Ann: Goulamina Drilling Returns Positive Results, page-155

weathered means the grade/quality has been reduced/corroded due to the weather/rain.

kinda like paint on a house... the pain on the outside is exposed to rain/sun and fades away quicker.
the paint on the inside will tend to last longer due to better conditions.

as such, the spodument lithium closer to the surface is "weathered" due to water/rain and the grade is corroded.
the deeper you go, the less weathering there will be.