Here's a "World Affairs" report from 1982 where the Pentagon
said one of the main motives of the Soviet invasion of Afghanistan
was it's strategic minerals and metals.

"The Russian Rope: Soviet Economic Motives and the Subversion of Afghanistan"

Journal article by Abdul Tawab Assifi
World Affairs, Vol. 145, 1982

"Geographically, Afghanistan is a landlocked, mountainous country that historians refer to as the heart of Asia.

Geopolitically, it has been a country forgotten by the rest of the world. Geostrategically, Afghanistan has the position of a fulcrum, the significance of which will be felt when it is displaced or destroyed.

The Soviet move into Afghanistan was designed by Kremlin leaders to tip the strategic balance of power. World conditions had provided them with a long-sought opportunity to achieve several objectives with one stroke.

A hazy realization has emerged regarding Soviet military and geopolitical motivations in Afghanistan, but as yet, almost no attention has been paid to the economic aspects.

Yet, just as the Soviets spent decades preparing the way for a political takeover and military control, so they also spent decades preparing for the economic exploitation of Afghanistan's many resources, especially its highly valuable mineral resources."