Geological and geochemical controls on the silver content (fineness) of gold in gold-silver deposits

Morrison, Gregg W.; Rose, Williams J.; Jaireth, Subhash

doi:10.1016/0169-1368(91)90009-v

Cited by 169 publications

(87 citation statements)

References 51 publications

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“…This mitigates against a supergene origin. Furthermore, the silver-rich (39-41 wt %) gold is consistent with that found in epithermal environments (10-55% Ag: Morrison et al, 1991) and contrasts with orogenic deposits (referred to in Morrison et al, 1991 as 'slate belt' type), which are typically associated with gold with ≤ 20 percent Ag (Morrison et al, 1991). The silver content of bedrock gold from Croagh Patrick varies considerably between outcrops (medians are 5.3 and 14.9 wt % Ag: Chapman et al, 2000).…”

Section: Orogenic Goldsupporting

confidence: 72%

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

et al. 2011

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The Bohaun gold deposit in County Galway, western Ireland is located in the Caledonian orogenic belt, proximal to a major accretionary boundary. The mineralization is hosted in a normal extensional fault, representing a zone of repeated extension and vein infilling of open cavities, within clastic sedimentary rocks, which have only reached the late diagenetic zone of low-grade metamorphism.The mineralization consists of undeformed veins, stockworks and breccias. Quartz dominates all the veins and commonly displays multiple growth stages and a variety of classic epithermal textures, including comb, plumose and banded, indicative of low confining pressures. The veins have a simple mineralogy comprising quartz-sericite-chlorite with minor sulfides, hematite, rare visible gold, with a high silver content (up to 41 wt %), and late barite and dolomite.Vein quartz is associated with two fluids compositions: Type I with moderate to high salinity (8.0-23.6 wt % NaCl equiv) and Type II with low to moderate salinity (0.0-7.8 wt % NaCl equiv). CO 2 contents in both fluid types are <3.5 wt percent. The majority of fluid inclusion homogenization temperatures range from 126 to 257 o C and geologically constrained depth estimates (≤5 km) indicate maximum fluid inclusion trapping temperatures between 175 and 245°C.The geodynamic setting of Bohaun, which comprises a deformed continental margin of an allochthonous terrane, is typical of orogenic gold deposits. However, an orogenic association for the Bohaun mineralization is discounted on the basis of fluid inclusion evidence, atypical vein textures, gold grain composition and undeformed veins. While vein textures locally resemble those found in epithermal quartz-adularia mineralization, there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks. The origin of the gold at Bohaun remains enigmatic. However, we favor an association with widespread Carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine type fluid, in a similar fashion to red-bed Au-Pd mineralization. Bohaun provides further evidence that low temperature brines may be capable of generating economic grade gold mineralization, independent of other fluids.3

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Section: Orogenic Goldsupporting

confidence: 72%

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

et al. 2011

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“…The variable and broad range of fineness of the gold grains found in the area is typical of low-sulfidation epithermal-type deposits (Morrison et al 1991). This suggestion agrees with the composition of gold particles described from a primary gold-bearing deposit in the area, La Estancia at (Fig.…”

Section: The Sources Of Placer Goldsupporting

confidence: 86%

Morphological and Chemical Study of Placer Gold From the San Luis Range, Argentina

Márquez-Zavalía¹,

Southam²,

Craig³

et al. 2004

The Canadian Mineralogist

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The composition of placer gold from the San Luis Range, Argentina, is poorly documented, although these deposits have been known since the 18 th century. A detailed study of gold grains collected from different streams in the La Carolina gold district was carried out. We performed 502 analyses on 191 grains. The core composition of the gold grains is heterogeneous, with an average chemical composition of 58 to 74 wt.% Au, 25 to 40% Ag, up to 0.5% Hg, and less than 0.4% Cu. The rim of each grain is generally richer in gold than the core, and the contact between rim and core is very sharp but of irregular shape and complex texture. There are populations with different chemical composition, but the compositional differences among rounded grains and those having a more delicate morphology are not invariably either significant or consistent. All sources are restricted to deposits associated with Tertiary volcanic rocks in the area, and the variable and wide range of fineness points to low-sulfidation epithermal-type deposits as the most likely source. The sharpness of the core-rim contact may result from either electrochemical processes active in the stream or stream sediments, or Au and Ag dissolution processes followed by Au precipitation in surface or near-surface environments. The presence of overgrowths of gold on gold grains suggests that there was secondary precipitation of gold after the host grain was formed. Gold grains do grow in low-temperature geochemical settings, and we believe that their composition becomes more gold-rich as the grains move downstream or are buried in the sediments. Anthropogenic Hg was detected in some of the streams, and even where there are several sources for it, we believe that all of it is coming from the amalgamation processes used by local inhabitants to recover the panned gold.Keywords: gold grains, gold rims, placer gold deposits, San Luis Range, Argentina. SOMMAIREOn connaît mal la composition de l'or dans les placers aurifères de la chaîne de San Luis, en Argentine, quoique ces gisements sont connus depuis le dix-huitième siècle. Nous présentons les résultats d'une étude détaillée de pépites d'or prélevées de divers cours d'eau du district aurifère de La Carolina. Nous avons effectué 502 analyses sur 191 grains. Le coeur des grains est hétérogène, contenant en moyenne entre 58 et 74% Au, 25 et 40% Ag, jusqu'à 0.5% Hg, et moins de 0.4% Cu. La bordure de chaque grain est généralement plus riche en or que le coeur, et le contact entre ce liseré et le coeur est très franc, mais aux contours irréguliers et à texture complexe. Nous décelons des populations différant en composition, mais les différences parmi les grains arrondis et ceux ayant une morphologie plus délicate ne sont pas inmanquablement significatives ou conformes. Les sources de minéralisation sont limitées à des gisements associés au volcanisme tertiaire dans la région, mais la pureté de l'or varie beaucoup, de sorte que des gisements de type épithermal à faible sulfuration semblent rendre compte de la prov...

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“…Taking into consideration the fineness values [Au/(Au + Ag)] ϫ 1000, concentrations in wt.%] compiled by Morrison et al (1991) for different types of epithermal deposits, the values measured in the Palai-Islica deposit, in the range from 714 to 986, agree very well with those proposed by these authors for andesitic environments (720-980), such as at Palai-Islica. These values are intermediate between those proposed for other environments: 0-880 in "adularia-sericite"-type environments, 900-1000 in "acid-sulfate" environments, and 520-969 in "Au-Te-Se" environments.…”

Section: Chemical Composition Of the Gold Grainssupporting

confidence: 82%

The Three Generations of Gold in the Palai-Islica Epithermal Deposit, Southeastern Spain

Rosúa¹,

Morales-Ruano²,

Hach-Alí³

2002

The Canadian Mineralogist

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In the Palai-Islica deposit, in southeastern Spain, gold is found associated with sulfide mineralization (particularly Fe sulfides).Grains of Au-Ag alloy (gold of types A and B) occur with pyrite in mineralized veins, and grains of native gold (type C) are associated with areas of massive silicification at the uppermost levels of the deposit. The content of "invisible" gold in the various Fe sulfides is practically nil. Of all the textural varieties of pyrite studied, unzoned medium-to coarse-grained pyrite is the only one bearing gold. A clear distinction can be established between three types of gold (A, B and C), each with a different genesis. Grains of type-A alloy deposited as a result of variations in the thermodynamic parameters of the system, mainly a decrease in sulfur activity, whereas the appearance of the type-B alloy was mainly controlled by electrochemical factors. Type-C gold may have been produced from colloidal solutions in significantly different geochemical conditions. The chemical evolution of the alloys is characterized by Ag enrichment as precipitation continued. Type-A grains [mean Au/(Au + Ag) = 0.861] were the first to form, encased in the pyrite, with a relatively low Ag content and barely any zonation, followed by gold of type B, overgrowing pyrite, with a higher average Ag content [Au/(Au + Ag) = 0.756] and commonly zoned, with later zones richer in Ag. Finally, type-C native gold has practically no Ag [on average, Au/(Au + Ag) = 0.988].Keywords: gold, gold-silver alloy, epithermal deposit, Palai-Islica, Spain. SOMMAIREDans le gisement de Palai-Islica, de la partie sud-est de l'Espagne, l'or est associé avec la minéralisation en sulfures, de fer surtout. Les grains d'un alliage Au-Ag (or de types A et B) sont associés à la pyrite dans les veines minéralisées, et les grains d'or natif (dits de type C) sont associés aux zones de silicification massive dans les parties supérieures du gisement. La teneur en or "invisible" des divers sulfures de fer est quasiment nulle. De toutes les variétés texturales de pyrite étudiées, seule la génération de pyrite en grains non zonés et à granulométrie moyenne à grossière est aurifère. On peut établir une distinction nette entre les trois types d'or (A, B et C), chacune ayant une différente genèse. Les grains d'alliage de type A ont été déposés suite à des variations des paramètres thermodynamiques du système, surtout une diminution de l'activité du soufre, tandis que la formation des grains d'alliage de type B était surtout régie par des facteurs électrochimiques. L'or de type C pourrait avoir été produit à partir d'une solution colloïdale dans des conditions géochimiques nettement différentes. L'évolution de l'alliage Au-Ag est marquée par un enrichissement en Ag au fur et à mesure que la précipitation progressait. Les grains de type A [en moyenne, Au/ (Au + Ag) = 0.861] ont été les premiers à se former, étant encastrés dans la pyrite, avec une teneur relativement faible en Ag, sans zonation importante, et ont été suivis par les grains de ty...

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Geological and geochemical controls on the silver content (fineness) of gold in gold-silver deposits

Cited by 169 publications

References 51 publications

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

Morphological and Chemical Study of Placer Gold From the San Luis Range, Argentina

The Three Generations of Gold in the Palai-Islica Epithermal Deposit, Southeastern Spain

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