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Boroo

Mongolia

Main commodities: Au
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The Boroo gold deposit is located some 110 km to the WNW of Ulaanbaatar in northern Mongolia (#Location: 48° 44' 36"N, 106° 9' 51"E).

Northern and eastern Mongolia is composed of numerous terrains that were accreted onto the southeastern edge of the Siberian craton during the several Paleozoic orogenic pulses.

The Boroo deposit lies within the North Khenteii Mountains terrain within this accretionary complex. This terrain, is dominated by northeast striking (~45°) master faults, which may have tens of km of cumulative sinistral displacement, and continue for hundreds of kilometres into Russia. A well developed fabric of ~330° striking faults and fracture zones occur as apparent extensional features between the northeast striking master faults. Regional dyke swarms fill both the ~330° trend and a north striking fabric, as well as the dominant northeast trend.

The general Palaeozoic tectonic history of the North Khenteii Mountains terrain can be summarized as follows:
i). during the Early Palaeozoic, the Late Proterozoic to Lower Palaeozoic Haraa sedimentary basin closed as the result of a craton facing compressional event, resulting in extensive fields of deeply eroded igneous complexes ranging in age from Early Paleozoic to Early Mesozoic age intruding older metasedimentary rocks;
ii). this was immediately followed by intrusion of the late orogenic, Early Paleozoic (~520 to 450 Ma) Boroo Complex leucocratic granites and various dykes which,
iii). predated a long-lived episode of uplift and erosion spanning the Middle Palaeozoic;
iv). minor rejuvenated basins developed during the Devonian and Carboniferous as evidenced by shallow-water carbonate and coarse detrital, terrigenous sedimentary rocks (molasse) of this age overlying the Haraa and Boroo Complex;
v). felsic volcanic and sub-volcanic complexes, tentatively assigned to the Devonian (295±20 K-Ar age on whole-rock, Rb-Sr age of biotite 305±30 Ma) are found near major lineament intersections;
vi). the terrain was rifted and intruded by felsic plutono-volcanic complexes of Permian age (K-Ar on whole-rock 288.4±14 Ma), which in turn were uplifted and eroded leaving only sparse occurrences of volcanic tuff and other outflow facies rocks;
vii). Upper Mesozoic terrigenous clastic facies sedimentary rocks (coal bearing) occur within northwest elongated pull apart basins formed by the last period of tectonism involving the northeast striking wrench faults and the extensional ~330° fabric. Placer gold accumulations have been recorded in these rocks.

The Boroo gold deposits and resources are hosted in the nearly flat lying Boroo fault zone, that commonly separates overlying metasedimentary rocks of the Haraa Series from underlying Boroo Complex granitic rocks. The bedrock geology at Boroo is dominated by the Haraa meta-sediments and Boroo Complex, cut by late, near-vertical dykes and lensoid intrusives, ranging from felsic aplites to intermediate diorites. The Boroo fault is cut by these dykes.

The Boroo fault forms a distinctive 1 to 70 m thick zone of mixed granite, metasedimentary rocks and localised blocks of diorite.

Within the mine area, the Haraa Series turbidite sequence comprises highly deformed Cambro-Ordovician shale, siltstone and fine sandstone intruded by Palaeozoic granitoids of the Boroo Complex. Numerous dacite, rhyolite and aplite dykes intrude both granitic rocks and metasedimentary rocks. The sedimentary rocks are intensely folded and have been subjected to low-grade (lower greenschist facies) regional metamorphism, overprinted by localised zones of contact (albite-epidote homfels facies) metamorphism at the margins of granite intrusions.

The Boroo Complex granitoids (plagio-granite, biotite granite, biotiteamphibole granite, leucogranite) are characteristically equigranular and exhibit prominent pink feldspar phenocrysts, and volumetrically dominant in the central and northern areas of the Boroo deposit. Dykes of granite-porphyry and diorite are found throughout the deposit area and vary in thickness from cms to several m. They most commonly occupy north-northwest trending vertical fault zones and form a network of almost vertical and gentle-dipping narrow bodies.

Gold mineralisation comprises disseminated, fine-grained gold-pyrite-arsenopyrite mineralisation accompanied by intense quartz-sericitic-iron carbonate alteration, locally overprinted by auriferous quartz veins. Disseminated mineralisation commonly carries fine-grained (micron-scale) gold at low to medium (1 to 5 g/t) but very consistent gold grades, while quartz veins carry coarse-grained gold and erratically high (tens of g/t) and very high (hundreds of g/t) gold concentrations. Albite has been recorded as an alteration mineral in granite, although the role of Na in the Boroo deposit genesis remains uncertain.

The total combined length of the alteration zone, or halo, that surrounds the anomalous (>100 ppb) gold values is more than 2500m, while the width is at least 400 m, and thickness is locally as much as 100 m. The ore grade mineralisation (>1 g/t Au envelope) within the alteration halo is a function of host rock permeability and varies from 5 to10 m in homogeneous metasedimentary rocks to 20 to 30 m in heterogeneous granite, diorite and metasedimentary rocks. Individual ore bodies may be up to 1 km in length. The deposit forms a single, large, contiguous zone of gold-sulphide mineralisation developed within Palaeozoic metasedimentary rocks and granite, occuring as a flat lying to slightly northwest dipping layer, which probably follows a regional scale thrust or detachment fault.

The sub-horizontal Boroo fault structural zone is interpreted to be the main control for Au and As mineralization in the Boroo deposit. It has a shallow northwest dip interpreted to follow a regional scale thrust fault, detachment fault or a regional scale shear zone that has been rotated to have a flat configuration.

Field investigations demonstrate that the main ore lens was a thrust sheet separated by upper and lower bounding shears/faults. The main ore body consists of an envelope, up to 30 m wide, of sheared and brecciated material.   Internally the envelope has strong brittle deformation, narrow climbing shears, occasional 2 m thick near horizontal lensoid quartz veins and more commonly, small, very narrow, mostly subhorizontal rather than sub-vertical, tensional vein arrays.

Two main styles of gold mineralization are recognised, associated with intense quartz-sericitic alteration, as follows:
i). Gold-sulphlide zones which host the highest proportion of the gold mineralization at Boroo. These zones occur in two distinct but spatially overlapping stages, namely:
 a). Gold-pyrite-arsenopyrite-quartz mineralisation in thin, irregular veinlets (stockwork), breccias, or disseminated within a zone of pervasive quartz-sericitic alteration. At depth, near the outer contacts, the granite hosts arsenopyrite-rich veinlets with disseminated pyrite and arsenopyrite accompanied by a well developed quartz-albite-chlorite(epidote) alteration assemblage.
 b). Gold-quartz-sericite-pyrite-carbonate disseminated mineralisation commonly overprinting and enriching the gold-pyrite-arsenopyrite-quartz mineralisation, and locally occurring as the only mineralization style.   Typically the quartz-sericitic alteration assemblage is composed of 55% quartz, 25% sericite, 15% pyrite and 5% carbonate, with sub-microscopic gold grains associated with the pyrite.
ii). Shallow-dipping, mineralised white quartz veins with a sulphide mineralogy dominated by a light-coloured almost white pyrite, sometimes rimmed by chalcopyrite with tetrahedrite and galena. Visible gold has been recorded in the quartz veins although it usually occurs with sulphides.

A peripheral halo of propylitic alteration is well developed in the Boroo district, with epidote clusters, veining and veinlets having been observed as far as 3 km to the north of the Boroo deposit.

Published ore reserve and mineral resource figures are as follows:
    10.3 Mt @ 3.52 g/t Au = 36.25 t Au    (Initial pre-mining probable reserve, 2004);

    13.39 Mt @ 2.8 g/t Au = 37.5 t Au    (Proven + probable reserve, 2005, Centerra Gold Website);
    2.65 Mt @ 2.4 g/t Au = 6.35 t Au    (Measured + indicated resource, 2005, Centerra Gold Website);
    2.56 Mt @ 2.0 g/t Au = 5.12 t Au    (Inferred resource, 2005, Centerra Gold Website).

Remaining ore reserves and mineral resources at December 31, 2011 (Centerra Gold Inc., 2012) were:
    Proven + probable reserves - 9.658 Mt @ 1.0 g/t Au; (reserves are in addition to resources)
    Measured + indicated resources - 4.916 Mt @ 1.5 g/t Au;
    Inferred resources - 7.323 Mt @ 1.0 g/t Au.

Remaining ore reserves and mineral resources at December 31, 2013 (Centerra Gold Inc., 2014) were:
    Proven + probable reserves - 2.364 Mt @ 0.6 g/t Au; (reserves are in addition to resources)
    Measured + indicated resources - 4.916 Mt @ 1.5 g/t Au;
    Inferred resources - 7.323 Mt @ 1.0 g/t Au.

The most recent source geological information used to prepare this summary was dated: 2005.    
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.


Boroo

  References & Additional Information
   Selected References:
Cluer J K, Kotlayar B, Gantsetseg O, Togtokh D, Wood G and Ulrich T,  2005 - Geology of the Boroo Gold Deposit, Northern Mongolia: in Seltmann R, Gerel O and Kirwin D J, (Eds.), 2005 Geodynamics and Metallogeny of Mongolia with a Special Emphasis on Copper and Gold Deposits: SEG-IAGOD Field Trip, 14-16 August 2005, 8th Biennial SGA Meeting; CERCAMS/NHM, London   IAGOD Guidebook Series 11,  pp 105-117


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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