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Brookfield

Nova Scotia, Canada

Main commodities: Ba
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The small Brookfield barite deposit is located within southern Nova Scotia in eastern Canada. It occurs near the contact of terrestrial sediments of the Early Carboniferous (Tournasian) Horton Group and carbonates and evaporites of the succeeding Visean Windsor Group.

The Horton Group is made up of the Horton Bluff and overlying Cheverie Formations. South of the deposit area, grey siltstone and sandstone beds of the Horton Bluff Formation dominate within an E-W trending anticline where they are overlain by predominantly fine-grained, red siltstones and shales with only minor grey strata. At the Brookfield deposit, red beds of the Cheverie Formation dominate. A prominent outcrop of Windsor Group rocks occurs along a ridge west of the deposit where a section of gypsum beds is exposed.

Within the open pit there is evidence of structural stacking of the grey sandstones, siltstones and limestones of the Horton Bluff and red siltstones and shales of the Cheverie Formations, as well as Windsor Group rocks, with contacts marked by fault gouge and calcmylonites.

Early flat lying thrusts that predate barite mineralisation, a steeply dipping, E-W to NE-SW trending mineralised fault, and late block faults are all observed within the pit.

The fault zone that hosts the barite vein mineralisation is defined by numerous alternating barite-siderite veins (striking at 70 to 80°) and is the major structure within the pit. It varies in width along strike, averaging around 25 m and is a zone of brittle deformation with internal cataclasis. Veining is complex with numerous crosscutting relationships. Individual veins may occupy subparallel, dilational features within the fault zone as well as occurring as sheeted vein systems. Episodic deformation is indicated by undeformed, coarse, crystalline barite cutting mylonite textured barite.

In addition to the barite veining, the balance of the fault zone is occuppied by variably altered sedimentary wall-rock material, with the proportion of wall rock and barite and/or siderite changing throughout. Rotation of the stratigraphy suggest dextral movement on the mineralised the fault.

The dominant lithology within the open-pit is a red-brown siltstone which has been altered to an olive-green or grey siltstone adjacent to barite mineralisation. Isolated barite ±siderite filled veins are sandwiched by millimetre to multicentimetre wide, green alteration selvages overprinting the red-brown host rock. Where multiple subparallel veins are observed, the zone resembles a sheeted stock-work system.

The size of the mineralised zone varies, with a maximum dimension of 25x100 m in plan view and an overall moderate plunge to the SE. Mining of this zone produces an average 35 percent barite, 25 percent siderite and 40 percent wall rock. This ore is concentrated to produce a 97.5% barite concentrate. The ore zone is highly irregular, comprising veins, pods and disseminations mixed with variable amounts of altered and deformed host rock. The main barite zone is surrounded by a halo if thin barite veins with associated alteration extending up to 70 m outwards from the ore zone.

The best mineralisation is coarse white barite occuring as as discontinuous pods. Barite dominates the ore zone, with siderite being the next most abundant vein fill, while quartz fills the remaining open space, often as euhedral crystals. Rare, late stage calcite veins crosscut the red shales. Base and precious metal (Pb, Zn, Cu, and Ag) sulphides are absent.

Five different textural and colored varieties of barite are recognized.

i). Coarse (centimetre sized), white, elongate barite crystals which dominate the ore and are often oriented parallel to each other and to the vein-wall rock contact. More rarely barite has a fibrous texture with growth perpendicular to the vein-wall rock contact.
ii). White barite intergrown with siderite. Siderite is texturally late and fills open space in barite. In some cases however, coarse siderite euhedra are aligned along a plane, as if filling a late open structure in the barite. Barite veins commonly have a selvage of siderite at the vein-wall rock contact.
iii). Massive to crystalline, laminated white to orange barite which occurs throughout the fault zone. Thin, mm to cm wide septae define the laminations within the barite, and are developed subparallel to vein-wall rock contacts and to the bedding and fabric of the wall rock. The layering in the barite is of tectonic origin, related to deformation and flow of the barite while the more competent siderite occurrs as augens. Rarely, fragments of grey limestone is found within this barite.
iv). Aphanitic orange barite, which occurs with massive laminated barite, usually as E-W oriented lenses. These lenses are several cms in width and are sub-parallel to the fabric of the host rock and to the layering in massive laminated barite. Microscopically, this barite has a fine-grained, recrystallised texture and a well-developed mylonitic fabric enclosing relict siderite fragments with shear sense indicators.
v). Disseminated barite within altered sandstone proximal to more abundant barite mineralisation. This style may occur within the main mineralised fault zone or peripheral to it.

Total production from the Brookfield depsosit from the early 1980s to 2005 has been around 125 000 t of >97.5% purity barite.

The most recent source geological information used to prepare this summary was dated: 2006.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Kontak D J, Kyser K, Gize A and Marshall D,  2006 - Structurally Controlled Vein Barite Mineralization in the Maritimes Basin of Eastern Canada: Geologic Setting, Stable Isotopes, and Fluid Inclusions: in    Econ. Geol.   v101 pp 407-430


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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