Skarn Deposits

Other kinds of deposits are related to intrusions. Skarns form at the contact between an intrusive rock and a carbonate rock or a clastic sediment rich in carbonate. They are zones with irregular shape, and have a characteristic mineral composition – calcium, magnesium and iron silicates. Skarns may contain iron, gold or zinc, but are particularly important as hosts of copper, molybdenum, tungsten or tin ores. They frequently occur at the margins of the intrusions that create porphyry copper deposits.

Granite-Hosted Deposits

Deposits of tin, tungsten, molybdenum or uranium commonly occur in and around granites, either veins or very coarse granite dykes, called pegmatites. These coarse-grained, granitic bodies are a source of many rare minerals, several of which can be cut into gems. Pegmatites are commonly found as dykes in a large mass of plutonic rock of finer grain size. These deposits can also be useful sources of fluorite or gemstones. The tin deposits of southwestern Britain are an example of these deposits.

Besides pegmatites, minable quantities of emeralds, rubies and sapphires have been found in crystalline limestones, mica schists, syenites and other rocks. 

Replacement Deposits

Replacement deposits are bodies of rock that have been “replaced” by ore. To be more precise, the ore-forming fluids have migrated out through the porous rock (usually, though not always, a sedimentary rock) and left behind enough mineralization in dissemination, fracture fillings, and true replacements to make ore. Vast low-grade gold deposits in the southwestern United States, such as Carlin in Nevada, are excellent examples of this kind of deposit.

Replacement deposits are often affected by supergene processes. The primary, unoxidized ore frequently have gold bound in pyrite, which prevents many metallurgical processes from extracting the gold. The weathered ores, in which the pyrite has been destroyed by oxidation, have free gold that is extracted more easily.