Core Shack › Big Hill (Broken Hill Cobalt Project)
Big Hill (Broken Hill Cobalt Project)
Stratiform cobaltiferous pyrite
The cobalt here is invisible, because it is hiding inside somebody else's crystal lattice.
Drillhole viewer
Every hole GSNSW holds within 500 m of the deposit centre. Drag to orbit, scroll to zoom, tap a collar for its details. Names are masked.
Collars on a local grid in metres from the deposit centre. The white trace is surveyed; the rest are drawn vertical.
What the core actually looks like
Hyperspectral mineralogy and core photography from the Geological Survey of NSW National Virtual Core Library. Shortwave infrared, dominant mineral group per 250 mm interval.
Cobalt without a cobalt mineral
Big Hill, part of the Broken Hill Cobalt Project in the Pyrite Hill–Big Hill area west of Broken Hill, is described by GSNSW as stratiform cobaltiferous pyrite. Read that phrase carefully, because it is doing a lot of work.
There is no cobalt mineral to look for. The cobalt substitutes into the pyrite structure, sitting in for iron. The rock that carries the grade looks exactly like the rock that does not: pyrite.
For a grade guessing competition this is the hardest possible case, and the most instructive one. Whatever you estimate here, you are not estimating it by eye. You are reasoning from setting, from sulphide abundance, and from what you know about how these systems concentrate metal.
Laid down, then rebuilt
The GSNSW record classes Big Hill as a turbidite basin-hosted stratiform base metal deposit with a reduced sedimentary-exhalative genesis — metal deposited on or near the sea floor from hot, reduced, metal-bearing brines venting into a basin.
Ore texture is logged as layered, relation to host rock as stratiform: the mineralisation follows the bedding because it was part of the bedding. Then the Broken Hill Block did what it does, and the gangue tells you so — plagioclase, biotite, garnet and quartz is a high-grade metamorphic assemblage, not a sedimentary one.
Alongside cobalt, GSNSW lists nickel, silver, sulphur, lead and gold. Pyrite, chalcopyrite, galena and sphalerite make up the sulphides.
Forty years of nothing, then batteries
The drilling history is short and it is a commodity story. The GSNSW-held holes at Big Hill were drilled across the 1970s and into 1980 — a handful of diamond holes, chasing an induced polarisation anomaly, in an era when cobalt was a by-product nobody was targeting.
Then the record goes quiet for roughly four decades. The core that has been hyperspectrally scanned here dates from 2017, which is precisely when the battery-metals cycle turned cobalt from a footnote into a reason to re-log old ground.
What this kind of deposit looks like
An idealised model of the deposit type, built from the GSNSW description — shape, structure, mineralisation style and alteration. Drag it around.
What to look for in the tray
- Pyrite abundance and habit — because the cobalt is in it, not beside it.
- Layering and bedding-parallel sulphide, the stratiform signature.
- Garnet and biotite in the gangue: the metamorphic overprint on a sedimentary orebody.
Grade guessing competition
Have a crack at the Big Hill (Broken Hill Cobalt Project) tray
Running across both days at the Geological Survey of NSW booth — booths 20 and 21, Exhibit Room B.
- Look at the core. Prospect and deposit type are on the card; hole ID and assays are not, on purpose.
- Write your estimate on an entry slip at the booth. There is a category per commodity.
- Entries are open day 1 and day 2.
- GSNSW hold the assays until the prize presentation on day 2. A bottle of wine per category, supplied by the conference committee.
The GSNSW record
Straight from the Geological Survey of NSW mineral occurrence database, reproduced as supplied.
- District
- Pyrite Hill-Big Hill
- Deposit size
- Large
- Major commodities
- Co, Ag, Sulphur, Pb
- Minor commodities
- Au, Ni
- Ore mineralogy
- pyrite (chalcopyrite, galena, sphalerite)
- Gangue
- plagioclase (biotite, garnet, quartz)
- Ore texture
- layered
- Relation to host rock
- stratiform
- Mineralisation style
- massive, breccia fill
- Deposit class (NSW)
- Turbidite (volcanic) basin-hosted stratiform base metal (Zn-Pb-Ag-Au-barite)
- Deposit genesis (NSW)
- Reduced sedimentary-exhalative systems
When it was drilled
What is on this page, and what is not. Drillhole names are masked and shown as sequential identifiers. There are no assay results here, and no map coordinates — collar positions are plotted on a local grid in metres from the deposit centre, rounded to 5 m. That is deliberate: the competition only works if nobody can look up the answer from their phone while standing at the tray.
Everything else — deposit type, commodity, mineralogy, alteration, drilling history — is open-file information published by the Geological Survey of NSW.
Source: Geological Survey of NSW (MinView), supplied 21 August 2026. Reproduced with permission. MetaGeo sponsors the Core Shack at Mines and Wines 2026.