Gold occurrences in the Stanly County area were first found in 1799. Gold-bearing quartz veins, typical of those found in many parts of the Appalachians, have been studied at the Cotton Patch, Mountain Creek, Parker, Gold Hill, and Coggins mines. The veins are sharply defined, near-vertical, and lie subparallel to schistosity in the Andesitic Tuffs Unit of the Upper Volcanic Sequence or Millingport Formation. Lower greenschist grade regional metamorphism at 483 m.y. has slightly altered the tuffs but little affected the veins. The veins are >95% milky to glassy quartz with minor chlorite, and trace amounts of magnetite, ilmenite, pyrite, and native gold. Gold fineness values and copper contents for the deposits range from 656-999 and 0-1.53 wt % respectively. Study of the fluid inclusions reveals homogenization temperatures of 194°-219°C (uncorrected) and saltinites of 0-5.0 wt% NaCl equivalent. A presumed pressure correction of 0.5-1.0 Kbar at the time of formation correspond to an increase of 40°-84°C for the original trapping temperature. The subparallel nature and sharp contacts of the veins, absence of metamorphic foliation in the vein minerals, and the low salinities indicate a post-peak-of metamorphism age for the veins. The volume of fluid (10¹³ liters) required to deposit the quartz but the small quantity of gold (5000 troy oz.) suggests that the veins were deposited by recirculating meteoric water which was heated at depth and which scavenged gold from a small volume (0.03 km³) of the underlying and adjacent rocks. / M.S.

No description available.

M.Sc., Faculty of Science, University of the Witwatersrand, 1992

[Truncated abstract] Mineralogical and geochemical characteristics were identified for regolith overlying two Australian Au deposits that discriminate mineralized and associated hydrothermally-altered rock from weathered rock that was not hydrothermally-altered. Mineralization was lithologically controlled within a previously unrecognized diorite dyke at the lower Proterozoic mesozonal Groundrush deposit, Tanami region, Northern Territory. Although hydrothermal alteration effects within the dyke were subtle and obliterated by weathering, Ti/Zr ratios clearly discriminated the diorite dyke from visually indistinguishable but generally unmineralized dolerite. In contrast, the Carboniferous Vera Nancy low-sulphidation epithermal Au deposit, located in the Drummond Basin, northeast Queensland, comprises structurally-controlled quartz veins within a relatively chemically homogenous suite of andesitic lavas and subvolcanic intrusions. A zoned hydrothermal alteration system in the hangingwall of the main vein grades from a proximal silica-pyrite alteration zone through an argillic zone into regionally extensive propylitic 'background'. Deep chemical weathering has destroyed the minerals diagnostic of the different alteration zones in bedrock to leave a kaolinitic regolith overlying all alteration zones. However, the silica-pyrite alteration zone is identified in regolith by retention of the anomalous concentrations of Au, As, Sb and Mo present in bedrock, and mineralogical characteristics, determined from X-ray diffraction investigations, discriminated weathered argillic from propylitic alteration zones. ... Metasomatic reactions, including weathering reactions, are typically difficult to specify, as some reactants and products may be removed by fluids, and thus evidence for their involvement is absent from the observed assemblages. In addition, the range of possible reactions even for relatively simple systems is such that identifying the real reaction may be intractable without additional information. Linear algebra provides an approach to this problem. If minerals and aqueous phases are represented as columns in a matrix with elements as rows, any vectors in the null space of this matrix (if it is greater than 0-dimensional) provide coefficients to balance reactions between the phases. The 'Gale' vectors for a set of phase are the row vectors of any basis for this null space. The relationships between phases are clarified through examination of these vectors in d-dimensional Gale vector space, where d is the dimension of the null space. The hyperplane normal to any vector in Gale vector space separates the space into reactant and product half-spaces. The geometric relationships between the Gale phase vectors describe all the possible reactions. Because changes to parameters (e.g. volume, mass, density) can be determined for each possible reaction, Gale analysis can be used to identify reactions consistent with these constraints. Gale analysis of weathering at the Vera deposit indicated that all the possible weathering reactions producing kaolinite, goethite and quartz from illite, pyrite and siderite resulted in minor reductions in volume and mass only, whereas acid-neutral weathering of propylitic rocks exhibited greater mass losses, consistent with observation and geochemical interpretation.

[Truncated abstract] The Laverton Greenstone Belt (LGB), located in the northeastern part of the Eastern Goldfields Province (EGP) of the Yilgarn Craton, Western Australia, has a total contained gold endowment of over 690t. An important feature of the gold deposits in the LGB is their close spatial association with granitoids, with many gold deposits located adjacent to, or hosted by, granitoids. Recently-proposed genetic models for Archaean orogenic gold deposits have emphasised the role of granitoids in the formation of ore-deposits, but differ significantly in the nature of that role. Some models suggest that the granitoids are a source of ore-fluids and solutes, whereas others suggest that granitoids exert an important structural control on gold mineralisation. Such competing genetic models for gold mineralisation variably propose either a proximal-magmatic or distal-metamorphic, or less commonly distal-magmatic, source for goldbearing fluids, or mixing of fluids from multiple sources. Isotope geochemistry and geochronological studies are used to constrain the source and timing of auriferous fluids at nine gold deposits in the LGB in an attempt to differentiate between conflicting genetic models. To overcome the lack of detailed deposit-scale geological constraints inherent to any regional study, hypotheses generated from regional datasets are tested in a detailed case-study of the Wallaby gold deposit. The Pb-isotope compositions of ore-related sulphides from deposits in the LGB plot along the line representing crustal-Pb in the Norseman-Wiluna Belt of the EGP, with individual deposits clustering with other nearby deposits based on their geographic location. This trend is similar to that recorded in the Kalgoorlie-Norseman region in the southern EGP, and is consistent with a basement Pb reservoir for gold-bearing fluids. As such, data are consistent with a similar fluid source for all gold deposits. The Nd and Sr isotopic composition of goldrelated scheelite in the LGB clusters very tightly. The inferred ore-fluid composition has a slightly positive εNd, similar to ore fluids at other gold deposits in the EGP for which a proximal magmatic source is highly improbable. As such, Sr and Nd data are consistent with a similar fluid source for the gold deposits analysed in the LGB, but cannot unequivocally define that source. The median S, C and O isotopic compositions of ore minerals from all nine different gold deposits studied in the LGB fall in a very narrow range

No description available.

A portion of northern southeastern Alaska known as the Juneau gold belt is composed of a disparate assemblage of lithotectonic terranes which range in age from Paleozoic and perhaps older to Cretaceous. Five progressive deformational events associated with contractional tectonism began in the mid-Cretaceous time and continued well into the Tertiary. Widespread plutonism occurred in the region from Cretaceous through Tertiary time. Gold-bearing quartz vein systems in the Juneau gold belt formed within a 160-km-long by 5- to 8-km-wide zone along the western margin of the Coast Mountains, Alaska. The vein systems are localized in second and third order shear zones spatially associated with terrane-bounding, mid-Cretaceous thrust faults. Mesoscopic structures integrated with ⁴⁰Ar/ ³⁹Ar ages from vein sericite are interpreted to indicate that a fluid cycling event along the entire belt occurred between 56.5 and >52.8 Ma. Structural analysis of the vein orientations and geometries are interpreted to indicate that mineralization developed under a near-field stress regime of subhorizontal contraction along a west-southwest to east-northeast trending axis. The axis of extension plunged steeply to the southeast. Slight variations in the interpreted stress axes may have been the result of variations in fluid pressure. Post-mineralization deformation was associated with a dextral transpressive regime along the Denali-Chatham Strait fault system. Gold vein mineralization occurred during the latter stages of orogenesis. Fluid flow and subsequent vein development was temporally associated with changes in plate motion during Eocene time. Veining was also synchronous with exhumation and voluminous plutonism immediately inboard of the gold belt. These interacting tectonic events likely facilitated fault-valve action and vein development along now exhumed shear zones.

No description available.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, / The Ventersdorp Contact Reef(VCR) is a major gold-bearing reef in the Witwatersrand Basin. It occurs between the overlying Klipriviersberg Group lavas and the underlying Central Rand Group sediments, and was strongly altered by hydrothermal fluids circulating in the Witwatersrand Basin. A detailed study of the mineralogy, geochemistry of rocks and minerals, physicochemical conditions, stable isotopes and ages of hydrothermal alteration zones associated with the VCR were carried out at Western Deep Levels South Mine, South Africa. ( Abbreviation abstract) / Andrew Chakane 2019

Orogenic gold mineralisation in schist terranes with few or no contemporaneous igneous intrusions is poorly understood. It is proposed in this thesis that the structural evolution of such terranes controls the generation of hydrothermal fluid pathways and thus the location of orogenic mineral deposits. Gold mineralisation in the Otago Schist, New Zealand and the Klondike Schist, Canada occurred in the latter phases of greenschist facies metamorphism as well as after metamorphism during Paleozoic-Mesozoic exhumation. In Otago, gold mineralisation occurred at a number of different times and structural levels as the schist belt was exhumed and rocks were brought up through the brittle-ductile transition. In Klondike Schist, gold mineralisation occurred in relatively brittle rocks after a period of regional compression and crustal shortening caused by the stacking of thrust sheets. Gold mineralisation in both schist belts is not associated with any coeval igneous activity. The earliest stage of gold mineralisation in the Otago Schist occurred in the Jurassic when mineralising fluids were progressively focussed into late metamorphic ductile shear zones such as the Hyde-Macraes Shear Zone (HMSZ), east Otago and Rise and Shine Shear Zone (RSSZ), central Otago. Both of these gold-bearing mineralised zones occur along mappable structural discontinuities or boundaries that separate structurally, metamorphically and lithologically distinct blocks. The HMSZ occurs in the hangingwall of an underlying low angle normal fault that juxtaposes mineralised lower greenschist facies rocks on to unmineralised upper greenschist facies rock. The RSSZ occurs in the footwall of an overlying low angle normal fault that juxtaposes unmineralised lower greenschist facies rocks on to mineralised upper greenschist facies rock. The two shear zones did not form as part of a single homogeneous structure. There are several other prospective late metamorphic boundaries that are different from later brittle faults that disrupt the schist. Late metamorphic gold mineralisation is characterised by both ductile and brittle structures, foliation-parallel shears, disseminated gold with sulphides in deformed schist and minor steeply dipping extensional veins. This style of mineralisation is the most prospective but can be subtle in areas without quartz veins. Hydrothermally altered rocks are enriched in gold, arsenic, tungsten and sulphur with minor enrichment of bismuth, antimony, mercury and molybdenum. Disseminated mineralisation in the HMSZ is associated with hydrothermal graphite however there is no hydrothermal graphite in the RSSZ. The next stage of gold mineralisation occurred in the Cretaceous during post-metamorphic exhumation of the schist belt and is characterised by steeply dipping, fault-controlled quartz veins, silicified breccias and negligible wall rock alteration. Most post-metamorphic veins strike northwest such as the ~25 km long Taieri river gold vein swarm, but there are other stibnite and gold mineralised structures that strike northeast (e.g., Manuherikia Fault system) and east-west (e.g., Old Man Range vein systems). The latest recognised stage of gold mineralisation is controlled by structures related to the initiation of the Alpine Fault in the Miocene and is characterised by steeply dipping quartz veins with abundant ankeritic carbonate in veins and ankeritic carbonate with gold in altered rocks. Hydrothermally altered rocks are enriched in arsenic, carbon dioxide and sulphur with minor enrichment of antimony. Gold-bearing veins at Bullendale, central Otago are of this type and are associated with a broad alteration zone. Gold-silver and gold-silver-mercury alloys occur in both Caples and Torlesse Terranes of the Otago Schist. Almost all mercury-bearing gold occurs in east Otago vein systems and mercury-free gold occurs in central and northwest Otago veins, irrespective of host terrane. There is no relationship between depth of vein emplacement and mercury content of gold. The Klondike Schist was emplaced as a series of stacked thrust slices in the Jurassic and thrust-related fabrics are preserved in all thrust slices. Strongly deformed carbonaceous schist horizons are spatially associated with thrust faults and graphite within these units is concentrated along spaced cleavage surfaces. Kink folding is best developed in the uppermost slices of Klondike Schist and overprints thrust-related fabrics. Gold-bearing veins formed in extension fractures controlled principally by pre-existing weaknesses such as kink fold axial surfaces. Normal faults correlated with a period of Late Cretaceous regional extension crosscut kink folds and offset gold mineralised veins. The main stage of mineralisation occurred after major regional compression and thrust stacking and before Cretaceous normal faulting. Gold-bearing veins are widely dispersed throughout the uppermost slices of Klondike Schist and are considered to be a sufficient source for Klondike gold placer deposits. Disseminated gold with pyrite is associated with gold-bearing veins in some Klondike Schist and this disseminated mineralisation expands the exploration target for these veins. Disseminated gold with pyrite, without quartz veins, occurs in some schist lithologies and is associated with chlorite alteration and weak silicification. The arsenic content of gold-mineralised Klondike Schist is much lower than mineralised Otago Schist and background concentrations of arsenic are much lower in Klondike Schist as well. No shear-related mineralisation has been discovered in Klondike Schist but due to its relatively poor exposure, this belt remains prospective for this style of mineralisation.