Summary:
The Pegmont lead-zinc deposit is a body of stratiform lead-zinc sulphides contained within a metamorphosed silicate facies BIF. It is considered to be an example of a BHT deposit.
Pegmont is enveloped by a variable but large volume of late or postorogenic fracture/vein- controlled alteration. Williams et al identified two stages of alteration. Stage 1 veins typically are composed of quartz ± K feldspar ± tourmaline ± biotite ± rutile ± ilmenite associated with K feldspar ± muscovite ± tourmaline, or biotite ± muscovite ± garnet alteration. Stage 2 quartz ± chlorite ± calcite ± ferroan dolomite ± hematite ± sulphide veins are associated with fine-grained muscovite (illite-phengite) + chlorite ± carbonate alteration.
A largest amphibolite (hornblende-plagioclase) body is poorly exposed in sub-crop on the edge of the mining lease, called the “Lease Amphibolite”, it is a flay lying, highly continuous body that transgresses the BIF without any apparent displacement. The Lease Amphibolite displays textural zoning, which may reflect chilled margins and compositional layering. The southeast edge of Zone 3 is defined where the amphibolite intersects the Lens B BIF, to the southeast of this the Lease Amphibolite is below the BIF's and to the northwest it is above the BIF's. Drill data density in the Lease Amphibolite is less than the mineralization.
The Squirrel Hills Granite, part of the Williams Batholith, is over 100 km north-south and up to 25 km wide. It has intruded the project sequence in the northern boundary of the Pegmont lease. Present as several low outcrops of tors exhibiting spheroidal and onion skin weathering. The southern contact is inferred from airborne magnetics. The Squirrel Hills Granite is a non-foliated, porphyritic granite, composed of biotite and hornblende with coarse K feldspar. The Yellow Water Hole Granite, also part of the Williams Batholith, approximately 18 km from Pegmont, is dated at 1493±8 Ma, postdating main tectonism that affected the sequence.
Lead-zinc mineralization at Pegmont is contained within BIF's. The BIF’s consists of banded quartzmagnetite-fayalite-garnet- grunerite-hedenbergite-sulphide. Apatite, gahnite, and graphite are common minor minerals. Bedding is typically on a scale of 1 to 5 mm. In fresh rocks, the main sulphide minerals are galena, sphalerite, with subordinate pyrrhotite, pyrite and chalcopyrite. In contrast to the almandine garnet in the hangingwall and footwall, the garnet in the BIF is manganiferous spessartite, reflecting the higher whole rock MnO content of the BIF.
The overall morphology of the stratiform mineralized banded iron formation at Pegmont is a flat, gently easterly dipping sheet. The known mineralization extends approximately 2 km along strike and approximately 1 km in the down dip direction to the southwest. Mineralization is known to extend to a depth of 350 m below surface but remains open down dip.
The principal lead and zinc mineralized BIF is termed Lens B, it is around 2 to 8 m thick. At surface, the Lens B BIF is present in banded ferruginous, jaspery, manganiferous gossans. Outcrops occur in two areas termed the Mount Lucas Load and Burke Hinge Zone (BHZ). Additional oxide mineralization has been intersected at the Bonanza Lode, which is about 2 km to the northeast of Mount Lucas. Work by Scott and Jones shows that there has been considerable depletion of Mg, Ca, Na, K, S, Ag, Cd, and Zn during weathering. The least mobile elements have undergone residual concentration up to the profile, whereas concentrations of other elements are either unaffected by weathering or vary irregularly, in many cases reflecting local variations within the ore horizon. On oxidation, galena changes either through a pyromorphite ± cerussite assemblage to plumbogummite / corkite or directly to coronadite.
Oxidation reaches depths of 25 m below surface. The surface gossans display delicate boxworks after fayalitic olivine and are composed of goethite, clay, and secondary lead minerals such as pyromorphite, plumbogummite, plumbojarosite, and / or beaverite, together with isomorphous hydrated lead, iron, and copper sulphates. No secondary zinc minerals have been observed. Between 0.5 and 2% graphite has been noted. A yellow-green fibrous vein mineral contained in the gossans is thought to be nontronite.
The lead content of outcropping gossan is the same as, or slightly higher than that of un- weathered ore. Although zinc is strongly depleted at surface (×10), its greater abundance (×2 depletion) only 15 m below the surface reflects the truncated profile and immaturity of the gossan.
In the partially oxidized transition zone, between 25 – 40 m below the surface, galena, sphalerite, magnetite (invariably showing alteration to hematite and goethite), manganese minerals including pyrolysite, and small amounts of pyrite and graphite have been described along with montmorillonite and kaolin, garnet, mica and quartz, and minor amounts of siderite, amphibole, chlorite, and talc.
The un-weathered Lens B BIF consist mainly of galena and sphalerite associated with a finely laminated assemblage consisting of dominant magnetite and spessartite with subordinate iron-magnesium-manganese silicates and apatite. Gangue minerals are apatite, olivine (fayalite), garnet (spessartine – almandine), amphibole (hornblende and grunerite), clinopyrozene, biotite, and greenalite.
Approximately 10 to 40 m below Lens B is a second BIF horizon, Lens C, generally only weakly mineralized and when present is around 1 m thick and is separated from the Main BIF by garnet-bearing quartzite and schist. Lens C becomes more important in Zone 5 where it is present at thickness of around 4 m.
In Zone 5 there is indications of a further four mineralized BIFs below Lens C, to date they have been intersected on the south western most drill section, in an interpreted anticline position.