Skarn Deposits of Monmouth Township, Ontario: Contact Metamorphism and Mineralization

Skarns in Monmouth Township, Ontario formed where igneous intrusions met carbonate-rich country rock, creating mineralized contact zones that host crystal-bearing assemblages valuable to prospectors, rockhounds, and mineral collectors.

Question: Are the fluorite mines in Madoc Skarn deposits?

No, skarn deposits are a different type of geological formation involving contact metamorphism and this is not the case in Madoc. While other mines in the broader region, such as the nearby Marmoraton Iron Mine, were skarn-type deposits, the specific fluorite mines in Madoc were vein-type deposits.

Left: Garnets from the Marmoraton Iron Mine - an enormous skarn that now exists as a water filled crater with endless heaps of tailings.

Skarn formation in Monmouth Township, Ontario

Skarns in Monmouth Township formed where intrusive igneous bodies interacted with carbonate-rich host rocks, creating contact metamorphic environments that concentrated economically and collector-grade mineral assemblages.

Today we’re diving into the fascinating world of skarns, with a general focus on those located in the Bancroft area of Ontario and a specific focus on Monmouth county. This region—rich in mineral history and geologic complexity—hosts some of the most intriguing skarn systems in the province, and possibly even the country. Monmouth Township is a place well known for pegmatites and skarns for which it has become the self dubbed mineral capital of Canada. Skarns in Ontario are known for hosting a variety of minerals, including industrial minerals like wollastonite and potentially valuable metals. The area west of Bancroft is particularly noted for its complex and intriguing skarn systems. This region is famous for a wide variety of minerals, with almost 90% of all mineral types found on Earth located in and around the community.

A skarn is a course grained metamorphic rock that has formed by the replacement of carbonate bearing rocks during regional or contact metamorphism or metasomatism. Skarns are rich in calc silicate minerals that have formed when super-heated fluids interact with the country rock. Skarns are typically associated with granitic plutons in the vicinity of a fault that intrudes into limestone or dolostone – hence the exchange of super-heated fluids and the formation of the skarn within the intrusion or without.

A skarn forming within the intrusive body needs a permeable area within the intrusion and then there is circulation of super-heated fluids into the country rock and back into the intrusion where the calc-silicate minerals develop. This is called an endoskarn.

The york River skarn as shown above is an exoskarn. Exoskarns form outside the intrusion. After high temperature crystals form inside the intrusion, the left over water leaves the fissure in a process called “boiling” and skarn materials form outside the intrusion , usually in limestone or dolostone. Exoskarns are the most common skarns in the Bancroft area, the York River skarn being the classic example. This skarn formed in dolomitic marble adjacent to an intrusion of nephylene syenite. By its signature minerals this is clearly a magnesium skarn, the magnesium content of the dolostone being what chemically defines it from standard limestone.

But in the Bancroft area, skarns are much more than textbook features, they’re historical mines and exploration targets waiting to be decoded by those who are interested in the minerals that they give rise to. What rockhound would’nt want to tap into a skarn and haul out lovely skarn-based crystals, zircon, garnet, quartz and diopside.

Notable Skarn Occurrences in Monmouth Township, Ontario

There are several notable skarn deposits in the Bancroft area and as is seen by our success at Dark Star, many that are yet to be discovered. Those that jump to mind are ...

York River Skarn: A world-famous locality, find it along the east bank of the York River north of Highway 28. Famous for grossular garnet (hessonite), vesuvianite, diopside, wollastonite, and fluorescent zircon. You will note that all of these crystals entail some component of magnesium in their structure.
Dyno Mine: Another significant skarn location for mineral collecting. The site was mined 1958 – 1960 to a depth of 525 meters.
Bessemer Mine: Historically mined for iron, this site also exhibits classic skarn features and darkened garnet clusters in with calcite on the otherwise hard grey country rock.
Rankin mine: close to the bessamer mine and yielding disseminated magnetite in a 1900 foot long ore body.
Saranac Skarn: This is just south of the 118 on the track leading to the well known zircon showing. It is known for its graphite.
Silver Crater Mine: A notable site for diverse mineral finds.

Each of these represents a unique variant of the skarn-forming process, but all share a few important traits.

Defining Geological Features of Monmouth Township Skarns

Monmouth Township skarns share common traits such as calc-silicate mineral assemblages, sharp lithological contacts, and structural controls that reflect intense contact metamorphism and fluid-driven mineralization.

Now to be a little more specific to the Dark star claims, Monmouth’s skarns are mostly calcic skarns, meaning they’re rich in calcium-bearing minerals. These formed when granitic and syenitic intrusions forced their way into surrounding limestone and marble, releasing superheated fluids. The resulting chemical exchange transformed the original rocks, generating mineral-rich skarn zones.

Importantly, these skarns have historically been mined for a variety of resources, including:

Uranium (bicroft and Dyno mine)
Iron (Bessemer and Rankin Mines)
Fluorite
Apatite
Rare Earth Elements (REEs) (Silver crater mine)

Metals in particular are concentrated in skarns, because by their very nature, skarns are an ideal natural mechanism for the concentration of ores. The super heated liquids that develop from the boiling off of the intrusive magma are rich in metals and the faults within which these fluids flow act as depositional highways.

Metals deposit in varying proximity to the fault highway, typically iron and tungsten right beside the fault and Uranium and REEs out in the retrograde zones. A skarn develops in proximity to carbonate rocks (limestone and dolostone) because those rocks are highly reactive to the super-heated fluids unlike silicate rocks which are relatively inert.

When acidic metal bearing fluids meet carbonates along the fault that allows fluid transfer, calcium is released and saturated fluids become super-saturated thus raising the PH which now super-saturates the fluid and metals precipitate as oxides, sulfides and silicates. It is for this reason that carbonates in tandem with the initial metal bearing fluid and the fault highway are amongst the most efficient ore traps in nature.

The mineralogical diversity of the skarn process is a defining feature, and in Monmouth, the zoning of its skarn bodies offers a clear record of the thermal and chemical gradients that shaped them.

Skarn Zoning and Mineral Layering in Monmouth Township

Distinct mineral zoning within Monmouth skarns records changing temperature and fluid chemistry away from intrusive contacts, allowing geologists and prospectors to interpret skarn evolution and target mineral-rich horizons.

One of the most fascinating aspects of skarns is their concentric zoning. Imagine an onion-like structure forming around the intrusive body. Each “shell” reflects different physical and chemical conditions during formation.

Proximal Skarn Zone Near Igneous Intrusions

The proximal skarn zone forms immediately adjacent to the intrusive body and is characterized by high-temperature calc-silicate minerals that reflect intense contact metamorphism and early-stage metasomatic mineralization. This is the highest-temperature, lowest water-activity zone. It features coarse-grained textures and is dominated by pyroxenes such as hedenbergite and diopside, alongside grossular and andradite garnets. These minerals form where metasomatism was most intense.

Retrograde Alteration Zone in Skarn Systems

Retrograde skarn zones develop as cooling fluids overprint earlier high-temperature minerals, producing hydrous mineral assemblages that can remobilize metals and enhance crystal development in Ontario skarn deposits. As the system cools and fluid chemistry evolves, retrograde minerals take over. This zone hosts amphiboles like actinolite and tremolite, along with quartz, magnetite, and sulfides like pyrite. Interestingly, this is also where ore minerals—such as gold, copper, and tungsten—tend to accumulate, often in association with magnetite or sulfide veins.

Skarn Front and Distal Contact Zone

The skarn front marks the outermost extent of metasomatic alteration, where skarn minerals grade into unaltered country rock and provide important clues for tracing mineralized contact zones in the field. The original marble or limestone is only partially altered in the skarn front.

The formation of these zones is largely controlled by fractures, which act as highways for fluid flow. These fractures determine not only the extent of metasomatism, but also the symmetry and directionality of the skarn body. You’ll often find that the geometry of the skarn mirrors the structural fabric of the host rocks and as we speculate by the crystals that we are finding on our quartz claim, we appear to have cut into a skarn as our trench pierces a Cliffside and runs direcly into the retrograde zone. For us there are the quartz and base metal markers that confirm this.

The Monmouth Skarn Project: Geological Mapping and Exploration

The Monmouth Skarn Project documents skarn development, host rock relationships, and mineralization patterns to better understand contact metamorphic systems and guide responsible mineral exploration in eastern Ontario.

Among the most promising modern developments is the Monmouth Project, located just south of Highway 503. Here, exploration has revealed a sizable skarn system that dips steeply and extends beyond 750 feet in depth, stretching over 1,200 feet in length. This is no minor occurrence—it represents one of the more substantial uranium-bearing skarns in southern Ontario.

Drilling data has shown significant uranium concentrations in skarns, especially within the retrograde zone, consistent with what we see in other uranium-rich skarns. The depth and extent of the Monmouth skarn make it a prime candidate for continued exploration, particularly given rising global interest in nuclear energy and the demand for stable uranium supplies.

Amongst the Bancroft crystal collecting sites this one stands out above the others as a skarn for the quality and diversity of its mineral specimens. Here Mick's father (James) examines the location.

Geography: Situated on the east bank of the York River in Dungannon Township, approximately 400 meters north of Highway 28.
Status: Most of the locality now falls within the boundaries of Egan Chutes Provincial Park. As of 2025, mineral collecting is strictly forbidden within the park.

2. Geological Context

Rock Type: It is a tactite (a type of skarn), which is a multimineralic carbonate rock formed through contact metamorphism and metasomatism.
Formation: It occurs where igneous intrusions (specifically nepheline syenites and granitic rocks) met and altered the surrounding country rocks, such as dolomitic marble.
Appearance: The skarn rock itself is typically white and green, composed of roughly 50–60% wollastonite and diopside.

Right: an example of a copper skarn in Iran. You can see that as proximity to the intrusion decreases, so too does the heat. The path of fluid travel is marked by the fissure and so you can see the extent of the skarn increases where the path of fluid travel is greater. Copper leaves the intrusion and deposits in the skarn zone, in part because of the higher ph an marble (the country rock).

Above: An old train once served the spur line to Bessemer - sparks burned a bridge along the line and led to some delays. The sulphur content of the mine's ore eventually closed it.

Right: Enormous tailings piles around the Bessemer open pit in which I have found epidote and garnets.

Far right: A relic from the Dynomine.

Left: Grossular garnets and diopside from the york river skarn

Right: hematite coated quartz from the Dark Star Quartz claim.

Above Left: Chalcopyrite, (Cu,Fe)S2, a common economic ore that is found in skarns.

Above right: lab grown bismuthinite Bi2S3, a great indicator mineral for gold and often found in its natural form (non-descript silver metal) in skarns. The Childs Mine is well known for this metal in a skarn.

Identifying Skarn Deposits Within Their Geological Setting

Recognizing skarn in its geological setting requires understanding host rock composition, intrusive contacts, structural controls, and surface expressions that guide prospectors toward mineralized skarn environments in Ontario.

1) look for marble and the accompanying minerals of green epidote, pyroxene, dark red to brown garnets.

2) Look for a bleached layer of rock that possibly extends well past the skarn mineralization. fractures and bedding planes could well extend the bleach-halo well past what you might expect.

3) Look for fluid escape structures, namely faults, chimneys and pipes, structures that would have relieved the pressures of a cooling intrusion and along which super-heated waters would have flown.

4) The presence of economic minerals, especially those that have formed with sulphides e.g. chalcopyrite CuFeS2, molybdenite MoS2 , bismuthinite B2S3, galena PbS

Above: Grossular and andradite garnets are common in skarns. Garnets in skarns often exhibit oscillatory zoning, where the composition changes rhythmically between andradite-rich and grossular-rich layers, reflecting fluctuating physical and chemical conditions (such as oxygen fugacity, pH, and fluid composition) during their formation by hydrothermal fluids. This makes garnet a valuable indicator mineral for geologists exploring for specific metal deposits

Bessemer Mine drops down in a huge hole to a depth of 236 feet. Today that hole is full of water. They were mining magnetite from one of the Bancroft area's largest skarns. Miners report the terror of hungry wolves howling incessantly around the camp during the winter. Nobody could come or go except by train and nobody ventured too far into the woods. At its greatest depth the mine then bent like a "J" and continued under nearby Little Mullet Lake, it closed after one of the all to common accidents when a young miner fell from the ore skip and died.

Bessemer Skarn, Ontario – Field Video

article - Geological contacts and rock transitions

see article on pegmatites