Summary:
The Tom and Jason deposits are examples of stratiform, strata-bound sediment-hosted, exhalative (“SEDEX”) zinc-lead-silver-barite deposits (Goodfellow et al., 1993; Leach et al., 2005; Goodfellow et al., 2007; Goodfellow, 2007).
Zinc-lead-silver-barite mineralisation at the Tom deposit varies from well laminated and stratiform (parallel to sedimentary layering) to a brecciated stockwork zone adjacent to the Tom normal fault. The Tom West and Tom East zones, both of which are exposed at surface, are interpreted to have formed one continuous strata-bound controlled lens prior to folding and faulting of the Tom Sequence, whereas the Southeast Zone is interpreted to have formed in a separate sub-basin to the main graben structure hosting the Tom West and Tom East zones (Goodfellow, 1991). All three zones have been affected by folding, with evidence for the possible development of a crenulation cleavage as opposed to the chaotic folding of laminae due to soft-sediment deformation. Ferroan carbonate alteration and quartz veining are common in footwall conglomerates near vent facies at Tom West.
The Tom West Zone dips 60° to the southwest, has a strike extent of approximately 1 km and extends up to 400 m down dip. It is about 40 m thick at its widest point and breaks into two discrete layers in the centre at depth. Contacts vary from transition over <1 m or are faulted and abrupt. The highest-grade portion of the Tom West Zone occurs along the southern and near surface portion of the zone where Pb+Zn grades exceed 10% with elevated silver. The Tom West Zone hosts the bulk of the resource at the Tom deposit.
The Tom West Zone can be divided into a series of mineralization facies (after Goodfellow, 1991; 2007) consisting of:
• Vent facies – Stockwork of pyrite, pyrrhotite, galena, sphalerite, with minor chalcopyrite, arsenopyrite and tetrahedrite with a gangue of ferroan carbonates, quartz and barite subdivided into five types, including an upper high-grade zone with 15–30% Pb+Zn, Ag between 150 g/t and 200 g/t and a low Zn/(Zn+Pb) ratio.
• Pink facies – Interbedded barite, chert, cream-coloured sphalerite, fine grained pyrite and black Bacarbonate, overprinted by pink and yellow sphalerite resulting in locally high grades in the range of 10–30% combined Pb and Zn.
• Gray facies – Interbedded pink sphalerite, fine grained galena and pyrite, white to pale gray barite, pale grey chert and grey to white Ba-carbonate/Ba-feldspar, typically with grades in the range of 4–5% Pb+Zn with negligible Ag.
• Black facies – Black mudstone and chert interbedded with barite, witherite (Ba-carbonate) and finegrained sphalerite, galena and pyrite, typically with grades in the 4–10% Pb+Zn range and a high Zn/(Pb+Zn) ratio.
The Tom East Zone occurs near the hinge of the anticline that has folded the originally planar deposit, and which plunges northward in this area. It consists of interbedded high-grade sphalerite, galena, barite and chert thought to have formed within the same stratigraphic interval as Tom West (McClay and Bidwell, 1986).
The Tom Southeast Zone is not exposed at surface, and consists of a tabular, stratiform body 0.5 m to 6 m thick with a strike length of approximately 400 m and a down-dip extension of at least 350 m dipping 60–70° to the east. It is located near the nose of the southeast-plunging Tom anticline on its eastern limb. Mineralisation consists of finely laminated sphalerite, galena, pyrite and black cherty mudstone (Goodfellow, 1991).
The Jason Main Zone is located on the northern limb of the east-plunging Jason syncline, while the Jason South Zone occurs on the southern limb. The South Zone consists of two separate horizons whereas the Main zone is defined by a single horizon. These two separate zones are likely connected through the hinge of a syncline, but this has yet to be demonstrated through drilling. These horizons can be divided into several distinct mineralisation facies (zones), including (after Turner, 1991):
• Pb-Zn-Fe sulphide facies – Massive, banded sphalerite-galena and galena-pyrite overlain by debris flow deposits containing clasts of earlier deposited massive sulphides.
• Barite-sulphide facies – Interbedded fine-grained sphalerite, galena, barite, chert and ferroan carbonate forming the bulk of the mineralisation at Jason.
• Quartz-sulphide facies – Interbedded sphalerite, pyrite, quartz and carbonaceous chert with quartzcelsian (barium feldspar) bands in the lower lens.
• Massive pyrite facies – Massive pyrite beds interbedded with sphalerite, galena, chalcopyrite, pyrrhotite and quartz located near the Jason Fault.
• Ferroan carbonate facies – Massive beds of siderite and ankerite up to several metres across with irregularly distributed galena, sphalerite, pyrrhotite, pyrite, quartz, muscovite and pyrobitumen; spatially associated with a breccia pipe.