1. Dry Valleys, Russia
Dry Valleys (Cyxoлйor in Russian, Sukhoy Log in English, belonging to Irkutsk Oblast) layered gold mine is located in Irkutsk, eastern Russia The Baikal-Patom Mountains of the region. The area is composed of mid- and late-Lifidian terrigenous and terrigenous-carbonate sedimentary metamorphic rocks. These rocks were accumulated in cratonic margin basins resulting from the formation of nearly SN-trending intracontinental rift systems. The development of the basin ended with stabilization of the continental margin, without signs of oceanization of the continental crust. This area developed in a rift movement environment, evidenced by the obvious uplift of the Moho surface, which resulted in the crustal thickness in the area being reduced to 36km, while in adjacent areas it was 42-45km.
Tectonically, the Dry Valley deposit is located within a complex syncline zone. The ore body occurs in an inverted anticlinal fold. An interlayer thrust fault zone can be traced in this anticlinal fold, as well as some advanced faults, cleavage zones, extensional fissures and small folds.
The terrigenous carbonaceous rocks surrounding the mineralization are the Late-Middle Fidaihomolxin Formation, with a total thickness of 750-850m. This group can be divided into 3 subgroups based on lithology. The upper subgroup mainly consists of fine-grained quartz-sericite-chlorite mudstone and siltstone, with occasional fine-grained sandstone. All rocks in this subgroup are rich in organic matter (ranging from 2 to 3 to 5 to 7 by volume), with the highest organic carbon content in the central part of the subgroup, where gold mineralization occurs. The middle subgroup of the Homolshin Formation is sandier, while the lower subgroup is similar to the upper subgroup, except that the organic carbon content of the middle and lower subgroups is lower.
In general, the Dry Valley deposit is located in the core of an inverted anticlinal fold, and its location is controlled by advanced fault zones and the distribution of the most carbon-rich terrigenous rocks. In vertical sections, the deposit is characterized by disseminated, nested-vein-like and vein-like quartz-carbonate-sulfide mineralization distributions. This kind of mineralization is not strong in the overlying and underlying rocks of the ore body (the upper ore zone and the lower ore zone), but is the strongest in the ore body, with the number of sulfides, newly generated quartz and carbonates reaching 5 to 7 (volume). The structural-tectonic characteristics of the ore and the relationship between the gangue and newly formed sulfide mineral aggregates and the original sedimentary rocks indicate that the mineralization is due to multiple metamorphisms and metasomatism in a carbonaceous medium with a high content of non-ferrous and precious metals. And formed. The most strongly expressed are multistage metasomatic carbonation (formation of magnesian and mafic carbonates in early stages), quartzization and pyritization.
Ore minerals occupy a small volume (3-5) in the ore, but the mineral composition is extremely diverse. B.B. Gistrel et al. (1997) studied that there are at least 75 kinds of ore minerals (including variants), which belong to natural metals, metal solid solutions and intermetallic compounds, sulfides, arsenides and sulfur arsenides. Tellurides and sulfur tellurides, selenides, bismuthides, antimonides, phosphates, tungstates, halides and oxides. Unlike many gold deposits (including gold deposits related to black shale), the Dry Valley deposit ores account for an absolute majority of pyrite, while the less common minerals are mainly Ni- and Co-rich sulfides and arsenides. and sulfur arsenide. Minerals that are often seen but do not form large aggregates include galena, sphalerite, rare earth minerals - phosphates (monazite) of Ce, Nd and La and aluminosilicates and phosphates of Y, Gd and Dy. . The rarer minerals are Cuba ore, arsenopyrite, molybdenite, chertite, NiFe2S4 and Ni3FeS4 minerals, wolframite, and scheelite.
Common minerals are mainly: pyrite, pyrrhotite, pentlandite, goethite, arsenite and arsenite; there are many kinds of minor and rare minerals, with Te Mainly compounds with Ag, Au and Bi. Tellurium gold ore, telluride silver ore, sulfur gold and silver ore, sulfur telluride silver ore, hexagonal telluride silver ore, and orthorhombic telluride gold and silver ore were identified in the Dry Valley deposit. These minerals generally occur as submicron inclusions in pyrite crystals that occur with natural gold. Copperite and sphalerite are also rare minerals. The discovery of rare earth minerals in gold sulfide ores at the Dry Valleys deposit is of great significance. Rare earth minerals are mainly phosphates of La, Nd and Ce (monazite-monazite), which often appear in carbonate gangue materials accompanying mineralization.
The general distribution pattern of platinum group metals in Dry Valley deposits is based on the analysis results of more than 400 samples taken from vertical sections of the overlying and underlying rock strata of the gold ore belt. Among the platinum group metals, platinum is the main one, with other platinum group metals appearing occasionally and in lower amounts. Higher Pt content (higher than 0.1g/t) appears in hydrothermal metasomatic rocks and sulfides in the profile. The entire profile contains ore, but the content above 1g/t is concentrated in the upper ore zone connected to the gold ore body. part. The Pt content in the submineral zone is relatively high, but its distribution is irregular. In other words, part of the platinum mineralization coincides with the section with the highest Au content, and part of it extends beyond the section with the highest Au content. The distribution of other platinum group metals also has this trend, but the Pd content is generally one order of magnitude lower than Pt. The remaining platinum group metals only appear occasionally, but their highest content (such as Rh up to 0.8g/t) is generally the same as the highest content of Pt. —To.
In terms of organic geochemistry: ① The average organic carbon content in the gold ore zone of the deposit is close to 0.7, with local variations of 0.2 to 5; ② In ordinary samples, there is no difference between the organic carbon content and the total precious metal content Correlation; ③ The organic matter is dominated by kerogen, which is some unstructured graphite-like material without functional groups, which is formed during the metamorphic transformation of the original sedimentary organic matter; ④ There is soluble organic matter in the organic carbon component. It may be the remnant of the original sediment; ⑤ There is a gas phase in the organic matter component. Due to the presence of organic compounds (amides, polymer compounds, etc.), they can form complexes with precious metals, but research has not found complexes with precious metals. Insoluble carbonaceous materials (as rock-forming components of ore-hosting rocks, particulate dispersed phases, equivalent to various disordered cryptocrystalline graphites and graphites) have two morphological types: drop-shaped (i.e., clastic form) and honeycomb-shaped (equivalent to adsorption form). Identification using Auger spectroscopy and X-ray photoelectron spectroscopy did not detect the presence of platinum group metals, but X-ray photoelectron spectroscopy showed that gold exists in an uncharged state (Auo) in carbonaceous matter. This means that the carbon particles are enriched with gold in its metallic (native) state, and the gold particles may be extremely fine and adsorbed by the activated surface of the carbon particles. It is speculated that platinum group metals are also in a similar situation.
Russian scientists used electron probes to analyze more than 1,000 pyrite, pyrrhotite, pentlandite, chalcopyrite, goethite, arsenite-arsenite series Mineral particles, but since the detection limit of most particles is lower than 0.01, no platinum group metals were found, only in a few pyrite, pentlandite, other nickel sulfide, arsenide and sulfur arsenide particles. Local enrichment of platinum group metals was found.
Through heavy sand research, it was found that the mineral samples with the smallest particle size and the largest density contained higher platinum group metal content. In a super-heavy concentrate sample with a particle size of -0.06mm, the total amount of platinum group metals reaches 9.9, or 99,000g/t. Further research shows that in the ultra-heavy particle size, platinum group metal minerals mainly exist as free particles and are rarely associated with ore-forming sulfides. Platinum group mineral particle sizes range from 0.5μm to 10μm, although larger particles do exist. The smallest platinum mineral particles are generally equiaxed and nearly round. Some particles with a size exceeding 10 μm are in the shape of irregular skeletons and have an internal structure, much like gold agglomerates. The platinum minerals associated with pyrite are irregular in shape and partially dendritic. Through research on more than 40 Pt-containing mineral phases, it has been proven that natural platinum and Pt-Fe-Cu series metal solid solutions are the main ones. The most common mineral phase is natural platinum with low Fe and Cu content. Natural platinum may appear as separate particles or in association with pyrite. In addition to natural platinum, there are also Fe-poor and Cu-rich solid solutions of Pt, which may be equivalent to the Pt3Cu type phase. This type of solid solution appears essentially as free particles. There are also metal solid solutions that are relatively Cu-poor and rich in Fe, which are equivalent to equiaxed platinum ore (Pt3Fe) or tetragonal platinum ore (Pt, Fe) in terms of composition. The Cu/Fe ratios in a few mineral phases are approximately equal and may be equivalent to Pt3 (Cu, Fe) solid solutions.
In addition, a few palladium mineral phases have been found. Moreover, Pd does not exist in natural platinum or in the solid solution of Au. Instead, it forms intermetallic compounds, such as galena and sphalerite. Yellow telluride palladium ore - telluride palladium ore (Pd, Ag) (Te, Bi) type Pd and Ag bismuth telluride.
A large number of natural metal minerals have also been discovered in Dry Valley deposits. In addition to natural gold, natural silver and natural platinum, there are also natural iron, chromium, tungsten, titanium, lead, tin and copper. All these natural metals are found mainly in heavy ore concentrates, often in association with other metallic minerals. Natural metals occur in irregularly shaped particles, much like the shapes of platinum group metal minerals. Particle size ranges from one or two microns to one or twenty microns.
In terms of origin, the Dry Valley deposit is related to the metamorphism and metasomatic transformation of C-containing sedimentary rocks with geochemical specificity. According to B.K. Nemerov's research, the background content of precious metals, non-ferrous metals and rare metals in the Homolshin Group, the ore-hosting surrounding rock composed of C-containing terrigenous sedimentary metamorphic rocks, is generally abnormal, which makes the ore-forming elements Enrichment occurs after activation has occurred. Γ·M·Warshal and others also pointed out that the platinum group metals in the C-containing materials of the ore-hosting rocks may be aggregated in the form of complexes in the O-containing functional groups of the C-containing materials. After the ore-hosting rocks are exposed to heat or acid, Precious metals can be converted into volatile compounds.
The mineralization process of the Dry Valley deposit is actually very complicated. Russian scientists have found that it has gone through at least three mineralization periods (I, the same diagenetic period; II - metamorphic period; III - hydrothermal - metasomatism stage and its various stages and sub-stages). Among them, the mineralization stage (Ⅲ) can be divided into: ① pre-mineralization stage; ② mineralization (gold mine) stage; a early high-temperature sub-stage; b gold-producing medium-temperature sub-stage; c late low-temperature sub-stage; ③ mineralization later stage. The main mineralization stage of platinum group minerals is earlier than the gold mineralization stage (Table 3-2).
Table 3-2 Mineral assemblages of various mineralization stages in the hydrothermal-mesosomatic period of the Dry Valley deposit
Isotope geochemical studies show that (Table 3-3), the organic carbon in the mineral zone The isotope composition of organic carbon and carbonate C has become significantly heavier (11.64‰), while the isotopic composition of organic carbon and carbonate C in the underlying rocks has become lighter again (-13.2‰), with the maximum value (-0.66‰) at Within the ore zone, the composition becomes lighter in the upper ore zone and lower ore zone, which are -8.68‰ and -2.28‰ respectively. This means that the heavier total C isotope composition of the ore belt may be caused by the contribution of carbonate carbon. Since the total carbon content in all rocks is almost equal, this indicates that the carbonates in the mineral zone are mainly formed by the oxidation of organic carbon. It appears that it is this process that promotes the appearance of natural metals in early mineralization stages. S isotope research results also show that the isotope composition (δ34S‰) of the ore belt itself becomes abnormally lighter, while the sulfur isotope composition of the rocks above and below the mine becomes heavier, and the sulfur isotope composition from early pyrite to late pyrite also becomes heavier. This shows that there are at least two factors affecting isotope fractionation: ① The mineralization occurs in a wide temperature range; ② There is an endogenous source that brings S into the mineralization zone, and mantle sulfur accounts for a large proportion of it. .
Table 3-3 Isotope geochemical characteristics of rocks and ores in the Dry Valley deposit
2. Brazilian Serra Leste gold
Serra Leste gold The platinum-palladium deposit is located about 30km northeast of the town of Curionpolis, Barra State, Carajas Province, Brazil. It was formerly known as the Serra Pelada deposit. This area is a copper-gold mineralization belt characterized by greenstone belt-type gold deposits and copper deposits. Therefore, Hilla List is unique as a sedimentary rock ore-hosting gold deposit. The deposit occurs in the folded strata of shallow metamorphic sedimentary rocks of the Archean Rio Fresco Formation. The ore body is located in the contact zone between metamorphic carbonaceous siltstone and dolomitic marble, and is structurally located at the hub of the inclined syncline (Figure 3-1).
Figure 3-1 Geological profile of the Gilalist precious metal deposit in Brazil
(According to Tallarico et al., 2000)
The main minerals in the ore include quartz (10 ~60), kaolinite (1~20), goethite (1~15), hematite (1~40), manganese oxide (1~15), muscovite (1~30), none Fixed carbon (1~10), trace minerals include tourmaline, carbonate minerals, chlorite and magnetite. The useful metal mineral areas are mainly Pd-Pt-(Hg) minerals and Cu-Co-Ni sulfides ( Tallarico et al., 2000). In terms of mineral content, the mineral composition in ores varies greatly.
Because the ore body has suffered strong oxidation on the surface, very few primary sulfides remain, mainly pyrite, chalcopyrite, arsenopyrite, covellite, bornite and galena. Chemical exploration shows that Ni (up to 1000×10-6) and Cu (up to 4000×10-6) are abnormally obvious, indicating that copper-nickel sulfide minerals may have originally existed.
The currently known mineralization of platinum group elements is mainly palladium, and the mineralization of palladium is related to Au-Ag-Pd alloy (Au~94, Ag~3, Pd~3) or Pd-Hg mineral (such as potarite and atheneite), but independent palladium minerals are rare, and equiaxed platinite is the only platinum group mineral identified so far. Gold is mainly natural gold, and secondary growth in the oxidation zone can form nugget gold weighing 62kg.
As for the origin of the Shila Lister gold, platinum and palladium deposit, there is currently not enough research. Due to the low degree of regional metamorphism, the frequent occurrence of actinolite-calcite mineral pairs indicates that the peak metamorphic temperature can reach 550°C. Therefore, Tallarico et al. (2000) believed that diorite intruded in metamorphic rocks may play an important role, while carbonaceous metamorphic siltstone only plays a role as a geochemical barrier. However, Tallarico et al. (2000) also pointed out that issues such as where precious metals come from and the age of mineralization have not yet been resolved. Archean intrusive rocks, metamorphic mafic volcanic rocks in the lower part of the Rio Fresco Formation, and diorite may have contributed gold and platinum group elements. The intrusion of diorite may be the final cause of the formation of the ore body, but Diorite itself is not exposed on the surface, so more direct signs of ore prospecting need to be found.