# The nugget effect

> coarse-gold scatter is geology, not QC failure

Source: https://geodb.io/guide/topic/qaqc_nugget_effect/ · Category: QAQC
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.