Summary:
The Woodlawn deposit is a stratiform volcanogenic massive sulphide (VMS) deposit that is hosted within the central part of the mid Silurian to early Devonian Goulburn Basin: a deep water, back-arc basin which developed within Ordovician to early Silurian sediments of the Lachlan Fold Belt that hosts numerous metalliferous deposits. Woodlawn lies on the eastern limb of the asymmetric north-northwest plunging Woodlawn Syncline. Mineralisation for base metal (copper, zinc, lead) and precious metal (silver, gold) is hosted in regionally metamorphosed (greenschist facies) fine to coarse-grained felsic to intermediate volcanic rocks, volcanogenic sedimentary rocks and minor carbonaceous shale, known as the Woodlawn Volcanics.
Three mineralised horizons (Lower, Middle and Upper) hosting 11 known massive sulphide lenses occur within a 400 m × 600 m wide and 900 m deep northwest plunging corridor which remains open at depth.
The updated and new domains occur within the Lower, Middle and Upper horizons. New domains were assigned to Lower, Middle and Upper horizons based on spatial orientation and proximity to known lenses. Updated domains comprised massive sulphide and stringer mineralisation extensions laterally or down plunge within B,C, D, J, H and I lenses.
Exploration drilling by Develop targeting extensions to the north of existing corridor has resulted in the discovery and interpretation of two new massive sulphide lenses increasing the number of known massive sulphide lenses from 11 to 13. The two new lenses have been provisionally assigned to the Lower Horizon based on proximity to B lens however further work on mineralogy and geochemistry is required to confirm the assigned horizon.
Mineralisation domains were interpreted primarily on geological and mineralisation characterisation models defined by downhole geological contacts, and were based on lithology, sulphide characterisation (and distribution), grade tenor, structural model and review of historical void geometries. Using this approach, two key mineralisation styles were interpreted, massive sulphide and stringer mineralisation. It was noted these styles were also historically documented by Heron and recognised by Entech during the site visit and review of drill core photographs. The two mineralisation styles comprise the following assemblages:
* Polymetallic mineralisation: fine- to medium-grained, massive (and banded) pyrite–sphalerite–galena–lesser chalcopyrite, with the gangue mineralogy including iron, talc, quartz, chlorite, phlogopite, muscovite and barite.
* Copper mineralisation: includes pyrite–chalcopyrite, lesser pyrrhotite as well as chlorite, quartz and calcite as massive sulphide and stringer veins.
Lithology and structure are considered the predominant controls on base and precious metals, and gangue (iron) mineralisation at the Woodlawn deposit.
- Zinc, lead and copper mineralisation is primarily associated with the polymetallic assemblage in the massive sulphide lenses. The mineralisation often comprises massive pyrite and has splays and thickened zones, which may be associated with faulting. Massive sulphide mineralisation may contain assays grading above 20% zinc, with copper and lead grades of several percent.
- The copper-rich assemblages are spatially located coincident within the massive sulphide footwall, or as stringer veins proximal to the footwall or hanging wall of the massive sulphides. It was noted by Entech that the stringer mineralisation style occurred primarily in felsic and metasediment hosts.
- Gold and silver mineralisation is associated both with massive sulphide and stringer mineralisation styles. The tenor of these metals was primarily related to their location within the horizon (Lower, Middle or Upper) and not by mineralisation style.
Several northwest-trending faults impact the strike and dip continuity of known lenses. Entech noted multiple instances of lenses structurally offset by these faults both in documentation and mapping of underground drives. Entech used historical (Heron) structural modelling to ensure interpreted mineralisation continuity accurately represented localised lens offsets. Extrapolated faults with sparse drill hole support from the historical structural model have been superseded by new drilling information for mineralisation extensions at depth. In areas where mineralisation remained open at depth, down plunge continuity was extended to a maximum of 40m from the last drillhole intersection and lateral continuity (up or down dip) was extended < 5m from the nearest drillhole. Lateral continuity assumptions have been made based on geometries and extents of known lenses.
In all, 17 existing domains were updated and 12 new domains interpreted. New and updated massive and stringer mineralisation were grouped as per historical nomenclature into lenses B, C, D,K, H and I. The mineralised lenses are grouped by Lower, Middle and Upper Horizons as follows:
* Lower: B,C and J lenses;
* Middle: D and Kate (K) lenses;
* Upper: H and I lenses.
Dimensions
The mineralisation extent of the Woodlawn deposit comprises three mineralised horizons (Lower, Middle and Upper) hosting thirteen known massive sulphide lenses occur within a 400 m × 600 m wide and 900 m deep northwest plunging corridor which remains open at depth. Across-strike widths vary from 1 m to <35 m.
The MRE for zinc, lead, copper, silver and gold on which this Table 1 is based has the following extents:
• Above 1800 mRL;
• From 8750 mE to 10050 mE;
• From 18950 mN to 19850 mN.
Production
Production from July 1, 2019, during the 9 months preceding the suspension of operations. On 25 March 2020, Heron announced the suspension of operations at Woodlawn, only 5 months after the sale of the first concentrate produced from the new process plant.
| Commodity | Product | Units | 2020 | Avg. Annual | LOM |
|
Zinc
|
Payable metal
|
M lbs
| | 79 | 481 |
|
Copper
|
Payable metal
|
M lbs
| | 26 | 176 |
|
Silver
|
Payable metal
|
M oz
| | | 4.8 |
|
Copper Equivalent
|
Payable metal
|
kt
| | 20 | |