Summary:
The Kurmuk Project lies within the Neoproterozoic volcano-sedimentary Tulu Dimtu shear belt at the northern end of the East African Orogen.
Dish Mountain
The Dish Mountain gold deposit is situated within a northeast-trending volcano-sedimentary succession. Four main rock groups in order of abundance are recognized by Allied from mapping and drilling:
• Foliated mafic igneous rocks comprising meta-pyroclastics, lavas and intrusives that strike to the northeast and dip moderately to the northwest.
• Foliated metasediments intercalated with the meta-volcanic rocks comprising:
- dominantly quartz–mica (+feldspar+chlorite) and variably carbonate-bearing siliclastic rocks dominated by pelites and phyllites, with lesser, psammo-pelite and psammites
- relatively uncommon chemical sediments, which are important marker horizons and range from silica-dominant to carbonate-dominant.
• Variably deformed ultramafic rocks.
• Syn to post tectonic intrusives ranging in composition from gabbro, diorite, tonalite to granite.
Most faults strike to the northeast, parallel or sub-parallel to the regional structural fabric.
The Dish Mountain gold deposit is interpreted as peripheral to a mafic dominant, bimodal, eruptive centre, which produced mainly tuff and ash pyroclastics with lesser lavas separated by periods of low energy sedimentation. The gold deposit forms a sigmoidal, lozenge shaped body in plan and is postulated to be hosted within accommodation features associated with a major northeast-trending thrust, informally named the Dish Mountain Fault.
Gold mineralization is related to late-stage, discordant extensional quartz > dolomite >> pyrite (+chloritetourmaline-gold) veins and adjacent dolomite-muscovite-pyrite altered selvages within broad dolomitemuscovite alteration haloes which may be anomalous in gold. The vein sets typically range from 1 m to 10 m in thickness and form stacked arrays.
The three main mineralized lode orientations recognized are:
• West-dipping lodes located on the eastern side of Dish Mountain associated with the interpreted Dish Mountain Fault
• Sub-vertical chert lodes
• Flat-lying lodes.
The-flat lying and west-dipping lodes comprise quartz veins that individually are thin (2–5 cm) but are laterally extensive and as a result of the structural environment, occur as 5–10 m thick stacks of quartz veins with pelites between the veins. The chert bodies at Radio Hill are sub-vertical, massive banded silica and quartz that are 10 m wide and strike for several hundred metres; they are truncated by a fault half way along the Dish Mountain deposit.
Ashashire
The Ashashire deposit is located about 15 km south of Dish Mountain within the same northeast-trending volcano-sedimentary succession comprising:
• A steep, southeast-dipping, mafic-dominated volcaniclastic footwall sequence with jasperoidal and chert horizons and numerous thin granite and tonalite intrusive rocks
• Siliciclastic sediment-dominated package comprising fine-grained psammites, pelites and psammopelites with minor carbonate sediments separated by mafic units
• A mafic-dominated hangingwall sequence that comprises chloritic siltstone and basalt.
The gold deposit forms a series of linear bodies along a ridge crests in plan. The gold mineralization is related to late-stage, discordant, quartz > dolomite >> pyrite (+chlorite-tourmaline-gold) veins and adjacent dolomite-muscovite-pyrite altered selvages within broad dolomite-muscovite alteration haloes which may be anomalous in gold.
The two key controls on the mineralization interpreted are:
• Competency contrast boundaries, with the mineralization hosted in zones typically associated with granitoid lenses and the margins of mafic bodies, adjacent to less competent schistose siliclastic rocks that typically dip steeply to the northwest
• Mineralized quartz veins developed sub-perpendicular to the steeply dipping lithological domains, with the dominant vein set dipping shallowly to the west-northwest.
The Ashashire gold deposit is a late-stage structurally controlled sheeted-vein (some minor stockwork) gold system. The currently defined gold system is restricted to an 010° to 020° trending 2.5 km long x 200 m wide corridor of mineralization segmented by several major oblique sinistral shear zones. Individual lodes are hosted in a variety of rocks comprising lower greenschist facies mafic and sedimentary rocks and granite dykes. Three main mineralized zones have been identified (Perch, Gin Dish and Scorpion), separated from each other by regional north-northeast trending shear zones.
Gold mineralization has intense muscovite-dolomite-pyrite (±chlorite) alteration adjacent to mineralized veins and a proximal to distal intense carbonate alteration halo. At a local scale, lithological controls and faults are important in controlling formation of vein arrays, possibly grade (i.e. mafic rocks may be higher grade) and plunge of ore systems. Vein arrays are predominantly 20° to 45° west-northwest dipping (dip direction 290°) (Groves and Francis-Smith, 2020).
Mineralization at Ashashire is connected throughout the deposit, from Perch in the north, through centrally placed Gin Dish to the southern Scorpion, and extends to 300 m depth, the current limit of drilling. True thickness of the stacked ore lenses begins at Perch at 50 m, attains 100 m at Gin Dish and then reduces to 30 m at Scorpion; it is noted that barren country rock lies between the ore lenses in the stack.
Mesothermal gold provinces are characteristically associated with regional structures along which allochthonous terranes have been accreted to continental margins or arcs. A recurring sequence of transpressive deformation, uplift, late kinematic mineralization, and maturation of magmas to high-K types is consistent with thermal re-equilibration of tectonically thickened crust. In this model, thermal re-equilibration of underplated and subducted (and/or obducted?) oceanic lithosphere and sediments in a transpressive regime, over time scales of 10–40 Ma, is interpreted as a necessary precursor to gold mineralization. Hydrothermal fluids are released along boundary faults and their splays during uplift: uniform temperature, low salinity and mole% CO2 signify uniform source conditions, whereas variable O, C, Sr, and Pb isotopic composition of fluids reflects lithological complexity of the source regions and conduits.