Having played board games since the 1960s (the original Avalon-Hill games a then Simulations Publications/Poltroon Press games) I will repeat what I've said elsewhere that making a computer game into a board game shows an utter lack of appreciation for what can be done with computer games. The complexity that would make a board game practically unplayable (take a look at the Europa series of WWII board games for a historical example of Gargantuan Complexity run wild) can be handled by the computer and made practically invisible to the gamer - IF you start out recognizing that the computer game has access to resources of information-handling that in a board game have to be dumped onto the gamer.
Still CIV series sells as a simple game when from MODs we know it could be much more.
People talk about the trend in the game industry about new games being easier and more casual, CIV6 with colorfull cartoon design, poor AI and the policy of "MOD it yourself" tell us that CIV dont really need to be much more that a digital board game.
When Copper is visible on the map, each Copper Resource worked after, say, Bronze Working Tech provides +1 Religion OR +1 Culture (+1 Religion only after you have a founded Religion - it adds pyrotechnics and fumigants to your ceremonies, but only after you have established ceremonies)
I use Bronze Working because the use of Tin-Copper alloy Bronze comes at about the same time as earliest indications of the use of Sulphur as a fumigant, and it presupposes the ability to 'cook' or smelt copper ores and pyrites to release Sulphur in small but useful quantities.
So Copper mines produce Sulfur when you have Bronze tech or is the resulting Religion/Culture yield representing it?
Anyway, in the first case produce from copper a second resource that only gives Religion/Culture just add a step for the same bonus. In the second scenario Copper had ritual value by itself since before Bronze working.
In the early Medieval (beginning of the 9th Century CE IRL) with Alchemy Tech Sulphur becomes visible on the map as a Deposit of gleaming and smoking yellow. The deposits will be located within the radius of action of Volcanoes, active or dormant, can be worked with Mines. IF worked before Gunpowder is discovered (which, if the game allows an Alternate Tech Tree instead of the Euro-centric Tech they've followed so far, would allow primitive Gunpowder at this same time as a Pyrotechnic additive to ranged weapons [fire arrows for crossbows and bows, catapults, etc] giving them a bonus attack factor) the Sulphur gives +2 Science and +1 Food (not really 'Food', but more population because the medicinal use of Sulphur lowers the death rate, increasing the population)
This is the regular CIV model.
With Gunpowder Tech (Late Medieval ina Euro-centric Tech Tree), Sulphur becomes a Resource for gunpowder Units - but since it can still be obtained without Deposits, you can accumulate 1 'point' of Sulphur to build units every turn for every 2 Copper Deposits worked, but the cost of the gunpowder unit is increased by 25%. The gamer doesn't have to calculate this - he/she just knows that it's taking Forever for his copper workings to get enough Sulphur to build one Bombard, while his neighbor with the bright yellow deposits around Mount Tahoma is cranking out Bombards and Arquebusiers like crazy.
Strategic resources are supposed to promote player actions to obtain it by trade, expansion or war, so INTERACTION with other players and the map.
Still if the objetive is to not limit the production of units just give natural Sulfur a huge discount to gunpowder units, not even the game need to calculate that.
In the Industrial Era, with Industrial Chemistry, new 'deposits' of Sulphur become visible as the salt dome deposits become available, but at about the same time smokeless powder is developed and the (Magazine) Rifleman and (Modern) Artillery units no longer require Sulphur. But, each point of Sulphur per turn now provides bonuses to Food production on Farms and Plantations (Fertilizers) and a bonus to Production in Factories (Sulphur in industrial processes of all kinds).
Obsolescence for new units is the regular system. Bonus for food works but is kind of redundant with its base yield and similitud to Niter role.
Now ERA DESIGN, Sulfur even if not 100% historical become in another reason expand/trade for resources in the colonial time of the European Colonial Empires and the Asiatic Gunpowder Dynasties. For later eras build Industrial districts that produce synthetic Sufur, Niter, Dyes and Rubber is a good way to represent how colonial resources were remplazed by the industrial version and actually see your cities growth to metropolis on the proper eras.
By late game sulfur is still usefull by their yield/bonus, but now the free tiles are needed to serch for oil, uranium, aluminum and titanium.
