Flat Mines area — includes historically mined Flat Mines North, East, South, and Nababeep deposits that are being advanced for redevelopment under the Okiep Copper Project (OCP) framework.
New Okiep Mining Company (Pty) Ltd (NOMC) holds the primary mining right for the Flat Mines project. Orion Minerals Limited (ASX: ORN / JSE: ORN) is the majority shareholder of NOMC, holding 60.63%. The remaining 39.37% is held by the Industrial Development Corporation of South Africa (IDC), a South African state-owned development finance institution.
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Summary:
The Flat Mines (FM) Project is located in an area commonly referred to as the Okiep Copper District and includes four deposits: Flat Mines North (FMN), Flat Mines East (FME), Flat Mines South (FMS) and Flat Mines Nababeep (FM-Nab).
Copper mineralisation is predominantly hosted within the Koperberg Suite of the mafic to intermediate intrusive bodies. The Koperberg Suite intruded as dykes, plugs, sills and shallow plunging tubular bodies into the Khurisberg Subgroup and Little Namaqualand Suite. More than 1,700 of these basic bodies is known to exist. The bulk of the mafic bodies intruded along east-northeast and east-west trending structures known as ‘steep structures’. Major rock types of the Koperberg Suite include anorthosite, leuco-diorite, biotite-diorite, leuco-norite, norite, glimmerite and orthopyroxenite. The most common arrangement of mafic bodies is that of irregular, steeply north dipping dykes, emplaced along antiformal steep structures. Intrusions are composites of multiple phases of intrusion with different mineral endowment.
The copper mineralisation in the FM Project occurs in mafic intrusives as irregular shaped lenses with the mining cut-off often not a sharp contact but based on an economic grade cut-off.
The Flat Mines (FM) Project MR is largely underlain by Concordia Granite with some areas of Rietberg Granite and scattered outcrops of metasediment, anorthosite and diorite of the Koperberg Suite. Diorite is the most prospective rock type in the area and these mafic rocks form a very distinctive curvilinear trend towards the east. Diorite as sills (FMN) and dykes (FME and FMS) are less common and are predominantly hosted within the anorthosite.
Flat Mines North (FMN)
Copper mineralisation at FMN is hosted predominantly in anorthosite, biotite-diorite and glimmerite. The mafic rocks occur on the northern flank of a broad east-west striking antiform in Concordia Granite gneiss. The mafic rocks intruded the Concordia granitic gneiss in a lit-par-lit style to form a sill complex consisting of inter-banded mafic rocks and granitic gneiss. The highest copper grades occur within the biotite-diorite and glimmerite where it intruded close to the base of the sill complex.
The geometry of the FMN deposit does not conform to the general attitudes of mineralised bodies in the Okiep Copper District. The deposit strikes for 600m north-south with a shallow plunge to the north and then makes a dogleg in the north to continue eastward on an east-west strike direction. FMN has been divided into three mineralised bodies (MB) namely: FMN-MB1, FMN-MB2 and FMN-MB3.
FMN-MB1 occurs in a channel like feature at the base of the sill complex. This has resulted in the formation of a 250m long, 40m wide and 15 - 50m thick sausage shaped mineral deposit that plunges at 45° northwest for the initial 80m and then flattens out to near horizontal for the next 130m. The plunge direction changes from 345° to due north after 400m and then increases again to 45° for 400m, where the FMN-MB1 mineralisation pinches out. Mineralisation reaches a maximum depth of 120m below surface on the 640m (AMSL) level in FMN-MB1. The mineralisation envelope interpreted for the Mineral Resource estimation was broadly defined on a 0.5% Cu cut-off with exceptions to maintain continuity.
Mineralisation continues again 90m to the north of FMN-MB1 on the 660m level. FMN-MB2 continues for 250m along the same north trend as FMN-MB1 but plunges shallowly at 2° to 25°. The mineralised body attains the same irregular sausage shape, albeit thinner than in FMN-MB1, at 10 - 15m thick and 25 - 50m wide. Mineralisation is hosted by dark biotite rich diorite within a larger anorthosite body. The mafic body is more constrained than the sill complexes hosting FMN-MB1and MB3.
At FMN-MB3, the strike of the mineralisation abruptly changes to east-west, plunging shallowly towards the east for 250m at a depth of 170 to 250m as an irregular lens-shaped body up to 20m thick. Copper mineralisation in this area is hosted by glimmerite and biotite-rich diorite developed at the base of a northerly dipping, 170m thick sill complex.
Flat Mines East (FME)
At FME, the main mafic intrusive massif is approximately 970m long and 20 to 80m wide. The intrusive strikes at 65° ENE and dips 50° - 75° WNW. The mafic body occupies an ENE trending steep structure within the Concordia Granite and Wolfram Schist. Unlike at FMN and FMS, diorite is not as volumetrically important as norite, which hosts the bulk of the mineralisation. The volume proportion of norite in the mafic body increases to the east. Anorthosite is more erratically mineralised with lower grades.
Drilling has defined three mineralised bodies namely: FME-MB1, FME-MB2 and FME-MBW. Mineralisation is concordant with the steep dip of the main mafic intrusive body. Significant zones of unmineralised tabular or lenticular xenoliths of Concordia Granite occur in both FME-MB1 and FMS-MB2 mineralised bodies.
FME-MB1 is the main mineralised body and occurs over a strike length of 500m. Mineralisation starts at 115m below surface and continues down dip for a maximum of 250m. The width of the mineralisation reaches up to 80m.
FME-MB2 is a smaller body developed structurally above and in the northeastern section FME-MB1. Mineralisation strikes east-west and dips towards the north at 70°. The top of the mineralisation is 25m below surface and it extends down dip to 200m below surface. The total strike length for FME-MB2 is 125m and the maximum true width is approximately 20m.
FME-MBW is a mineralised zone that has been broadly defined by wider spaced exploration drilling. Mineralisation here is predominantly hosted by diorite.
Flat Mines South (FMS)
Interpretation of drilling indicates that a single mineralised body is present at FMS (FMS-MB1) which has an irregular but continuous east-west strike. The interpreted mineralised body has a strike length of 570m, down dip extent of 530m and a maximum width of 30m. The overall dip of the mineralised body is approximately 40° - 60° but steepening to 80° in the deeper sections. The thicker anorthosite tends to be poorly mineralised and more biotite rich and hybrid-diorite varieties carry blebby rather than disseminated sulphides.
Flat Mines Nababeep (FM-Nab)
At FM-Nab, mafic rocks of the Koperberg Suite consist mainly of anorthosite, biotite diorite and biotite schist. The anorthosite does not host economic copper mineralisation. The FM-Nab mafic intrusive complex outcrops continuously over 1,300m, striking at 70° east of north.
The eastern end of the complex terminates against the Flat Mine Fault while the mafic rocks in the west pinches out approximately 820m WSW of FM-Nab. The dyke-like complex can be up to 100m wide and is vertical to steep northerly dipping. It is commonly intercalated with Concordia granite. In the vicinity of the shaft, 50m of Concordia granite separates the complex into a northern, 35m wide, biotite diorite and biotite schist-dominant dyke that host the FM-Nab deposit and a southern anorthosite-dominant dyke.
Surface mapping, drilling and mine plans shows copper mineralisation to occur over a strike length of 180m with the mineralised body terminating against NNE striking D4 faults on both the eastern and western sides. Mining reached a depth of 150m from surface with an average width of less than 4m. From underground mapping and sampling the mineralisation was reported as lenticular with erratic grades. The mineralised biotite diorite and biotite schist is segmented by unmineralised anorthosite. The best mineralisation is confined to the contact zones with the highest grades developed in the eastern part of the mine where a fairly massive body of high-grade ore was developed.