Journal of Geoscience and Environment Protection, 2014, 2, 66-71
Published Online June 2014 in SciRes. http://www.scirp.org/journal/gep
http://dx.doi.org/10.4236/gep.2014.23009
How to cite this paper: Nikitenko, E. M. et al. (2014). Natalka Gold Deposit. Journal of Geoscience and Environment
Protection, 2, 66-71. http://dx.doi.org/10.4236/gep.2014.23009
Natalka Gold Deposit
E. M. Nikitenko, T. I. Mikhalitsyna, M. I. Fomina, O. T. Sotskaya
North-East Interdisciplinary Scientific Research Institute N. A. N. A. Shilo, Far East Branch, Russian Academy of
Sciences (NEISRI FEB RAS), Magadan, Russia
Received April 2014
Abstract
Studies were taken of ore-bearing rocks, ores and gravity concentrates from Natalka gold deposit
by a set of complex methods including mineralogical, petrographic and geochemical research,
gravity concentration, and electronic microscopy. The major form of Au found is native: free gold
is larger and dispersed, in the form of microinclusions. The obtained results are the basis for an
effective technology to extract gold and sequence of profitable mining of Natalka deposit.
Keywords
Yana-Kolyma Fold System, Natalka Gold Deposit, Mineralogical and Petrographic Characteristics,
Chemical Composition, Gravitational Concen tr ate s
1. Geological Position of the Natalka Gold Deposit
The Natalka gold deposit is one of the largest in Russia. Subsurface ore mining had been conducted at the
mine until May 30, 2004, and was stopped because of its non-efficiency.
The geological position of the Natalka deposit is determined by its association with the margin of the as-
sumed pluton in the zone of the Ten’ka deep fracture and caused by the geodynamic situation of the collision
stage of the Yana-Kolyma folded system development. The Yana-Kolyma folded system, included in the in-
frastructure of the Verkhoyansk-Chukotka folding, has the regional north-western orientation and is repre-
sented by the association of Permian, Triassic, and Lower Jurassic hemipelagic and pelagic terrigenic rocks
interpreted as a complex of underwater debris cone containing underwater rockslide horizons.
Sedimentary rocks at some places have higher grades of the syngenetic carbonaceous substance.
The deposit area is attached to the south-eastern part of the Yana-Kolyma folding system and located in
the south-western wing of the Main Ayan-Yuryakh Anticlinoirum. The main structural element of the anti-
clinorium is the Ten’ka Anticline; its part in the basins of the Kulu and Nelkoba rivers is distinguished as the
Ten’ka Ore District and includes three ore clusters: the Degdekan (Tokichan and Degdekan deposits), the
Omchak (Natalka, Omchak, and Pavlik deposits), and Pioneer (Rodionovskoye, Igumenovskoye, and Klin
deposits). The geological structure of the area comprises terrigenic Permian and Triassic deposits forming
the section of about 5 - 7 km in total thickness. Very few Late Cretaceous volcanic formations lie on Per-
mian strata with angular and stratigraphic discordancies. Magmatic formations in the area are divided in
three complexes: the Late Jurassic complexes of dykes with mostly medium composition and batolith-like
granitoid intrusives (the Kolyma type) and the Cretaceous complex uniting rocks from gabbro to leucocratic

E. M. Nikitenko et al.
67
granits (the Omsukchan and the Okhotsk types). Dislocations with breaks of continuity in the area are quite
numerous, clearly manifested, and to a great extent determine the structural plane of the area.
Dominating are longitudinal dislocations, mostly of the north-western strike and relatively folded struc-
ture. The most important longitudinal fracture is considered the Omchak fracture, associated with the forma-
tion of all gold deposits of the Omchak Ore Cluster. In the northern part of the area, the large latituinal In-
yakan-Kolyma fracture has been distinguished; it is of significance for the gold metallization formation and
refers to the category of ore-concentrating open structures. At its intersection with the Omchak fracture, the
Omchak dome-circular structure has been marked; here the Natalka gold deposits have been localized. The
deposits refer to the type of vein-veinlet and stockwork zones. Metallogenically, they refer to the pyritear-
senopyrite type of the gold-quartz less-sulfide formation.
The previous research of the essential ore composition mostly involved the deposit deep horizons (+600 m) in
underground works. However, with the start of open-pit works at surface sites (horizon +940 m), the research of
essential ore composition and gold distribution in subsurface zones at the first batch of the deposit exploration
come into the spotlight.
Within the current research framework, large-volume ore samples presenting most characteristic metallization
types were taken and studied by a set of methods including mineralogical, petrographic, and geochemical re-
search, gravity concentration, and electronic microscopy.
2. Mineralogical and Petrographic Characteristics of the Ore-Bearing Rock at the
Natalka Deposit
Ore-bearing rocks are hydrothermally metamorphized sedimentary and igneous-sedimentary deposits of the
Permian age (aleurolites, sandstones, gravelites with inclusions of the tuffaceous material) divided in the Pioneer
(P1-2pn), Atka (P3at), and Omchak (P3om) suites (Byakov & Vedernikov, 1990). The leading structural and
morphological type of metallization is the vein-veinlet one. Ore bodies are zones of contiguous quartz veinlets
with metasomatic silicification sites and rare lenses and veins of quartz. The sulfides in ores do not exceed 1 - 3
vol.%, seldom 5 vol.%. 95% - 99% ore minerals are represented by arsenopyrite and pyrite. The deposit refers to
the pyrite-arsenopyrite mineral type of the gold-quartz formation (Goncharov et al., 2002).
The Natalka deposit area is traditionally divided in 3 sites (North-Western, Central, and South-Eastern),
which differ in their geological structure and location.
The Central site is attached to the south-western wing of the Natalka syncline and adjacent to its central lock
part. Ore-bearing rocks are well sheeted and cleavage, with veinlets of the quartz-feldspar composition from less
than 1 mm to 1 cm thick, seldom thicker. Veinlets form subparallel bundles and clusters of various orientation.
In the clastogenic part, debris of albits and fine-grained aggregate of the quartz-albite composition dominate.
Albitization of potassium feldspars, limonite development (to 3%), and the reduction of sericite in the cement
with chlorite increase is observed. Exposed is the intensive development of carbonate as the dispersed impreg-
nation on the basic rock matrix as well as of irregular grain aggregates filling the inter-grain space and replacing
the debris. Ore mineralization (0.01% - 1.5%) is unevenly distributed. Among ore minerals, arsenopyrite, pyrite,
chalcopyrite, pyrrotine, galena, sphalerite, marcasite, and native gold are identified. Two generations of arseno-
pyrite (short- and fine-prismatic) have been distinguished. The short-prismatic arsenopyrite is associated with
the deposition of the main part of native gold. Gold grain sizes vary from 0.001 to 0.2 mm.
The North-Western site of the deposit is attached to the north-western wing of the Natalka syncline, where
rocks are mostly monoclinally bedded with a steep (50˚ - 60˚) fall to the north-east. Rock is represented mostly
by tuffaceousslates containing numerous interbeds of sandstones and fine-pebble conglomerates with
quarts-feldspar veinlets developing. Ore zones have cross deposition related to the dominating stretch of
ore-bearing rocks and deviate from it for 20˚ - 30˚.
The South-Eastern site of the deposit is attached, respectively, to the southern part of the Natalka syncline;
here bedrocks have flat tilty dip. At the site mostly clayey slates are developed. Concerning the mineral compo-
sition, rocks are distinguished with the reduced number of feldspars and the development of limonite (to 6%).
The number of ore mineralization on the two sites of the deposit decreases to 1%; mineralization is unevenly
distributed. The ore mineral composition is analogous to that at the Central site. The distinguishing characteristic
is the absence of marcasite, sporadic ore occurrence (South-Eastern site), and the development of the oxidation
zone (chalcosine, covellite, and scorodite). Native gold of 0.1 - 0.07 cm is deposited in quartz interstitions and

E. M. Nikitenko et al.
68
interlocks with shor t-prismatic arsenopyrite.
3. Chemical Composition of Ore-Bearing Rocks at the Natalka Deposit
By the chemical composition all of the analyzed ore-bearing rock samples are found to be similar. The major
components are earth silicon and aluminum, with the total average content of 78.02%. The highest amount of
earth silicon (75.71%) is marked within the Central site. The average contenst of SiO2 by sites are: South-East-
ern—63.66%, Central—64.34 %, North-Western—64.07%. The amount of earth silicon distinguishes the origi-
nal rocks as persilicic. There are also tributary components—Fe2O3, FeO, Na2O, K2O, CaO, MgO; their total ave-
rage content is 15.15%, including alkali elements Na2O and K2O—5.88% avg. (3.69% and 2.18%, accordingly).
Cumulate alkalinity fluctuates from 4.97% to 6.75%, which corresponds the higher alkalinity of the rock
composition. In most samples of the base ore, Na2O is dominating over K2O; however, at the flanks of the de-
posit, inside ore-bearing rocks, the K2O slightly exceeds the Na2O content, due to the increased amount of the
hydromicaceous material in the rock composition. Maximum alkali content (5.37%) is found in the Central site
rocks. Average contents by sites are: South-Eastern—Na2O—3.49%, К2O—2.57%; Central section— Na2O—
3.73%, K2O—2.25%; North-W e st —Na2O—3 .77%, К2O—2.25%. In relation to the FeO/Fe2O3 content, the
samples show significant fluctuations from 0.17 to 4.18 wt. %; most samples of the base ore are characterized by
FeO avg. (2.61 wt. %) dominating over Fe2O3 avg. (2.23 wt. %), which suggests the reducing conditions of the
rock formation
Al2O3 (8.85% - 15.47%), K2O (1.04% - 3.31%), and MgO (1.00% - 2.20%) variations reflect the changes in the
percentile content of mica and chlorite in the ore sample. Fluctuations in the Fe2O3 (0.91% - 4.89%) content are
mostly triggered by the sulfides amount variability. The increased up to 4.47%) CaO content reflects the presence
of calcite and apatite. The high content of Na2O in the rock is representative for albitemetasomatites. The P2O5
content levels fluctuate from 0.09 to 0.23%, which is related to a phosphate-containing mineral (apatite) presence.
While comparing absolute contents of rock-forming oxides, that reflect the base ore composition, a number of
the most informative petrochemical modules were examined (Yudovich & Ketris, 2000). For the modules cal-
culation, the oxide weight contents (%) were used, and the calculations were made in accordance to “Standard
UK” (Yudovich & Ketris, 2000).
One of the universal indicators, intended for quantitative estimation of two most important supergene proc-
esses (desalinisation and hydrolysis) is hydrolyzate module (HM) – (TiO2 + Al2O3 + Fe2O3 + Fe O + MnO)/SiO2.
The HM value in most base ore samples was found to exceed 0,30, which allows to attribute these formations to
clay rocks and greywackes; the Central site samples showed HM level – 0.20 - 0.30 (avg. 24,6), appropriate for
clay silicites (cherty-silica shales), polymictic quartz sandstones, and aleurolites; in a single ore sample HM
level – 0.17, to be appropriate for low-shale volume silicites (silica-cherty shales), oligomictic quartz sandstones,
and aleurolites (HM = 0.10 - 0.20).
For a more detailed characteristic of hydrothermally converted rocks, additional petrochemical factors were
used, which took into account ferruginosity, femicity, titanium, and alkalinity.
The ferrous module (FM)—(FeO + Fe2O3 + MnO)/(Al2O3 + T iO2) —reflects the correlation between ferrous
and aluminous hydrolysis products. The FM value for most ore samples is significantly less than one and fits in
the 0.30 - 0.39 interval, which corresponds to normo-ferruginous siallites, with a standard content of basic
petrogenic oxides. The FM module ((FeO + Fe203 + MgO)/Si02) is very important for recognizing petro- and
pyrogenic deposits—such rocks are almost always labeled with FM < 0.10. On the scale of the femic FM mod-
ule, ore samples fit the 0.06 - 0.13 interval, with an obvious attr ac tion of micaseous-chloritic rock variations to
its lower boundary. In greywackes and polymictic quartz sandstones, due to a content increase of iron and mag-
nesium within the chlorate-carbonated cement, the interval of FM median values rises to 0.12 - 0.13. Based on
the normalized alkalinity value—(Na2O+K2O)/(Al2O3), ore samples from the North-Western and South-Eastern
sites do not exceed the 0.20 - 0.40 limit, which usually corresponds a standard mixture of hydromicas and feld-
spars. However, in the rocks enriched in feldspar clastolites, this value significantly increases to 0.40 - 0.50 and
over 0.50 (Central site), which is explained by a sharp increase of sodium with in albite-oligoclase.
The data from the cluster analysis of basic petrogenic elements used, the following regularities have been ex-
posed: three groups of components clearly distinguish themselves: 1) Au-SiO2-Na2O, which emphasizes the
quartz-feldspathic veinlet composition within the rock, with gold associated with them; 2) TiO2-Al2O3-P2O5-
MnO -K2O, which testifies to titanian, phosphate, and argillous minerals in the rock composition; 3) FeO-MgO-

E. M. Nikitenko et al.
69
Ca, which reflects the iron-magnesium-calcic tenor of carbonates.
Statistical adaptation of the results of the ore-bearing rock atomic emission spectrum analysis allowed to es -
tablish the petrogenic nature of Pb, Cr, As, W, Mo, Sn, Cu, Ag, Zn, Ni, Co, Au accumulation. The R-type clus-
ter-analysis dendrogram shows four separate geochemical assemblages: 1) Cr -Mo -As; 2) Co-Zn; 3) W-Ag-Au; 4)
Cu-Ni-Pb-Sn. The group-forming elements have a hybrid petrogenic specialization; the 3-rd group distinctly re-
flects the gold ore specialization. All geochemical microelement assemblages of hold positive interrelationship.
When analyzing concentration distribution of the mineralization-indicating elements in relation to the geo-
chemical background of the Natalka deposit (Goncharov et al., 2002), the following has been found: the South-
Eastern site—As from 5 to 116 geophones, W—4, Ag—2, Au—1 geophone and lower; Central site—Au—7 - 9
geophones, As to 128, W—5 - 6, Ag—2 geophones; North-Western—As from 11 to 68 geophones, W—3 - 5,
Ag—1 - 3, Au – content varies from 1 to 7 geophones.
4. Technological Middling Research Resu lts
Concentrates were prepared from 3kg of ore charge by gravity concentration, using centrifugal vibratory con-
centrator (CVC-100-2М); they were “mounted” into the epoxide blocks and polished. Quantitative evaluation of
the concentrated product material composition was performed on the FEI QEMSCAN hardware and software—
based on Carl Zeiss SEM EVO-50.
The QEMSCAN automated system expedites obtaining quantitative mineralogical properties: relative mineral
content in a sample, mineral associations, grain size, and mineral grain-opening degree. The research was con-
ducted in the particle mineral analysis (PMA) mode, displaying separate mineral particles, as well as in the trace
mineral search (TMS) mode, displaying only the particles that contain specific required stages. The measure-
ment conditions were as follows: accelerating applied voltage 5 kV, electrical current in the Faraday cup 5 nA.
For the separate particles composition research, the QUANTAX (Bruker) quantitative analysis system was used,
included in the QEMSCAN complex.
The total of three centrifugal concentrated samples was examined. Samples № 1 and № 2 characterize the
veinlet-impregnated ore that contains ~1-3% sulfides extracted from the Central section of the deposit at the
depth of 12 - 15 meters. Sample № 3 characterizes the impregnated ore containing ~ 3% sulfides extracted from
the South-Eastern section at a depth of 250 meters, in a 11 (122.6 - 125.6 М) cross cut.
The quantitative mineralogical analysis results of the gravitational concentrates show that the concentrates
vary in composition as well as in the sulfide amount (Table 1).
Arsenopyrite is the major ore mineral; pyrite has subordinate spreading. Other sulfides do not exceed 1%, ex-
cept galena in Concentrate 1. The same concentrate contains big amounts (~31%) of slime-forming materials,
which can negatively affect the gold recovery during the following cyanidation. With particles smaller than 0.1
mm, up to 40% of gold occur in loose (open) condition; the rest, in the form of attachments or inclusions, is as-
sociated with other minerals. Approximately 10.5% of gold is associated with arsenopyrite and pyrite; no more
than 2%, with galena; approximately 3%, with quartz; and 3.5%, with rock-forming minerals. The content of the
isomorphic silver impurity in gold fluctuates from 16 to 19%. Gold microinclusions in sulfides are quantitatively
subordinate.
The gravity concentrates contain free native gold, closely associated with sulfides. In Concentrate 1, the gold
grade is ~7% (see Table 1), mostly in a loose condition, in relatively large grains; microinclusions in sulfides
are quantitatively subordinate. The loose gold particles are irregularly shaped, reaching to 200 µm in size.
Dispersed micrograins of gold occur as oval and pseudoisometric microinclusions in arsenopyrite; less often,
in arsenical pyrite. Loose gold fills sulfide microfractures and defects; gold grains vary from 0.3 to 15 µm in
size; gold fineness fluctuates from 820 to 850‰.
For the first time in the Natalka ores, telluric silver (hessite) has been found. Hessite has been discovered in
the gravity concentrate obtained from the vein-disseminated ore from the Central site. Grains vary from 2 to 5
µm in size, their shape is oval and isometric. The hessite content is close to theoretical.
Concentrate 1, which comes from the Central site ore, shows 0.65% presence of cobaltite isometric grains, 20
to 35 µm in size. Cobaltite is closely associated with galena, as cobaltite grains almost constantly contain 0.5 to
10 - 15 µm galena inclusions. Cobaltite itself constantly contains 2% - 5% nickel (Ni) and 3% - 5% iron (Fe)
impurities.

E. M. Nikitenko et al.
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Table 1. The mineral composition of the gravitational concentrates.
Mineral
The mineral composition of the gravitational
tt
Concentrate №1 Concentrate №2 Concentrate №3
Gold/Electrum
6.86
0.90
0.23
Arsenopyrite 24.65 75.38 92.49
Pyrite 5.17 9.45 2.09
Gal e n a 2.41 0.18 0.06
Cob altite 0.65 – –
Sphalerite – 0.10 0.08
Chalcopyrite – 0.14 0.06
Limonite 0.55 0.15 0.17
Goethite – 0.04 0.08
Magnetite – – 0.04
Monazite 0.15 0.05 –
Gar n et 0.22 0.09 –
Rutile/Anatase 2.95 0.59 0.31
Zircon 0.57 – –
Ba r yt e 0.46 0.08 0.05
Apatite 0.72 0.05 0.05
Biotite 3.58 0.39 0.29
Muscovite 3.07 0.59 –
Calcite 3.37 0.07 –
Pyroxene 0.68 – –
Amphibole 0.50 0.22 –
Albite 9.88 2.36 0.29
K-feldspar 8.39 1.84 –
Quartz 17.13 2.25 0.23
Others 8.05 5.09 3.48
Total 100.00 100.00 100.00
5. Findings
The richest in useful components (Au, Ag) ore-containing rocks at the Central site are characterized by maximal
Na2O and SiO2 content. The feasible exploration sequence for the Natalka deposite consists of the following
stages: first, the Central site, then the North-Western, and finally, the South-Eastern site.
The revealed features of the mineral-petrographic composition show that the effective gold extraction from
the Central site ore is possible through gravity concentration in a series of stages and the consequential extrac-
tion of magnetics based on the size class. Working the South-Eastern site ore, it is essential to consider the
presence of the oxidation zone minerals, including high contents of limonite; therefore, on the flotation concen-
tration stage it is important to select the right floatation reagents. To avoid overgrinding of the soft clay slates
from the South-Eastern site ores, as well as of heavily sheeted Central site ores, especially while working the
upper levels, grinding mill operating conditions should be closely monitored.
A new data on different mineral compositions of the concentrate samples, various gold associations with other
minerals in them, and different degrees of the gold grain exposure has been obtained.
The major form of Au found is native: free gold is larger and dispersed, in the form of microinclusions. The
main minerals, containing dispersed microscopic gold microinclusions, are arsenopyrite and arsenical pyrite.
A new exploration form of silver (telluride, or hessite) has been exposed; also, relatively wide cobaltite dis-
tribution of has been demonstrated.
References
Byakov, A. S., & Vedernikov, I. L. (1990). The Permian Deposits Stratigraphy in the No r th -Eastern Rim of the Okhotsk

E. M. Nikitenko et al.
71
massif, the Central and South-Eastern Parts of the Ayan-Yuryakh Anticlinorium. Preprint. Magadan: NEISRI FEB AS
USSR, 69 p.
Goncharov, V. I., Voroshin, S. V., & Sidorov, V. A. (2002). Natalka Gold Ore Deposit. Magadan: NESC FEB RAS, 250 p.
Yudovich, Y. E., & Ketris, M. P. (2000). Basics of Lithochemistry. Saint-Petersburg: Science, 479 p.