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BY-NC-ND 3.0 license Open Access Published by De Gruyter Open Access April 7, 2016

Solid Inclusions in Au-nuggets, genesis and derivation from alkaline rocks of the Guli Massif, Northern Siberia

  • Sami N. Dvorani EMAIL logo
From the journal Open Geosciences

Abstract

A total of 112 Au-nuggets, collected from alluvial placer deposits of the Ingarinda River from the Guli massif, located in northem Siberia, Russia, were investigated. The Guli massif consists of a huge dunite-clinopyroxenite complex (the largest complex in the world), an alkaline to highly alkaline rock suite (melilite, nephelinite, ijolite) enveloping the dunite and carbonatite intrusions, associated with disseminated schlieren type chromitite and Au-Ag, Pt placer deposits. The nuggets are characterized by various sizes and shapes and show chemical compositions Au, Au-Ag and AuCu, typical for a derivate of carbon-atites and/or ultramafic complexes. A great variety of oxide, silicate, REE-minerals, carbonate and sulphide inclusions have been detected in the nuggets, which are identical in mineralogy and chemical composition to mineral constituents of the alkaline to highly alkaline rock suite surrounding the Guli dunite core complex thus, considered as the source for Au-nuggets.

1 Introduction, geological settings

The Guli dunite-clinopyroxenite massif is located in the Maimechia-Kotui province in the northern part of the Siberian Platform (Taimyr Province, Russia). According to geophysical data, the Guli massif occupies an area of 2000 km2 at an intersection of two paleorifts: the meridional Taimyr-Baikal and the latitudinal Yenisei-Kotui pa-leorift [1]. The Guli massif consists of an ultramafic complex, an alkaline suite, and carbonatite intrusions (Fig. 1). Ultramafic rocks are composed of dunite with occasional highly disseminated to schlieren-type chromitite occurrences, wehrlite and magnetitic clinopyroxenites. Dunite is predominant within an area of approximately 450 km2. The Guli placer-forming sediments are related to the Ingarinda, Saby, Gule and Selinga rivers, and their tributaries. Recent and Upper Quaternary alluvial, fluviatile and terrace beds are productive for platinum group minerals (PGM) and native gold [2-7]. The precious metal nuggets are particularly located between unconsolidated sediments and bedrock. It is suggested that the placer deposits were formed during unroofing and disintegration of rocks of the Guli massif after Early Cretaceous times [5-7].

Figure 1 Geological map of the Guli massif.
Figure 1

Geological map of the Guli massif.

This Study represents a detailed mineralogical and geochemical investigation of representative composite Au nuggets from the Ingarinda River. These results are compared with a detailed study of the alkaline rock sequences located between the dunite core complex and the carbonatite intrusion, in order to find the source rocks for the Au-nuggets.

2 Methods

A detail mineralogical, petrological and geochemical investigation was carried out, on the basis of 112 Au-nuggets and five selected drill cores (Z2, Z3, Z4, G28 and Z12). 112 Au-nuggets were collected from Ingarinda river placer deposits. The drill core locations were selected from the periphery of the dunite core complex in the direction of the carbonatite intrusion. The rock suite investigated is composed of dunite at the periphery of the dunite core complex, showing the typical cumulate texture of the olivine melanephelinites, clinopyroxenite and carbonatite.

Sample preparation and investigation were completed at Leoben University (Montanuni Leoben) in Austria. Microscopy investigation used an OLYMPUS BX 40 and BX 60 Microscope with different magnitudes, Raman spectroscopy used a Jobin Yvon Raman LABRAM spectroscope frequency equipped with a double-Nd-YAG laser (100 mW, 532.2 nm), He-laser (633 nm) and CCD detector matrix. Laser focusing and sample viewing was performed through an OLYMPUS BX 40 microscope with reflected and transmitted light. Mineral chemistry composition was determined by electron microprobe techniques (ARL SEMQ 30; 15 kV; 15 nA) using certain standards (Mineralogy and Petrology department). XRF and ICP-MS equipment at the Analytical Chemistry Department used for major and trace elements analyses of the host rocks chemistry.

3 Morphology and size of the gold nuggets

Morphometric and morphological analyses of gold nuggets have been used as indicators of the transport mode, the transport distance and also for the source the gold might have been derived [8, 9]. The grain size of nuggets is mainly a function of transport distance and/or the period of time a particular particle has remained in transport. Thus, grains bigger than 20 mm are supposed to be deposited a few km from the source area, whereas particles with a grain size between 3 and 20 mm could have been transported 10-15 km away from the source area [10].

In this study, 112 nuggets were investigated with respect to their grain size and morphology. The investigation was carried out using a binocular equipped with a digital camera. Grain size was also determinate (i.e. equivalent circle diameter).

The main proportion of the gold nuggets reveal a middle dimension dm (dm = (L + W)/2) between 0.25 and 1.0 mm, 10% of the nuggets show a dm between 0.125 and 0.25 mm, 38% are bigger than 0.25 mm and 39% show a dm between 0.5 and 0.1 mm (Fig. 2). Similarly, 96% of the nuggets show a maximum length (L-max) smaller than 2 mm with 79% of them having an L-max between 0.25 and 1.0 mm.

Figure 2 Morphometry properties of gold nuggets: a = dm [(L +W)/2], b = L-max [maximum length].
Figure 2

Morphometry properties of gold nuggets: a = dm [(L +W)/2], b = L-max [maximum length].

The majority of the nuggets show an ECD (= the equivalent circle diameter, defined as L. W, according to Bonev [11]) between 100 and 500 μm (59.05%), 40.95% have an ECD (Equivalent circle diameter) between 500 and 2000 μm. The ratio between length and width of the nuggets (i.e. the anisotropy 2 - D = L/D) is > 1.0,12% of the nuggets show a 2-D ratio between 1.0 and 1.1, very typical for highly spheroidal particles, 34.48% have a 2-D ratio between 1.3 and 1.6, and 6.25% of the nuggets reveal a 2-D ratio between 1.8 and 2.

The shape of the gold nuggets was determined by roundness and sphericity using the classification scheme of Krumbein [12]. The majority of the nuggets show a roundness (R) between sub-angular (R = 0.3; 18.75%), rounded (R = 0.5; 42.75%), and well-rounded ( R =0.7; 19.74%). The sphericity (S), shown in Fig. 3; lies predominantly between 0.3 and 0.7.

Figure 3 Morphometry properties of gold nuggets: Roundness (R) of grains from 0, 1 to 0, 9 und sphericity (S) 0, 3 to 0, 9.
Figure 3

Morphometry properties of gold nuggets: Roundness (R) of grains from 0, 1 to 0, 9 und sphericity (S) 0, 3 to 0, 9.

The size and morphological characteristics indicate a transport distance of around 20 km from the source area.

4 Chemical composition of the gold nuggets

The chemical composition of the nuggets was determined using an ARL-SMQ electron microprobe, equipped with a LINC EDS. Analysis conditions were a beam current of 15 kV at 20 nA, using mineral standards for quantitative analyses. Representative compositions of gold nuggets are given in Table 3.

Table 1

Abundance of sulfide inclusions in the different nugget classes.

III-AII-BII-CII-DIII
cuprostibitecuprostibiteparadocrasitepentlandite
chalcopyritehessitechalcopyritecobaltine
chalcosinenickelinecubanite
bornitegalenapentlandite
pentlanditepyrrhotiteparkerite
nickelinebornitepyrhotite
cobaltinebornite
Table 2

Comparison of typical mineral inclusions in the nugget with those characteristic of the alkaline rock suite.

GOLD NUGGETSHOST ROCK
ilmenitepentlanditeilmenitepentlandite
Ti-magnetitepyrrhotiteTi-magnetitepyrrhotite
Cr, Ti-magnetitegalenaCr, Ti-magnetitegalena
perovskitechalcopyriteperovskitechalcopyrite
olivinenickelineolivinepyrite
sphenebreithauptitesphenesphalerite
Ti-garnethessiteTi-garnetK-sulfide
clinopyroxenechalcosineclinopyroxenerutile
aegirineborniteaegirinenepheline
Na-amphibolecubaniteNa-amphiboleTi-clinohumite
phlogopiteparkeritephlogopite
serpentinezirkeliteserpentine
chloritechevkinitechlorite
calcitecalcite
dolomitedolomite
apatitefeldspar
apatite
Table 3

Representative microprobe analyses of gold nuggets.

12345678910
Saple wt%G632G63-4G65-3G65-4G65-9G71-6G71-13G74-3G77-4G77-6
Au81.989.6674.0282.3397.1495.9974.8972.1274.2696.11
Ag2.827.90.7113.611.470.930.2425.730.190.17
Hg0000000000
Cu13.12.4824.834.020.152.9524.181.2525.652.56
Pd00.04000.0700000
tot98.5399.4799.5699.9698.8399.8799.3199.1100.198.84
Finenes at%967925990958985990997737997998
Au63.0780.9748.6168.8196.7389.8549.8358.6448.1992.1
Ag3.9712.020.8520.772.671.590.290.230.230.3
Hg0000000000
Cu32.966.9450.5410.410.468.5649.8751.5951.597.6
Pd00.07000.1300000
tot10010010099.9999.9910099.99100100100
11121314151617181920
Saple wt%G98-1G98-11G49-1G49-2G49-7G35-1G35-3G46-2G58-2G67-7
Au55.4693.229069.7188.8986.9997.9688.6895.1195.03
Ag44.914.2610.0629.285.8113.352.510.415.585.05
Hg0000000000
Cu0000.110000.340.140.01
Pd000.010000000
tot10097.47100.199.194.7100.3100.599.43100.8100.1
Finenes at%553956899704939867975895945950
Au40.3492.383.0356.4489.3478.195.5581.5589.9591.08
Ag59.667.716.9543.2910.6621.94.4517.489.648.84
Hg0000000000
Cu000.030.280000.970.410.03
Pd0000000000
tot100100100100100100100100100100

Argentian gold (containing 5-20 wt. % Ag), electrum (containing 20-60 wt.% Ag), tetra-auricupride and pure gold (< 5 wt.% Ag) have been found amongst the Guli nuggets, as single and composite grains. The composite nuggets are complex in composition in terms of electrum (e.g. with varying Au:Ag ratios) with oriented exsolu-tion lamellae of tetra-auricupride and an outer pure gold rim, quite characteristic of Guli nuggets [2, 3]. Cu in Guli nuggets was first reported by [4, 13]. The maximum Cu-content detected in the nuggets in this study is 4 wt.% in Au-Ag-Cu composite grains and up to 3.26 wt.% in single grains. Characteristically, the distribution of Au, Ag and Cu is very inhomogeneous from core to rim and no regular zonation cold be detected. Pure gold (i.e. > 98 wt.% Au) typically forms up to 5 μm thick irregular outer rims. Less commonly, pure gold has been found as irregular patches within the core portions of the nuggets with a sharp contact to the host. These patches are often related to micro fractures (Fig. 4a), indicating that the pure gold was formed by selective removal of Ag and Cu along the micro fractures. Pure gold has also been detected as single nuggets.

Figure 4 Microphotograph under reflected light, parallel Nichols, tetra-auricuprid and electrum complex texture (a); Argentiangold (b); masiv electrum (c) masiv tetra-uricuprid (d) pure gold rim less than 5 μm in size.
Figure 4

Microphotograph under reflected light, parallel Nichols, tetra-auricuprid and electrum complex texture (a); Argentiangold (b); masiv electrum (c) masiv tetra-uricuprid (d) pure gold rim less than 5 μm in size.

5 Classification of the gold nuggets

On the basis of compositional characteristics, the Guli nuggets investigated can be classified into the following groups:

  1. Au-Cu, Au-Cu +Au-Ag.

    11.43% of the nuggets investigated belong to this class. The nuggets are defined by tetra-auricupride as single mineral phase, or tetra-auricupride with electrum in composite nuggets. (G3; G26; G63; G65; G70; G71; G74; G77).

  2. Au-Ag nuggets

    78.86% of the investigated nuggets belong to this class. Depending on the following, sub-classes can be distinguished:

  3. AuAgx (x > 1 and Cu-content < 0.05 wt%).

    Fineness from 407 to 660. (G5; G96; G97; G98.)

  4. AuAg-Au1.9Ag (Cu-content 1.63 wt%).

    Fineness from 663 to 790. (G11; G15; G21; G25; G29; G32; G33; G36; G40; G49; G53; G68; G69; G99; G103; G104).

  5. Au2Ag-Au3.9Ag (Cu-content 3.26 wt%)

    Fineness from 774 to 870. (G1; G2; G4; G9; G10; G13; G22; G24; G30; G35; G37; G39; G41; G44; G47; G51; G54; G56; G75; G76).

  6. Au4Ag-Au5.9Ag (Cu-content 1.45 wt%)

    Fineness from 878 to 920 (G20; G27; G38; G45; G46; G55; G60; G72)

  7. AuXAg (x > 6 Cu-content 0.46 wt%).

    Fineness from 915 to 951 (G6; G8; G12; G14; G16;G17; G18; G19; G42; G52; G58; G59;G67

6 Solid inclusions in the Guli gold nuggets

The detailed microscopic study of the gold nuggets revealed a great variety of solid inclusions. Quite commonly mineral phases are attached to the gold nuggets at the rim. In these cases the attached mineral phase is termed "inter-growth" in the following detail.

An overview of all the solid inclusions found in the Guli nuggets is illustrated in Table 2. The abundance of solid inclusions with respect to the gold nugget classification is summarised in Fig. 5. The investigation of these inclusions and their compositional characteristics should provide some indications of the source of the nuggets.

Figure 5 Abundance of silicate, oxide inclusions and rare inclusions in the different gold nugget classes. ol = olivine, amph = amphibole, chl = chlorite, Fs = feldspar, ilm = ilmenite, sph = sphene, chr = Cr-magnetite, dol = dolomite, qtz = quartz, cpx = clinopyroxene, serp = serpentine, phl = phlogopite, mt = magnetite, per = perovskite, grt = garnet, cal = calcite, ap = apatite, mel = melilite.
Figure 5

Abundance of silicate, oxide inclusions and rare inclusions in the different gold nugget classes. ol = olivine, amph = amphibole, chl = chlorite, Fs = feldspar, ilm = ilmenite, sph = sphene, chr = Cr-magnetite, dol = dolomite, qtz = quartz, cpx = clinopyroxene, serp = serpentine, phl = phlogopite, mt = magnetite, per = perovskite, grt = garnet, cal = calcite, ap = apatite, mel = melilite.

Oxides

Oxide inclusions occur in all gold classes distinguished.

Ti-magnetite to Cr-magnetite, perovskite and ilmenite had been found as inclusions in the gold nuggets. Perovskite was found intergrown with gold, ranging in size up to 5 μm. Representative microprobe analyses are shown (Table 4, 5). Ilmenite (up to 50 μm in size) is an inclusion mineral in gold containing between 1.2 and 1.84 wt% MnO and up to 3.37 wt% MgO. Magnetite forms the main oxide inclusion mineral in gold. Four types of magnetite were detected on the basis of its chemical composition: a) magnetite with some Al and Mg impurities, b) Ti-magnetite with up to 21.1 wt% TiO2) Ti-Cr-magnetite, characterized by up to 8.1 wt% of TiO2 and 3 wt% of Cr2O3, and c) Cr-magnetite, containing up to 27.4 wt% Cr2O3. They are small in size (up to 2 μm), and commonly associated with dolomite and serpentine.

Table 4

Representative microprobe analyses of inclusion in gold nuggets.

12345678910
Pro.NamSIG38-2SIG72-21SIG11-4SIG45-9SIG36-4SIG36-5SIG38-5SIG45-8SIG56-1SIG38-3
Min Namesphsphololcpxcpxcpxcpxcpxcal
SiO231.4730.4039.5241.1754.0951.7054.5050.0852.290.08
Al2O30.060.440.000.241.673.060.722.301.000.00
TiO239.5931.950.050.001.712.440.732.661.420.04
FeO1.121.3815.3911.386.777.0428.166.659.630.00
MgO0.000.0044.3748.7215.8513.791.6915.1312.170.00
MnO0.020.020.250.300.120.080.000.140.270.05
NiO0.000.000.240.050.050.100.020.000.020.00
Cr2O30.050.020.000.000.050.370.280.030.020.00
K2O0.000.010.000.010.000.020.010.020.000.00
Na2O0.390.000.000.000.370.4813.140.561.320.06
CaO25.6228.490.140.2419.9421.901.3122.1121.6254.58
Total98.3292.71100.16102.11100.62100.98100.5699.6899.7654.81
Inclu/Interginclinclintintinclinclintintintincl
Oxy554466666
Si1.041.081.001.001.971.901.991.871.97
Al0.000.020.000.010.070.130.030.100.04
Ti0.980.850.000.000.050.070.020.070.04
Fe0.030.040.320.230.210.220.8640.210.30
Mg0.000.001.671.760.860.750.0920.840.68
Mn0.000.000.010.010.000.000.000.000.01
Ni0.000.000.000.000.000.000.000.000.00
Cr0.000.000.000.000.000.010.000.000.00
K0.000.000.000.000.000.000.000.000.00
Na0.020.000.000.000.030.030.940.040.10
Ca0.911.080.000.010.780.860.060.880.87
XMg0.000.0084.5088.1481.0078.0010.0080.0069.00
Cations2.993.073.003.003.963.984.04.024.02
111213141516171819
Pro.NamSIG42-2SIG32-2SIG65-1SI-G1-20Pro.NamSIG45-7SIG45-16SIG99-2MAG19-10SIG103-3
Min NameilmilmpervTi-mtCr-Ti-mtmtchrchr
SiO20.050.591.580.10SiO20.270.416.450.160.21
Al2O30.000.000.000.00TiO221.148.110.063.961.48
TiO249.4950.5054.4056.62Al2O30.091.751.072.440.00
FeO47.8342.631.870.35Cr2O30.0012.190.0227.4122.93
MgO0.003.070.000.00V2O30.000.000.000.000.00
MnO1.840.00.020.00FeO74.9866.1875.5457.9068.08
NiO0.090.020.100.00MnO2.431.510.223.591.39
Cr2O30.000.030.000.00MgO0.392.980.671.940.94
K2O0.000.010.010.07CaO0.010.030.190.060.01
Na2O0.160.000.870.77ZnO0.000.000.000.000.00
CaO0.010.0738.6733.18
Total99.4798.1297.5291.09Total99.4493.2584.3197.4695.04
Inclu/Interginclinclintintincl/intintinclintintincl
Oxy666Oxy323232
Si0.000.030.070.00Si0.080.120.110.050.06
Al0.000.0000.00Ti4.671.841.510.870.34
Ti21.921.902Al0.0310.030.840.00
Fe2.061.800.070.01Cr0.002.910.016.335.51
Mg0.000.230.000.00V0.000.000.010.000.00
Mn0.080.050.000Fe(iii)6.478.5312.706.999.69
Ni0.000.000.000.00Fe(ii)11.958.219.527.167.62
Cr0.000.000.000.00Mn0.600.390.020.890.36
K0.000.000.000.00Mg0.171.340.050.850.43
Na0.020.0200.080.07Ca0.000.010.010.020.00
Ca0.000.001.931.75Zn0.020.020.020.000.00
XMg0.000.110.000.00
Cations4.094.054.063.94Cations24.0024.0024.0024.0024.00
Table 5

Representative microprobe analyses of inclusion in gold nuggets.

20212223242526272829
Pro.NameSIG40-6SIG103-1SIG37-2SIG1-9MAG14-6MAG14-36SIG72-6SIG72-27SIG44-4SIG27-1
Min NameserseramphamphamphamphchlchlNepouiteMelilit
SiO237.4539.9641.8559.0156.8854.7637.6238.8226.4338.67
Al2O34.830.976.340.980.822.5812.0613.762.3412.46
TiO20.100.040.660.150.090.520.110.040.150.59
FeO7.078.1216.293.629.5512.647.256.617.190.14
MgO37.0635.8911.3621.5115.7315.2429.3828.282.3911.39
MnO0.050.070.070.060.510.250.150.150.082.66
NiO0.680.050.810.000.090.000.290.1332.300.00
Cr2O30.000.170.060.130.000.000.030.050.070.01
K2O0.050.021.560.260.510.480.660.540.042.24
Na2O0.000.000.286.178.267.470.160.050.000.64
CaO0.090.071.105.356.073.290.270.330.2230.27
Total87.3885.3680.3897.2498.5197.2387.9888.7671.2199.07
Inclu/Intergintintinclinclinclintinclinclinclincl
Oxy28282323232366914
Si7.217.867.428.128.077.907.127.213.973.51
Al1.080.221.310.160.140.432.662.980.411.32
Ti0.010.010.090.020.010.060.020.010.020.04
Fe1.141.342.420.421.131.521.151.030.900.01
Mg10.6310.523.004.413.333.288.307.830.541.54
Mn0.010.010.010.010.060.030.020.020.010.20
Ni0.110.010.120.000.010.000.040.023.900.00
Cr0.000.030.010.010.000.000.000.010.010.00
K0.010.010.350.050.090.090.160.130.010.26
Na0.000.000.101.652.272.090.060.020.000.11
Ca0.020.010.210.790.920.510.050.070.042.95
XMg90.0089.0055.0091.0075.0068.0088.0088.0037.0099.00
Cations20.2220.0115.0315.6216.0315.9119.5919.329.809.95
303132333435
Pro.NameSIG40-3SIG9-11SIG72-10SIG53-6SIG71-6SIG103-
Min NamegrtphlphlfsK-fsdol
SiO235.2945.3541.6458.6465.190.03
Al2O35.6011.2012.8025.8213.010.00
TiO23.000.130.040.070.040.00
FeO18.704.424.020.930.281.40
MgO0.0032.2026.880.000.0023.26
MnO0.050.000.040.000.020.26
NiO0.000.180.030.010.000.00
Cr2O30.030.000.060.000.000.02
K2O0.025.058.250.4816.400.01
Na2O0.150.680.575.490.660.00
CaO36.430.180.0010.340.0031.14
Total99.2799.3984.33101.7895.6056.12
Inclu/Intergintinclinclintintincl
Oxy2422223232
Si6.055.995.9010.3912.66
Al1.121.732.115.342.95
Ti0.390.010.000.010.01
Fe2.680.490.480.140.05
Mg0.006.345.680.000.00
Mn0.010.000.000.000.00
Ni0.000.020.000.000.00
Cr0.000.000.010.000.00
K0.000.851.490.114.06
Na0.050.170.161.890.25
Ca6.700.030.001.960.00
XMg0.0093.0092.000.000.00
Cations17.0115.6215.8319.8419.97
36373839
ProbeMAG21-17MAG21-18MAG21-23MAG2
Min-NameChevkinitChevkinitZirkelitZirkelit
SiO223.1419.840.130.08
Al2O36.673.380.000.00
TiO211.7617.5528.8433.38
FeO18.5220.749.288.50
MgO0.960.710.000.00
MnO0.200.480.170.17
NiO0.100.050.070.07
Cr2O30.000.000.000.00
K2O0.930.130.000.00
Na2O0.000.000.000.00
CaO2.844.456.648.14
ZrO25.7310.5622.3023.69
La2O30.490.390.620.69
Ce2O33.923.445.354.65
Nd2O33.123.255.004.69
Sm2O30.910.611.041.22
Total79867985
Oxy222277
InclInclInclInclIncl
Si4.783.960.0100.01
Ti1.832.641.7501.84
Al1.620.800.0000.00
Cr0.000.000.0000.00
Fe(ii)3.203.470.6260.52
Mn0.030.080.0120.01
Mg0.300.210.0000.00
Ni0.030.010.0080.01
Ca0.630.950.5740.64
Na0.000.000.0000.00
K0.240.030.0000.00
La0.010.010.0060.01
Ce0.300.250.1580.12
Nd0.230.230.1440.12
Sm0.060.040.02890.03
ZrO0.581.030.87710.85
Cations13.8313.7204.194.16

Silicates

Silicate inclusions are common in all distinguished classes of gold nuggets.

Quartz

Quartz forms small (up to 2 μm in size), typically single inclusions in gold. However, in one nugget (G36) quartz was found in a two-phase inclusion associated with clinopyroxene.

Titanite (sphene)-inclusions and intergrowths

Titanite is found in polyphase inclusion and dominantly intergrown with gold, associated with amphibole, calcite, chlorite, clinopyroxene and Na-pyroxene, phlogopite and pentlandite (Fig. 8D). Titanite in the Guli gold nuggets contains up to 1.38 wt% FeO identical to those titanites in rocks of the alkaline suite (i.e. up to 2 wt% FeO) of the Guli Massif.

Figure 6 Composition of clinopyroxene inclusions (full circles) compared to those in rocks of the alkaline suite of the Guli Massif (open circles). Classification of clinopyroxene [14].
Figure 6

Composition of clinopyroxene inclusions (full circles) compared to those in rocks of the alkaline suite of the Guli Massif (open circles). Classification of clinopyroxene [14].

Figure 7 Composition of amphibole inclusions compared to those in rocks of the alkaline suite of the Guli massif (samples G28; Z2; Z3; Z4). Classification of amphibole [16].
Figure 7

Composition of amphibole inclusions compared to those in rocks of the alkaline suite of the Guli massif (samples G28; Z2; Z3; Z4). Classification of amphibole [16].

Figure 8 Microphotograph under reflected light (A; C; D; E; F) and backscattered image (G; H), showing typical inclusions in Au-nuggets.
Figure 8

Microphotograph under reflected light (A; C; D; E; F) and backscattered image (G; H), showing typical inclusions in Au-nuggets.

Olivine

Olivine (50 μm in size) occurs intergrown with gold nuggets, associated with clinopyroxene and magnetite (Fig. 8A). The composition of olivine included in gold is characterized by Fo 84-88, somewhat lower than those from the dunites of the Guli Massif (i.e. Fo 84-93, [15], almost identical to those from the cumulate dunites enveloping the Guli dunite complex (i.e. Fo 84-87), but higher than the olivine Fo in melanephelinites from the alkaline suite (i.e. Fo 78-82). Cr-contents are very low (i.e. around 0.06 wt% Cr2O3), Ni up to 0.33 wt% NiO, MnO up to 0.3 wt% and CaO in the range of 0.24 wt%.

Clinopyroxenes

Clinopyroxenes are abundant mineral inclusions and are intergrown in/with the gold nuggets. Diopside (Mg# = 69-83) clearly predominates over augite (Mg# = 58-83) and aegirine (Figure 6, classification after Morimoto [14]). Most are mono-phase inclusions of diopside, subordi-nately augite and aegirine, rarely polyphase inclusions of diopside + olivine, diopside + serpentine + chlorite, diopside + magnetite + serpentine + chlorite occur (Fig. 8). Clinopyroxenes contain Ti and Al concentration (i.e. up to 4.23 wt% TiO2 and 4.18 wt% Al2O3), aegirine contains up to 13.1 wt% Na2O. The composition of clinopyroxene inclusions in the gold nuggets is identical to that in rocks of the alkaline suite of the Guli massif.

Amphibole

Amphiboles form quite common (although less abundant than clinopyroxene) inclusions and intergrowths with the gold nuggets. According to the amphibole classification [16], all amphiboles belong to the sodic-calcic and sodic-alkaline family, where katophorite, winchite-richterite and ribeckite-arfvedsonite series can be distinguished (Fig. 7). Amphibole inclusions are predominantly part of the richterite-arfvedsonite series, whereas those intergrown with gold belong to the ribeckite-arfvedsonite series. Amphibole inclusions are almost identical in composition compared with those from clinopyroxenite dykes and melanephelinites from the Guli massif, typical mineral assemblage occur in alkaline rocks [17].

Phlogopite

Phlogopite (around 3 μm in size) forms subordinate monophase inclusions in class II gold nuggets. Phlogopite shows TiO2 contents between 3.2 and 0.13 wt% and NiO from 0.85 to 0.18 wt%. The composition of phlogopite inclusions is identical, if compared to those from clinopyroxenite dykes and melanephelinites.

Other minerals

Melanite, as well as calcite and dolomite (around 2 μm in diameter) form accessory monophase inclusions in gold nuggets, apatite, plagioclase, and K-feldspar were found intergrown with gold.

Serpentine - Chlorite

Serpentine is dominantly intergrown with gold, and/or occurs together with clinopyroxene in polyphase inclusions. Serpentine intergrowths contain low concentrations of chromium and titanium (i.e. up to 0.17 wt% Cr2O3, and 0.18 wt% TiO2) and 0.68 wt% NiO. The content of aluminum and iron is about 4.83 wt% Al2O3 and 4.30-8.12 wt% FeO. Subordinately nepouite, with up to 32.3 wt% NiO, was found.

Chlorite displays very similar occurrence compared with that of serpentine. Chlorite intergrowths and inclusions contain low titanium and chromium (i.e. up to 0.19 wt% TiO2 and 0.10 wt% Cr2O3). Nickel concentrations are up to 2.6 wt% NiO).

Both serpentine and chlorite are considered as secondary minerals phases, although it is suggested (e.g. as with many monophase inclusions) that these minerals were generated already in the host rock, prior to nugget formation

REE-minerals

In nugget G21 (class II) two lath-shaped (width of the laths < 3 μm) REE containing minerals were found in polyphase inclusions. Zirkelite ((Ca, Th, Ce) Zr(Ti, Nb)2O7) is associated with chlorite (Figure 10-H), and chevkinite ((Ce, La, Ca, Na, Th)4 (Fe2+Mg)2 (Ti, Fe3+)3Si4O22) together with magnetite and biotite (Fig. 7G). Zirkelite inclusion revealed La and Sm concentrations from 1.0 to 1.2 wt% Sm2O3 and 0.6 to 0.7 wt% La2O3, respectively. The neodymium and cerium contents of zirkelite are 4.7 to 5.4 wt% Ce2O3, 4.7 to 5 wt% Nd2O3, respectively. Chevkinite contains a total REE concentration of 7.7 to 8.4 wt%, also dominated by Ce (i.e. 4 wt% Ce2O3).

Sulfide-inclusions

Sulnde inclusions are restricted to gold nugget classes I and II. They form inclusions only, except Pb, and are represented by spherically shaped dominated monophase, subordinately polyphase inclusions composed of different sulfide phases only (Table 6). Their size commonly does not exceed 10 μm. The abundance of sulfide inclusions with respect to the nugget classification is illustrated in Table 1. The following could be distinguished:

Chalcosine inclusion of 3.0 μm in size, in tetra-auricupride only.

Table 6

Representative microprobe analyses of sulphide inclusion in gold nuggets.

SampleSG25-1SG25-3SG32-1SG32-2SG44-1SG44-2SG48-1SG53-4SG71-4SG72-2SG77-2SG70-3ANSSG2-3ANSSG3-2ANSSG9-1
Wt%
Fe000.030.0261.7936.8500.04026.067.820.073.389.7130.6
Cu0.7000.08025.28000.24069.3348.7166.0134.47
S0012.48038.3835.960.030.1712.3332.7821.870.0317.0624.3934.65
Pb30.0499.386.9800.170.0100.041.2900.151.6100.060
Te000.2831.330.0800000.0600000
Sb62.450.8500.030.040.0460.050.883.90.410.0742.33000
As4.9600.110004.9657.46038.9905.2645.4100
Ni0.02000.010034.9341.3230.2100.051.0115.5200
Au000000000000000
Ag Bi00068.5300000 51.260 00 00 0000
Co000017.6900
Zn000000.030
Tot97.54100.1599.88100100.4698.2399.9799.9199.2398.399.2999.0299.06100.299.48
At%
Fe000.070.0547.9930.2700.05021.617.310.13.438.8125.09
Cu0.15000.15018.25000.32056.9663.2752.6125.05
S0047.87051.9351.450.080.3632.4147.3535.620.0830.1838.5449.08
Pb19.9898.5651.6200.040.0200.010.5200.040.640.010
Te000.2727.810.0300000.020000
Sb70.71.4400.020.010.0242.690.492.700.0328.700
As9.1200.180005.7351.6700.2505.7934.3700
Ni0.05000.020051.547.4243.3830.760.041.4214.9900
Au00000000000000
Ag Bi00071.9600000 20.670 00 00 000
Co000017.0200
Zn00000.020
Tot100100100100.01100100.0110010010099.99100100100100100
FormSb2(Pb.As)PbPbSAg2TeFeSCuFe1.65S2.8Ni(Sb.As)NiAsNi3Bi2S2(Ni.Fe)9S8(Cu.Fe)2SCu2(Sb.As.Ni)(Co.Ni)AsSCu5FeS4CuFeS2
MinParadokrasitepoorGalenaHessitePyhotiteCubaniteBreithauptiteNickelineParkeritePentlanditeChacosineCuprostibiteCobaltineBornitechacopyrit

Nickeline 2 μm in size, included in tetra-auricupride and in Au2Ag.

Pentlandite forms anhedral crystals up to 6.7 μm in size and occurs together with breithauptite and parkerite in the gold nuggets.

Breithauptite up to 10 μm in size with the composition NiSb. Antimony (Sb) is replaced by arsenic with up to 7.82 wt%.

Bornite, galena and hessite appear as mono phase sulfides with a size below 2.6 μm in a gold (Au1, 1Ag) nugget, excluding bornite which occurs below 20 μm in size.

Parkerite (around 1.4 μm in size) appears in AuAg nuggets, associated with breithauptite and pentlandite.

Chalcopyrite is the most common predominant monophase, rare polyphase (i.e. associated with pent-landite and pyrrhotite) sulfide inclusion.

Pure lead forms intergrowths with gold only, associated with magnetite, paradokrasite and cuprostibite.

Paradokrasite and cuprostibite are detected together with pure lead and magnetite, with sizes below 2 μm.

Table 7

Representative microprobe analyses of mineral from host rocks.

123456789101112
ProbeZ2-1Z2-2Z4-1Z4-2G28-1G28-3Z4-3G28-3Z2-3Z2-4Z3-1Z4-4
M. Nameamphamphamphamphcpxcpxcpxgrtolololol
SiO242.9857.9960.5753.2854.6453.7150.1034.8844.1140.5640.6140.64
TiO24.060.330.161.014.330.351.994.100.020.060.080.10
Al2O38.980.290.083.120.330.131.821.710.000.000.000.09
Cr2O30.090.030.020.020.000.070.310.030.000.040.000.02
FeO14.681.551.706.1826.603.305.2924.6713.9318.2018.6317.97
MnO0.170.140.170.120.130.150.110.140.270.330.450.33
MgO10.3823.3922.3218.100.8916.2015.200.6942.0538.9442.7041.45
NiO0.050.090.060.030.000.000.000.000.400.120.210.20
CaO10.115.684.277.180.6125.5223.7132.290.250.120.190.24
Na2O3.566.056.726.6613.390.460.380.100.000.000.000.07
K2O0.900.600.830.410.000.010.000.000.010.020.000.01
Total96.0496.1496.9096.11101.2299.9098.9198.61101.0498.39102.87101.12
Oxy23232323666124444
Si6.558.058.297.642.0131.981.882.921.081.051.011.02
Al1.450.050.010.360.00.020.120.080.000.000.000.00
Al0.170.000.000.170.0140.020.000.090.000.000.000.00
Fe(iii)0.000.000.000.000.00.080.101.370.000.000.000.00
Cr0.000.000.000.000.000.000.010.000.000.000.000.00
Ti0.470.030.020.110.120.010.050.260.000.000.000.00
Fe(ii)1.870.180.190.740.8190.020.070.350.280.390.390.38
Mn0.020.020.020.010.0040.000.000.010.010.010.010.01
Mg2.364.844.553.870.0490.890.850.091.531.501.581.55
Ni0.010.020.010.010.000.000.000.000.010.000.010.01
Ca1.650.840.631.100.0241.010.952.890.010.000.010.01
Na1.051.631.781.850.9560.030.030.000.000.000.000.00
K0.180.110.140.080.000.000.000.000.000.000.000.00
Cations15.7715.7615.6515.944.04.034.038.072.922.952.992.98
XMg55.7696.4295.9083.935.9589.3283.7884.3379.2380.3480.44
131415161718192021222324
ProbeG28-4G28-5Z3-2Z3-3Z3-4G28-6Z4-5Z2-5Z4-6Z2-6G28-7Z3-4
M. Namesphmtchrchrilmperserpserpphlphlphlphl
SiO232.770.070.090.090.030.0845.8944.5539.6141.1342.4242.45
TiO239.600.228.037.5358.4555.180.040.021.375.240.380.19
Al2O30.100.074.574.500.360.240.000.0510.8412.629.550.91
Cr2O30.000.1521.0222.650.040.000.000.000.000.050.020.02
FeO1.2596.4558.1854.8431.101.104.534.5816.0510.1313.5618.09
MnO0.020.240.370.378.040.090.170.000.420.100.620.12
MgO0.000.386.346.658.760.0934.5638.5019.7019.5019.2524.49
NiO0.030.100.180.290.000.050.170.000.090.060.050.00
CaO23.400.140.010.030.1942.770.080.120.000.000.000.00
Na2O0.190.000.000.360.000.210.060.060.201.130.120.35
K2O0.000.000.010.010.000.020.230.028.167.559.456.41
Total97.6397.8298.8097.32106.9799.8385.7387.9096.4497.5195.4293.03
Oxy532323266282822222222
Si1.050.040.020.030.000.008.698.285.845.806.266.53
Al0.000.051.491.480.020.010.000.011.882.101.660.17
Al0.000.000.000.000.000.000.000.000.000.000.000.00
Fe(iii)0.049.9810.2010.200.000.000.000.000.000.000.000.00
Cr0.000.084.594.980.000.000.000.000.000.000.000.00
Ti0.940.111.671.581.960.000.010.000.150.560.040.02
Fe(ii)0.0013.063.242.561.161.900.720.711.981.191.672.33
Mn0.000.130.090.090.300.040.030.000.050.010.080.02
Mg0.000.362.612.760.580.009.7610.674.334.104.235.62
Ni0.000.090.070.120.000.010.050.000.020.010.010.00
Ca0.960.100.000.010.010.000.020.020.000.000.000.00
Na0.040.000.000.190.002.100.020.020.060.310.030.10
K0.000.000.000.000.000.020.060.001.541.361.781.26
Cations3.0324.0024.0024.004.030.0019.3419.7215.8615.4315.7716.04
XMg33.434.1093.1593.7468.6477.4471.6870.71

7 Conclusion

The main part of the gold inclusions consist the mineral phases clinopyroxene, aegirine, amphibole, phlogopite, spinelle ilmenite, perovskite and sulphide (Table 2). This minerals assemblage (i.e. sphene associated with amphibole, calcite, chlorite, clinopyroxene and Na-pyroxene, phlogopite, and pentlandite (Fig 8D) is typical for alkali-ultramafic sequence in form of veins and schlieres in cumulate ultramafic series.

The chemistry of determined solid inclusions in gold nuggets and the mineral phases in carbonatit-alkali-ultramafic show a very similar.

Sulphide inclusions in gold exclude the possible formation of the gold under sedimentary conditions.

A large number of the paragenesis: olivine clinopyroxene gold and chromite gold are typical and known from alkali-ultramafic rocks.

Furthermore suggests (it can be concluded), that means gold source from alkali-ultramafic sequence can be obtained.

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Received: 2014-9-11
Accepted: 2016-1-10
Published Online: 2016-4-7
Published in Print: 2016-4-1

© S. N. Dvorani, published by De Gruyter Open

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

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