This setting will enable you to purchase and either prospect or place industries which create raw materials.
Normally, you are only allowed to purchase industries which process materials, e.g. Factories, Sawmills, etc. When this setting is enabled, you are also allowed to build raw-material producing industries such as coal mines, iron ore mines, etc.
When the prospecting construction method is selected, you will be able to purchase a new industry of the selected type, but the industry will appear in a random location on the map. There is a chance that prospection may fail, which means that the money will be spent, but the industry will not be constructed.
This does not affect which types of "naturally occurring" industries show up, just which ones you are able to purchase.
Construction of primary industries is disabled by default. When enabled, the list of industries you can purchase will include industries which produce the raw materials, with appropriate pricing. Raw industries are are about 8x more expensive to build at a specific location than to randomly prospect. You can enable this setting through the Advanced Settings -> Economy -> Industries -> Manual primary industry construction method window or use the following syntax in Openttd.cfg:
[construction] raw_industry_construction = 2
0 = none, 1 = as other industries, 2 = prospecting
Valid options: [ 0 | 1 | 2 ]
It can also be set in the console using the command (single player games only):
set raw_industry_construction <value>
List of fundable Primary Industries
|T||S||_||_||Coal Mine||£ 4,921,875||Produces Coal to be sent to Power Plants.|
|T||S||_||_||Forest||£ 4,687,500||Produces Wood to be sent to Sawmill.|
|T||_||_||_||Oil Rig||£ 5,625,000||Obviously this may only be built at sea.|
|T||S||R||_||Farm||£ 5,859,375||Produces Grain and Livestock to be sent to Factories.|
|T||S||R||_||Oil Wells||£ 5,156,250||Produces Oil to be sent to Oil Refineries.|
|T||_||_||_||Iron Ore Mine||£ 5,156,250||Produces Iron Ore to be sent to Steel Mills.|
|_||S||_||_||Gold Mine||£ 4,875,000||Produces Gold to be sent to Banks.|
|_||_||R||_||Copper Ore Mine||£ 4,804,687||Produces Copper Ore to be sent to Factories.|
|_||_||R||_||Diamond Mine||£ 4,992,187||Produces Diamonds to be sent to Banks.|
|_||_||R||_||Fruit Plantation||£ 5,273,437||Produces Fruit to be delivered to Food Processing Plants.|
|_||_||R||_||Lumber Mill||£ 395,507||Is technically classified as secondary industry.|
|_||_||R||_||Rubber Plantation||£ 5,109,375||Produces Rubber to be delivered to Factories.|
|_||_||R||_||Water Supply||£ 4,664,062||Produces Water to be delivered to Water Towers.|
With inflation, the cost of specific industries will rise (or have already risen). The costs shown are the costs to build the industry assuming you can choose the location where you want the industry to be built. If the prospecting method is enabled, the costs are divided by a factor of 8 (approximately).