Summary:
DEPOSIT TYPES
The Blanket orebodies comprise up to 10% of precious metals. The Blanket mineralisation is hydrothermally emplaced and associated with the regionally developed D2 deformation characterised by areas of high strain wrapping around relatively undeformed remnants of the original basaltic flows. Wider orebodies occur within the more ductile tensional high strain areas. Two main types of mineralisation are recognised at Blanket, namely Disseminated Sulphide Reefs (DSR) and quartz-filled reefs and shears. A third type of mineralisation may be evidenced in the form of auriferous sulphide minerals as a replacement of the iron-rich minerals along the hinges of the folds in BIF.
MINERALISATION
Mining at Blanket occurs over a 3 km strike that includes from north to south, the deposits of Lima, Eroica, Sheet, AR Main, AR South, Feudal, Blanket Section (Blanket 1 to Blanket 6) and Jethro. The main Blanket underground workings are connected to Lima by a 2 km long haulage which follows the strike of the main fabric.
Wall rock alteration typically comprises silica-pyrite-muscovite within a broader carbonate alteration halo. Quartz-carbonate altered rock forms the most commonly recognised alteration assemblage. Gold is deposited at crustal levels within and near the brittle-ductile transition zone at:
• depths of between 6 km and 12 km;
• pressures between 1 and 3 kilobars; and
• temperatures between 200ºC and 400ºC.
The deposits may have a vertical extent of up to 2 km, demonstrate extensive down-plunge continuity, and lack pronounced zoning. The ore mineralogy is dominated by gold, pyrite and arsenopyrite. Subordinate minerals such as galena, chalcopyrite, pyrrhotite, sphalerite, tellurides, scheelite, bismuth and stibnite also occur. Sulphide mineralogy commonly reflects the litho-geochemistry of the host rock with arsenopyrite being the most common sulphide mineral in metasedimentary host rocks and pyrite or pyrrhotite being more typical in metamorphosed igneous hosts. The gangue and alteration mineralogy are dominated by quartz and carbonate (ferroan dolomite, ankerite, siderite, calcite) with subordinate albite, fuchsite, sericite, muscovite, chlorite and tourmaline.
Two quartz-filled shear zones are mined, namely the Blanket Quartz Reef (“BQR”) and the Eroica Reef, which have long strike lengths but are not uniformly mineralised although continuous pay shoots of over 100 m on strike are seen. Gold grade fluctuations are more extreme in the quartz reefs than in the DSR type reefs but on average these quartz shears have higher grades and are used as a sweetener of ore to the mill.
BLANKET QUARTZ REEF
The BQR strikes some 500 m on surface and is up to 5 m wide, diminishing with depth, dipping 55°W. It displaces the DSR type orebodies with an apparent reverse movement of up to 250 m. Mineralisation in the BQR is not uniform and comprises native gold with galena. Arsenopyrite is more dominant down-dip. Economic mineralisation is restricted to three 90 m pay shoots.
LIMA
Lima is situated 2 km north of the Blanket Section and an underground haulage links the two mines. Like the Blanket Section orebodies, the Lima orebodies developed in very high-strain areas. The main shoots are the Hanging Wall and Interlimb. Mineralisation in the Hanging Wall limb comprises pyrite with subordinate arsenopyrite in cleavage planes within pervasive biotite/chlorite alteration. The Interlimb is characterised by a centrally silicified core with pyrite and arsenopyrite constituting the main sulphides.
EROICA
The Eroica orebody lies approximately 1,300 m north of the Blanket Section orebodies and renowned for its high native gold content. It dips at 65°W and has a strike length of 300 m in a northerly direction. The Eroica orebody is hosted in a high-strain area where the shear is up to 15 m wide. Brown carbonate alteration characterises the shear in strong association with biotite development. The orebody is defined by thin silicified stringers that develop into swells of up to 5 m in width. The silicification shows pinch and swell both on strike and down-dip, resulting in a series of dismembered silicified pods developed within a particular shear. The biotite and carbonate alteration, together with the silicified stringers, form marker links between the dismembered pods. Finely-disseminated arsenopyrite, pyrite and pyrrhotite are associated with the gold mineralisation.
SHEET
The Sheet orebody lies about 500 m south of Eroica and is a typical example of a fault controlled mineralisation. It comprises of at least three stepped-out, sinistrally displaced and highly silicified hornblende-chlorite schist fault blocks . The orebody was subjected to both strike and “east-west” dip faulting, resulting in major bifurcation of the orebody up-dip into highly fractured North and South orebodies with variable dips. As with the other DSR orebodies at Blanket Mine, the less disturbed Sheet orebody downdip extension has a 60-65°W dip and a fabric that is sympathetic to the north-northwest regional shear. The orebody can attain widths and strike lengths of up to 15 m and 60 m respectively. Mineralisation is associated with finely disseminated arsenopyrite. The orebody was mined between 230 m Level and 870 m Level on separate dismembered shear zones.
AR OREBODIES
AR is a “Z”-shaped mineralised zone and consists of two separate orebodies (AR Main and AR South) with widths up to 30 m as a result of tectonic thickening from faulting and folding. The mineralised zone has no known surface expression and appears to form a ‘peak’ under the regional dolerite sill just above 9 Level some 500 m north of the Blanket Section orebodies. From this point the body splits into two the ore shoots of AR Main and AR South, which plunge 55°W and 58°SW respective.
AR Main
AR Main is a DSR-type orebody and occurs within a broad shear envelope in pillowed metabasalts, which is generally irregular in plan and bounded by shears that assist in defining the limits of the mineralisation. At 750 m Level, a shear disrupts the bodies causing the plunge to flatten to the west. The orebody strikes between 40 m and 60 m with an average width of 30 m at the centre of the envelope (MSA, 2011).
AR South
AR South plunges southwest, trending towards the Blanket 2 orebody at depth. AR South is also developed within a broad shear zone and is more pipe-like than AR Main. Its maximum thickness is approximately 50 m and high grade sections are defined by silicification and arsenopyrite.
FEUDAL
Occurring in the hanging wall of the Blanket-Lima strike about 900 m northwest of Jethro is the almost mined out “outlier” Feudal orebody. The orebody is interpreted to be located at the focal point of two Blanket-Lima strike transgressing shears, namely the Blanket-Feudal and Jethro-Feudal shears. The known orebody (surface outcrop to 7 Level for a 200 m lift) is now mined out.
BLANKET SECTION
The Blanket Section comprises six orebodies, namely Blanket 1 to Blanket 6, which occur some 500 m south of the AR orebodies. On average, the orebodies dip 80°SW. The reef has a shallower dip than the DSR bodies, but plunges in the same direction so that it progressively advances towards them with depth, displacing Blanket 2, 3, 1, 4. Blanket 2 reappears on the footwall of the BQR and is established on the 630 m Level through to the 870 m Level.
JETHRO
The Jethro orebody is located some 400 m south of the Blanket Section. The north-south striking Jethro orebody dips near vertical in a westerly direction and tends to roll over locally.