Group 7

York River Skarn Zone

Earth 231 Dr.Tom Edwards

By:

Andrew Wilson 00000000 Erin Hamilton 00000000 Shannon Robinson 00000000 Merih Tekeste 00000000

1. Introduction

The Bancroft area is known by rockhounds (like us!) and mineralogist as a world-class

classic mineral collecting area. It is especially known for radioactive minerals and

nepheline bearing rocks. This paper discusses the geography, geology and the

mineralogy of Bancroft’s York Skarn Zone.

2. Geographical Location

The York River Skarn Zone is located on the east side of the York River. It is 320 metres

north of Highway 500 and 11 km east of Bancroft, Dungannon Township in the Hastings

County in Ontario (www.ontariominerals.com). When driving from the intersection of

Bridge Street and Hastings Street (Hwy 62), drive east along Hwy 28E for 10.9 km to the

York River. Immediately after the bridge over the river turn right and drive north for

approximately 400m on the east bank. Figure 1 Geographical location of region – the

star indicating our site.

Figure 1. Geographical Location

3. Regional Geological Location and Setting

“The Northern Hastings region lies within the Grenville Structural Province of the

Canbadian Shield. The Grenville is characterised by metamorphosed, and

contorted, sedimentary and igneous intrusive rocks, which are today seen as

lenses of marble, bands of flat lying quartz, gneisses and crumbly paragneisses.”

Bancroft and District Camber of Commerce 2002

The oldest rocks found in the Bancroft area are volcanic in origin, dating to

approximately 1.3 billion years ago. Sediments that were created from the erosion of this

material were deposited as clay, silt, sand and gravel, which later formed shales and

sandstones and later still re-crystallized into banded gneisses. Limestone slowly formed

in the shallow warm sea basins between influxes of classic sediment eroded from the

land. This limestone later recrystallized to form marble. Molten rock masses or magmas,

relatively rich in iron and magnesium, moved upward within the crust but did not reach

the surface. They slowly cooled to form masses of gabbro and diorite, which are rich in

dark minerals such as amphiboles and pyroxenes. Erosion of the overlying rocks exposed

these intrusions at the surface. Molten intrusive rock masses rich in sodium and

potassium cooled to form masses of syenite. The formation of syenite could have also

been explained due to the alterations earlier in sedimentary rock and reactions with

marble. Some syenites and associated pegmatites are rich in nepheline and contain

corundum, sodalite and cancrinite and a variety of other minerals. Then, another mass of

intrusive rock masses relatively rich in potassium and silica moved upward in the crust,

but did not reach the surface before they cooled. Some cooled into masses of granite,

mainly feldspar and quartz and some altered earlier sedimentary rocks to a granitic

appearance. Figure 2 illustrates the geological composition of the area.

Figure 2 .Geological composition.

4. Geology of Skarn Zone

The marble and nepheline gneiss in either direction along the road from the skarn zone do

not exhibit any of the minerals that were identified in the skarn zone. These minerals

seemed to be created through a metasomatic process. Metasomatism is described as the

creation of calcium, iron, manganese and magnesium silicates by the reaction of a host

carbonate-rich rock and hot ion bearing magmatic fluids. Figure 3 shows a block

diagram of the geology of the York River Skarn Zone.

A large igneous intrusion is located to the east of the protolythic-limestone (marble) unit.

{ map reference} A large mafic boulder, see in figure 4, was found at the top of the hill

above the skarn outcrop. This boulder was assumed to be representative of the intrusive

igneous body.

The calcium and magnesium ions that were used to create the minerals found in the skarn

zone came from this mafic igneous body.

Various sized vessuvianite crystals were found in the marble. The massive diopside

contained traces of calcium and magnesium rich garnets. Spinel crystals were found in

the marble. Crystallized diopside was found under ledge along a fracture. Brucite and

talc were also found on fractured ledges.

The marble was formed in a previous metamorphic event and is underlain by nepheline

gneiss at a 45° angle dipping downward into the skarn zone. Figure 5 shows the contact

between the marble and the nepheline gneiss. The regional metamorphism was caused by

the igneous body that also fuled the metasomatism. Local groundwater and the water that

was released by this metamorphic event were important to this metasomatism process.

The water leached the ions from the igneous body and transported them to the skarn zone

where it reacted with the marble to form new minerals. These metasomatic minerals

formed in cracks and pore spaces in the marble unit. The largest and best-formed

minerals formed in fractures – where ion-rich water is in good supply. In some situations

mineralization occurs as a halo around a fracture where the minerals decrease in size the

further they are from the fracture.

Figure 3. Block Diagram

Igneous Intrusion

Marble

Nepheline Gneiss

Figure 4. Mafic Boulder

Figure 5. Marble / gneiss contact.

Marble

Gneiss

The ion rich water combined with the heat from the intrusion and the dolostone created

the minerals that are found today. The creation of diopside from the thermal

metamorphism of dolostone is an example of this.

5. Vesuvianite Ca10(Mg,Fe)2Al4(SiO4)5(Si2O7)2(OH)4

Vesuvianite has tetragonal crystallography and is often vertically striated. It is

usually found as crystal for but may also be found as granular masses. The cleavage of

vesuvianite is poor and has a hardness of 6.5 and a specific gravity of 3.35-3.45.

Vesuvianite has a vitreous luster to resinous and appears usually as a green or brown

colour. At the York River skarn zone the vesuvianite occur as small-disseminated

crystals less than 1mm. The reason for this is explained in the proceedings of the report.

Vesuvianite is a sorosilicate with some substitution of Na for Ca+; Mn2+ for Mg;

Fe3+ and Ti for Al; and F for (OH). The structure of vesuvianite is shown below.

Isolated SiO4 tetrahedra as well as Si2O7 groups occur. Three fourths of the Ca is in 8-

coordination and one-fourth is in 6-fold coordination with oxygen.

Vesuvianite is usually formed as the result of contact metamorphism of impure

limestones. Vesuvianite is usually associated with other contact minerals, such as

grossular garnet, wollastonite and diopside. (Klien, 2002. pg.509)

Photo above shows an example of vesuvianite found at our site showing the small

crystals embedded in marble. The size of the crystals grew progressively along the

marble outcrop.

6. Spinel MgAl2O4

Spinel has isometric crystallography and is usually in octahedral crystals or in

twinned octahedrons. Spinel has a hardness of 8 and a specific gravity of 3.5-4.1. It has

a non-metallic vitreous luster. The black spinel that was found at the York River skarn

zone was of a black colour and is referred to as ferroan spinel. Ferroan spinel is an

intermediate between spinel and gercynite.

The spinel group is based on an arrangement of oxygens in approximate cubic

closest packing along (111) planes in the structure. The figure above illustrates this. The

cations that are interstitial to the oxygen are in octahedral and tetrahedral coordination

polyhedra with oxygen. Spinel is a common high-temperature mineral occurring in

contact metamorphosed limestones and metamorphic argillaceous rocks poor in silica. It

occurs as an accessory mineral in many dark igneous rocks. (Klein, 2002. pg582)

Photo above shows an example of the small crystals in the embedded marble discovered

at our site. There were also some larger crystals in the smaller matrix (phenocrysts) as

seen in the lower right hand corner of sample.

7. Brucite Mg(OH)2

Brucite is a hydroxide with hexagonal crystallography. Crystals of brucite are

usually tabular and may show small rhombohedral truncations. Brucite has perfect

cleavage in one plane that is parallel to the octahedral sheets. The follia are flexible but

are not elastic. This mineral is sectile. Brucite has a hardness of 2.5 and a specific

gravity of 2.39. The luster is pearly on the base but is vitreous to waxy everywhere else.

The colour can vary from white, grey, and light green. At the York River skarn zone

green brucite was found.

The structure of brucite consists of Mg2+ octahedrally coordinated to (OH)- with

the octahedra sharing edges to form a layer. Because the hydroxide group is shared

between three adjoining octahedra the hydroxide group is neutral. For this reason, the

layers in the brucite structure are held together by only weak bonds. The Structure of

brucite is shown above. (Klein, 2002. pg393). Brucite is found associated with dolomite

and crystalline limestone. At our site we found brucite formations perfectly displaying

cleavage in flexible flakes. The brucite appeared in fractures of the rock unit.

8. Hessonite (Grossular) Ca3Al2(SiO4)3

This isometric mineral forms hexoctahedral prisms and is commonly identified by

its hardness of 6.5-7.5. It has a vitreous luster and is brown-orange. This mineral occurs

during contact metamorphism of impure limestone. Instances of hessonite were found in

the massive diopsite.

9. Diopside CaMg(SiO3)2

This monoclinic inosilicate mineral is usually green, has a hardness of 5-6,

vitreous luster and cleavage planes at essentially 90˚. It is often prismatic but can also

occur as massive forms. It is of the pyroxene family and is closely associated with

hedenbergite and augite. It occurs in metamorphic rocks and is frequently associated

with Calcite and grossular. This is consistent with what was found at our site since much

of the massive diopside occurred alongside Calcite and the crystalline diopside was

intermingled with hessonite. Diopside is commonly formed through the metamorphism

of Mg-rich limestones and dolostones, but is also known to form during igneous

processes.

CaMg(CO3)2 + 2SiO2 CaMgSi2O6 + 2CO2

The York River Skarn Zone provides a variety of diopside occurences - likely from

igneous and sedimentary metasomatism - as well as in prismatic and massive forms.

This photo shows the massive diopside with calcite and hessonite intermingled.

10. Wollastonite CaSiO3

This triclinic inosilicate mineral is often grey/white, has a hardness of 5-5.5 and is

usually massive or fibrous. Although not readily observable, it has two perfect cleavages

at approximately 84˚. Of the pyroxenoid group, it has a vitreous luster and may give a

pearly appearance on cleavage surfaces. Wollastonite primarily occurs with contact

metamorphism of limestone:

CaCO3 + SiO2 CaSiO3 + CO2

It is also associated with epidote, vesuvianite, Calcite and a few others but unfortunately,

we did not discover any wollastonite.

11. Chondrodite Mg5(SiO4)2(F,OH)2

This monoclinic nesosilicate usually occurs as small yellow crystals. It can also

be red and has a hardness of 6-6.5 and a vitreous, resinous luster. Chondrodite most

frequently occurs in association with the metamorphism of dolomitic limestone. In

regards to skarn formations, it is found with wollastonite and further metamorphosed

forms of limestone such as monticellite. Unfortunately, we were unable to find any

samples at the site.

WORKS CITED

1. ‘Geology and Mineral Collecting Sites of the Bancroft Area’ . Ministry of Natural

Resources. 1981.

2. Klein, Cornelis. The 22nd edition of the Manual of Mineral Science. John Wiley &

Sons inc. Toronto: 2002.

3. ‘Mineral Capital of Canada’ . Bancroft and District Chamber of Commerce.

www.bancroftdistric.com. Sunday, November 17, 2002.

4. Wilton, Derek. ‘Skarn Deposits’ . Part 1. Nov 22, 1999.

5. www.ontariominerals.com. 2002.