Summary:
Marymia area covers the northeast-southwest trending Plutonic (Marymia) Greenstone Belt, occurring in the central portion of the Marymia Inlier. The Belt is interpreted as a regional-scale fold-thrust belt. Rocks of the Marymia Inlier consist mainly of granite and gneiss with enclaves of meta-greenstone (including mafic and ultramafic igneous rocks, BIF and sedimentary rock precursors) metamorphosed at upper amphibolite to granulite facies. The meta-greenstones may represent high-grade metamorphic equivalents of the Plutonic Greenstone Belt to the north-east. Rocks of the Peak Hill Schist area include quartz-sericite schist, quartzmuscovite schist and quartz-muscovite-biotite-chlorite schist, which have been variously deformed and contain a range of mylonitic textures and discrete mylonitic units.
The area hosts 39 known gold deposits mined throughout the history of mining across the Marymia Dome. Gold mineralisation occurs in a large number of deposits and prospects in the Belt, with the main deposit at the Plutonic Gold Mine on the southern end (outside the tenement holdings). Mineralisation regularly occurs as shallow dipping, layered parallel lodes, although steep lodes and minor quartz-vein-hosted deposits also occur. Regionally within the greenstone belt, mineralised host rocks vary from amphibolites to ultramafics and banded iron formation (BIF). Lateritic and supergene enrichment are common throughout the Belt and have been mined locally such as at the Skyhawk deposit. Biotite, arsenopyrite, and lesser pyrite/pyrrhotite are common minerals generally accepted to be associated with gold mineralisation.
Deposit Type
Catalyst is exclusively exploring for gold deposits. The deposits in the Marymia Project area are classified Archean Orogenic Greenstone gold deposits. The gold mineralisation is predominantly structurally controlled and occurs within a series of north-east trending corridor hosted in a variety of lithologies ranging from ultramafic and mafic volcanic rocks, metasediments, felsic intrusive, volcanoclastic units, and banded iron formations.
The gold deposits are hosted in rocks that are generally highly deformed (four or more fold events) and metamorphosed (up to amphibolite facies). Mineralisation is early in the paragenesis, (syn- to post-D1 isoclinal folding) with folded mineralisation commonly observed. The literature review identified several structural themes that appear to control mineralisation:
- Proximity to major E-W trending fault. The entire belt is cut by several of these and forms major bounding faults to mineralised trends. The MMR fault has already been identified at Plutonic as a major cross-basin fault. Evidence of two folding events – gold tenor increases with complex fold interference patterns suggesting that the areas must be subject to more than early fold and thrust deformation.
- Evidence of NE and N-S trending faults – the former is evident in magnetics, but the latter is quite subtle and only obvious when looking at Max Au.
There is no “preferred” host rock, but mineralisation is interpreted to be precipitating where there are rheological, chemical and permeability contrasts. This is obvious when there is a mapped lithological change, but not in the mass of basalt. It is quite likely that there are changes in basalt chemistry and texture (i.e., going from Mg to Fe rich, usually translating from massive/variolitic to pillowed/breccia facies) that are not reflected in the map. The large volume of volcaniclastics sandstones and conglomerates which host mineralisation on the eastern trends suggests that there may be remnant volcanic centres that are the source of these sediments which are often very good buttresses to strain if subvolcanic domes and massive flows remain.
Trident
The Trident gold deposit is a structurally controlled, orogenic, mesothermal (amphibolite metamorphic facies) gold deposit hosted by ultramafic rocks that are part of strike extensions to the Plutonic Gold Mine stratigraphy. The gold deposit is specifically hosted by, shallow to moderate dipping, ultramafic tremolite – phlogopite (mica) schist, immediately overlying serpentinised ultramafic units, derived from higher MgO ultramafic volcanics.
High-grade gold zones are best developed within the shallow dipping ultramafic tremolite – phlogopite schist where it is bent into a concave flexure, in the hangingwall of steep, north-westerly dipping fault structures (see (Figure 7-2). Vertical “dragging” movement against these steeply dipping faults appear to have played a role in dilating the cleavage of the ultramafic schist, resulting in mineralisation and alteration between the dilated cleavage planes. The steeply dipping faults also host gold mineralisation.
Gold mineralisation is associated with potassic, phlogopite mica, and alteration has a low proportion of quartz and sulphides, including minor pyrrhotite, pentlandite, chalcopyrite and, directly associated with gold, bismuthinite and rare bismuth tellurides. Rarely observed gold grains (in microscopy) are predominantly fine (<50 micron) but free and/or attached to, and rarely occluded within sulphide grains.
Trident West
Trident West is the up-plunge surface representation of the Trident underground deposit. Like Trident, mineralisation is hosted by, shallow to moderate dipping, ultramafic tremolite – phlogopite (mica) schist, immediately overlying serpentinised ultramafic units, and with a hangingwall of thrusted granite-gneiss that has been eroded away at Trident West.
Gold mineralisation at Trident West is orogenic, hosted within a sheared contact zone in ultramafic rocks. High grade ‘shoots’ of mineralisation are associated with flexures in the mineralised host shear zones between steeply dipping structures.
High-grade gold zones are best developed within the shallow dipping ultramafic tremolite – phlogopite schist where it is bent into a concave flexure, in the hangingwall of steep, north-westerly dipping, fault structures. Gold mineralisation in fresh rock (Trident) is associated with potassic, phlogopite mica, alteration and has a low proportion of quartz and sulphides, including minor pyrrhotite, pentlandite, chalcopyrite and, directly associated with gold, bismuthinite and rare bismuth tellurides. Rarely observed gold grains (in microscopy) are predominantly fine (<50 micron) but free and/or attached to, and rarely occluded within sulphide grains. The Trident West Mineral Resource estimate is predominantly oxide and transition mineralisation and is demarcated from the Trident underground resource by the optimised open-pit boundary.
Dimensions
Mineralisation extends over a strike length (East – West) of approximately 250 m and down-dip up to 200 m. Mineralisation currently extends to a depth of approximately 100 m below surface.