Summary:
ATO sits regionally within the Devonian through Late Jurassic Mongol-Okhotsk tectonic collage that has been emplaced along a transform-continental margin of the North Asian Craton (NAC). A number of Late Jurassic-early Cretaceous broad, gold-bearing mineral belts have been recognized in eastern Mongolia. ATO is located north of the Main Mongolian Lineament (MML), and midway along the NNE trending 600km long Onon base and precious-metal province that crosses eastern Mongolia. Though ATO presently represents the only well-explored gold deposit in this part of Mongolia, a large number of minor gold occurrences have been recognized throughout the region.
The geology of the ATO Project region consists of metamorphosed Devonian sedimentary rock overlain by a volcanic and sedimentary sequence of Permian age and remnant scraps of probable Jurassic volcanoclastic units, intruded by Jurassic plutons ranging from diorite to granite in composition and including rhyolitic phases mainly as dykes.
Mineralisation: The ATO deposit is an epithermal gold and polymetallic deposit of transitional sulphides in breccia pipes in a Mesozoic continental rift zone in eastern Mongolia. It could be characterised as an intermediate sulphidation system. Up to 2017 exploration focussed on three gold, silver and base metal mineralised sub-vertical pipes (Pipes 1, 2 and 4) spaced ~300 m apart on a WNW trend. Another pipe (Pipe 3) exists just west of the others but is not mineralised. Subsequently a fourth pipe-like body (Mungu) was found ~600 m to the north east of Pipes 1, 2 and 4). The pipes have been emplaced into stratified rocks. The three pipes are elliptical in shape with the long axis oriented toward the north east. Each have approximate surface dimensions of 300 * 150 m. The pipes taper to depth vertically. Mungu is a north east plunging system of tall lenticular lodes. Pipes 1 and 2 are near paleo surface, epithermal (hot spring) emplacements and the upper parts of mineralized breccia pipes. Pipe 4 is slightly buried without the surface mineralisation.
The deepest hole into Pipe 1, inclined 60°, is ~700 m long. Silica cap rock (pink) has a variable thickness in Pipe 1, generally tapering from a maximum thickness of about 40 m under the topographic high point of the pipe to less than 1 m near its margins. However, bottom surface of the cap rock is highly irregular, showing sharp undulations with underlying quartz-veined Middle-Late Jurassic gravel and coarse pebbly sandstone, some blocks of which are totally engulfed by massive silica.
The pipes also are cut by a number of minor faults, both steeply dipping and shallow dipping. Some narrow flat-lying post-mineral diorite dikes also have been emplaced along faults that offset margins of the pipes.
Pebbly conglomerate and pebbly sandstone were being shed from both nearby mostly Early Permian highlands elevated during emplacement of Early Jurassic magmatic rocks, as well as apparent high walls of an enclosing oval collapse feature. Continued deposition of Jurassic strata then covered the pipes after cessation of mineralization.
Deposit type: ATO’s mineral deposit type is that of multiple surface epithermal deposits with intermediate sulphidation (feeder) pipes below. This implies a specific shape where the top part (near or at current surface) would represent a wide thinnish roughly circular accumulation of mineralisation in country rock around an original surface ground-water-interacting hydro-thermal or fumarole vent system. Below that would be a tall root-shaped breccia pipe, flared at the top and narrowing downwards, through which the magmatic or meteoric fluids rose above a lower hot igneous body. The pipe would be vertically veined and/or brecciated.
Mungu
Mungu deposit is hosted in Lower Permian age volcano-sedimentary rocks and overlying Upper Permian sedimentary rocks and is itself cut by late diorite dykes. Post-mineralization diorite porphyrite dykes are abundant in the area. Weakly to moderately chloritized, black green coloured diorite porphyrite dykes with rare pyrite dissemination occur at depths of 150-180 m. They are in massive structure, have undergone little fracturing, and are consistently continuous along dip with average thickness of 10-15 m in almost horizontal position. They are branched out in some parts in varying directions. Also, light green coloured, weakly sericitized, strongly fractured and deformed diorite dyke have been found that undergone clay alteration and have a small thickness (up to 1 m). This post-mineral dyke is found close to a fault that displaced the orebody along a horizontal plane. These dyke plays destructive role in deposit settings.
Mungu is a structurally controlled epithermal gold-silver system with localized bonanza grades, and an Ag:Au ratio approximating 10:1. Mineralization occurs in brecciated zones controlled by NE trending structure and tiny dark coloured quartz-sulphide veinlets developed along the big fault zones. Ore body has almost linear shape, almost vertical and directing to NE by azimuth 035°. Trend of mineralization is plunging to NE by azimuth 035 and dip angle 40°. The mineralized bodies separated by late postmineralized dykes into parts and most significant dyke occur in 150 m depth below the surface, lies almost horizontally. Mungu orebody is elongate to the NE direction and continued about 700 m along the structure and orebody thickness varies up to 200 m in plan.
Mineralization consists of variable concentrations of mainly pyrite with arsenical rims, plus minor amounts of base metal sulphides and rare silver sulfosalts as veins, disseminations, and breccia fill in a relatively steep, narrow structure which has been traced over a lateral distance of about 700 m and vertical extension about 350 m depth from surface, mainly within dacitic rocks. Quartz veining is a minor component of the mineralization, and banding and other typical epithermal textures are essentially absent. Argillic alteration forms as envelope with tens of meters wide. About 5-40 m wide zone of low-grade mineralization with localized narrow zones of high-grade mineralization at depth has yielded bonanza grades in both gold (to 172.88g/t) and silver (to 1500.0g/t).