The Occurrence of High-grade Gold Pockets in Quartz Reefs at the Gwynfynydd Mine, Wales, United Kingdom: A Geological Explanation for the Nugget Effect

Abstract

Gold-bearing quartz reefs commonly show extremely erratic and unpredictable grade variation, although gross geological continuity may be good. This type of variation is often described as being nuggety or having a high nugget effect and can be measured quantitatively using the semivariogram. Understanding of geological features such as reef texture and structure will provide improved models for the interpretation of assay data, drill core descriptions, etc. In this paper, a case study from the quartz-hosted gold occurrences in the Gwynfynydd mine, United Kingdom, is described. The work provides a starting point for models of lateral variation and demonstrates the types of structural and textural features that may be sought to give clues to the prediction of gold distribution in similar deposits.Gold deposition at Gwynfynydd is primarily associated with the reef footwall. The host veins form a discrete group of structures that may have been emplaced early in the sequence of quartz veins forming the reef. This potentially exposes them to a maximum number of dissection events during repeat dilations of the lode. Gold was deposited in the local porosity at extreme concentrations, equivalent to kg/t grades, when the host fissure had become filled with a porous framework of crystalline quartz and sulfides. Distribution of porosity varied laterally along the vein as the result of the formation of clump-like growths of minerals from the vein walls. The growth style is relevant to the distribution of gold although it may not be relevant to the occurrence of gold in a particular vein. The preservation of pores with some connectivity late in the vein fill sequence may be important in permitting continued but slow fluid flows. This facilitates effective fluid reaction with wall rock-derived methane, thus changing the relative rates of gold and quartz deposition in favor of gold.The textural studies explain the first-order control of nugget distribution at Gwynfynydd but do not allow for prediction and optimization of the resource estimation process. This is principally because of the very low geological continuity and poor predictability of the high-grade pockets. Nonetheless, the work does provide a clear geological explanation for the erratic nature of the gold. It indicates textures and structures that can be used to determine geological continuity of the goldbearing elements within the gross reef envelope.

Approaches to reporting grade uncertainty in high nugget gold veins

“…These are usually wider more complex structures ranging from 20 to 100 m in width. Vein systems typically contain both barren and productive segments with gold grade varying laterally, vertically and across the body (Dominy et al, 2003b;Platten and Dominy, 2003). Vein formation generally has a complex and extended history, related to the emplacement of relatively barren and gold-rich vein elements at different stages.…”

Importance of good sampling practice throughout the gold mine value chain

“…• primary gold particle distribution is often erratic (high GNE), with localised clustering effects (Dominy and Platten 2007;Platten and Dominy 2003) and and Platten 2007). These clusters, which may be distributed on the centimetre-scale or greater, give rise to high assay variability in field samples.…”

Author's Reply to "Comments on Classification and Reporting of Mineral Resources for High-Nugget Effect Gold Vein Deposits," by M. Vallee

“…At the extreme end of the spectrum, individual highly restricted veins can grade 10 000's g/t Au within larger dominantly barren vein packages (Platten and Dominy, 2001;Dominy et al, 2003a). The discontinuous and restricted nature of these high-grade carrier veins clearly contributes to a high-nugget effect.…”