Limestone (a carbonate rock) is chemically reactive, so when hot fluids from an intrusion — a body of magma that pushed into older rock and cooled underground, also called a pluton — hit it, dramatic things happen. Two related deposit types result, and both depend on a carbonate host rock.

Skarn — right at the contact

Where the hot intrusion actually touches the limestone, the rock is cooked and chemically rebuilt into a new rock of distinctive minerals — a skarn. Depending on the system, the metal dumped here can be copper, iron, tungsten, zinc, or gold. Skarns hug the intrusion–limestone contact.

CRD — farther out in the limestone

CRD stands for Carbonate Replacement Deposit. Farther from the intrusion, the same fluids travel along the limestone and replace it with massive sulfide ore — typically lead, zinc, and silver. "Replacement" means the ore literally takes the place of the dissolved limestone, often as nearly-solid bodies, chimneys, and sheets.

Think of it as a distance series from the heat source: skarn hugs the contact; CRD forms in the carbonate beyond it; and even farther and cooler, low-temperature lead-zinc deposits called MVT (Mississippi Valley Type) form from basinal brines, not directly from the intrusion.