Chalcedony Veins Research Articles

A 4,000-year-old shaman’s stone cache at Casita de Piedra, western Panama

During new excavations at the preceramic rockshelter of Casita de Piedra in western Panama, a cache of 12 unusual stones was recovered near the back wall, dating to between 4800 and 4000 cal bp. The stones include quartz, pyrite, a chalcedony vein nodule, a bladed quartz and jarosite aggregate and a human-modified dacite cylinder. Based on the unusual lithic types and the context of the cache, we suggest that these stones once belonged to a ritual specialist, such as a healer or shaman. Special stones are frequently mentioned as being an important component of a shaman’s ritual paraphernalia in ethnographic records of various historic Native American groups throughout Central and South America, including the Bribri and Cabecar of southeastern Costa Rica and western Panama (formerly known as the ‘Talamanca’). The cache of stones recovered at Casita de Piedra may represent the earliest material evidence in Central America of shamanistic practice.

Chalcedony vein horizons and clastic dikes in the White River Group as products of diagenetically driven deformation

Chalcedony veins occur as local stratabound arrays at multiple levels within the finer-grained sediments of the White River Group, making up to 2%–3% of the outcrop volume. The veins are commonly deformed by small folds, faults with well-developed striae, and various fold-fault combinations, and they also exhibit striae and slickenslides on vein walls. These indicate significant vertical shortening of the veins. The combination of a stratabound distribution and vertical shortening is consistent with an origin by diagenetically driven deformation, where changes in clay and/or silica phases drive syneresis and associated dewatering and compaction. In this way, the chalcedony veins bear similarities in origin to stratabound polygonal normal fault systems seen in fine-grained marine strata. Smectite clays, silica phases, and clinoptolite in the White River Group are associated with diagenetic reactions that could produce syneresis. At different localities, vein strike distributions vary from being statistically random to highly organized. These distributions are also consistent with a syneresis origin, with local stress fields organizing the distribution into multiple coeval directions in some cases. Chalcedony veins locally occur inside and parallel to clastic dikes, clearly indicating that the veins were emplaced at the same time as or after the dikes. Thin-section textures from dike-vein composites indicate that vein formation occurred while the clastic fill was unlithified and still mobile. These relationships, along with common orientations when in proximity, link clastic dike and chalcedony vein formation. Dikes also show complex strike orientation distributions that differ by locality. Internal dike features indicate multiple fill events with intervening lithification. Evidence for vertical dike shortening suggests synchronous or later compaction. The clastic dikes are also postulated to result from syneresis. We suggest that chalcedony vein formation, silica mobilization, local uranium mineralization, and clastic dike formation are part of diagenetically driven fracture development that produced a fluid flow network, initiating feedback relationships among diagenesis, dewatering, fluid migration, and associated compaction. Given that the clastic dikes occur within the Sharps Formation, the event was Miocene or later.

First evidence of epithermal gold occurrences in the SE Afar Rift, Republic of Djibouti

The geology of the Republic of Djibouti, in the SE Afar Triangle, is characterized by intense tectonic and bimodal volcanic activity that began as early as 25–30 Ma. Each magmatic event was accompanied by hydrothermal activity. Mineralization generally occurs as gold–silver bearing chalcedony veins and is associated with felsic volcanism. Eighty samples from mineralized hydrothermal chalcedony, quartz ± carbonate veins and breccias were studied from ten sites representing four major volcanic events that range in age from early Miocene to the present. The most recent veins are controlled by fractures at the edges of grabens established during the last 4 Myr. Gold in excess of 200 ppb is present in 30% of the samples, with values up to 16 ppm. Mineralogical compositions allowed us to identify different types of mineralization corresponding to different depths in the hydrothermal system: (1) surface and subsurface mineralization characterized by carbonate chimneys, gypsum, silica cap and quartz ± carbonate veins that are depleted in metals and Au; (2) shallow banded chalcedony ± adularia veins related to boiling that contain up to 16 ppm Au, occurring as native gold and electrum with pyrite, and tetradymite; (3) quartz veins with sulfides, and (4) epidote alteration in the deepest hydrothermal zones. Samples in which pyrite is enriched in As tend to have a high Au content. The association with bimodal volcanism, the occurrence of adularia and the native Au and electrum in banded chalcedony veins are typical of epithermal systems and confirm that this type of mineralization can occur in a young intracontinental rift system.

Chrysoprase from Gumigil, Queensland

Chrysoprase from Gumigil, near Marlborough in central east Queensland, occurs in weathered ophiolite of the Marlborough Block, in veins up to 20 cm thick in the lower parts of an approximately 50 m-thick regolith profile. Serpentinised harzburgite saprolite hosting magnesite and chalcedony veins with rare chrysoprase grades upward to a plasmic zone of chalcedony, nontronite and chlorite, with a lateritised zone above consisting of nodular or vermiform ferruginous duricrust, boxwork chalcedony and kaolinite. Gem chrysoprase and the poorer quality so-called ‘wetstone’ are dominantly chalcedonic quartz, coloured by willemseite, which occurs as 30–100 nm-wide zones largely aligned on polycrystalline quartz grain boundaries. Individual willemseite flakes (possibly single crystals) range from 2 to 12 nm thick. The quartz grainsize in chrysoprase is finer than in wetstone, and the willemseite domains in chrysoprase are smaller and less abundant, amounting to about 5% in chrysoprase and 15% in wetstone. Willemseite composition varies slightly between samples, with an average formula (Ni2.7Mg0.3)Si4O10(OH)2.

Geology, geochemistry, and genesis of Axi: A Paleozoic low-sulfidation type epithermal gold deposit in Xinjiang, China

Axi is a low-sulfidation type epithermal gold deposit hosted in Paleozoic subaerial volcanic rocks in the western Tianshan orogenic belt, Xinjiang, China. The resource is more than 50 t gold at an average grade of > 4.4 ppm. The deposit occurs in the Tulasu volcanic fault-basin in the Paleozoic active continental margin on the northern side of the Yili-Central Tianshan plate. The host rocks are andesitic volcaniclastic rocks of the Paleozoic Dahalajunshan Formation, and the orebodies occur as veins in annular faults of a paleocaldera. Mineralization at Axi can be subdivided into five stages: quartz and/or chalcedony vein, quartz vein, quartz-carbonate vein, sulfide vein and carbonate vein. There are two types of ore host: quartz vein and altered rocks. Ore minerals are native gold, electrum, pyrite, marcasite, arsenopyrite, hematite, limonite, and trace amounts of pyrargyrite, polybasite, naumannite, cerargyrite, sphalerite, chalcopyrite, tetrahedrite, galena, pyrrhotite and clausthalite; gangue minerals are mainly quartz, chalcedony, illite, calcite, siderite, dolomite, adularia and laumontite. The main wall-rock alteration is silicification and phyllic alteration, carbonatization and propylitization. The deposit is characterized by an enrichment, relative to crustal abundance, of Au, Ag, As, Sb, Bi, Hg, Se, Te and Mo, depletion in base metals (Cu, Pb, and Zn), and a low Ag/Au ratio (0.5–3.7). Three types of fluid inclusions were recognized in quartz from the major mineralization stages: liquid aqueous inclusions, liquid-rich two-phase inclusions and small amounts of vapor-rich two-phase inclusions. Microthermometric measurements indicate that the final ice melting temperatures are − 0.3 to − 4.4 °C, corresponding to salinities of 0.5–6.9 wt.% NaCl equiv. (2.2 wt.% NaCl equiv. in average). The peak temperatures of ice melting varies from − 0.4 to − 1.9 °C, corresponding to salinities of 0.7–3.1 wt.% NaCl equiv. Homogenization temperatures range mainly between 120 and 240 °C, with an average of 190 °C and a maximum of 335 °C. The fluid density is 0.73 to 0.95 g/cm 3 and thus the estimated maximum mineralization depth is about 700 m. Hydrogen and oxygen isotopic compositions of the ore fluids lie within a narrow range: δD H2O is − 98 to − 116‰ and δ 18O H2O − 1.8 to 0.4‰. 3He/ 4He ranges from 0.0218 to 0.138 Ra, with an average of 0.044 Ra, indicating that He derived predominantly from crust with negligible mantle He in the ore fluids. By contrast, the 40Ar/ 36Ar ranges from 317.7 to 866.0, suggesting that crust-derived radioactive 40Ar⁎ accounts for 7.0 to 66%, and atmospheric 40Ar about 43 to 93% in the ore fluids. Hydrogen, oxygen, carbon, sulfur and noble gas isotopes indicate that the ore-forming fluids of the Axi gold deposit consisted predominantly of circulating meteoric water. Ore-forming metals may have derived mainly from the host volcaniclastic rocks of the Dahalajunshan Formation and basement rocks. The occurrence of adularia, platy calcite, and quartz or sulfide aggregates as pseudomorphs after bladed calcite in ore veins, and occurrence of aqueous liquid, and liquid-rich and vapor-rich two-phase inclusions, indicates that boiling of the ore-forming fluid have occurred, leading to supersaturation of the hydrothermal solution and deposition of ore metals. This is the main mineralization mechanism for quartz-vein type ores in Axi. The ore-forming fluid was buffered to a near-neutral pH in a reduced environment during mineralization. The preservation of this Paleozoic Axi deposit and its discovery required a rapid accumulation of sediments in the basin after formation of the deposit, and minimal amount of erosion after Late Cenozoic uplift.

Gold-bearing hot spring systems of the northern Coast Ranges, California

Gold-mercury mineralization in the Knoxville and Sulphur Creek districts of the Coast Ranges of northern California fits the emerging hot spring classification of epithermal ore deposits. Quartz and chalcedony veins, which show hydrofracture texture, are capped by subaerial sinter deposits. All the deposits appear to be very young. The McLaughlin deposit and the Sulphur Valley and Elgin deposits contain hydrocarbons in their ore. Gold-bearing quartz of the San Quentin ore shoot of the McLaughlin deposit is isotopically heavy (delta 18 O = 24 ppm) and appears to have formed near 150 degrees C from isotopically heavy waters. Such waters are presumed to have moved both hydrocarbons and precious metals to the surface sinter of the deposit.Hot springs composed of isotopically heavy fluids issue today from the centers of the Sulphur Valley and Elgin deposits. The hot spring fluids carry small concentrations of hydrocarbons and transport gold, silver, and mercury to the surface. Ag/Au weight ratios of the precipitates formed by the hot springs are about 1:8. Gold grades of the hot spring precipitates range from 0.5 to more than 10 ppm; the mean value for 20 samples is 3.3 ppm. Measurements show that the waters carry 1 ppb gold in solution. Thermodynamic calculations indicate the gold is probably present as a bisulfide complex. The chemical and isotopic characteristics of the hot spring fluids appear to be similar to the fluids inferred to have formed the McLaughlin deposit. In these two gold districts, nonmeteoric waters are responsible for the formation of epithermal gold deposits. A sedimentary or metasedimentary source for the ore-forming fluids is indicated by chemical and isotopic data and is consistent with the local geology.

Geological features of some Paleozoic epithermal gold occurrences in northeastern Queensland, Australia

The geological characteristics of several newly discovered Paleozoic epithermal gold deposits and occurrences in northeastern Queensland are described. They include the Pajingo, Wirralie and Yandan deposits as well as several weakly mineralized alteration systems. All have well-preserved epithermal characteristics and occur clustered around the northern margin of the continental Drummond Basin. Here a thick sequence of predominantly terrestrial sediments and acid to intermediate volcanic products was deposited between the Late Devonian and Middle to Late Carboniferous. All of the occurrences have adularia-sericite alteration mineral assemblages and textural features which suggest formation at shallow depth. They include colloform banding in quartz and chalcedony veins, comb-textured quartz veins, platy textures in quartz pseudomorphing carbonate minerals, vein textures which indicate repetitive sealing, silica-cemented breccias which are suggestive of hydraulic fracturing, and the presence of silica sinter deposits. Limited fluid-inclusion data indicate depositional temperatures of 300°C or less. Potassium-argon age dating of alteration minerals from three of the occurrences indicates ages of formation of 294–329 Ma.

Structural analysis of the main uraniferous chalcedony vein at G. El Missikat environs, Central Eastern Desert, Egypt

Abstract This study deals with the structural analysis of joints in the uranium-bearing vein deposits and the surrounding enclosing granite rocks in G. El Missikat pluton, Central Eastern Desert, Egypt. The study includes the different structural features such as the intensity of joints, their opening and length for the different sets taken on a systematic pattern through and along the upper and lower contact of the uraniferous chalcedony vein, and their correspondence in the subsurface mining working. The relative movement of the chalcedony vein is interpreted from slickensides striation along the enclosing fault plane. The analysis of microfolds along the fault planes also indicates the movement picture. Photolineaments are traced showing the main fracturing intensity trend in the granite pluton. The study reveals that the main fracturing surface sets trend ENE-WSW and NW-SE stained by surface uranium mineralization. The first set in the subsurface mining working is found rotated anticlockwise relative ...

Petrographic and isotopic studies of the altered acid volcanics of the Tolfa-Cerite area, Italy: the genesis of the clays

AbstractAlunite-kaolinite deposits occur in argillized zones in acid volcanic rocks at Tolfa. Over seventy samples were examined by one or more of the following methods: thin section, DTA, X-ray, IR, TG, SEM, chemical, and hydrogen and oxygen isotope analysis. Kaolinite and dickite, of high structural order, are the dominant clays with micamontmorillonite and halloysite subordinate. In many samples kaolinite and dickite coexist; their relative amounts are variable but the dickite content tends to increase with depth. SEM studies show that dickite crystals—up to 30 µm—are bigger than kaolinite.The D/H and18O/16O ratios of six clays, a whole rock, two biotites from fresh host rock, two chalcedony veins and three local meteoric waters indicate that the clays formed in a meteoric-hydrothermal environment of acid hot spring type at about 80°C. The fresh volcanic host rocks are strongly enriched in18O relative to ‘normal’ igneous rocks due to derivation from or exchange with18O-rich sedimentary rocks at depth prior to hydrothermal and solfataric activity. In the absence of present-day geothermal activity the life of this hot spring system was less than 4 m.y.

Uranium deposits of the northern part of the Boulder Batholith, Montana

Uranium minerals and radioactivity anomalies occur in many silverlead veins and chalcedony veins and vein zones in the Boulder batholith of southwestern Montana. Pitchblende has been identified in a few silverlead veins. These silver-lead veins occupy shear zones along which there is no evidence of large-scale lateral displacement. The wall rock adjacent to the veins is intensely silicified and sericitized quartz monzonite and granodiorite. The veins have yielded substantial quantities of lead, silver, zinc, and gold. The silver-lead veins consist principally of galena, sphalerite, tetrahedrite, chalcopyrite, and pyrite in a gangue of light- to dark-gray quartz, altered rock, gouge, and subordinate chalcedony and carbonate minerals. No anomalous radioactivity nor uranium minerals have been found in similar veins in prebatholithic rocks of the area.Chalcedony veins and vein zones, some of which are uraniferous, are distinctly different from the silver-lead veins and, with a single exception, are known only in the batholith. The chalcedony vein zones consist of one or more discontinuous stringers or veins of chalcedony and microcrystalline quartz in silicified and sericitized quartz monzonite and granodiorite, and in less strongly altered alaskite. Only small amounts of silver ore have been produced from these chalcedony veins and vein zones. All of the veins are early Tertiary in age, but the silver-lead veins probably are older than the chalcedony veins. Uranium is closely associated with chalcedony and microcrystalline quartz in both types of veins. This association suggests that all of the uranium in the area is of the same age. If so, some of the silver-lead veins must have been re-opened during the period of chalcedony vein formation.

On the occurrence of sandstone dikes and chalcedony veins in the White River Oligocene

On the occurrence of sandstone dikes and chalcedony veins in the White River Oligocene