Paragenesis in the Hollinger veins
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Abstract:
The ore bodies of the Hollinger Mine occur in an area of complexly folded and highly altered lava flows intruded by quartz-feldspar porphyry. The ore bodies consist of quartz veins and associated pyritized wall-rock.The investigation is concerned with the paragenesis of the minerals of the ore bodies, particularly the relationship of gold to quartz. Results are given of a microscopic study of thin and polished sections of vein material and wall-rock, supplemented by underground study of the ore bodies.Most of the gold occurs in a quartz-ankerite-pyrite type of ore body and was introduced later than the major period of quartz mineralization. In the veins, gold appears to be genetically related to a pale green sericite and was localized around and in inclusions of such minerals as pyrite, scheelite, arsenopyrite and ankerite. In the wall-rocks, gold is associated with pyrite but is believed to be later than it. A quartz-calcite mineralization was later than most of the gold.Data accumulated during the investigation indicate that the veins were formed predominantly by the filling of fractures successively reopened to the waves of mineralization.Keywords:
Paragenesis
Sericite
Ankerite
Arsenopyrite
Chalcedony
Wall rock
Speleothems of silica are far rarer than those of calcite but occur in a range of types including stalactites, stalagmites and flowstones. This study has found a wider range and far greater number of silica speleothems on the quartz sandstones of the Sydney region than the small number of previous accounts had suggested. Speleothems on the Sydney region sandstones are composed of multiple layers of amorphous opal‐A and cryptocrystalline chalcedony. Silica slowly dissolved from detrital and diagenetic quartz and kaolinite clays of the host arenites is redeposited as opal‐A at the sandstone surface when groundwater evaporates. This amorphous silica converts over time by Ostwald‐type paragenesis to the cryptocrystalline form, but the expected intermediate opal‐CT phase has not been detected. The crystallisation of chalcedony at earth‐surface temperatures is generally believed to take an extremely long time and its presence makes these speleothems very significant, especially as it is reported in only a small number of silica speleothems elsewhere. Furthermore, a similar paragenetic silica‐'ripening' mechanism may also be involved in the low‐temperature earth‐surface formation of other crystalline silica deposits such as silcrete duricrusts and pedogenic quartz. Additional closely coupled laboratory and field investigations into the processes that control silica paragenesis under earth‐surface conditions are sorely needed.
Chalcedony
Paragenesis
Stalagmite
Pyrophyllite
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Sericite
Ankerite
Gangue
Carbonate minerals
Overprinting
Rhodochrosite
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Sericite
Chalcedony
Arsenopyrite
Marcasite
Ore genesis
Magmatic water
Breccia
Stockwork
δ34S
Stibnite
Hypogene
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The ore genesis of gold deposits in Xiaoqinling, located in the south margin of the North China Craton (NCC), remains highly debated. Either a magmatic source or a metamorphic origin has been advocated for the ore-forming fluids. In this paper, we report new 40Ar/39Ar dating, in-situ pyrite trace elements and C-H-O isotopic data for the Cangzhuyu deposit, hosted in quartz veins within the Archaean metamorphic rocks to constrain the ore-forming processes. Four stages of mineralization and alteration can be recognized in the deposit, including Stage I: quartz-pyrite veins; Stage II: pyrite-quartz-ankerite veins; Stage III: polymetallic sulfides-ankerite-quartz veins; Stage IV: quartz-carbonate veins. Sericite from stage III gold-bearing veins has a 40Ar/39Ar plateau age of 130.7 ± 1.3 Ma, which is synchronous with the regional gold mineralization event in the Xiaoqinling district. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) trace element analysis shows that Stage I and Stage IV pyrite show similar compositions and rarely contain lattice gold. In contrast, gold mostly occur as native gold or Au-Te-Bi mineral inclusions within Stage III pyrite, which are consistent with their high Ni, Te, Bi contents. The ore-forming fluids equilibrated with ankerite have C-O isotopes of −6.6 to −4.1‰ and 8.8 to 10.8 ‰, respectively. They are consistent with H and O isotopes of sericite that δDfluid ranges from −61.1 to −60‰ and δ18Ofluid ranges from 8.2 to 7.2 ‰, implying a magmatic source. We hence suggest that the Cangzhuyu gold deposit was formed from mantle-derived magmas in a setting of the destruction of NCC and lithospheric thinning.
Ankerite
Sericite
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Sericite
Ankerite
Felsic
Breccia
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Summary The paragenesis of a suite of hydrothermal minerals (sulphides, baryte, rhombohedral carbonates, quartz, sericite, and chlorite) at four mines in this district is discussed. Of particular interest are the conversion of pyrrhotine to marcasite, which is shown to take place by loss of iron, the replacement of blende by chlorite, and the possible formation of primary goethite. The occurrence of gudmundite is recorded for the first time in the British Isles.
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Paragenesis
Marcasite
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Ankerite
Sericite
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For long-term performance assessment of obsidian, we present computation chemistry and experimental data sets on glass dissolution and associated ionic release obtained after treating specimens under near hydrothermal conditions (for 1-64 h time intervals). The EQ3/6 simulated results indicate appearance of quartz (chalcedony) and other mineral species. Besides quartz, SEM-EDX of the precipitate shows presence of smectite having two distinct microstructures which consist of small and large lamellae and their formation is linked with the release of Na, K and other ions accountable for increase in the pH and conductivity of the leachates. The former structure represents paragenesis of an earlier formed alteration product. These observations largely correspond to the simulated results, suggesting its suitability in geological repository.
Chalcedony
Paragenesis
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