Summary:
The mineralisation at Ikkari is considered to be orogenic-style with gold mineralisation associated with low sulphidation alteration.
Overall, the mineralisation trends at approximately 065° strike and has a strong sub-vertical control. However, within the mineralised halo different grade zones have varying morphology and plunges on a local scale.
Mineralisation at Ikkari occurs in several styles, but in all cases, gold distribution is correlated to the abundance of disseminated pyrite and intensity of veining, which are in turn considered to be principally controlled by lithological contacts, fold geometry and brittle fracturing. The style of mineralisation is principally controlled by the host lithology with significant controls on mineralisation localization including:
• Brittle-fracture regime in intensely albite-altered felsic sediments that controls veinlets of gold associated with fine-grained pyrite and magnetite. Given that this style of mineralisation is limited to the albite-altered sediments it is most prevalent in the north-western portion of Ikkari where the felsic sediments form a large block. It also occurs in larger felsic intercalations within the komatiite domain;
• Lithological contacts: notably intensely chlorite-sericite-siderite-magnetite-pyrite-(±fuchsite)- altered sediments with felsic sediments, quartzite and conglomerate, and siltstones;
• Complex and concentrated short-wavelength (metre-scale) parasitic folding of narrow felsic and siltstone sedimentary intercalations within intensely chlorite-sericite-magnetite-altered ultramafic rocks that appears to further focus fluid flow and pyrite deposition, particularly at fold hinges. Intense, irregular carbonate-quartz veining is frequently developed in these zones and mineralised higher in grade; and
• Within and at the margins of hematite-carbonate hydrothermal breccias, that have a sub-vertical expression and overprint folding and cross-cut lithological contacts. Where these breccias host intense disseminated pyrite, bonanza gold grades are commonly seen.
Ikkari is unusual among orogenic gold deposits in the width of mineralisation when compared to the strike. In typical orogenic gold systems, the strike of mineralisation is an order of magnitude greater than the width, however, at Ikkari the strike length of the mineralisation is only two to three times the width and this is be attributed to multiple, stacked mineralised zones perpendicular to the strike. These stacked zones are interpreted to arise from the structural interleaving of diverse lithologies pre-mineralisation in D1, with no evidence to support post mineralisation thickening. From the northwest to southeast across Ikkari, at least four subtly different mineralised zones can be described:
• Within the large felsic block to the northwest of Ikkari, a brittle-fracture regime in intensely albite-altered felsic sediments. This coalesces towards surface and exhibits a moderate northern dip in close proximity to the carbonaceous shale. At depth, and in the east, these brittle fracture zones separate into at least two, narrower, vertical trends. These mineralised zones are separated from each other, and the subsequent mineralisation trend to the south, by largely barren sericite and weakly albite-altered felsic sediments. In the domaining for resource estimation this is termed the "Northern Felsic Domain";
• At the contact between the northern felsic block and the komatiite domain in the west and then stepping off this contact to the east to be entirely within the komatiite domain, is the next zone of mineralisation. In the west, mineralisation occurs on both sides of the felsic-komatiite contact with the intensely albite-altered felsic sediments hosting an intense silica-pyrite brittle fracture to breccia regime, whilst to the south of the contact and along strike to the east, in the komatiite domain, mineralisation is most commonly related to first intercalated felsic or phyllitic sediment encountered, the contacts of this and fold hinges within. In this, the strongest zone of mineralisation, mineralisation is commonly pervasive throughout the intercalated sediment rather than focused on its contacts;
- To the east, away from the large felsic block, barren talc-chlorite-altered komatiite occurs to the north of this mineralisation, separating it from the converging carbonaceous shale. Further east still, this mineralisation trend is terminated by the cross-cutting carbonaceous shale. Where the mineralisation trend occurs in close proximity to the carbonaceous shale it exhibits a northern dip consistent with the shale but elsewhere the dip is more vertical, and the apparent plunge is approximately 30° to the east. In the domaining for resource estimation this istermed the “Contact Domain”
• Further south still are several parallel mineralisation trends within the komatiite domain are characterised by a decreasing gold tenor and lateral extent towards the south/southeast. Mineralisation is primarily associated with contacts to intercalated felsic or phyllitic sediments within the komatiites and enhanced at the fold hinges of these intercalations. Mineralisation in this portion of the deposit plunges back to the WSW at approximately 15° which is consistent with the S2 fold hinges measured throughout the komatiite domain; and
- The opposite plunge of this mineralisation in comparison to the trend north of it, creates diverging mineralisation trends to depth in the west and converging mineralisation trends towards surface in the central-eastern portion of the deposit. To the south of this trend, and where the trends diverge, talc-chlorite-altered barren komatiites separate the mineralisation trends. However, where mineralisation trends are in closer proximity, no talc-altered komatiite is preserved, and weakly mineralised iron-metasomatised chlorite-sericite-siderite assemblages, the distal alteration product of the komatiite domain, separates the mineralisation trends; this is also the case in the poorly mineralised / barren gaps between the mineralisation trends of this type. In the domaining for resource estimation this is termed the “Internal UM Domain.”
• To the south of the E-W fault array, within the low-strain talc altered komatiites, laterally discontinuous felsic intercalations host mineralisation at the contacts to the komatiite in a similar style to those described above. However, here the mineralisation is more discontinuous, and the proximal komatiite does not exhibit extensive iron metasomatism as the mineralisation trends further north.
It is considered that all the gold mineralisation is related to the same (oxidised) fluid event that was introduced along a complex brittle-ductile permeability meshwork. Sites of gold deposition are structurally controlled but locally dependent on the availability of a geochemical reductant that allows deposition of pyrite and associated gold. Such iron-rich reductants at Ikkari are likely to include magnetite and chlorite, formed during an earlier iron-metasomatic alteration and/or syngenetic pyrite that may have been present in the intercalated siltstones. The presence of a pre-existing reduced fluid cannot be excluded. The spatial association of high-grade gold zones to apparently later, largely post deformation hematite-carbonate breccias is indicative of a later gold-bearing fluid phase also being present.