Niger

Total: ASM populataion numbers in 1999 (440,000) and 2016 (450,000) are inconsistent as no growth is unlikely. Most likely 1999 is overestimated and 2016 underestimated not taking into account the ongoing gold rush. Given the 2021 estimates of CEDEAO it appears reasonable to raise the estimation to at least 600,000
Min/max: Maintaining 450,000 as lower boundary. Upper boundary deducted from 2021 CEDEAO estimations as follows: With a population of 24 million and a household size of 6 we can assume 4 million households. 10% would be 400,000 households. With 2 persons per ASM household engaged in ASM (man and elderst son), up to 800,000 ASM miners are believed possible. → 450,000 - 800,000
Female participation: 0
Gold: All ASM appears to be ASGM → 600,000

Reported increase of ASGM to 10% of population by https://www.agenceecofin.com/gestion-publique/3107-90444-selon-la-cedeao-10-de-la-population-du-niger-du-mali-et-du-burkina-faso-dependent-de-l-orpaillage-artisanal

 10 % de la population du Niger, du Mali et du Burkina Faso dépendent de l’orpaillage artisanal sur plus de 1000 sites informels », indique Sédiko Douka, Commissaire de la CEDEAO chargé de l'Energie et des Mines, dans une interview accordée au quotidien d’État nigérien ‘’Le sahel’’, mercredi 28 juillet dernier. Considéré comme l’activité minière la plus préoccupante particulièrement au niveau des pays du Sahel, « l’orpaillage clandestin est quasiment à la même proportion que l’exploitation industrielle pratiquée par les grands groupes légalement constitués et reconnus avec des permis formels », souligne Sédiko Douka.

(IGF-18): Table 3

Directly working in ASM: 450,000
Estimated number of dependants: 2,700,000
Main Minerals: Gold

https://doi.org/10.4000/echogeo.14933 describes the ongoing gold rush in Saharan countries, particularly in Niger
Data from the Ténéré desert and the Aïr massif:

All these Saharan rushes seem to have the same characteristics, but it would seem that none has the magnitude of those experienced in northern Niger.
... absence of women on the sites ...
As gold panning is an informal activity, it is difficult to assess the number of people who engage in it. According to several informants, it is probably more than 70,000 people who went to the sites in search of gold (very able-bodied men mainly between 14 and 50 years old). 

(HIL-16):

Directly working in ASM: 450,000
Estimated number of dependants: 2,700,000

(UBC-14): Gold-only data produced by statistic modelling:

Expected ASM population: 291,000 
Estimated ASM gold production: 1 t/a

(ILO-99): Employment: 440,000
Number of mines: 150
Illegal mines: > 95%

Videos:
https://www.youtube.com/watch?v=u7mH-UtTQB8&list=PLW0k64ZQOWlqP4iQyLrS9_NHkrUi411cy

Articles:
https://journals.openedition.org/echogeo/14933
https://www.agenceecofin.com/gestion-publique/3107-90444-selon-la-cedeao-10-de-la-population-du-niger-du-mali-et-du-burkina-faso-dependent-de-l-orpaillage-artisanal