Plumbago Creek

Geological setting and mineral system

Plumbago Creek sits in the Emu Creek Block of the southern New England Orogen, where the Permian Bruxner Park Adamellite (Pc_b, NSW Seamless Geology / MinView) intrudes a Carboniferous–Early Permian limestone–shale–chert sequence and marmorises the carbonate lenses. Government regional mapping classifies the iron bodies as probable contact skarns formed by reaction between the granite and the limey Permian sediments (Brownlow 1989, R00004511, p. 11). Seven modern airborne layers resolve a coherent ~1,000 m × 400–500 m magnetic body at the historic workings, with a discrete K–U radiometric alteration halo and no coincident gravity high — consistent with an alteration-hosted replacement system, not a massive magmatic Fe core. The ≥5 discrete pods interpreted at depth along the NW–SE corridor are a geophysical inference from upward-continued magnetics: no historical hole has reached them — total subsurface investigation is 77 m of costean and adit, none beyond ~4 m below surface.

The grades — what the system has already demonstrated

Result (depths / widths, R-number)
Result (depths / widths, R-number)	Confirmation status / disclosed discrepancy
53.6–64.0 % Fe, hard silicified ironstone, over 4.88–20.12 m of continuous costean channel (samples 1–5); Mn 0.09–0.44 %.  BHP 1960, GS1960/024 (R00028669).	Government-lodged primary assay (BHP Newcastle Works), no discrepancy flagged. Channel is measured along the costean floor, not vertical depth (BHP penetrated ~4 m at most). Confirms a high-grade oxide but not its polymetallic tenor — no base/precious metals were assayed.
4.20 % and 7.33 % Mn with 0.135 % / 0.238 % P, southern adit faces (samples 6–7).  BHP 1960 (R00028669).	Same primary source; a one-to-two order-of-magnitude Mn rise outboard of the magnetite–haematite core — a recognised redox-margin vector in iron-oxide systems.
99 to 99.8 % CaCO₃ across five diamond holes (surface / industrial result).  Whitehouse Market Survey, GS1974/245 (R00022434).	Reported as related by the Whitehouse principals in the 1974 Market Survey — not the drill-log assay record. The original 1973–74 Hayes logs are a Phase 1 records-recovery target (synthesis Appendix D, VW-06).
33.28 g/t, 11.64 g/t and 4.30 g/t Au (rock samples 38700, 94, 95), 3–7 km E on the same granite contact.  Griffiths 1986 (R00015020).	District-significant adjacent-ground surface gold; sample 94 carries Sn 28 ppm, Cu 430 ppm, Zn 61 ppm — Sn–Cu–Zn pathfinders diagnostic of skarn / IOCG-affinity systems. Not on the project area; contextual, not a project grade.

Exploration history — nineteen recorded campaigns, documented exits

Period · operator (licence) · model targeted · documented departure
Period	Operator (licence)	Model targeted	Documented departure (R-number)
1960	BHP — A to P 2098 (R00028669)	Iron ore (steelmaking)	Iron-ore decision on a coarse 3-traverse magnetometer: “no major continuation of the ore bodies” — not a polymetallic assessment (R00028669).
1971	Fitzpatrick (R00024830)	Limestone	Quarry-scale limestone survey completed; commodity-specific brief.
1973–74	Whitehouse Mines / Langley (R00013586, R00022434)	Limestone purity + dimension stone	Five limestone diamond holes; programme completed and handed to production operators.
1978/79–86/87	David Mitchell-Melcann chain	Limestone production (Tabulam)	Market exit, not a resource exit — see §4 (Brownlow 1989, R00004511).
1980–86	Griffiths syndicate — EL 1656 (R00015020)	Polymetallic / skarn vectoring	Six-year field programme; capital exhausted in 1986 before the cable-tool drill could test the 1982 geophysical targets at the contact.
1988–91	Etcorp — Mallanganee EL (R00001761)	Industrial magnetite (coal-washing)	Adjacent Emu Creek skarn; failed coal-washing test; skarn never assayed for Au / Cu / W / Sn.
1989	Brownlow — GS1989/309 (R00004511)	Regional industrial-mineral synthesis	Government synthesis; classified the iron bodies as probable skarns (see §1, §4).
1990s–2018	Tabulam ML series	Limestone production	Production wind-down; 2008/09/17 reports record no work; two Unimin testwork trials negative for landscaping / specialty use.
2007–2023	Malachite (EL 6701), Ardent (EL 4760), Sentinel (EL 9080), Syndicate (EL 9153)	Desktop polymetallic / held ground	Four modern holders — desktop reviews only, zero new field samples in ~18 years; relinquished on JV or company-strategic grounds.
Every field campaign found the mineralisation its brief sought; every departure is documented as economic, market or strategic — not a negative geological finding. Government surveys (Raggatt 1939, Kenny 1950, Byrnes 1973) and pre-tenure lime workings are omitted for space; the full nineteen-campaign chronology is in Synthesis §4.1.

Why the grade is still available — in the operators’ own words

Wrong commodity, wrong model.  Each operator found what its brief sought and left for reasons that were economic, market or strategic. BHP (1960) closed the ground on an iron-ore economic frame after a coarse magnetometer reported “no major continuation of the ore bodies” (R00028669) — a conclusion modern airborne magnetics directly contradict. The Tabulam limestone producers left on the market: “operations have now ceased, apparently because of competition from deposits in southern Queensland” (Brownlow 1989, R00004511). Brownlow’s own regional mapping had already classified the iron bodies as probable contact skarns — the polymetallic reading was recorded, then foreclosed by the iron-only economics of the day.

The analytical gap.  CAADIA’s audit of the GSNSW open-file record finds that in 87 years no sample from the project area has ever been assayed for Cu, Au, Ag, Pb, Zn, REE, U, Bi, Mo, Co, Te, Sn or W: the entire historical assay inventory is Fe, SiO₂, Al₂O₃, CaO, MgO, Mn, P, S and Ti. The polymetallic tenor of the system has therefore never been measured, only inferred. Two independent lines confirm the pathfinders exist in the district: Griffiths’ adjacent-ground gold rock chips carry Sn, Cu and Zn (R00015020), and Etcorp’s heavy-mineral concentrates on the same skarn belt carry scheelite and cassiterite (R00001761).

The untested target.  Murdoch Geophysics (1982) outlined IP and drill targets on the Bruxner Park Adamellite contact for the Griffiths syndicate; the cable-tool drill reached site in 1985 but the licence ended in 1986 before those targets were drilled — the one formal drill recommendation on the system was never actioned. The result, in the synthesis’s precise formulation: not one hole has tested the ~1,000 m coherent airborne magnetic body beneath and along the ironstone; historical work sampled only the surface oxide, never beyond ~4 m below surface. 

The miss, precisely. No one tested this system and found it wanting — no one tested it at all. Each operator assayed only for the single commodity in its brief (iron, then limestone), so the base- and precious-metal content of the ironstone was never once measured; and the only formal drill target ever defined — the magnetic body at the granite contact — was never drilled before the licence lapsed. The polymetallic case was left open not by a negative result, but by the absence of the two tests that would settle it: a multi-element assay and one hole.

System scale — dimensions, and the model-class lens

Footprint (dimensions only).  The coherent magnetic body measures ~1,000 m × 400–500 m and resolves at depth into ≥5 discrete pods en-echelon along a NW–SE structural corridor; the mapped limestone–ironstone belt runs ~10 km NW. Historical sampling tested nothing below ~4 m vertical, whereas contact-skarn / IOCG replacement systems of this class typically extend hundreds of metres in depth — the tested interval is a thin surface skin over an essentially undrilled body.

Model-class analogue signature (belongs to the deposit class, not the project).  The synthesis states typical IOCG proximal-core signatures for the deposit class as Cu 100–10,000 ppm, Au 0.1–5 g/t, Ag 1–20 g/t, Bi 5–200 ppm and LREE 500–5,000 ppm (Synthesis §5.5.2), against unmineralised-metasediment backgrounds of Cu <50 ppm, Au <5 ppb and REE ~50–150 ppm. These are attributes of the model class used to set diagnostic thresholds — they are not measurements at Plumbago Creek, where none of these elements has yet been assayed

On value.  The synthesis makes no in-situ or price-based valuation argument, and none is offered here; the commercial case rests on multi-commodity exposure (Cu, Au, REE, U, Co, Bi, W, Sn) hedging single-commodity risk, on a high-purity CaCO₃ industrial floor, and on candidacy for NSW critical-mineral drilling co-funding at the drill stage (Synthesis §8.4).

The targets

Primary — the ~1,000 m magnetic body (Target A).  Its known limitation is stated plainly: airborne magnetics alone cannot distinguish a mineralised skarn / IOCG core from barren magnetite or hornfels, because both are magnetic. The designed test resolves this in three steps — a Phase 1 Th/U + pathfinder ICP-MS diagnostic on retained BHP pulps discriminates iron-oxide provenance; Phase 2 gradient-array IP adds a sulphide (chargeability) discriminator; and holes PLB-DD01 (300 m) and PLB-DD02 (250 m) drill the body below oxide with full multi-element assay to test it directly.

Secondary.  Target B — the southern high-Mn / high-P redox margin (BHP samples 6–7), tested by PLB-DD03 (200 m) for a sulphide-bearing core.  Target C — the granite–limestone contact skarn, tested by PLB-DD04 (200 m), which also confirms the high-purity limestone floor at depth.

Testing the thesis — gates, kill criteria, cost to answer

Phase + cost · scope · gate & kill criterion
Phase + cost	Scope	Gate & kill criterion
Phase 1 — pulp re-assayA$3,000–14,000	ME-MS61 + ME-MS81 + Au multi-element ICP-MS on seven retained BHP 1960 ironstone pulps; ~3–4 weeks.	Binary. Gate: Th/U <1 with Cu/Au/REE elevated → proceed to Phase 2. Kill: Th/U >3 with pathfinders at background → IOCG hypothesis closed; project re-frames to the high-purity limestone floor. The kill answer for the whole polymetallic thesis costs under A$14,000.
Phase 2 — ground geophysics + soilA$55,000–100,000	Ground magnetics, gradient-array IP and a soil grid to rank pods and fix collars; ~6–10 weeks.	Gate: coincident IP + soil anomaly on a flanking pod → drill. No anomaly → defer drilling, re-rank targets; spend stops before the A$0.5M+ drilling commitment.
Phase 3 — diamond drillingA$545,000–815,000(central ~A$680k; ~950 m)	Four HQ/NQ diamond holes: PLB-DD01 300 m + DD02 250 m (Target A), DD03 200 m (B), DD04 200 m (C). QAQC: CRMs, blanks and field/pulp duplicates ≥1-in-20; chain of custody; half-core and residues lodged at the Londonderry Core Library.	Per-target kill criteria (Drill Design v1.0 §13): Target A barren magnetite → reverts to an iron-only prospect; B no bedrock sulphide → surface redox feature; C no skarn → limestone floor stands on grade. By design, “a small number of well-placed holes can either advance or retire each target on evidence.” Candidate for NSW critical-mineral drilling co-funding.

Technical Manager: Dr Tim McConachy FAusIMM (1974), Senior Fellow SEG, former Rio Tinto Chief Geologist.

Data room (under NDA): technical synthesis v4.1, Phase 1 drill programme design, source register and figure set.

Contact: Doug Alcock, Managing Director, CAADIA Metals Pty Ltd — dalcock@caadiametals.com.au

No JORC (2012) Mineral Resource, Ore Reserve or Exploration Target is declared or implied. Historical results are compiled from GSNSW open-file (DIGS) records, have not been verified by a Competent Person for public reporting, and are provided for context only. The IRGS / distal-replacement interpretation is a working hypothesis under test; alternative interpretations of the geophysical data exist. Earn-in terms are indicative and non-binding, subject to contract, due diligence and grant of the Exploration Licence.