercalated tholeiitic mafic volcanic rocks and silicified, ferruginous sedimentary rocks locally referred to as BIF. It is stratigraphically overlain by massive ultramafic intrusive and extrusive rocks, high-Mg tholeiitic basalt, and minor felsic volcanic rocks, which have been intruded by post-kinematic granodioritetonalite plutons to the west and east.
In the Nannine area, there are two major changes in the orientation of the greenstone belt. North of Aladdin, the volcanic-sedimentary package trends north. Between Aladdin and Caledonian, the rocks trend north-northwest. South of Caledonian there is a return to north-trending stratigraphy. This kink matches the convex western margin of the Norie Pluton and appears to reflect broadly east-west compression and competency contrast between the granite and adjacent volcanic-sedimentary rocks.
Reedy’s
The Reedy gold deposits occur within a north-south trending part of the Meekatharra - Wydgee Greenstone belt composed of volcano-sedimentary sequences and separated multiphase syn- and post-tectonic granitoid complexes. Structurally controlled gold mineralisation occurs at the sheared contacts of dolerite, basalt, ultramafic schist, quartz-feldspar porphyry, and shale.
The Reedy gold deposits occur within two lineaments or structural corridors. The western lineament corresponds to the Reedy Shear Zone along which gold mineralisation extends over 15 kilometres. The second lineament to the east sits on a structural corridor called the Turn of the Tide Shear Zone. It corresponds to the northern extension of the Mount Magnet Shear Zone. Both shear zones are located on either side of the Culculli Granitoid complex.