1. Introduction
The liberation of mineral particles from host lithologies by weathering generates a mineralogical and geochemical footprint whose extent is governed by transport in the prevailing surficial environment [
1,
2]. Spatial variations in chemical response or mineral abundance can act as vectors to the in situ source [
3], and the approach can be particularly powerful when based on specific mineral-ore deposit style relationships, e.g., kimberlite [
4], magmatic Ni–Cu–PGE e.g., [
1,
5], and gold [
6,
7]. Particular attention has been given to specific erosional products of copper porphyry mineralization, e.g., magnetite [
8,
9], apatite [
10,
11,
12], and tourmaline [
13]. These minerals are useful because subtle differences in their chemical composition may be linked to specific settings within a mineralized system, and their physical durability and chemical stability ensure longevity in the surficial environment, such that the geochemical/mineralogical anomaly is not ephemeral.
The presence of detrital gold in surficial sediments is generally accepted as the best evidence for a gold-bearing source, and consideration of the gold morphology and geomorphological process may permit speculation on the likely location of the source [
14]. While the characterization of dispersion trains of gold particles in till has successfully been employed as vectors to source [
6,
15], the composition of gold particles and implications for source type have not found routine application in exploration. In contrast, an increasing number of academic studies have sought to utilize the compositional signatures of placer gold to either illuminate the evolution of economically important placers or speculate on the nature of the source(s). Several placer mining districts in Russia have been the focus of robust studies [
16,
17,
18,
19,
20,
21], in which distinct signatures of sub-populations of gold have been identified through the study of large numbers of gold particles. Similar approaches have been adopted in remote areas of geological complexity, e.g., South America [
22] and Northern Pakistan [
23,
24]. If gold compositional studies are to find regular application in exploration projects, readily available compositional templates describing the generic features of gold from different deposit types are essential, but these are rarely generated in studies where the focus is a specific placer. In contrast, other studies have sought to identify generic compositional signatures that can subsequently be applied more widely [
25,
26,
27,
28], and while some clear diagnostic signatures for gold from different deposit types emerged, there are two main knowledge gaps. First, as more data are collected, the potential compositional ranges in gold corresponding to specific deposits are extended; e.g., even large studies of gold particles from different magmatic hydrothermal systems [
29] subsequently proved inadequate as compositional templates [
30]. Similarly, early attempts to ascribe distinguishing features to gold from a wider range of deposit types [
31] were completely revised [
32] after a further period in which several relevant studies were published. Second, our understanding of the compositional characteristics and range of gold from some specific deposit types is underrepresented, either as a consequence of a lack of focused studies or because the small particle size of gold commonly associated with some deposit types precludes collection by standard field techniques.
Placer gold is widespread in British Columbia, Canada (BC), as evidenced by the large amount of historical mining activity [
33]. However, in many placer gold-producing areas, the in situ source(s) remain undiscovered. Surface exposure is commonly obscured by surficial deposits, and exploration approaches using indicator minerals have found favor [
34]. Parallel studies in Yukon, Canada, have demonstrated the potential for placer gold compositions to establish source type and hence contribute to an improved understanding of regional metallogeny [
28,
35]. The presence of detrital gold, however, is not confined to sites of current or historic placer working, and exploration activities on all scales could collect gold particles and benefit from the interpretation of their compositional signature.
The Cordilleran Orogen that underlies BC is a complex assemblage of terranes that vary considerably in terms of age, composition, and tectonic history. The summary presented here is based on two references that address both tectonic history and metallogeny [
36,
37]. The region includes “pericratonic” terranes that display a largely continental affinity, some of which (e.g., Yukon–Tanana and Kootenay terranes) are thought to have originally been part of the Northwestern Laurentian margin, as well as continental margin arc terranes (e.g., Stikine and Quesnel terranes), and terranes such as the Cache Creek and Slide Mountain terranes that represent rock units formed in a mainly oceanic environment. These various terranes were assembled into their current configuration through a series of tectonic events that ranged in age from the latest Paleozoic through Early Tertiary time and included both collisions of exotic terranes against the Laurentian margin and each other as well as tectonic shuffling along major, late, dextral (and minor sinistral), crustal-scale strike-slip faults. Individual terranes comprise varying proportions of volcanic and sedimentary rocks and commonly include intrusive rock units that are coeval and comagmatic with the volcanic rocks. In addition, late and post-accretion intrusions are present within most terranes and locally crosscut many of the terrane boundaries. The metamorphic grade that has affected many of the terranes is generally low to moderate.
In addition to the geological complexity of the BC Cordillera, this region also displays a wide range of mineral deposit styles, including many variations on intrusion-related mineralization (porphyry, skarn, epithermal), as well as volcanogenic massive sulphide (VMS) and sedimentary exhalative (SEDEX) deposits and base and precious metal carbonate replacement deposits. The location of the localities mentioned in the text is provided in
Figure 1. Gold (and silver) represent the major economic commodities in many of the deposit types in BC, including several subtypes of mainly late-tectonic orogenic gold deposits (e.g., Cariboo, Bralorne, Cassiar, Atlin, and Zeballos camps); epithermal vein deposits (Blackdome, Silbak Premier, Brucejack); and some rare gold-rich VMS deposits (Eskay Creek) [
38]. Gold is also an important by-product in a wide variety of other deposit types in BC, including Cu–Au skarns (Hedley), Cu–Au alkalic porphyry deposits (Mt. Milligan, Mt. Polley, Copper Mountain, Galore Creek), and some calc-alkaline porphyry deposits (e.g., Red Chris, Kemess, Highland Valley). Gold is present in at least trace amounts in a very large proportion of mineral deposit types in BC, highlighting its potential to be used as a discriminant between deposit styles.
Gold particles exhibit compositional and microtextural features that are a consequence of their genesis and subsequent residence in their hypogene setting. These features persist post-liberation and erosion and have utility in interpreting the origins of detrital gold particles. This subject has been discussed in detail previously [
39,
40], and a brief overview is presented here.
Gold is almost always an alloy of Au and Ag, although other minor metals such as Cu and Hg may be detectable by electron microprobe (EMP) analysis. Some gold particles are compositionally homogeneous, but many are heterogeneous due to the presence of microfabrics caused by alloys of different compositions (usually variations in Ag) and/or inclusions of other minerals [
32]. The origins of various microfabrics have been classified according to the time of formation with respect to the initial mineralizing event using a dual approach of compositional and crystallographic study [
32]. In this way, it has been possible to ensure that the analysis of gold particles generates data pertaining only to the ore-forming stage rather than that resulting from subsequent modifications in residence within either the hypogene or surficial environments.
Differences between the mineralogy of different types of gold mineralization (e.g., low- and high-sulphidation epithermal deposits, calc-alkalic porphyries, and gold from orogenic deposits) are well known, and these are reflected in the suite of mineral inclusions observed in polished sections of gold particles from these different deposit types [
32]. Furthermore, the physico-chemical environment of gold precipitation influences the Au–Ag ratio of the resulting alloy [
41], together with the concentrations of other minor metals such as Cu, Hg, and Pd [
32]. Consequently, the broad controls on ore fluid and mineralization environment (ore deposit type) have a major influence on the gold signature, with further variation arising as a consequence of specific conditions that influence alloy composition. In addition, the temporal and spatial evolution of a hydrothermal mineralizing event can generate a compositional range between gold particles within the overall population, and therefore a sufficient amount must be analyzed to generate a robust compositional signature of gold from a single mineralizing event. The term ‘sample population’ is used to denote a population of gold particles collected from a specific site. In the overwhelming majority of cases, a sample population exhibits a compositional range, which is effectively a proxy for the stability of the mineralizing environment or an indication of multiple mineralizing episodes.
Gold particle studies may consider sample populations collected either from in situ or placer sources. In situ mineralization may comprise multiple episodes that may or may not have been emplaced under similar conditions. Thus, it is possible that gold from a single in situ locality may exhibit more than one signature [
42,
43]. Erosion and transport of gold from a single locality generate detrital gold whose composition is faithful to that of the source, but populations of placer gold may contain particles from multiple sources. In order to establish the nature of the contributing gold types, sufficient particles must be available, and these must exhibit sufficient diagnostic criteria to permit interpretation. Despite these challenges, various recent studies have identified groups of compositional characteristics that are exhibited by gold from specific deposit types. Examples include the Pd–Hg inclusions (and alloy) signature of gold from alkalic Cu–Au porphyry systems in BC [
26] and the Bi–Te–Pb–S inclusion signature of gold formed in calc-alkalic porphyry systems in Yukon [
27]. Gold signatures from mineralized orogenic systems are characterized by a broader array of features within which particular inclusion associations commonly occur, namely a simple base metal signature associated with sulphides ± (sulpharsenides or tellurides) ± sulphosalts [
28].
Much of the early pioneering work on gold composition was carried out in BC between 1985 and 1993 [
44,
45,
46,
47,
48]. These studies focused on the relationships between the alloy compositions of gold from known lode sources in Southern and central BC and those of the surrounding placers. The origins of gold in the Fraser River were discussed in terms of potential contributions from the Cariboo Gold District (CGD) and Bridge River area, and a similar approach was applied to the Coquihalla drainage. Two main compositional groups were identified on the basis of Cu and Hg levels in the Au–Ag alloy. The high Cu group was attributed to gold associated with ultrabasic lithologies, whereas the presence of Hg was interpreted as indicative of orogenic gold sources. Within these groups, there were compositional overlaps that could not be resolved through the study of alloy compositions by EMP alone. Nevertheless, examination of microfabrics within the high-Cu population greatly refined the characterization of gold with an ultrabasic association [
49].
A study of over 2000 gold particles from placer and lode settings in the CGD [
43] augmented the alloy composition data previously reported [
47] with both inclusion data for those samples and new material collected for the study. Comparison of mineralogical descriptions of lode occurrences with mineral suites helped refine the classification of gold types in the Wells–Barkerville area, in particular distinguishing between a low-Ag type associated with cosalite inclusions occurring around Wells and a more Ag-rich regional type with an inclusion suite dominated by pyrite and arsenopyrite. The correlation of the Ag contents of these gold types with bulk fineness data from historical placer mining activities permitted the evaluation of the most economically important gold types.
Gold compositional studies in the Northern Cordillera in BC, Yukon, and Alaska have also established generic compositional signatures associated with gold from specific mineralizing environments. Gold from alkalic Cu–Au porphyries in BC yields a Pd–Hg signature [
26], while similar work in Yukon showed that gold from calc-alkalic systems shows a Bi–Pb–Te–S signature in the inclusion suite [
27]. A perceived disadvantage to this approach was the number of gold particles required to establish the signature, and consequently [
50] investigated whether the larger range of detectable elements afforded by the use of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) could generate a consistent signature from fewer gold particles. The aim was to evaluate whether the small number of gold particles generated in stream sediment surveys could find utility in an indicator mineral context. This work developed during the time that the large degree of heterogeneity of trace and ultra-trace elements within gold was becoming clear, and it is now apparent that analysis of only a few particles could produce highly unrepresentative results [
40].
The large numbers of gold particles from BC analyzed and inspected prior to the present study revealed internal microfabrics and alloy compositions that were entirely compatible with the detrital model of placer gold, i.e., a model in which eroded gold particles remain largely intact within fluvial sediments. Nevertheless, it is important to note that other workers have reached different conclusions through consideration and interpretation of different information. The apparent discrepancies between both the particle size and bulk fineness of gold in lodes and placers in the Cariboo Gold District have been cited as evidence for gold nugget growth in the supergene environment [
51,
52,
53]. These assertions resonate with the widely held perception that gold is chemically active in surficial environments to the extent that placer gold may be compositionally distinct from that recovered from the proximal lodes owing to an entirely different genesis. The argument for gold growth in the placer environment has also been advocated more recently in a number of papers, e.g., [
54], that propose that the commonly observed micron-scale precipitation of gold onto pre-existing particles through biogenic activity is an ongoing process that results in particle size increases. If gold modification in the surficial environment is widespread and bulk compositions are indeed modified, the potential use of gold as an indicator mineral would be fatally undermined. The subject has recently been discussed at length in a study [
39] that considered over 40,000 sections of gold particles from localities worldwide and concluded:
- i.
Gold particles can increase their mass in specific supergene (not fluvial) settings of circumneutral groundwaters where both Au and Ag are transported as thiosulphate complexes.
- ii.
Hypogene gold exhibits specific microfabrics and inclusion assemblages, and the identification of these features in placer gold particles confirms a detrital origin.
- iii.
Gold from the overwhelming majority of placer localities globally exhibits such features, whereas microfabrics consistent with a process of nugget growth have not been recorded in any of the 40,000 placer particles studied.
On the basis of the scale and detail encompassed by this study, we assert that the internal compositions of placer gold particles are faithful to those within the lode source and therefore represent a platform on which to develop a robust indicator methodology.
In this study, we have demonstrated the close correlation between mineral inclusion suites and the mineral assemblages associated with gold in different ore deposit types. In tandem with substantial new alloy and inclusion data describing gold from many localities, we have developed compositional templates for gold from orogenic, low-sulphidation epithermal, and alkalic porphyry settings. There have been substantial advances in characterizing gold from other magmatic hydrothermal and orthomagmatic environments, facilitated both by the data set at our disposal and petrographic studies of auriferous mineralization. The Synthesis of these data sets has generated deposit-specific compositional templates against which ‘unknowns’ may be compared. In this way, it has been possible to identify the type(s) of source mineralization for some gold localities where this was previously unknown.
2. Materials and Methods
The major objective of producing a database of gold compositions depends on access to sufficient populations of gold particles representing both geological and geographical spread within the province. This project has taken advantage of gold collections from both the University of British Columbia (UBC) and the University of Leeds (UoL), and the geographical spread of gold sample populations examined in the study is shown in
Figure 1.
The database describing gold from localities where the deposit type is known comprises 11,520 particles from 133 localities. The UBC collections comprise placer and hypogene gold collected over several years in the 1980s and 1990s. Polished sections of both placer gold populations from specific localities and Au-bearing ore samples were analyzed by EMP at UBC during this period. The initial database was augmented in two ways during the present study. Firstly, the inclusion suites present in each population of gold particles were established by visual examination on the scanning electron microscope (SEM; see below). The incidence of inclusions varies considerably [
27,
32], and in many cases, the number recorded in sample populations was insufficient to underpin rigorous classification. Secondly, the UBC collections contained additional particles from numerous localities, and these were mounted and analyzed in the present study to improve the quality of the final data set. The remit of the present project to generate a compositional template against which other gold samples may be compared requires gold samples whose deposit type provenance is unambiguous. Samples of placer and lode gold in the UoL collections relate to either locality-specific studies (Cariboo Gold District: [
43]; Atlin, [
55]) or deposit-type-specific studies (gold from alkalic porphyry systems, [
26]). Lode samples are vital in this regard, but in many other cases, the source type of gold placer samples can be established with near certainty, particularly where the signature of the placer gold corresponds to that of proximal lode gold [
26,
43]. In other cases, the deposit type from which placer gold is derived remains unclear, and such sample populations cannot be used to generate compositional templates. Similarly, placer samples from some (commonly large) drainages may contain gold particles from two or more different deposit types. Around 30% of the gold particles in the UBC collections fall into this category (e.g., gold from the Fraser and Coquihalla river main valleys), because at the time of collection, the drivers for gold collection were to investigate variation in gold signatures between localities rather than to identify compositional signatures for gold from specific deposit types. For the purposes of the present study, the data set has been divided into sample populations where the source deposit type can be ascribed with confidence and others where, although the source deposit type is unclear, there is sufficient compositional information to establish a compositional signature. A full table showing details of the localities for which deposit types may be confidently ascribed is presented in
Appendix A, and the data are summarized in
Table 1. The data set comprises 11,840 gold particles from 160 localities.
Gold from orogenic settings has the strongest representation in the data set, and this is an inevitable consequence of the amenability of orogenic gold to form placers. It is also clear that several deposit types (high-sulphidation epithermal, VMS, intrusion-related gold, and skarns) are poorly represented. In some cases, it has been possible to partially alleviate this issue by studying samples of polished blocks of ore, where the association of gold with coeval minerals can be used to predict elements of the inclusion signature. In addition, there is a bias in the whole data set according to previous studies in the Province that targeted gold from the Cariboo Gold District [
43] and the sample suites describing gold from alkalic porphyry deposits [
26].
The suite of samples for which provenance is unknown comprises a total of 2916 gold particles from 41 localities, and details are provided in
Appendix B. However, only 8 of these yielded a sufficiently large inclusion suite to permit comparison with deposit-specific compositional templates (
Table 2). In addition, sample populations from Bonaparte Mine, Granite Ck, Lilloet, Peers Ck, and Fairless Ck exhibited compositional characteristics that could be informative, and these are mentioned in the text.
Polished blocks were inspected using the secondary electron (SE) and back scattered electron (BSE) facilities of a Quanta 650 FEG scanning electron microscope (SEM). Liberated or detrital gold particles are characterized through a combination of alloy profiles (determined by EMP) and inclusion assemblages (determined by visual inspection in both (SE) and (BSE) modes). Both approaches require particles to be sectioned and polished. Alloy analyses of most of the UBC sample suite were carried out at UBC, and all other analyses were carried out at UoL. The compatibility of results from the two analytical facilities was previously established by duplicate analyses of populations of gold particles from localities in Yukon [
42]. All analysis regimes included Au, Ag, Cu, and Hg, but the early studies did not include Pd. An overview of the analytical conditions used for gold analysis for the full element range has been described previously [
32]. All analyses quoted are mass%.
A summary of the workflow from gold collection to sample preparation is provided in
Figure 2. The first stage in the sample characterization was a visual inspection of all gold particle sections. These studies were carried out at UoL using a SEM. Mineral inclusions were identified and chemical analyses generated using the energy dispersive spectrometer (EDS) facility. Mineral speciation was interpreted by comparing the spectra with those of reference minerals. In some cases, a small degree of substitution was observed (e.g., Cu in acanthite or Sb in galena). In these cases, a record was generated that influenced the scoring system used in the generation of radar diagrams, as described previously [
28].
5. Conclusions
This large-scale regional study has greatly increased our understanding of the range of compositional signatures of gold within the complex geological settings in British Columbia. Although the geochemistry and mineralogy of gold grains in the region are certainly far from simple, gold alloy compositions of populations of gold particles together with inclusion suites of opaque minerals have in many cases proven capable of discriminating between gold derived from different types of source deposits. In some cases, the project outcomes have confirmed those of previous work, but new insights have also been generated, principally by combining studies of in situ gold-bearing mineralization with compositional studies of detrital gold particles.
It is important to consider the full range of compositional data available when aiming to identify deposit-type signatures. In this study, the Ag content of the Au–Ag alloy has not proved useful in the majority of cases, as there is substantial overlap in the compositional ranges of gold from different deposit types and between samples from different localities of the same deposit type. Nevertheless, extreme values may prove informative. Where detectable, concentrations of minor metals (Cu, Hg, and Pd) can be useful as strong indicators of gold genesis, but in many cases they are below detection by EMP. The study of the relationship between the mineralogy of the inclusion suite in detrital gold particles and the ore mineralogy itself has proved far more illuminating.
Detailed petrographic studies of samples of gold-bearing ore from various deposit types have confirmed the relationship between ore mineralogy and the compositional signatures of gold particles in their erosional products. The range of well-constrained compositional templates for gold from orogenic and low-sulphidation epithermal systems shows strong similarities with mineralogical associations of gold with other coeval minerals within ore samples of those deposit types. This outcome supports two important claims about gold compositional studies. The first confirms that the inclusion suites of detrital gold from locations where the source is unknown can be used to elucidate the type of that source mineralization, and comparison of deposit-specific inclusion suites with those of gold from other localities in the Canadian Cordillera in neighboring Yukon confirms the generic nature of the signatures. The second has been brought into sharp focus in the present study and shows that an understanding of the mineral associations of gold in a specific deposit type may be used to predict the inclusion suite of any associated detrital gold liberated by erosional processes. Consequently, although there is limited inclusion data describing mineral suites in detrital gold from high-sulphidation, skarn, and intrusion-related deposits, their signature is predictable. Where small inclusion suites are available for comparison, they support this hypothesis.
For some localities in BC where the source of detrital gold is unknown, it has been possible to apply compositional templates to elucidate the source type. The success of this approach depends entirely on the quality of the sample population of the unknown sample in terms of the number of particles available and the degree to which they are representative of the whole population. Donated samples may not fulfill either of these criteria, and projects such as this generally demand dedicated sampling campaigns. This is especially important in regions such as BC, where gold from different source types may be present in a single drainage system, with the result that the detrital gold inventory may contain gold with different compositional signatures. An understanding of the various compositional ranges of gold from different deposit types permits discrimination between sub-populations, an accurate interpretation of local gold metallogeny, and an aid to the design of focused exploration campaigns.
The outcomes of the project can already underpin the examination of new sample suites where the source deposit types are unclear. At the very least, it is possible to clearly discriminate between gold from orogenic and magmatic hydrothermal systems as well as that associated with the ultramafic rocks present at several different localities. Further study is required to gain a better generic compositional template for gold from some deposit types and to identify the compositional nuances between gold from deposit types formed by magmatic hydrothermal systems.