Summary:
Mineralisation at Kharmagtai is classified as porphyry style.
Mineralisation
There are three main styles of mineralisation at Kharmagtai:
• porphyry stockwork mineralisation
• tourmaline breccia mineralisation
• epithermal mineralisation.
Mineralisation at Kharmagtai is directly related to typical porphyry-style vein and hydrothermal breccia assemblages. These assemblages demonstrate both spatial zonation and temporal overprinting relationships commonly associated with porphyry Cu-Au systems, with multiple overprinting phases of intrusions and mineralisation (‘telescoping’ characteristics).
All mineralisation occurrences across the Kharmagtai Project area demonstrate some (if not all) of the aforementioned mineralisation characteristics.
The principal minerals of economic interest in all Kharmagtai deposits are chalcopyrite, bornite and gold, which occur primarily as infill within the veins and breccia cements, as well as minor chalcocite and gold is frequently intergrown with chalcopyrite and bornite.
Mineralised zones at Stockwork Hill, White Hill, Copper Hill and Zaraa are associated with paragenetically early-stage quartz veins that were intensely developed in and around quartz diorite intrusive rocks. The vein systems manifest as both sheeted vein arrays and stockwork zones, demonstrating clear structural and temporal controls on vein domain morphology.
Late-stage sulphide only veins (chalcopyrite ± pyrite ± bornite) overprint the quartz-sulphide vein assemblages and are commonly associated with higher Cu-Au grades. Visual overprinting relationships indicate that these sulphide-only veins both predate and are locally synchronous with the late-stage tourmaline and sulphide-rich hydrothermal breccias. At the deposit-scale, sulphide mineralisation is zoned from a bornite-rich core outward to chalcopyrite-rich and then outer pyritic haloes, with gold grades closely associated with chalcopyrite and bornite abundance.
Porphyry stockwork mineralisation
The porphyry deposit model is well understood. While each deposit is different in detail, most follow the typical porphyry theme whereby the copper sulphides are broadly associated with sheeted to stockwork quartz, pyrite, chalcopyrite and bornite veins which are surrounded by disseminated pyrite.
At Kharmagtai this pattern stands, although as the mineralisation is structurally controlled many of the deposits form as sheeted veining within wall rock rather than wrapping around a causative intrusive. There are discussions amongst the geology group at Xanadu whether any causative intrusive has been drilled to date, which has significant implications for the potential scale of the project.
In the standard porphyry model the bulk of copper mineralisation occurs early in the intrusive history and copper input wanes over time. At Kharmagtai there appears to be multiple copper events with an early system producing a broad halo of copper bearing quartz veining which has been overprinted by later stage chalcopyrite veins (c-veins). Examples of this are the Southern Stockwork Zone at Stockwork Hill and the Copper Hill deposit, where high-grade copper and gold occurs as earlier b-veins have been re-opened and crosscut by chalcopyrite-only veining.
Sulphide species zonation within porphyry deposits is also well understood. In the standard model the core of a deposit is bornite mineralisation, grading outwards/upwards to chalcopyrite mineralisation with a broad halo of barren pyrite. This pattern presumably represents a down temperature chemical process and a relative lack of copper versus sulphur and iron in the system.
At Kharmagtai the zonation is broadly consistent with the accepted sulphide species zonation, although bornite mineralisation is only recently being drilled in the lower portions of Stockwork Hill and to date the other five deposits have very limited bornite. This strongly suggests the drilled portions of the deposits are only the tops of the system and the greater part of the system is yet to be drilled.
Copper to gold ratios of the porphyry stockwork mineralisation average 1% Cu to 1 g/t Au in the early stockwork, 1% Cu to 2 g/t Au in the higher-grade C-vein upgrade and 1% Cu to 3 g/t Au in the bornite zone.
Tourmaline breccia mineralisation
Tourmaline breccia (TBX) mineralisation occurs throughout the lease. However, the only mineralised tourmaline breccia of potentially economically significant size occurs at Stockwork Hill. The tourmaline breccia body at Stockwork Hill crosscuts the earlier porphyry mineralisation. The breccia is variably mineralised with a larger body of weakly mineralised breccia containing lozenges of much higher grade at the margins of the breccia.
Copper to gold ratios within the tourmaline breccia average 1% Cu to 0.5 g/t Au although the silver content of the TBX is generally higher than the stockwork mineralisation.
Epithermal (carbonate base metal vein) mineralisation
The final stage of mineralisation at Kharmagtai consists of carbonate base metal veins which form within late-stage structures cutting all rock types and mineralisation styles. These commonly occur as 10 cm to 2 m wide veins containing calcite-quartz-siderite-pyrite-chalcopyrite-galena and sphalerite.
Within the vein system, veins often run to 50–100 g/t Au, although vein widths and continuity currently preclude economic interest. The chemical signature (Au-Cu-Ag-Pb-Zn-As) of the epithermal veins are useful fault markers and allow mapping of specific structures between disparate drill holes.
Dimensions
• Stockwork Hill: The strike length of the mineralised zone is about 1,350 m. Width is up to 800 m, traced down dip to 1,250 m. Mineralisation outcrops at the surface.
• White Hill: The strike length of the mineralised zone is about 1,800 m. Width is up to 830 m, traced down dip to 1,210 m. Mineralisation outcrops at the surface.
• Copper Hill: The strike length of the mineralised zone is about 630 m. Width is up to 150 m with apparent plunging to SW at about 40 degrees. traced down dip to 420 m dipping 70 degrees to SE. Mineralisation is outcropped at the surface.
• Zaraa: The strike length of the mineralised zone is about 1,300m. Width is up to 600m with apparent plunging to SW at about 60 degrees. traced down dip to 1,280m dipping. Mineralisation outcrops at the basement surface, beneath 35m of Palaeozoic cover.
• Golden Eagle: The strike length of the mineralised zone is about 400m. Width is up to 400m. traced down dip to 450 m. Mineralisation outcrops at the basement surface, beneath 35m of Palaeozoic cover.
• Zephyr: The strike length of the mineralised zone is about 1,030 m. Width is up to 310 m. Traced down dip to 350m. SE. Mineralisation outcrops at the basement surface, beneath 30m of Palaeozoic cover.