This is a working worldresourcemod.xml (it WILL add elementium nodes to fresh worlds)
<?xml version="1.0" encoding="utf-8"?>
<ResourceTypes>
<DataChecksum NoParse="1">
<Ignore>DispName</Ignore>
<Translate>DispName,Description</Translate>
</DataChecksum>
<!--**********************************************-->
<!--************** Core Resources ****************-->
<!--**********************************************-->
<ResourceType InternalName="ElementiumOre">
<DisplayName>Elementium Deposit</DisplayName>
<Type>ElementiumOre</Type>
<Description>Building a Mine here produces one Elementium every ten turns.</Description>
<LongDescription>Elementium is the stuff of legends. Even when located, it requires excessive amounts of labour to harvest, due to how little is found in the ground.</LongDescription>
<PreferredTerrain>Land</PreferredTerrain>
<NeedsToBeUnlocked>0</NeedsToBeUnlocked>
<HarvestType>Mining</HarvestType>
<IconColor>0,0,0</IconColor>
<Icon>Gfx//Icons//Icon_Metal.png</Icon>
<ClothIcon>gfx\\TacticalIcons\\Res_VentiOre1.png</ClothIcon>
<TileDesign>R_Venrti_01</TileDesign>
<ModelColor>32,0,0</ModelColor>
<Production>1.0</Production>
<Worth>5</Worth>
<Global>0</Global>
<Rarity>50</Rarity>
<Stored>0</Stored>
<RummagedPerTurn>1.0</RummagedPerTurn>
<Medallions InternalName="IronOre_Res_Medallions">
<All>Res_Ores_Plains.png</All>
</Medallions>
<OnSelectSFX>Click_IronOre_01</OnSelectSFX>
</ResourceType>
</ResourceTypes>
And this is a working (cross-faction) building to mine it.
<?xml version="1.0" encoding="ISO-8859-1"?>
<ImprovementTypes>
<DataChecksum NoParse="1">
<Ignore>DisplayName</Ignore>
<Translate>DisplayName</Translate>
</DataChecksum>
<!-- ************ -->
<!-- ** Stable ** -->
<!-- ************ -->
<ImprovementType InternalName="ElementiumMine_testmod">
<!-- Improvement Data -->
<DisplayName>Elementium Mine</DisplayName>
<Description>Elementium Mines produce one Elementium every ten turns.</Description>
<TileDesign>K_Stables_01</TileDesign>
<ConstructionTileDesign>K_Stables_Build_01</ConstructionTileDesign>
<SupportedTerrainType>Land</SupportedTerrainType>
<SupportedTerrainType>City</SupportedTerrainType>
<DrawnIcon>Gfx/TacticalIcons/K_Stable1.png</DrawnIcon>
<DrawnIconConstruction>Gfx/TacticalIcons/K_Stable1_C.png</DrawnIconConstruction>
<Thumbnail>Gfx/Medallions/Resource_Thumb.png</Thumbnail>
<Medallions InternalName="">
<All>Gfx/Medallions/Resource_Thumb.png</All>
<Border_All></Border_All>
</Medallions>
<!-- Prerequisites -->
<RequiresCity>False</RequiresCity>
<RequiresResource>True</RequiresResource>
<SupportedResourceType>ElementiumOre</SupportedResourceType>
<!-- Cost -->
<LaborToBuild>15.0</LaborToBuild>
<GameModifier>
<ModType>ConstructionResourceCost</ModType>
<Attribute>Gold</Attribute>
<Value>-500</Value>
<PerTurn>0</PerTurn>
</GameModifier>
<!-- Production -->
<GameModifier>
<ModType>Resource</ModType>
<Attribute>Elementium</Attribute>
<Value>0.1</Value>
<PerTurn>1</PerTurn>
</GameModifier>
<AIData AIPersonality="AI_General">
<AITag>Farming</AITag>
</AIData>
<!-- Base Improvement Modifiers -->
</ImprovementType>
Note that I've not updated or changed any of the icons, so it will look like a ventri ore when unbuilt, and like a kingdom stable when built. I ignored those things and just tried to get it working.