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The Gatsuurt gold deposit is ~90 km north of Ulaanbaatar in the municipality of Selenge Aimag and province of Mandal Soum in central northern Mongolia. It is 35 km SE of the Boroo mine, to which it is connected by a 55 km road (#Location: 48° 38' 3"N, 106° 37' 27"E).
The Gatsuurt Project lies within the North Khentei gold belt which forms part of the NE-trending North Khentei tectonic belt of north central Mongolia. This tectonic belt is bounded to the NW and SE by the regional Bayangol and the Yeroogol fault systems, respectively. These structures are thought to be sinistral strike-slip fault zones. Respectively, they separate the North Khento Belt from the Taryato-Selenge terrane, composed of a stable craton, overlain by a Palaeozoic marine platfrom to the NW, and the Khentei Terrane, composed of terriginous clastic and volcanic rocks in an Early- to Mid-Palaeozoic eugeosyncline to the SE. The regional extent of these faults, in conjunction with the associated volcanism and magmatism, suggest they are deep-seated features. Higher order structures, oriented ~20 and 130°, intersect the Yeroogol fault, forming intersections that are considered favourable for gold mineralisation. The Gatsuurt deposit is located along the NE-SW Sujigtei Fault which is part of the larger Yeroogol system.
The North Khentei tectonic belt is composed of three major lithotectonic components.
• Late Precambrian to Early Palaeozoic flysch and subsequent plutonism. Miogeosynclinal flysch includes the Precambrian Yeroo Series greenschist grade metamorphic rocks adjacent in the NW to the Yeroogol fault and the Lower Palaeozoic Kharaa Series sandstones, shales, siltstones, conglomerates, phyllites, quartz-sericite and sericite-chlorite schists, and some intermediate tuffs. Early Palaeozoic Boroo Complex (450 Ma to 520 Ma) biotite and biotite-hornblende granodiorite and granites have intruded the Yeroo and Kharaa Series east of the Bayangol fault zone. The intrusives are dislocated by renewed movement along faults such as the Sujigtei Fault at Gatsuurt.
• Mid to Upper Devonian continental volcanic and sedimentary rocks lie unconformably on the Yeroo, Kharaa, and Boroo rocks and are spatially confined to the Yeroogol fault system. Volcanic rocks consist of subvolcanic rhyolite porphyry, tuffaceous andesite lava and breccia. Sedimentary rocks consist of shale, sandstone, and conglomerate.
• Jurassic-Cretaceous and Tertiary coal-bearing sedimentary rocks and conglomerates.
The Gatsuurt deposit is located within the North Khentei gold belt, which has a long history of alluvial placer mining, including bedrock gold deposits. Historic placer workings as well as gold and arsenic anomalies in stream sediments are aligned roughly parallel to the Bayangol and Yeroogol fault systems. The Gatsuurt placer that overlies the Central Zone of the deposit, occurs in the Yeroogol fault system. Bedrock gold deposits within the trend include the Boroo Mine, Gatsuurt and Bumbat. The North Khentei gold belt deposits and occurrences are typically mesothermal and may comprise individual quartz veins with coarse gold and low-sulphide content, as well as disseminated fine gold in sulphidised rocks.
The Gatsuurt area is underlain by Permian rhyolite and Early Palaeozoic Boroo Complex granodiorite, granites and diorites, and by Lower Palaeozoic Kharaa Series metasedimentary rocks. These lithostructural units occur along, and are separated by, the Sujigtei subvertical fault. Volcanic rocks (rhyolite) are found on the NW side of the Sujigtei Fault, whereas Boroo granite occurs on the SE side. The granite contains fault-bounded 'xenoliths'of Kharaa Series metasedimentary rocks as well as volumetrically minor diorite dykes and xenoliths of other Boroo Complex rocks.
The Gatsuurt deposit comprises disseminated and vein style gold mineralisation in volcanic, granite and metasedimentary rocks in the structurally separated Central and Main zones which are both located adjacent to the Sujigtei Fault. The Central Zone lies on the SE hanging wall side of the structure, whilst the Main Zone is on the NW side ~400 m to the SW. The combined length of the two zones is ~1.5 km, with a combined width is at least 300 m.
The Central Zone is hosted within a complex assemblage of Boroo granite, Kharaa meta-sandstones and lesser diorite and rhyolite. The granite is the dominant host, although meta-sandstones may carry gold at contacts or in narrow structures, but are mostly low grade to barren. Diorite and rhyolite are typically barren. The altered granite has been subjected to cataclasis with textures that may be massive, fine to medium grained or brecciated. Mineralisation in the Central Zone is predominately a disseminated gold-sulphide assemblage. Younger veins and fine-grained silica alteration are locally superimposed over the disseminated mineralisation, with variable amounts of coarse native gold.
The Main Zone deposit is almost entirely hosted by the Permian(?) Zuunmod porphyritic rhyolite intrusive that occurs on the NW side of the northeast-trending regional Sujigtei Fault. In plan view, the zone of mineralisation measures approximately 500 x 100 m, extending for 500 m along the faulted contact of the rhyolite with the largely barren granite and Kharaa metasediments, and extending 100 m into the rhyolite.
Structural elements control the distribution of both gold mineralisation and prominent alteration zones in the Gatsuurt deposits. The Sujigtei Fault is the major tectonic structure controlling the distribution of gold mineralisation in the Central Zone, where it divides essentially barren volcanic rocks from granite that hosts the gold-bearing quartz and gold-sulphide mineralisation. The structure consistently strikes at 45°, with no apparent displacement, and dips subvertically to 85°SE. NE and SW of the Central Zone, the fault has a more northerly strike, suggesting that the Central Zone may lie at a jog/inflection in it’s regional trend. The fault is occupied by comminuted rhyolite gouge, clay and sometimes silica altered material, and/or 0.5 to 2 m wide breccia zones. Although additional fault planes have been intersected in the immediate hanging wall, parallel to the Sujigtei Fault, the gold mineralisation associated with these substructures is unevenly distributed and weaker, compared to that in the Sujigtei Fault.
There are two dominant orientations observed at the Main Zone, namely the 45° trending Sujigtei structure defining the SE margin of mineralisation and the 20 to 30° azimuth which is the orientation of the higher grade zones (>3 g/t Au) within the broader zone of quartz-sericite alteration and >1 g/t Au mineralisation. The northwest margin of the zone of mineralisation is defined by a general weakening in the sulphide content and degree of quartz sericite alteration.
It is possible that the Main and Central zones once formed a single deposit and that post-mineral sinistral movement along the Sujigtei Fault displaced them by some 750 m. However, the two zones have substantially different gold mineralisation and alteration characteristics, possibly reflecting vertical zoning in the pre-faulted deposit.
Six alteration styles have been recognised within the Gatsuurt deposits, largely confined to rhyolite and granite host rocks. Alteration assemblages are laterally zoned, and, in several instances, are due to multiple alteration events. The width of the zone of pervasive alteration varies, but is commonly from 50 to 80 m, asymmetrically decreasing in intensity outward from the Sujigtei Fault axis, over widths of tens of metres to the NW and locally up to hundreds of metres to the SE. Alteration is interpreted to be suggestive of mesothermal mineralisation, although the Black Quartz Zone that occurs on the immediate hanging wall of the Sujigtei Fault, could have been produced by either late stage lower temperature fluids as the system collapsed, or a later epithermal-style event channelled along the still active Sujigtei Fault. The alteration styles observed are:
i). sericitic; ii). siliceous; iii). K-feldspar or potassic; iv). propylitic; v). carbonatisation; and vi). argillic alteration
Gold mineralisation at Gatsuurt is associated with sericitic and potassic alteration styles. The core of the main alteration system comprises a sericite-K feldspar-quartz stockwork assemblage accompanied by pyrite.
In the core of the Central Zone, later quartz veins and stockworks that are rich in native gold were repeatedly superimposed. Propylitic chlorite and epidote alteration forms proximal and distal haloes SE of the Central Zone.
To the NW of the Main Zone, peripheral potassic alteration and quartz-carbonate-fluorite veins are found as proximal and distal alteration haloes.
The limits of the principal gold mineralised zones correspond to the boundaries between proximal and peripheral alteration zones.
Sericitic alteration, containing up to 60% sericite, may completely replace both phenocrysts and groundmass in rhyolites, granites, and even locally, in metasedimentary rocks, as well as occurring in veinlets, breccias and masses, or as cement with quartz and carbonate. The strongest sulphidation usually occurs inside strong sericitic alteration zones, and in the Central Zone, the southeastern boundary zone of strong sericitisation and sulphidation constitutes the ore zone itself. Sericite usually accompanies silica and potassic alteration in the Central Zone and silica alteration in the Main Zone. Weak sericitic alteration overprints propylitic alteration on the periphery of the Central Zone.
Silica alteration is represented by quartz veins, veinlets, stockworks and massive silica replacement zones with contrasting styles in volcanic rocks and granites. Thin white quartz veins with a limited lateral extent of up to 10 m, and a separate generation of high-grade gold-quartz veins, are found in the granite and metasedimentary rocks to the SE of the Sujigtei Fault, and are also significant components of the alteration/mineralisation assemblage in volcanic and granitic rocks. Volcanic rocks tend to have more quartz stockworks and replacements to the southwest. Silica replacement generally increases to the SW from the Main Zone, where the silica phase apparently carries only minor gold, with the exception of mineralisation in isolated structures. The finely crystalline quartz in the thin veins resembles chalcedony.
Potassic alteration occurs as pinkish microcline, which is coincident with gold-sulphide mineralisation, occurring as massive alteration adjacent to the Sujigtei Fault, peripheral to the system, and along some cross-cutting structures. It affects both volcanic rocks and granites, although only incipient peripheral alteration is found in the volcanic rocks, where it is accompanied by a dramatic increase in sericitic alteration.
The gold mineralised bodies of the Gatsuurt deposits are localised immediately adjacent to the Sujigtei Fault zone and sub-parallel structural zones, and like those structures, strike northeasterly with subvertical dips. The Main Zone is hosted within felsic volcanic rocks in the footwall of the fault zone, while the Central Zone is mostly hosted by intrusive rocks and, to a lesser extent, by metasedimentary xenoliths. At both the Central and Main Zones, the host rocks are variably altered by a quartz-sericite-K feldspar-pyrite-arsenopyrite assemblage. Gold is associated with three styles of mineralisation:
• Disseminated and fracture controlled stockwork sulphide veinlets,
• Pervasive silicified zones with fine-grained sulphides and occasional visible gold,
• Discrete white quartz veins with variable sulphide content and occasional visible gold.
In the Central Zone, gold distribution is more variable over short distances and has a much broader width than in the Main Zone, where the host geological environment is considerably more homogeneous, with a relatively uniform distribution of gold.
The sulphides typically comprise 2 to 6 vol.% and comprise comparable amounts of pyrite and arsenopyrite. Micron size gold has been detected as discrete particles within pyrite grains, whilst some 'lattice bound' gold may be present within arsenopyrite, particularly in the Main Zone.
In the Central Zone, gold mineralisation has been continuous traced over a strike length of ~900 m and horizontal width varying from 2 to >70 m, typically comprising broad mineralised bands of >1 g/t Au, enclosing higher grade lenses of >3 g/t Au which have variable lateral and vertical continuity. Gold mineralisation persists to depths of >360 m. Gold-bearing zones in the immediate eastern portion of the Central Zone, the South Slope target, are oriented at ~20° and coalesce with the dominant 45° fabric along their northerly strike extent, and might be considered to follow 'horse tail' structures. The Sujigtei Fault defines a relatively sharp northwestern boundary of the Central Zone, whilst the southeastern boundary corresponds to a gradual transition from a sericitic-potassic-silica to a propylitic alteration assemblage. The width of the quartz-sericitic-sulphidation zone SE of the Sujigtei Fault varies from 30 to >80 m and is, to a large degree, dependant on the specific lithologies adjacent to the Sujigtei Fault. The heterogeneity of the host rocks in the Central Zone has resulted in a much more variable distribution of gold mineralisation and grade compared to the Main Zone, which is hosted by rhyolite alone. The greater diversity of mineralisation in the Central Zone is also due, in part, to a greater abundance of superimposed mineralising events, which could be interpreted to suggest this zone was was the conduit through which hydrothermal fluids remained active during the entire mineralising chain.
The Main Zone resource is ~500 m SW of the Central Zone, and occurs along the footwall (northwest) side of the Sujigtei Fault. Gold mineralisation is hosted by strongly or pervasively altered rhyolite or rhyolite-porphyry, characterised by extensive micro-brecciation, veinlets and stockworks. In the gold mineralised zone, the porphyritic texture of the 'rhyolite-porphyry' is only rarely observed. Veinlets are filled with sulphide, quartz, sericite and carbonate assemblages, with rare fluorite. Reasonably continuous gold mineralisation extends over a strike length of ~400 m, widths of up to 70 m, and persists to depths of >250 m. Grades typically average 2.0 to 2.5 g/t Au. Gold emplacement is considered to post-date earlier quartz-sericite alteration, and is principally associated with disseminated fine to medium-grained acicular arsenopyrite and lesser pyrite, with combined sulphide levels of up to 5%. The grade of the gold is considered to primarily be proportional to the sulphide content, with coarse free gold being only very rarely observed, usually in association with centimetre scale quartz veining. Quartz veining is much less prevalent than in the Central Zone, with the content of 0.1 to 2.0 cm scale veins rarely >5%.
Reserves and Resources
Estimated ore reserves and mineral resources at December 31, 2013 (Centerra Gold Inc., 2014) were:
Proven + probable reserves - 17.129 Mt @ 2.9 g/t Au; (reserves are in addition to resources)
Measured + indicated resources - 5.098 Mt @ 2.4 g/t Au;
Inferred resources - 5.475 Mt @ 2.5 g/t Au.
This summary is drawn, and frequently paraphrased, from "Hendry, J.W., Roscoe, W.E. and Ross, D.A., 2006 - Technical report on the Gatsuurt Gold Project, Northern Mongolia; prepared for Centerra Gold Inc., by Roscoe Postle Associates Inc., Toronto, Ontario, Canada, 208p."
The most recent source geological information used to prepare this summary was dated: 2006.
Record last updated: 3/2/2016
This description is a summary from published sources, the chief of which are listed below.
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