Fact Sheet for Deposit Area 805

					Norsk versjon
	THE INDUSTRIAL MINERAL DATABASE
	Deposit Area 805 - 302
	(Last updated 16.mar.2021)

Name of Deposit Area : Bjørntvet

(Object Id : 0805,302,00,00)

Location

County :

Telemark (08)

Municipality :

Porsgrunn (0805)

Map 1:50000:

Porsgrunn (1713-2)

Map 1:250000:

Skien

Marking point:

Longitude:

9.6704710

Latitude:

59.1261620

EU89-UTM Zone 32

(Coordinates IS confirmed)

X-coord:

538378 m.

Y-coord:

6554293 m.

Commodity

Main Type:

Carbonates

Sub Type:

Limestone

Production

Activity:	Mining	Reserves:
Prod. method:	Open pit mining	Production:
Prod. status:	Active	Volume of dump:

Importance

Public: Regional Importance (reg. 21.02.2019)

Economical: Very significant , (Confirmed 21.feb.2019 by Havard Gautneb)

Products

Element/product Crude ore grade or quality

SEMENT 100%

Ownership

Type of ownership Owner Land- and title number

Concessionary (Mining rights) Norcem A/S

Operations

From - To Activity Comments

1955 Regular production Company/Institution :Norcem A/S

Mineralization

Era: Paleozoic Period: Silurian

Dating: Method:

Genesis: Sedimentary/metamorphic Form: Layer

Main texture: Structureless Min. distribution: Massive (>50 % ore minerals)

Main grain size:

Strike/Dip: Direction:

Plunge:

Stratigraphic classification of host rock

Era: Paleozoic Period: Silurian

Province: Oslo Region

Geotec.unit:

Tectonic complex:

Igneous complex:

Group: Formation:

Mineralogy

Relationship Mineral Amount

Not defined Calcite Major mineral (50-90%)

Not defined Quartz Subordinate mineral (1-10%)

Not defined Feldspar Subordinate mineral (1-10%)

Not defined Chlorite Subordinate mineral (1-10%)

Structures

Location: Type: Orientation(360 gr.): Relation to min.:

Host rock Primary layering

Information(s) in free text format

Free text

Bjørntvet kalksteinsbrudd ligger like ved bebyggelsen i Porsgrunn by. Avstanden sydover til Norcem´s sementfabrikk på Dalen er ca. 7 km. Forekomsten på Bjørntvet kom i drift i mai 1955. Kalksteinen er av Silurisk alder ( ca. 40 mill. år yngre enn "Dalen - kalken") og tilhører Steinsfjordformasjonen (tidl. "revkalken"). Kalksteinen er på langt nær så homogen som kalksteinen på Dalen, som tilhører Steinvik formasjonen. I tillegg er den mer uren enn den kalken som brytes i gruvene på Dalen. CaO - innholdet varierer fra 50 - 70% av glødet materiale. Mektigheten på Steinfjordformasjonen er ca. 115 meter, og det er drift på kalkstein fra hele formasjonen. Kalksteinsdraget stryker tilnærmet VNV med et ØNØ-lig fall på 13 - 18 grader. I motsetning til kalksteinen på Dalen finner man på Bjørntvet en rekke interne strukturer i kalksteinen, deriblant en rekke fossiler som gjenspeiler det yrende dyrelivet som fantes på den tiden da kalksteinen ble avsatt. Kalksteinsbruddet på Bjørntvet drives som dagbrudd og årlig brytes det ca. 1 mill. tonn. All produksjon går til sementfabrikken på Dalen. Ettersom kalkstein fra Bjørntvet er mer uren enn kalkstein fra gruven på Dalen, blir denne i hovedsak brukt til produksjon av standardsement hvor en ikke har de samme strenge kravene til innholdet av forurensninger som i høykvalitetssement (lav-alkali sement). I 2014 ble all drift nedlagt, bruddet blir benyttet som trase for for jernebaneverkets ny Eidanger trase. Det er lite sannsynlig at forekomsten vil kunne drives videre. Ref.: Carstens Halfdan red.: "--bygger i berge" Tapir Akademisk Forlag, 2000

Photo(s) from the Deposit area:

Photo no. 1 showing Panoramabilde over bruddet sett mot nord"

Photo no. 2 showing Tynnslipbilde av kalkstein"

Bibliography:

From NGU's Reference Archive:

Hetherington, Callum J.; Nakrem, Hans Arne; Batchelor, Richard A. , 2004

The Bjørntvet metabentonite: A new correlation tool for the Silurian of the southwst Oslo Region

;Norsk geologisk forening;TIDSSKRIFTARTIKKEL;Norsk geologisk tidsskrift; No.84 (4);239-249 pages

Abstract:

A three-layer horizon of silicate-rich rock exposed on the Bjørntvet limestone quarry in Porsgrunn has been identified as a metabentonite. Its total thickness is 75 cm, it has dark green-grey colour and each sub-layer is 25 cm thick. Sedimentary structures are visible and there is no evidence that the siliceous material was deposited by clastic sedimentation or intruded. The boundaries between each sub-layer and the contact between the metabentonite and the limestone are conformable, sharp and depicted by bands of white calcite. The metabentonite belongs to the Steinsfjorden Formation, an Upper Silurian unit of the Oslo region. The Steinsfjorden Formation, an Upper Silurian unit of the Oslo Region. The Steinsfjorden Formation is the youngest Wenlock unit exposed in the area and is conformably overlain by the Ringerike Group. In this area the Steinsfjorden Formation is approximately 140 m thick, and the metabontonite is of Late Wenlock to Early Ludlow age. Each sub-layer is dominated by quartz, calcite and plagioclase, with smaller quantities of orthoclase, biotite, muscovite, chlorite, epidote, amphibole and pyroxene. This mineral assemblage grew during contact metamorphism of the rock due to emplacement of the larvikite complex to the souteast. The whole-rock chemistry of the bands suggests that they are derived from calc-alkaline dacitic magma, erupted in a volcanic arc setting. Correlation between this metabentonite and other Silurian bentonites in Norway and Scandinavia is discussed.

Fieldsamples

Sample No. Sample type Miscellanrous

TE0100.01 Bedrock

Comment :Gjnnomsnitt av råstoffproduksjonen i perioden 2000/01/01 til 2001/03/33

No. of registrated oxide analyses = 1

18-05 Bedrock

Sampler :H. Gautneb

Comment :Grålig kalk Rytteråker fm, my fossiler av pentamerus

No. of registrated CaOMgO analyses = 1

No. of registrated oxide analyses = 1

19-05 Bedrock

Sampler :H. Gautneb

Comment :Grålig kalk Rytteråker fm, my fossiler av pentamerus

No. of registrated CaOMgO analyses = 1

No. of registrated oxide analyses = 1

NB! All analyse values are shown at the end of the printout.:

Analyse Results

from

Deposit Area 805 - 302 Bjørntvet

Analysis of acid soluble CaO and Mgo

Analyzed values Derived values

Sample No. Sample Type Acid soluble CaO Acid soluble MgO Dolomite Calcite Unsolved

18-05 Bedrock 49.53% .46% 2.10% 87.26% 10.64%

19-05 Bedrock 50.23% .57% 2.61% 88.23% 9.16%

Oxide analyses
( All values are in % )

Sample No. Sample Type SiO2 Al2O3 TiO2 Fe2O3 MnO MgO CaO

TE0100.01 Bedrock 24.42 6.91 3.11 2.96 59.56

18-05 Bedrock 2.34 0.62 0.04 0.66 0.05 0.78 52.96

19-05 Bedrock 23.71 6.06 0.31 2.52 0.04 2.18 36.46

------------------

Sample No. Na2O K2O P2O5 Sum

TE0100.01 0.57 1.27 99.00

18-05 -0.10 0.21 0.05 58.00

19-05 0.65 1.59 0.07 74.00

	Type of ownership	Owner	Land- and title number
	Concessionary (Mining rights)	Norcem A/S

	From - To	Activity	Comments
	1955	Regular production	Company/Institution :Norcem A/S

Era:	Paleozoic	Period:	Silurian
Dating:		Method:
Genesis:	Sedimentary/metamorphic	Form:	Layer
Main texture:	Structureless	Min. distribution:	Massive (>50 % ore minerals)

	Province:	Oslo Region
	Geotec.unit:
	Tectonic complex:
	Igneous complex:

Relationship	Mineral	Amount
Not defined	Calcite	Major mineral (50-90%)
Not defined	Quartz	Subordinate mineral (1-10%)
Not defined	Feldspar	Subordinate mineral (1-10%)
Not defined	Chlorite	Subordinate mineral (1-10%)

	Location:	Type:	Orientation(360 gr.):	Relation to min.:
	Host rock	Primary layering

18-05	Bedrock	49.53%	.46%	2.10%	87.26%	10.64%
19-05	Bedrock	50.23%	.57%	2.61%	88.23%	9.16%

Sample No.	Sample Type	SiO2	Al2O3	TiO2	Fe2O3	MnO	MgO	CaO
TE0100.01	Bedrock	24.42	6.91		3.11		2.96	59.56
18-05	Bedrock	2.34	0.62	0.04	0.66	0.05	0.78	52.96
19-05	Bedrock	23.71	6.06	0.31	2.52	0.04	2.18	36.46

Sample No.	Na2O	K2O	P2O5	Sum
TE0100.01	0.57	1.27		99.00
18-05	-0.10	0.21	0.05	58.00
19-05	0.65	1.59	0.07	74.00