The nugget effect

In coarse-gold (and other nuggety) mineralisation, wide field-duplicate scatter is often geology, not a QC failure. Two different effects hide under one name:

• Geological nugget effect (GNE) — real in-situ variability; no sampling protocol can remove it.
• Sampling nugget effect (SNE) — variability added by poor protocol (above all, insufficient sample mass).

QC that can't separate these will misattribute geology to the lab — Bishi: nuggety deposits "will almost certainly give challenges in attaining high precision… evaluated on its unique merits." There is no universal duplicate tolerance.

The landmark case

New Found Gold (2022): a suspected bias from 30 half-core duplicates triggered an independent review (Bloom & Srivastava). Result: no systematic bias — of 37 visible-gold samples, 17 were higher in one half, 20 in the other. Even with >12,000 screened-metallic assays on ≥3 kg samples, duplicates "may differ by a factor of five or more." The scatter was the deposit, not the lab.

A usable flag

Bishi's implementable rule: flag pairs with >100% variation AND at least one value above 3 ppm as likely nugget effect, and send them to pulp-metallic (screen fire) assay — flag and investigate, don't fail.

In geoDB

geoDB applies the standard RPD tolerances to all duplicate pairs — it has no nugget-effect flag, so in nuggety mineralisation expect real geology to show up as duplicate "failures." Before treating those as lab problems, look at the grades involved and your deposit style; per-element threshold overrides on the project are the current lever for deliberately widening tolerance on nuggety elements.