Malmyzh - Freedom, Valley, Flats, Central
Khabarovsk Kray, Russia
Super Porphyry Cu and Au|
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The Malmyzh porphyry copper-gold deposit is located 220 km NE of the city of Khabarovsk, Khabarovskiy Kray, in the Russian Far East (#Location: 49° 54' 59"N, 136° 53' 49"E).
Low grade gold mineralisation has been known in the Malmyzh area since the 1970s (Chernyavsky and Shavkunov, 1976) but had received little attention. Porphyry-copper characteristics were recognised at Malmyzh in the late 1990s and confirmed in 2005 by grass roots exploration undertaken by Phelps Dodge Corp., which was acquired by Freeport McMoRan Inc. in 2007. The deposit subcrops in low relief, forested hills flanking the Amur River valley, and is concealed beneath a veneer of soils and alluvium. Recognition of porphyry characteristics in the field involved the sighting of barren, peripheral limonitic D-veinlet stockworks exposed in a small gravel pit (Bowens et al., 2017). Porphyry-style mineralisation was intersected in 2008 in drilling by a consortium led by Lundin Group company Fortress Minerals that also included Eurasian Minerals Inc. In 2009 a management buy-out of Lundin assets resulted in the foundation of IG Copper, a private stock company. IG Copper, as manager, had consolidated ownership, advanced the project to discovery and earned a 51% share of the joint venture by 2011, with the remaining 49% held by Freeport-McMoRan Exploration Corporation (Chitalin et al., 2013; Canby, 2019). Subsequently, a preliminary Inferred Mineral Resource of 2.54 Gt @ 0.29% Cu, 0.14 g/t Au, or 0.37% Cu-eq. was delineated at a 0.20% Cu-eq. cut-off (Newall, 2015) in 4 of more than 15 mineralized 'porphyry centres' known in the immediate district. These centres are distributed over a NE-SW trending, 16 x 5 km corridor. The four centres contributing to the resources, from NE to SW, are known as Freedom, Valley, Flats and Central. In October 2018, the Malmyzh project was sold to the privately owned Russian Copper Company.
The Lower Cretaceous (~128 Ma) Malmyzh deposit is situated in the Amur Terrane, which lies within the northern part of the Mesozoic Sikhote-Alin orogenic system that extends northward from Vladivostok for >1500 km along the western Pacific coast.
For detail of the regional setting of this part of the Central Asian and Palaeo-Pacific orogenic belts see the separate Manchuria Overview record that will be available soon.
In the Malmyzh district, the Amur Terrane comprises a thick Lower Cretaceous flysh-like turbiditic sedimentary sequence that has been subjected to intense deformation, with tight folds and numerous fault zones. These rocks are intruded by a series of small, Early to early Late Cretaceous, magnetite-series plutonic stocks of the Lower Amur Plutonic Suite which were emplaced in a continental transform margin regime after the cessation of active subduction. The Lower Amur Plutonic Suite has been subdivided into three intrusive stages, namely: i). early gabbro, monzogabbro, monzonite, diorite and quartz diorite; ii). intermediate granodiorite, quartz monzonite, quartz monzodiorite and granodiorite-porphyry; and iii). late granite-porphyry. The early stage dominates and constitutes the bulk of the intrusive stocks that individually outcrop over areas of 2 to 12 km2 (Vaskin et al., 2009). Early stage diorite from the Malmyzh deposit yielded an age of 128±2 Ma (U-Pb zircon; Bukhanova, 2018). The intermediate stage intrusions have been dated at 100 to 90 Ma (K-Ar; Vaskin et al., 2009), whilst late stage quartz diorite-porphyry was dated at 99.3±1.6 Ma to 96.2±1.9 Ma (U-Pb zircons; Bukhanova, 2018).
Other significant mineralisation within the terrane includes the similar Poniy cluster of porphyry Cu-Au occurrences and Au veins some 60 km NE of Malmyzh and a cluster of large tin-tungsten to tin-polymetallic (Cu, Pb, Zn) deposits some 100 km NW of Malmyzh. The latter are associated with dominantly Late Cretaceous ilmenite-series, locally high-K monzonitic and granitoid intrusions (Vaskin et al., 2009; Gonevchuk et al., 2010).
More than 15 separate 'porphyry centres' have been recognised at Malmyzh, each defined by a cluster of roughly isometric Early to early Late Cretaceous dioritic to granodioritic porphyry stocks of the Lower Amur Plutonic Suite. These stocks intrude a Lower Cretaceous package of fine-grained, flysh-like siliciclastic rocks. Each 'porphyry centre' is a composite multiphase intrusive centre composed of small overlapping porphyry stocks that intruding one another, typically with subvertical contacts. Locally these stocks are dyke-like and have complex-shaped apophyses. Individual stocks and composite intrusions vary from 100 m in diameter to 1 km2 and more in area and define a 16 x 5 km, NE-SW aligned corridor, the limits of which are masked by younger cover (Chitalin et al., 2013). Some centres include bodies of magmatic (eruptive) breccias.
The structural trends defined by the mineralised corridor, dykes, breccia zones and alteration patterns are interpreted to indicate structural control by a wide NE-striking fault zone, its splays and branches, while the porphyry centres are regarded a being localised by intersections with transverse, NW-striking cross structures (Soloviev et al., 2019).
Four to locally five intrusive phases have been recognised at Malmyzh, apparently to representing the first and partially the second intrusive stages of the Early-early Late Cretaceous Lower Amur plutonic suite, as detailed above. These phases are, from early to late:
• Diorite to quartz diorite-porphyry ('hornblende quartz diorite porphyry' of Newall, 2015) which form larger intrusives. These rocks are a medium- to fine-grained mesocratic, grey to greenish-grey porphyry composed of 20 to 25 vol.% brownish-green amphibole, 3 to 5 vol.% brown biotite, 60 to 65 vol.% plagioclase, ~5 vol.% K feldspar and 5 to 12 vol.% quartz. Phenocrysts comprise 15 to 20 vol.% amphibole that are up to 20 mm long, with tabular zoned plagioclase and rarely quartz, surrounded by a fine- to medium-grained groundmass of plagioclase, K feldspar, quartz, amphibole and biotite. Accessory minerals include magnetite (locally up to 3 to 5 vol.%), titanite, apatite and zircon.
• Tonalite-porphyry ('quartz-phyric hornblende feldspar porphyry' of Newall, 2015) occurring as smaller stocks. These occur as a fine-grained grey to greenish-grey mesocratic porphyry, composed of 7 to 10 vol.% brownish-green amphibole, 8 to 10 vol.% brown biotite, 60 to 65 vol.% plagioclase, ~5 vol.% K feldspar and 10 to 15 vol.% quartz. Phenocrysts comprise to 60 vol.% of the rock, including zoned plagioclase up to 8 mm across, rare amphibole and quartz, and are set within a fine-grained groundmass of plagioclase, K feldspar, quartz, amphibole and biotite. Accessory minerals include magnetite (locally up to 3 to 5 vol,%), titanite, allanite, apatite and zircon.
• Microtonalite-porphyry ('hornblende feldspar porphyry' of Newall, 2015), which Soloviev et al. (2019) suggest may be a quenched facies of the tonalite-porphyry. It is a fine-grained, dark-grey to dark brownish-grey melanocratic, sharply-porphyry rock composed of 7 to 12 vol.% brownish-green amphibole, 8 to 10 vol.% biotite, 55 to 65 vol.% plagioclase, ~5 vol.% K feldspar and 10 to 15 vol.% quartz. Phenocrysts comprise zoned plagioclase and rarely rounded quartz, within a very fine-grained groundmass of plagioclase-K feldspar-quartz-biotite-amphibole. Accessory minerals include magnetite (locally up to 3 to 5 vol.%), titanite, apatite and zircon. This porphyry characteristically occurs as microdykes or 'vein-dykes' (c.f., Seedorff et al., 2005) that vary from 2 to 3 mm to as much as 10 to 20 mm in thickness. Some of the microdykes are from fluid magmatic melt, whilst others appear to be fracture-controlled, fluid-induced recrystallisation of the host rocks. They locally contain chalcopyrite in their cores that can be attribud to magmatic accessory minerals, and form separate small <10m across mineralised stockworks.
• Granodiorite to quartz monzonite-porphyry which also occurs as small stocks and dykes. A grey to pinkish-grey to pink fine- to medium-grained mesocratic porphyry, composed of 10 to 15 vol.% brown biotite, 40 to 45 vol.% plagioclase, 15 to 20 vol.% K feldspar and 20 to 25 vol.% quartz. Larger phenocrysts, mostly of quartz and plagioclase (andesine), with smaller plagioclase (andesine-oligoclase), are set within a fine- to medium-grained groundmass of K feldspar-plagioclase-quartz-biotite. Plagioclase phenocrysts have a distinct zoning from a core of andesine to andesine-oligoclase rims. Accessory minerals include magnetite (locally as much as 3 to 5 vol.%), titanite, rutile, apatite and zircon.
Some of these lithologies show distinct textural variations in different areas suggest there may multiple intrusive phases of similar composition, in agreement with Newall (2015), who suggested at least eight dioritic/tonalitic intrusive phases.
Clast- to matrix-supported phreatomagmatic, or eruptive breccias are found in some locations, frequently with microtonalite porphyry to milled clastic-magmatic matrix. In some locations (e.g., Freedom Zone) they form large bodies that may be hundreds of metres across (Newall, 2015).
A thin (<10 cm thick) intramineral mafic dyke interpreted to be either a lamprophyre (kersantite) or quartz-bearing monzogabbro porphyry, cuts tonalite porphyry. This mafic rock contains 2 to 5 vol.% apatite and ~10 vol.% magnetite, the latter of which contains bornite and chalcopyrite micro-inclusions (Soloviev et al., 2019).
The intrusives of the porphyry centres belong to a magnetite series, dominantly low-K calc-alkaline diorite-tonalite igneous suite, with a higher potassium content in the younger (quartz monzonite to granodiorite) intrusive phases. These metaluminous to peraluminous I-type igneous rocks exhibit slightly elevated Sr/Y. They exhibit a moderate enrichment in LREE, with a moderately steep overall REE pattern, and no Eu anomalies in the rocks of the early intrusive phases, whereas those of the late phases exhibit a distinct Eu minimum (Soloviev et al., 2019).
Mineralisation and Alteration
The composite porphyry stocks are surrounded by intense hornfels development and superimposed hydrothermal alteration halos, characteristically zoned from an inner potassic assemblage to peripheral propylitic alteration and overprinting phyllic zones (e.g., Chitalin et al., 2013). Potassic alteration has been shown to overprint the diorite- and tonalite-porphyry intrusive phases, although the microtonalite-porphyry dykes appear to postdate zones of potassic alteration. The granodiorite- to quartz monzonite-porphyry phase contains fragments of quartz veins regarded as associated with potassic alteration, but are cut by laminated quartz-magnetite and quartz-ankerite-sulphide veinlets of the propylitic and phyllic alteration stages. Local narrow intervals of argillic alteration found in some mineralised zones appear to be associated with faults. The main alteration types may be summarised as follows (after Soloviev et al., 2019):
• Potassic alteration, which is typically found in the deeper core of the alteration halos, usually within porphyry stocks, although it may also occur in the host metasedimentary rocks that have been altered to biotite hornfels. It occurs as broad zones up to several hundreds of metres across of pervasive replacement, associated with stockworks of veinlets and veins and their coalescing selvages. The zones of pervasive replacement are occur as patchy to massive aggregates of fine-grained quartz-biotite and quartz-K feldspar-biotite. Within the porphyries, mafic phenocrysts are replaced by fine-grained biotite (with minor magnetite and trace chalcopyrite), whilst plagioclase phenocrysts are replaced by K feldspar. Dark-green amphibole is also locally observed with calcite and plagioclase (oligoclase), indicating sodic-potassic alteration. Within metasedimentary hosts, fine-grained quartz-biotite-K feldspar ±plagioclase assemblages correspond to biotite hornfels. Pervasive potassic alteration zones encompass numerous A-type veinlets with a quartz core and predominantly K feldspar to K feldspar-biotite selvages (c.f., Gustafson and Hunt, 1975; Seedorff et al., 2005; Sillitoe, 2010). These veins range from barren to containing minor chalcopyrite, bornite and/or very rare molybdenite. The latter appear to be younger than the barren and chalcopyrite-bearing varieties. Minor magnetite is present in some veinlets which locally contain micro-inclusions of bornite and chalcopyrite, whilst trace hematite is also locally observed. Anhydrite occurs in quartz-K feldspar veinlets at depth. The copper grades in potassic alteration zones that have not been influenced by younger alteration events are generally <0.3% Cu (generally 0.1 to 0.2% Cu), whilst gold grades are typically <0.1 g/t Au (occasionally 0.1 to 0.2 g/t Au).
Potassic alteration assemblages are estimated to have formed from a homogenous high-salinity (57 to 78 wt.% NaCl and 13 to 12 wt.% KCl), high-temperature (>525 to 535°C), sodic-potassic aqueous-chloride fluid, under a pressure of 500 ±100 bars (Soloviev et al., 2019).
• Propylitic alteration is typically found peripheral to potassic alteration, both laterally and upward in most porphyry systems, with a gradational contact. However, at Malmyzh, the marginal and upper parts of potassic alteration zones are clearly overprinted by propylitic alteration, indicating a separate later hydrothermal propylitic stage. This is consistent with the recognition of two different types of propylitic alteration at Mamyzh (c.f., Wilson et al., 2003; Corbett, 2017). These are an 'outer' zone which mostly occurs outside of the porphyry stocks and forms an external halos surrounding potassic alteration zones. A second development, or 'inner' zone is more proximal to the porphyry stocks and overprints the outer and upper parts of potassic alteration zones. Both pervasive replacement and quartz stockwork veining are exhibited in this 'inner' zone. Pervasive alteration occurs as selective replacement of mafic minerals (amphibole and biotite), both primary-magmatic and potassic alteration products, with fine-grained chlorite, locally with associated magnetite and titanite. Locally, epidote and newly formed long-prismatic dark-green amphibole (ferroactinolite to actinolite) are also replaced. In contrast, feldspars are replaced by fine-grained quartz-magnetite ±albite ±calcite aggregate. Magnetite typically occurs as 3 to 5 vol.% fine background disseminations closely associated with chlorite and rutile, as well as thin (<5 mm) and hairline (<1 mm) veinlets and stockworks. The up to 10 vol.% magnetite content in zones of propylitic alteration is generally much greater than that in zones of potassic alteration. Apatite is also common, forming fine, ~2 to 3 vol.% background disseminations, particularly in close association with magnetite and fine-grained titanite.
Fine-grained, disseminated chalcopyrite and minor local bornite are found in zones of pervasive propylitic alteration, apparently replacing magnetite. Fine-grained, ~2 to 3 vol.% pyrite is also common, locally increasing to as much as 10 vol.% as aggregates accompanying chalcopyrite and magnetite.
A range of quartz stockworks and vein systems are common in the 'inner' propylitic zones, particularly those composed of quartz-chlorite ±epidote and amphibole. Thick, 5 to 20 mm laminated quartz-magnetite ±chlorite M-type veins and veinlets (c.f., Clark, 1993; Seedorff et al., 2005) are also found in the 'inner' propylitic zone containing minor bornite and chalcopyrite. A younger generation of quartz-sulphide-magnetite to quartz-sulphide veins cut and truncate laminated quartz-magnetite veins, containing less to no magnetite but more sulphides (chalcopyrite or chalcopyrite+bornite, locally also minor pyrite). These veins can also be laminated, and are planar to weakly curved, with no obvious selvages, although some have thin albite-rich halos (indicating sodic alteration), and are regarded as transitional AB- or a variety of B-type veining. They contain the bulk of the Cu mineralisation within the deposit. Locally, much thicker, 20 to 25 m quartz-magnetite-chalcopyrite-pyrite vein systems represent the principal Cu enrichment (e.g., in the Central Zone) with grades of >1% Cu and >1 g/t Au. Sulphide minerals (chalcopyrite, bornite) are commonly intergrown with chlorite, epidote and/or amphibole (ferroactinolite to actinolite) in these veins.
The propylitic alteration zones, particularly the 'inner' zone, appears to reflect the major mineralising event with the most significant enrichment in Cu. Gold mineralisation within the propylitic zone occurs as very fine-grained native gold-1 (fineness ~980‰) in magnetite. Lower fineness (850 to 760‰) native gold-2 and electrum (fineness 511–616‰) occur as micro-inclusions in chalcopyrite and may be associated with younger mineral assemblages.
Zones of 'inner' propylitic alteration overprinting potassic alteration contain 0.5 to 1.5% Cu (generally 0.3 to 0.7% Cu), whilst gold grades are typically 0.1 to 0.3, and locally up to 1 g/t Au. The 'outer' propylitic alteration is generally found outside of the porphyry stocks, replacing metasedimentary rocks, locally extending to >1 km from the porphyry stock margin. It is generally pervasive, and quartz (quartz-chlorite, etc.) veinlets are rare to absent. This alteration is weaker than that in the 'inner' zone and is characterised by relatively more abundant epidote and albite, at the expense of the chlorite and amphibole that are dominant in the 'inner' zone, while magnetite is only a minor constituent.
The early propylitic stage quartz-magnetite-chlorite assemblage is estimated to have formed from a 480 ±5°C, sodic-dominant aqueous-chloride fluid, under near-critical conditions, and at a pressure of ~500 bars. This was followed by two episodes of the most intense Cu and Au deposition that occurred under overlapping temperatures of ~380 to 250°C, but from compositionally distinct (sodic-potassic- to sodic-calcic-dominant) boiling to homogenous fluids, during the final propylitic and the early phyllic alteration stages, respectively (Soloviev et al., 2019).
• Phyllic alteration overprints the earlier alteration types described above. Where overprinting zones of potassic alteration, K feldspar and/or biotite are replaced by fine-grained aggregates of quartz-sericite to quartz-sericite-carbonate (calcite, siderite, ankerite and/or dolomite). Alteration of overprinted 'inner' propylitic chlorite and/or feldspar results in sericite and the development of a composite chlorite-sericite-quartz aggregate. Whilst phyllic alteration of igneous rocks produces the assemblage detailed above, quartz phenocrysts are unaffected. Zones of pervasive sericite (quartz-sericite-carbonate) alteration overprinting the 'inner' propylitic zones, contain finely-disseminated chalcopyrite and pyrite, locally in substantial concentrations.
A variety of vein types are evident in the phyllic zone, which include multi-directional quartz ±sericite veinlets similar to the AB- to B-type veins found in the other alteration zones. They contain chalcopyrite, but differ in the absence of K feldspar and biotite, and the presence of sericite carbonate in selvages compared to very similar veining in the potassic and propylitic zones. Quartz-sericite veins cut both quartz-K feldspar and laminated quartz-magnetite veinlets occurring in potassic and propylitic zones respectively. Predominantly pyrite veins are younger still, as are thicker planar D-type pyrite ±quartz ±sericite veinlets and veins. Zones of massive silicification, typically with ubiquitous pyrite, are also common. The latest native gold-3 (fineness ~925‰), which is typically associated with Bi and Te minerals, is restricted to phyllic alteration zones, whilst other gold in the phyllic zone occurs as Au and Au- Ag tellurides. Whilst chalcopyrite associated with pervasive sericite and quartz-sericite overprint of propylitic alteration, and/or in quartz-sericite veinlets, can locally result in enhancement of Cu and Au grades, in many other intervals, phyllic alteration, particularly that with high quartz and/or pyrite contents, causes Cu grade dilution. Pyrite occurs as fine dissemination of irregularly-shaped grains and dominantly cubic crystals in zones of pervasive alteration, with larger aggregates in zones of silicification and quartz-pyrite veinlets. Minor sphalerite and galena are also common in phyllic alteration assemblages (Soloviev et al., 2019).
The main copper-gold mineralisation at Malmyzh is, in general, coincident with the 'inner' propylitic alteration zones that overprint potassic alteration, with copper grades increasing with a greater chlorite content (Newall, 2015), and additional copper and gold grades in local zones of sericite replacement of the phyllic alteration stage (Soloviev et al., 2019).
In zones of disseminated sulphides and/or quartz-sulphide to quartz-magnetite-sulphide stockworks, mineralisation comprises chalcopyrite with minor bornite, and variable but generally minor pyrite. In higher-grade, >0.50% Cu mineralisation, the density of stockwork veining exceeds 30 veinlets per metre, and is typically in the range of 10 to 20 veinlets per metre for grades of 0.20 to 0.40% Cu. Quartz-sulphide veinlets are typically 3 to 10 mm thick and comprise from ~10 to 40 vol.% with sulphide contents that vary from 0.5 to 1 vol.% to 2 to 3 vol.% in zones of medium to higher grade mineralisation respectively. Gold grades are higher in zones with greater pyrite contents. Trace molybdenite occurs occasionally.
Newall (2015) suggested ~3 km of overlying rock had been eroded to the present surface. Soloviev et al. (2019), noted the occurrence of the high-sulphidation minerals alunite and diaspore in float rocks in the southern part of the deposit where down-faulting has preserved the uppermost parts of the porphyry-epithermal systems, suggesting the potential for high-sulphidation epithermal overprint mineralisation.
As of October, 2018, quoted Mineral Resources at the Freedom, Valley, Flats and Central deposits at Malmyzh were as follows (IG Copper website, viewed January 2020):
NI 43-101 compliant Inferred Mineral resource at 0.3% Cu eq. cut-off - 1.661 Gt @ 0.34% Cu, 0.17 g/t Au, 0.42% Cu eq.
0.55% Cu eq. cut-off - 220 Mt @ 0.51% Cu, 0.3 g/t Au, 0.67% Cu eq.
Russian GKZ Reserves (on balance) and Resources at 0.3% Cu eq. cut-off
C1+C2 Reserve - 1.39 Gt @ 0.41% Cu, 0.22 g/t Au, 0.52% Cu eq.
P1 Resource - 0.93 Gt @ 0.36% Cu, 0.14 g/t Au, 0.42% Cu eq.
Combined C1+C2+P1 Reserve+Resource - 2.316 Gt @ 0.39% Cu, 0.19 g/t Au, 0.48% Cu eq.
The most recent source geological information used to prepare this summary was dated: 2019.
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.
Bowens, T.E., Canby, V.M. and Ashleman, J.C., 2017 - The Malmyzh porphyry Cu-Au discovery, Khabarovsk Krai, Far East Russia: in Society of Economic Geologists, SEG 2017 Conference Proceedings, 2p.|
Khanchuk, A.I., Kemkin, I.V. and Kruk, N.N., 2016 - The Sikhote-Alin orogenic belt, Russian South East: Terranes and the formation of continental lithosphere based on geological and isotopic data: in J. of Asian Earth Sciences v.120, pp. 117-138.|
Soloviev, S.G., Kryazhev, S.G., Dvurechenskaya, S.S., Vasyukov, V.E., Shumilin, D.A. and Voskresensky, K.I., 2019 - The superlarge Malmyzh porphyry Cu-Au deposit, Sikhote-Alin, eastern Russia: Igneous geochemistry, hydrothermal alteration, mineralization, and fluid inclusion characteristics: in Ore Geology Reviews v.113, 27p. doi.org/10.1016/j.oregeorev.2019.103112.|
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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