The significance of copper concentrations in natural gold alloy for reconnaissance exploration and understanding gold-depositing hydrothermal systems

Abstract

Analytical studies of placer (detrital) gold grains, intended to track bedrock sources and styles of mineralization, mainly consider alloy Ag content and inclusion mineralogy to generate 'microchemical signatures'. Occasionally Hg and Pd help discriminate gold from different sources; Cu infrequently because electron microprobes seldom detect the low concentrations. Reporting of Cu has been largely confined to gold from porphyry-epithermal environments, and this has biased subsequent interpretations of observed Cu-bearing gold alloys.This study focuses on placer gold in the Mourne Mountains, Northern Ireland, where auriferous bedrock remains undiscovered and the complex geology is consistent with either orogenic or intrusion-related mineralization. Over 500 gold grains analysed average 0.17% Cu ranging to >1% Cu and show a wide variation in Ag content. Inclusion mineralogy mainly matches that of orogenic gold elsewhere in the host terrane; however, some signatures are suggestive of zonation within an orogenic hydrothermal mineralizing system, or possibly metamorphic-hydrothermal remobilized gold associated with subsequent intrusive activity. One locality in the study area has placer gold of a distinctive Cu-rich alloy composition containing Cu sulphide inclusions, an association noted elsewhere in gold derived from alkali Cu-Au porphyries.Consideration of Cu-bearing gold alloys worldwide indicates that previously proposed compositional correlations with deposit type are of limited value. We show that Cu contents to at least 0.8% are permissible within orogenic gold -the first time that such compositions have been clearly ascribed to orogenic mineralization. The result is particularly important considering on-going exploration in northern Canada which employs gold grain analysis to help define exploration targets.

The genesis of gold mineralisation hosted by orogenic belts: A lead isotope investigation of Irish gold deposits

“…The alloy composition and mineral inclusion suite of placer gold from south-west Donegal (Grampian terrane) and the Mourne Mountains (Southern Uplands terrane) tend to be similar to those for orogenic mineralisation found elsewhere in Ireland (Chapman et al, 2000a(Chapman et al, , 2000bMoles et al, 2013). However atypical compositions with high Cu contents and Cu-sulphide inclusions have been recorded at Balwoges in south-west Donegal and in the Mourne Mountains (Chapman et al, 2000a;Moles et al, 2013).…”

“…Ixer et al (1990) linked the second (Late Caledonian) stage of mineralisation with magmatic processes, and this may have been responsible for the more radiogenic and isotopically variable Pb (source 3). Furthermore, gold alloys with appreciable copper, such as those under discussion here, could be associated with remobilisation linked to intrusive activity (Moles et al, 2013). Data fields for feldspars from SET Late Caledonian granites (from both Britain and Ireland, but including the Leinster granite itself) are plotted in Fig.…”

“…However atypical compositions with high Cu contents and Cu-sulphide inclusions have been recorded at Balwoges in south-west Donegal and in the Mourne Mountains (Chapman et al, 2000a;Moles et al, 2013). Such copper bearing gold alloys are typically associated with non-orogenic styles of mineralisation and have been considered characteristic of magmatic associations (for example relating to porphyry and intrusion related gold; Chapman et al, 2011), but they can also be associated with zonation within an orogenic hydrothermal mineralising system or with remobilisation of orogenic mineralisation by later intrusive activity (Moles et al, 2013).…”

“…Balwoges), or indirectly (i.e. the Mournes Mountains) through remobilisation of preexisting orogenic mineralisation (see Moles et al, 2013). In relation to the Mourne Mountains placer gold, the radiogenic Pb is likely to have been derived from the Lower Palaeozoic sediments of the Southern Uplands which were derived in part from the more radiogenic SET crust (Hutton and Murphy, 1987;Murphy, 1987).…”

A new indicator mineral methodology based on a generic Bi-Pb-Te-S mineral inclusion signature in detrital gold from porphyry and low/intermediate sulfidation epithermal environments in Yukon Territory, Canada

“…Ternary diagrams constructed to show Au-(Ag × 10) and (Cu × 100) depicted largely mutually exclusive compositional fields for each population. However, these compositional fields relate only to magmatic-hydrothermal systems and cannot be used to differentiate between the low-Cu epithermal signature of epithermal gold and that of gold derived from orogenic systems (Moles et al 2013).…”