Chiney Layered Mafic-Ultramafic Intrusive Complex - Etyrko, Magnitnyi (V-Ti-Fe), Rudnoe, Kontaktovyi, Skvoznoe, Pravoingamakitskoe (Cu)
Zabaikalsky Kray, Russia
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The Chiney layered mafic-ultramafic intrusive complex hosts large resources of iron, titanium, vanadium and associated copper-nickel-PGE-Au. It is located within the western Aldan Shield, NE of Lake Baikal, in Zabaikalsky Kray of eastern Russia, and ~660 km NE of Chita, the capital of the Krai. The intrusion is ~10 km SSE of the Udokan sediment hosted copper deposit and ~40 km SSE of Novaya Chara railway station of the Baikal-Amur Mainline (BAM) loop of the Trans-Siberian Railway, and adjacent to the Chineskaya spur line,
(#Location: 56° 32' 26"N, 118° 33' 7"E).
For details of the stratigraphy into which the layered complex was emplaced, see the the separate Udokan record.
The Chiney layered complex intruded the ~12 km thick, deformed and metamorphosed Palaeoproterozoic clastic suite with lesser carbonate rocks of the Udokan Supergroup, which unconformably overlies ~3.0 Ga (Glebovitsky et al., 2008) Archaean basement of the Chara-Olekma block of the Aldan Shield. The mapped Palaeoproterozoic metasedimentary successions represent remnants of a much larger basin (Arkhangelskaya et al., 2004).
There are three large structures in the southern Udokan Basin, the Naminga and Katugin synclines and intervening Chiney Anticline (Chechetkin et al., 2000). The 80 x 10 km Naminga Syncline, which has a steep northern limb and an overturned southern limb, hosts the cupriferous sandstones of
the Udokan copper deposit. The meta-sedimentary sequence of the Katugin Syncline have been almost completely replaced by Palaeoproterozoic, 1876 to 1873 Ma (U-Pb zircon; Larin et al., 2000) granite of the Kodar Complex. The Chiney Layered Complex is located in the centre of the 10 to 12 km in wide Chiney Anticline. The location of the Chiney pluton appears to be controlled by the intersection of near east-west and NW-trending fault systems that are evident on satellite and geophysical imagery. The NW trending fault zone both bounds the Kodar-Udokan Basin, and controls the emplacement of Neoproterozoic gabbro and Mesozoic dykes. One of the key structures within this NW trending structural zone, the Ingamakit Fault Zone, bounds the Chiney pluton in the north and is locally up to a kilometre in width, coinciding with a belt of hydrothermally reworked folded and foliated rocks characterised by abundant chlorite-actinolite, carbonate and zeolite veinlets. Konnikov (1986) suggested that the northern part of the pluton has been downfaulted by this structure by 3 to 5 km and is now located beneath the Udokan copper deposit.
The exposed portion of the Chiney Intrusive Complex covers an area of ~120 km2 and comprises the four distinct Western, Central, Eastern and Southeastern blocks. The Western and Southeastern blocks host layered rocks with high concentration of titanomagnetite gabbro and the titanomagnetite ores. The Eastern and Central blocks are largely composed of gabbro, diorite and monzodiorite that overlie the ore bearing phases and contain numerous carbonate xenoliths derived from the country rocks embracing the intrusive complex.
The Chiney layered complex is an asymmetric lopolithic body whose basal contact dips toward the centre at 10 to 25° in the west, but is near horizontal in its eastern section, although the base in this limb is complicated by basins and swells. The northern contact is faulted and the maximum thickness has been estimated as 2.5 to 3.0 km. The Eastern limb is as described for the Eastern block above. Four main intrusive phases, or groups, have been identified from the western half of the intrusion:
• Group 1 is largely only preserved as xenoliths and remnants that comprise ~5% of the pluton's volume, predominantly in the eastern and northern segments. They are represented by massive anorthosite, quartz-biotite and hornblende gabbro, gabbro-diorite and monzodiorite to lenticular segregations of potassic granite. They occur as large sheet-like blocks and xenoliths that vary from metres to tens of metres in size, incorporated into the two succeeding groups.
• Group 2, high-Ti gabbro, the host to the Fe-Ti-V mineralisation. It occupies ~60% of the total pluton’s volume and ~85% of its eroded surface. It has distinct layering compared to the low-Ti varieties. The layers can be subdivided into high- and low-Ti bands, visually distinguished by the titanomagnetite content - referred to as the 'Titanomagnetite Gabbro Series' and 'Leucogabbro Series' respectively. In general, the individual layers lie parallel to the bottom of the intrusion and can be traced from west to east along the strike up to the River China for almost 10 km in its central part where the pattern is distorted. These two series may be described as follows:
- The Titanomagnetite Gabbro Series - has a maximum thickness of ~1000 m in the western part of the complex, and is characteristically finely layered due to fluctuations in the content of major minerals within separate layers, the result of gravity separation (Buddington et al., 1936; Wager and Brown 1968). In contrast, the leucogabbro series has a 'coarser' layering composed of 2 to 3 m thick anorthosite and leucogabbro bands within a background of massive gabbronorite and gabbro.
The Titanomagnetite Gabbro Series consists of various melanocratic (pyroxenite, melanorite), mesocratic (gabbro, gabbronorite) and subsidiary leucocratic gabbro rocks (leucogabbro, anorthosite) with gradual transitions between each. Some 10 to 80 vol.% titanomagnetite is found as a major mineral in all lithologies. Norite and gabbronorite, particularly where present as thin layers within the titanomagnetite gabbro series, have high-titanomagnetite contents and are a dominant component of the series.
The composition of the plagioclase within these rocks varies widely from An35 to An72, although the overwhelming majority are from An48-62. Andesine and oligoclase are commonly associated with interstitial quartz and K feldspar. Altered plagioclase crystals are saussuritised in their core and albitised in outer rims. Orthopyroxene occurs as prismatic crystals, frequently resorbed and rimmed by clinopyroxene. Orthopyroxene and plagioclase occur as inclusions in large clinopyroxene grains, producing a poikilophitic texture. Brown hornblende and sheet-like biotite inclusions develop along fractures and grain boundaries of clinopyroxene. Biotite with strong pleochroism occurs as rims to titanomagnetite, and as isolated irregular, up to 0.5 mm, segregations, whilst quartz is intersticial to major minerals. Apatite is found as thickened prismatic or elongated columnar crystals. Titanomagnetite averages 1 to 2 mm in size, occurring as elongated segregations of 5 to 6 crystals forming discontinuous chains. Plagioclase and some clinopyroxene crystals are similarly oriented, although clinopyroxene grains are mostly equant. Titanomagnetite-pyroxene rocks dominate at the basal sections of individual bands. Pyroxene is occasionally replaced with biotite or serpentinite but most frequently with hornblende. Titanomagnetite also occurs as small, up to 1 mm, inclusions in ortho- and clinopyroxene or as larger, up to 3 mm aggregates.
The composition of the Titanomagnetite Gabbro Series varies markedly laterally. From west to east, the section becomes more leucocratic, with markedly decreased titanomagnetite contents. It rests on low-Ti gabbro rocks in the western and central parts of the complex or is in immediately contact with the sedimentary rocks of the Udokan Supergroup.
The distinct small-scale layering and fluctuations of titanomagnetite content in the Titanomagnetite Gabbro Series are related to microrhythms that mainly occur in pyroxene and plagioclase rocks in its lower and upper parts respectively. In the latter case, massive and high-grade titanomagnetite ore is found in close association with adjacent barren anorthosite. The microrhythms, which commonly have distinct, sharp boundaries, vary in thickness from 2 to 3 cm, to as much as 1.5 m, with no regular trends or patterns. The central part of the series, between these upper and lower parts, is also generally characterised by fine banding and thin rhythms with similar sharp layers differentiated by their titanomagnetite content. Within these bands, titanomagnetite is either distributed more or less uniformly within a rhythm or as cumulates near its base. The visual contrast of rhythms is related to the titanomagnetite content of the rocks, with those that have diffuse boundaries being of lower grade. In the upper parts of microrhythms, rocks are generally leucocratic, coarse-grained, and often pegmatoidal. Chalcopyrite and pyrrhotite mineralisation are most frequently only hosted just in these leucocratic segregations.
Gongalsky and Krivolutskaya (2019) describe a well mineralised, 9 cm microrhythm at a depth of 642 m, which is devoid of xenoliths, veinlets or secondary
alteration of the rock-forming minerals represented by titanomagnetite, ortho- or clinopyroxene, or plagioclase. The content of titanomagnetite gradually decreases from the the floor (73%) to the roof (18%) microrhythm, whilst the pyroxenes first increases to the middle of microrhythm from 27 to 59%, and then decreases to 37% in its upper part. Plagioclase only appears as a major mineral in the middle of the rhythm, gradually increases up to 40 vol.% in the upper part. Biotite, chalcopyrite and pyrrhotite are subordinate, with total combined content that does not exceed 5 vol.%, mostly within the leucocratic rocks of the upper part of the microrhythm.
At the base of the microrhythm, titanomagnetite occurs as 3 to 4 mm multiangular cumulate grains, which higher, are clustered as small <1 mm grains and chains up to 5 mm long between pyroxene and plagioclase grains. At the base, orthopyroxene crystals have xenomorphic outlines and cement titanomagnetite crystals, changing upwards into an idiomorphic habit, with a size increases to 1.5 cm. Isolated xenomorphic plagioclase grains up to 1 mm wide occur sporadically in the lower third of the microrhythm with brownish-green reaction rims of fine-grained aggregates up to 0.1 mm in thickness where in contact with titanomagnetite. In the middle of the microrhythm, the plagioclase crystals are up to 2 mm in size, gradually increasing in quantity and size towards the roof where lenticular segregations of large tabular crystals appear, with small xenomorphic pyroxene grains localised between them.
The transition from melanocratic to leucocratic lithologies within particular layers is also seen at a larger scale, with the development of rhythmicity as macrorhythms that are tens to hundreds of metres in thickness, rhythms of a few metres, and microrhythms measured in centimetres and up to a few metres.
- The Leucogabbro Series corresponds to the interval within the layered complex where mesocratic gabbronorite is replaced by leucocratic varieties and abundant anorthosite as the dominant lithofacies. The series is as much as 1500 m thick, divided into a 500 m thick lower, 800 m thick middle and 200 m of the upper member, all of which differ in composition and structure.
The Lower Member is characterised by anorthosite layers and associated massive titanomagnetite ore within a sequence of massive mesocratic gabbronorite and gabbro. Titanomagnetite layers, which are the bases of rhythms, are of the order of 1.5 to 3.0 m thick, and as they form brittle layers, are strongly fragmented, undergoing intense bedding-plane deformations. Leucogabbro, spotty gabbro rocks and anorthosite bands have a mean thickness is 1 to 2 m, locally as much as 5–6 m. The boundaries of leucocratic layers most frequently are nearly parallel to the layering as a whole, with angular unconformities and crosscutting relationships less abundant.
The Middle Member is characterised by abundant leucocratic interlayers, frequently closely spaced, and hosted in barren gabbro. Anorthosite layers are commonly <1 m thick, locally reaching 2 to 3 m and have reasonably distinct boundaries. Some extend over 1 to 2 km strike lengths, although most are only a few tens or hundreds of metres long emplaced in an en echelon pattern. Leucocratic rocks also occur as short, irregular lenses or bodies with distinct contacts. Anorthosite often has associated spotty varieties in which the normal light-colored matrix contains dark spots with a composition close to gabbronorite. Quartz, which is xenomorphic to major minerals, occurs in both dark spots and light matrix. Leucogabbronorite consists of 60 to 70 vol.% plagioclase (An54–64), and 15 to 25 vol.% hypersthene + augite. Pyroxene crystals are commonly larger than those of plagioclase. Biotite occurs as up to 0.3 mm) sheets whilst apatite is found as elongated grains at boundaries of rock-forming minerals.
In the uppermost part of the leucogabbro series, the quantity of quartz increases and K feldspar appears such that the composition approaches that of quartz diorite although this end phase is of limited abundance. This phase has been attributed to assimilation of country rock sandstone of the Udokan Supergroup by gabbro resulting in the enrichment in alkali metals and silica (e.g., Lebedev 1962; Kulikov et al., 1981). This interpretation is questioned because the clastic country rocks themselves are not enriched in alkali and alkali-earth metals (Konnikov et al., 1981), whilst Mel'nikova et al. (1983) suggest the anomalous alkalinity is the product of fractionation of the initial mafic melt.
• Group 3 is largely coextensive with Group 2, both along strike and down dip, and comprises low-Ti norite and gabbronorite, with significantly lesser melanonorite and leucogabbro. These rocks generally contain <5 vol.% titanomagnetite, although extremely rarely, some intervals up to a few metres thick are enriched to grades of 10 to 20 vol.%. The rocks of Group 3 are predominantly in the lower part of the complex, below the high-Ti gabbroic rocks of Group 2, although they also occur as thin sills hosted in gabbroic rocks of the latter group. In addition, numerous crosscutting relationships are also evident between the two groups, emphasised by the orientation of plagioclase laths in norite at an angle to the trachytoid structure of adjacent titanomagnetite gabbronorite. Intrusive relationships recognised between the gabbroic phases of the two groups are interpreted to reflect repeated local injections. There is also evidence, especially in the southern segment of the pluton, of an unconformable relationship between the two gabbroic phases, where large irregular blocks of titanomagnetite gabbro are incorporated into the massive gabbronorite with leucogabbro interlayers. The contacts between the low and high Ti gabbro are sharp, although, in some cases, the norite offsets penetrate the titanomagnetite gabbro along the layering. Lenticular segregations of massive titanomagnetite ore that vary from a few cm to 1.5 to 2.0 m in thickness occur almost everywhere in the contact zone, irrespective of contact morphology. Considering the intrusive relationships between low- and high-titanomagnetite gabbro phases, the abrupt variations in thickness, irregular position within the section, and discordant strike and dip, Gongalsky and Krivolutskaya (2019) concluded that two phases represent multiple injections of different portions of temporally separated melt.
Group 3 includes a series of three, upward and eastward thinning macro-layered units containing numerous micro-layered rhythms that can be traced through the low-Ti gabbro rocks. There is also a compositional variation within the macro-layers, expressed as a variation of orthopyroxene composition from hypersthene to bronzite. Plagioclase does not show such distinct trends due to its narrow compositional range from of 50 to 58 mol.% An. The lower sections of each macro-layered unit contains the bulk of the micro-layered rhythms, each of which forms a separate ~1.5 to 2.0 m thick layer with distinct boundaries reflecting an abrupt change from leucocratic into melanocratic rocks. Gravity layering is controlled by primary accumulation of pyroxene at the base of each rhythm.
The norite is massive, with crystals that vary from 0.5 to 3.0 and 4 to 5 mm across, predominantly plagioclase and orthopyroxene, with subsidiary clinopyroxene, titanomagnetite, hornblende, biotite, quartz and K feldspar; accessory apatite, zircon and monazite; overprinted by secondary sericite, epidote, zoisite, carbonate, talc, serpentine, actinolite and chlorite. Plagioclase occurs as labradorite (An), forming perfectly faceted, 0.5 to 4.0 mm laths and is characterised by polysynthetic twinning and occasional vague zoning. Orthopyroxene crystallised as 3 to 6 mm prismatic crystals that are idiomorphic relative to plagioclase, although inverse relationships are also observed. Individual micro-rythms contain lower melanocratic rocks that contain bronzite, grading to leucocratic rocks in the upper parts containing hypersthene. Talc and serpentine develop along fractures. Clinopyroxene (augite) occurs as small 0.3 to 0.5 mm grains that are xenomorphic relative to the major minerals, and overgrow orthopyroxene in some cases. Biotite occurs as separate flakes replacing hornblende, whilst symplectic biotite-quartz intergrowths locally surround pyroxene grains. Quartz and K feldspar form micropegmatitic intergrowths in sporadic interstices. Scarce titanomagnetite grains are spider-shaped with associated biotite, zircon and apatite.
An increase in clinopyroxene produces a poikilitic texture, and as a consequence, in gabbronorite, where clinopyroxene prevails over orthopyroxene, this texture is dominant. Large clinopyroxene grains in gabbronorite differ in composition from sporadic equant augite grains in norite. The Group 2 low-Ti gabbroids are markedly different from the high-Ti gabbro of Group 3, the monzodiorite of Group 1, and lamprophyres of Group 4.
• Group 4 is composed of fine-grained gabbro and gabbronorite, as well as higher alkalinity quartz diorite and monzodiorite. They constitutes the marginal (i.e., lateral and vertical) facies of the intrusion, varying from a few to a few tens of metres, and up to 100 m thick in the SE and south of the complex. The rocks of the group are cut by fluid-magmatic breccia with lamprophyre cement that are of limited abundance, primarily restricted to the contact zone between gabbroic rocks of Groups 2 and 3 and sedimentary country rocks of the Udokan Supergroup. These breccias are most widespread in the eastern part of the complex, mainly controlled by near-horizontal faults at the base of the intrusion. They vary in thickness from 50 to 60 m to a few metres and then to complete pinch-out. These abrupt changes are observed at fault intersections, where some thin (10 to 15 m wide) vertical dyke-shaped breccia bodies are also developed, corresponding to these offsets that cut through both low- and high-Ti gabbroic rocks. These breccias persist into the country rocks for up to 300 m from the intrusive contact in the east, whilst to the west, they are noted at a distance of 1 km from the main intrusion. The margins between the lamprophyre breccias and the sedimentary rocks are distinct and sharp, often characterised by a migmatitic appearance. The magmatic breccias are interpreted to have been formed in crush zones, commonly at the contact between gabbroic rocks and sandstone, where lamprophyre was subsequently injected. Fragmental material varies from a few to as much as 70% in the central part of breccia bodies and includes gabbro, skarn, quartz crystals and numerous schlieren of mafic minerals (pyroxene and hornblende replaced by actinolite, talc and chlorite). The lamprophyres contains numerous curved and angular fused fragments of recrystalised hornfels after sedimentary protoliths, cemented by equigranular fine-crystalline rocks, mainly composed of sodic and intermediate plagioclase (An35–60), up to 40 vol.% biotite, actinolite, chlorite, and occasionally quartz and K feldspar..
The marginal facies of the complex have been dated as 1880 ±16 Ma (Ar-Ar; Polyakov et al., 2006) and 1867 ±3 Ma (U-Pb zircon; Popov et al., 2009), whilst the central portion was dated as 1850 ±90 Ma (Sm-Nd; Gongalsky et al., 2008). The titanomagnetite-bearing gabbro of Group 2 has been dated at 1858 ±17 Ma, although a Group 3 Norite is much younger at 1811 ±27 Ma (both SHRIMP-II zircon; Gongalsky 2012). The granites of the Kodar Complex, exposed ~20 km to the NE and SE have been dated between 1876 and 1873 Ma (U-Pb zircon; Larin et al., 2000), whilst the Ingamakit Complex that intrudes the Chiney Intrusive Complex to the west is 305 ±32 Ma Palaeozoic in age.
Two main ore types are represented within the Chiney Intrusive Complex:
Two main Fe-Ti-V deposits have been delineated within the Chiney Intrusive Complex, Etyrko and Magnitnyi in the western and southeastern blocks of the complex respectively, ~7 km apart. Both are hosted within the high-Ti gabbroic rocks of Group 2. Together they account for ~30 Gt of resources averaging 0.49% V2O5 (Maximum 1 to 1.5 wt.% V2O5) and 6.3% TiO2 (Gongalsky and Krivolutskaya, 2015). According to Seltmann et al. (2010) the titanomagnetite resources totalled 10 Gt @ 16 to 58 wt.% Fe, 0.22 to 1.23 wt.% V2O5, 3.35 to 15.4 wt.% TiO2.
The Etyrko deposit is composed of 'stratabound', mostly disseminated titanomagnetite, whilst Magnitnyi predominantly comprises cross-cutting ore veins and irregular bodies in gabbronorite. The 'stratabound' Etyrko deposit is regarded to represent syndepositional cumulate mineralisation, whilst the vein orebodies at Magnitnyi are superimposed on the host gabbro rocks, i.e., they are interpreted to be post-depositional.
Magnitnyi contains ~1.5 Gt of ore in at least 11 major and 23 minor orebodies, with ~89% of the total reserves and 86 to 92% of mineable metal reserves in the major orebodies. The minor orebodies are apophyses of major orebodies. The total iron grade gradually decreases westward from 39.4 to 28.9 wt.%. Titanomagnetite and ilmenite (the amount of the latter rarely exceeds 10% of the total) are the major ore minerals.
The stratabound orebodies at Etyrko are of two types, both of which can be traced for several kilometres along strike with an orientation that coincides with that of the gabbro host rocks, which dip at from 2 to 20°, They are:
• Disseminated mineralisation where titanomagnetite crystals define fine layers of the microrythmicity in the titanomagnetite gabbro series. This is the most important ore type. It is, in turn, subdivided into two sub-types, i). as accessory minerals with subhedral titanomagnetite and ilmenite grains within the host, and ii). early magmatic segregation massive and disseminated mineralisation that are generally thin, only of a few cm in thickness at the base of microrhythms, but comprise >80% titanomagnetite. The margins of the stratabound orebodies are generally diffuse and determined by assay as grade boundaries.
• Thin, up to ~0.5 m thick interlayers of massive titanomagnetite associated with chineyites of the leucogabbro series. Chineyites are anorthosites
enriched in titanomagnetite (Gongalsky and Krivolutskaya 1993). The layers associated with chineyite, leucogabbro or anorthosite are characteristic of the upper parts of macro-layered units that are composed of high-Ti gabbro rocks. The ore layers are commonly a few tens of centimetres thick, although their lateral extent may be several kilometres. The contacts with under- and overlying rocks are distinct, often emphasised by fracture systems parallel to the general layering of the intrusive complex.
The cross-cutting ore veins and irregular bodies of the Magnitnyi deposit can be morphologically separated into:
i). Extended lenticular lodes that are localised between the finely layered stratabound ore, and are related to pinch-outs of the host titanomagnetite gabbro rocks. They strike parallel to the general layering in the area (which at Magnitnyi strikes at 75° and dips at <10°NW) and have distinct boundaries with mineralised host gabbro rocks. Individual lenses are between 5.7 and 26.5 m in thickness. The transition to background gabbro from high-grade disseminated (and less frequently massive) ore containing up to 80% titanomagnetite, occurs over an interval of 10 to 15 m, reflecting the halo of disseminated titanomagnetite that surrounds the high grade mineralisation. In the southern part of the intrusive complex, the orebodies are hosted in the Group 3 low-Ti gabbro rocks as large xenolithic Group 2 blocks, which dip to the south in contrast to the general northward dip elsewhere.
ii). Massive titanomagnetite veins and ore lenses that are hosted by poorly layered low-Ti gabbro, with dimensions of 1 to 3 x 0.7 to 1.5 m. Their contacts with host rocks are sharp, emphasised by fractures oriented parallel to the lens surface and clearly expressed in topography. The host rocks are commonly barren and contain only ~5% titanomagnetite.
iii). Large isometric and irregular bodies; and
iv). Small round, oval or irregular massive titanomagnetite segregations that have sharp boundaries with host gabbro rock (Gongalsky 2010, 2015). Their mean size is a few centimetres to a maximum of a few tens of centimetres. This type of ore is not economically viable on its own.
Five main sulphide deposits related to the Chiney intrusive complex are known, from west to east: the Kontaktovyi, Skvoznoe, Pravoingamakitskoe, Verkhne-Chineyskoe and Rudnoe. According to Seltmann et al. (2010), the sulphide deposits contains reserves of 8.2 Mt of contained Cu (at an unspecified grade), with additional Ni, Co, Pt, Ag and Au. However, the grade at Kontaktovyi is quoted as from 0.27 to 2.85, averaging 0.8 wt.% Cu which is lower than the ore at Rudnoe, but it is more consistent. Gongalsky and Krivolutskaya (2015) quote grades of these deposits from Chechetkin and Kharitonov (2002)) as follows: Rudnoe - 0.73% Cu, 0.05% Ni, 0.008% Co, 0.26 g/t Pt, 1.24 g/t Pd, 0.2 g/t Au, 3.41 g/t Ag; and Kontaktovyi - 0.39 to 2.62% Cu, 0.03 to 0.2% Ni, 0.005 to 0.014% Co, 0.02 to 0.21 g/t Pt, 0.07 to 0.79 g/t Pd, 0.01 to 0.07 g/t Au, 1.5 to 4.9 g/t Ag.
These five deposits occur as lenticular and veined orebodies, with the highest concentrations of sulphide minerals near the basal contact of the intrusive complex, divided into endocontact and exocontact types. Of these, Rudnoe and Kontaktovyi are of the most potential economic value. These two deposits are summarised below. The Pravoingamakitskoe deposit is located ~4 km to the south of the Chiney intrusive complex, in immediate proximity to the
Skvoznoe deposit interpreted to be directly related to that intrusive:
Rudnoe occurs within gabbroic rocks of the endocontact zone of the eastern offset of the Chiney Intrusive Complex and in the exocontact zone with sedimentary rocks of the Udokan Supergroup. The mineralised zone is essentially 'stratabound' being traceable along the entire exposed ~9 km contact of the gabbro intrusion which dips at 6 to 12°NW. The mineralised zone varies from 3 to 65 m in thickness and has been tested to a depth of 240 m. Three styles of mineralisation are recognised:
• Endocontact-disseminated sulphides - which have a very non-uniform distribution, but contains 50 to 60% sulphides in the gabbro host. Mineralisation intensity generally increases toward the contact surface before abruptly pinching out in the adjacent country rocks. At Rudnoe the base of the intrusion is near horizontal and coincides with a marked increase in sulphide content. Mineralisation occurs as a near horizontal stratabound and lenticular bands that average 10 to 15 m in thickness, to a maximum of 50 to 60 m, with lateral extents of several hundred metres. The mineralisation is predominantly composed of chalcopyrite and pyrrhotite in variable proportions, with lesser pentlandite, sphalerite, pyrite, titanomagnetite and ilmenite, rare siegenite, galena, violarite, millerite, cobaltite, gersdorffite, safflorite, loellingite, argento-pentlandite, michenerite, merenskyite, sperrylite and other platinum group minerals. Two principal sulphide assemblages are recognised:
i). Chalcopyrite-pyrrhotite in variable relative proportions, making up more than 80% of the sulphide content, either as chalcopyrite or pyrrhotite predominating, or more commonly as a mixture of both. The boundaries between different sulphide combination assemblages is often sharp, over a few mm. In addition to these two minerals cobalt-bearing pentlandite with a Co content of as much as 6 to 7 wt.% Co, Co-bearing pyrite, and the linnaeite group of minerals and sphalerite occur (Krivolutskaya 1986). PGE minerals are also common in this assemblage (Tolstykh et al., 2004; Tolstykh 2008). Chalcopyrite occurs as grains of from 0.001 to as much as 3 or 4 mm in size. Pyrrhotite commonly encloses numerous pentlandite inclusions and contains trace Ni, Co and Cu. Pentlandite grains vary in size and morphology and may be up to 0.3 mm in size, and are replaced by violarite, pyrite and magnetite along the cleavage fractures. Sphalerite occurs as emulsion and fine crystals in pyrrhotite and chalcopyrite, occupying 10 to 15% of the host mineral volume. Galena is a common mineral but only forms very small grains. Pyrite occurs as small euhedral crystals among silicates, rims around pyrrhotite grains and veinlets in rock-forming minerals.
ii). almost 'pure' chalcopyrite with Co and Ni arsenides and sulphoarsenides. This assemblage is of limited abundance and includes the cobaltite-gersdorffite isomorphic series, loellingite and safflorite. Palladium minerals (michenerite, merenskyite, etc.) are associated with sulphoarsenides
and arsenides. In particular, the Pd-bearing phases have been detected in niccolite (Krivolutskaya 1986, 1989; Gongalsky and Krivolutskaya 1999, 2004; Tolstykh et al., 2004; Tolstykh 2008).
• Exocontact-disseminated sulphides are hosted by sandstones of the Udokan Supergroup sedimentary succession and comprise 20 to 30% sulphides. Orebodies have varying strikes and dips and a wide variations in thickness, generally of from 10 to 30 cm, although vertical quartz-chalcopyrite veinlets are noted as far as 250 m from the contact. In contrast to that of the endocontact, exocontact mineralisation is dominated by chalcopyrite and locally by chalcopyrite with bornite with common cubanite. Mineralisation is characterised by the prevalence of hexagonal pyrrhotite over monoclinic varieties and by a 2 to 3 wt.% Co admixture in pentlandite, whilst cobaltite-gersdorffite sulphoarsenides often replace sulphide aggregates. Arseno-hauchercornite, maucherite, nickeline, hessite, cubanite, mackinawite, millerite, Ni-Bi compounds, native lead, sperrylite and numerous compounds of Pd with Bi, Te, Sb, Sn, and As (klockmannite, melonite, sudburyite, froodite) have been identified among rare minerals (Krivolutskaya 1986; Krivolutskaya et al., 1997; Tolstykhet al., 2004). As with the endocontact disseminations, minerals from the linnaeite group are subdivided into varieties close to linnaeite, violarite and siegenite. Linnaeite occurs as flame-like segregations in pyrrhotite; siegenite forms separate up to 0.2 to 0.3 mm euhedral grains in gangue mineral aggregates, whilst violarite most frequently occurs as 0.2 to 0.3 mm thick veinlets in pentlandite and sporadically as individual grains in the gangue. Bornite is generally of limited abundance, and is most frequently associated with chalcopyrite and millerite, although it may locally prevail over chalcopyrite to form a special chalcopyrite-bornite ore type.
• Massive sulphides veins are found in exocontact rocks, predominantly sandstones, for up to 20 and 30 m and sometimes as much as 70 m from the lower contact of the intrusive complex (Gongalsky and Krivolutskaya 1993). They vary in thickness from 10 cm to 1.5 m with lateral extents of from 10 to 20 m and locally up to 50 m. The veins fill a system of horizontal fractures in the sandstones which parallel the basal contact of layered mafic-ultramafic complex with the host Udokan Supergroup sedimentary rocks. The vein margins are sharp. There is also abundant associated breccia ore, where sulphides (mostly chalcopyrite) cement 2 to 10 cm fragments of gabbroic rocks.
Mineralisation within these veins is composed of >95% chalcopyrite, with subsidiary millerite, pyrrhotite, pentlandite, linnaeite group minerals, magnetite, mackinawite, gersdorffite, cubanite and sphalerite; with rare nickeline, maucherite, arseno-hauchercornite and Pd minerals. Veins commonly have selvages that are enriched in Ni minerals or are composed of massive millerite.
Kontaktovyi is located close to the western contact of the Chiney Intrusive Complex where disseminated mineralisation reaches a maximum thickness of 70 to 80 m in a number of separate orebodies. The larger of these occur as stratabound lodes with lateral extents of hundreds of metres, accompanied by a number of smaller lenticular bodies. These mineralised bodies are conformable to the layering of the gabbro host rocks and parallel to the base of the intrusive complex, striking at 100 to 145° with a 40°NE dip. Ore block margins are determined as grade boundaries. Two lodes have been outlined: i). at the base of the intrusive complex, and ii). a second that is ~150 m higher in the intrusive complex.
Mineralisation is predominantly disseminated within hosts that are mainly leucogabbro, spotty anorthosite and leopard gabbro (comprising 1.5 to 2 cm spots on a gabbroic background). These rocks maintain a consistent composition, structure and texture throughout the mineralised zone, but have been subjected to intense albite, amphibole and epidote alteration. The intensity of mineralisation is proportional the degree of alteration. The spots within the lithologies of the intrusion are the result of micrographic intergrowth of plagioclase and pyroxene. In well mineralised intervals, sulphides replace mafic minerals while retaining the primary rock texture. Some 70 to 80% of the sulphide mineralisation is uniformly distributed through the host rocks as up to 2 to 4 cm diameter segregations, concentrated in the spots that occupy 40 to 50% of the rock volume.
The mineralisation comprises chalcopyrite and pyrite in a 1:1 ratio. Pyrrhotite, magnetite, millerite, violarite and sphalerite are rare, whilst titanomagnetite and ilmenite are next in abundance after the sulphides. Chalcopyrite occurs as irregular 0.6 to 0.7 mm grains, whilst larger euhedral cubic pyrite crystals contains numerous irregular chalcopyrite inclusions. Pyrite also contains admixtures of Co and Ni at twice the concentration in chalcopyrite. Extremely rare, 0.05 to 0.10 mm pyrrhotite grains are found among the gangue minerals. Titanomagnetite occurs as large, strongly decomposed, 5 to 8 mm clusters of 3 to 4 crystals with internal textures that are commonly roughly latticed. Magnetite is replaced with gangue minerals, and only an ilmenite lattice is left among the rock-forming minerals and sulphides. The relative amount of ilmenite does not exceed 10 vol.%.
Pravoingamakitskoe is located ~4 km to the south of the Chiney Intrusive Complex and is hosted within metasedimentary rocks of the Chitkanda, Alexandrov and Butun formations, all of which are members of the Chiney Group. The first two are predominantly composed of sandstone, while the third comprises mainly carbonate rocks. Pravoingamakitskoe is located immediately adjacent to the Skvoznoe deposit which is more closely related to the intrusive complex. The latter occurs as disseminated mineralisation in the exocontact zone of the intrusive, comparable to similar mineralisation at the Rudnoe deposit described above. Pravoingamakitskoe has been considered to be an analogue of the Udokan deposit, i.e., a stratabound, sediment hosted Cu-Ag deposit, although it differs markedly in stratigraphic position, mineralogy and the chemical characteristics of mineralisation.
The host, copper-bearing sandstone-siltstone-carbonate rocks, which are principally in the middle of the Chitkanda Formation, can be traced at surface for almost 10 km along strike, 4.5 km of which are considered to contain potentially economic mineralisation that has been traced for 400 to 500 m down-dip (Gongalsky and Krivolutskaya, 2015). A second mineralised horizon is developed within the Alexandrov Formation (Gongalsky and Krivolutskaya, 2015). At surface, significant mineralisation occurs as en echelon lenses that are up to 300 to 440 m in strike length, 1.3 to 4.0 and 15 to 38 m in thickness, with an average grade of 0.47 to 2.5 wt.% Cu. This interval is estimated to contain a potential 'economic resource' of 0.608 Mt of copper (Sekisov et al., 2014). This mineralisation mainly occurs as both massive and disseminated sulphides, with a gradational transition between the two, but with the latter being more extensive. Ore block boundaries are determined by grade cut-off. The host and deposit have been disrupted into a series of fault bounded blocks that are offset by hundreds of metres.
Mineralisation also occurs in quartz veins and lenses superimposed on the massive and disseminated sulphides, and is interpreted to be epigenetic and
hydrothermal in origin. Where studied in detail (e.g., the Bazaltovyi site at Pravoingamakitskoe) these veins and lenses are up to 1 m thick and a few tens of metres in lateral extent containing sulphide veinlets and segregations. The mineralisation is composed of pyrite-chalcopyrite to chalcopyrite-pyrite, with brecciated, massive and disseminated textures. Similar veins and sandstone-hosted disseminations containing chalcopyrite-bornite, cubanite-chalcopyrite, chalcocite-bornite and pyrite-chalcopyrite assemblages are present, but are less abundant. This mineralisation is characterised by high and variable Cu/Ni ratios of between 10 and 700. The greatest enrichment in Ni is in quartz veins that have elevated sulphide contents of as much as 20 vol.%, that includes higher contents of millerite and pentlandite. Most of the principal ore minerals at Pravoingamakitskoe have high Ni and Co concentrations, particularly pyrite, which contain concentrations of these elements of as much as 1.75 wt.% Ni and 1.48 wt.% Co. However, the pentlandite is a low Co variety (0.22 wt.% Co), which contrasts with the copper deposits within the Chiney Intrusive Complex, where Co ranges from 2 to 18 wt.%. The disseminated pyrite-chalcopyrite ore from some locations within the Pravoingamakitskoe deposit resembles that of the exocontact ore at the Rudnoe and Kontaktovyi deposits, as described above. In some parts of the deposit there are elevated concentrations of noble metals, reaching 2.2 ppm Pt, 6.2 ppm Pd and 0.4 ppm Au where up to 10 µm grains of the PGE minerals clausthalite, bravoite and bogdanovichite have been recognised (Gongalsky et al., 2007). However, in bodies of massive sulphide that are strongly enriched in copper, the noble metal concentrations are much lower (0.04 to 1.0 ppm Pt, 0.6 to 1.0 ppm Pd, and 0.1 to 0.4 ppm Au) than in the veins, except for Ag. The high content of silver is typical for this type of mineralisation, reaching anomalous values up to 371 ppm Ag. Millerite and pyrite contain up to 0.19 wt.% Ag.
The Chiney layered mafic-ultramafic intrusive complex is part of an extensive 1880 ±16 Ma to 1811 ±27 Ma bimodal magmatic event developed within the extensive Kodar-Udokan Rift Trough. The rift trough was developed in a long lived extensional regime and was filled with the ~12 km thick, deformed and metamorphosed Palaeoproterozoic clastic suite with lesser carbonate rocks of the Udokan Supergroup. 3D modelling of gravity data suggests the Chiney Intrusive Complex is developed above a much larger deep magma chamber that extends to depths of >20 km and is centred on a large scale domal antiformal structure - the Chiney Anticline. The multiple and diachronous injections of mafic melt phases that make up the Chiney Intrusive Complex suggests it is the result of repeated tapping of the deep, differentiating magma chamber. The Chiney Intrusive Complex hosts extensive cumulate Fe-Ti-V mineralisation in its lower half, and cumulate copper mineralisation at the base of the intrusion (e.g., at Kontaktovyi), with disseminated and vein copper sulphides in the underlying meta-sedimentary rocks of the Udokan Supergroup (e.g., at Rudnoe). However, at deposits such as Pravoingamakitskoe, sediment hosted mineralisation is restricted to sandstones of the Udokan Supergroup which may be traced for as much as 4 km from the contact with the Chiney Intrusive Complex. The Udokan sediment hosted copper deposit is developed ~10 km NNW of the intrusive complex and is one of a considerable number of Cu-Ag deposits and occurrences developed within several hundred kilometres within the same host unit, but also at other stratigraphic positions throughout the Udokan Supergroup. These observations would be consistent with a common source of Cu, derived from a large mantle input, such as the deep magma chamber below the Chiney Intrusive Complex and other similar intrusions in the Kodar-Udokan Rift Trough. Metals would then be extracted, transported and deposited via a number of processes, including as magmatic cumulates within the intrusive complex, hydrothermal fluids released into the sedimentary country rocks, or Cu leached from the source by later basinal fluids. The metals wold then be combined with sulphur, such as that derived by magmatic assimilation of intruded pyrite-pyrrhotite country rocks or via reaction with other sources within the sedimentary pile.
Except where noted otherwise, most of this record is drawn from Gongalsky and Krivolutskaya, 2015 - World-Class Mineral Deposits of Northeastern Transbaikalia, Siberia, Russia; Springer, Switzerland, 323p. ISBN 978-3-030-03559-4.
The most recent source geological information used to prepare this summary was dated: 2015.
This description is a summary from published sources, the chief of which are listed below.
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Gongalsky, B. and Krivolutskaya, N., 2015 - Mineral Deposits in Magmatic Rocks; Ch 5 - The Chiney Layered Pluton: Structure and Mineral Composition; Ch 6 - Chemistry and General Typification of Intrusive Rocks; Ch 7 - Titanomagnetite Ore; Ch 8 - Sulfide Mineralization - Chiney Pluton: in Gongalsky, B. and Krivolutskaya, N., 2015 World-Class Mineral Deposits of Northeastern Transbaikalia, Siberia, Russia, Modern Approaches in Solid Earth Sciences, Springer, Switzerland, v.17, Ch. 5 to 8, pp. 115-254.|
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