『Abstract
The style of gold mineralization (e.g., mesothermal, porphyry,
intrusion related, high-sulphidation epithermal, low sulphidation
epithermal) is normally established at an early stage in the gold
exploration process from the study of the mineralogical and lithological
examination of outcrop, but this approach is problematic in areas
of poor exposure. However, placer gold is frequently present in
local drainages, and may be characterized according to the alloy
composition and the suite of mineral inclusions. In the temperate
areas which are the subject of this study, this microchemical
signature of the cores of placer gold grains reflects that of
gold from the hypogene source. Thus, informed speculation on the
nature of the source bedrock mineralization is possible through
comparison of the microchemical signature of placer grains with
the generic characteristics of gold from different styles of mineralization.
A holistic approach involving consideration of the local geology
and microchemical signatures of populations of placer gold grains
can be used to infer the presence of epithermal gold mineralization.
epithermal gold deposits are characterized by a wide range of
Ag values in the electrum grains and a corresponding complex mineralogy
of the opaque inclusion suite. Having established that the placer
grains are likely to be derived from epithermal mineralization,
the sulphidation style may be inferred through comparison of the
sulphide mineral inclusion suite with the compositional fields
which define the sulphidation state of the mineralization. In
addition, suites of telluride minerals can be used to constrain
the possible sulphidation state of the mineralization using log
f Te2/log f S2
diagrams. Further useful information is available from the study
of non-opaque inclusions which can be used to indicate sulphidation
state and the type of wall rock alteration associated with the
mineralization.
Two case studies have been undertaken to illustrate the value
of this approach. The Eureka Creek locality in Yukon Territory,
Canada lies in an unglaciated region where bedrock is covered
by mainly residual overburden derived from local bedrock sources.
At Borland Glen in Scotland there have been several periods of
glaciation which have influenced geomorphology in particular by
forming deep thicknesses of till and rare bedrock exposures. In
neither case has outcropping mineralization been identified. This
study describes how consideration of compositional and morphological
data of placer gold together with an understanding of the geological
setting has indicated that both placers are derived from low-sulphidation
epithermal mineralization.
The Eureka Creek locality forms part of a goldfield with a recorded
production of roughly 150,000 oz. The microchemical signature
of the placer gold is consistent with the signature of the low-sulphidation
epithermal gold and consequently distinct from the major gold
mineralization found to the north in the Klondike District. Interpretation
of the opaque mineral inclusion suite using log f S2-temperature diagrams indicates a temperature
of formation near 200℃.
A similar approach has shown gold from a highly anomalous placer
occurrence in Borland Glen, Ochill Hills, Scotland to be of local
origin and most probably derived from an episode of low-sulphidation
epithermal mineralization unique to the region. Three phases of
mineralization corresponding to different mineral inclusion/gold
alloy compositions have been identified, only one of which has
the potential to be of economic importance. The sequence of deposition
of the three phases has been established. The mineralization from
which the placers were derived is hosted at least in part by propylitically
altered wall rock. The areal extent of the surface expression
of the most significant gold type has been investigated through
identification of gold grains which exhibit a diagnostic internal
texture of different phases of alloy compositions each associated
with specific inclusion assemblages. This approach has proved
an efficient tool in demarking a new target area within a region
where placer gold derived from sources of low economic potential
is also moderately abundant.
This study has highlighted the potential value of characterizing
the mineral inclusion suites of placer gold grains as part of
a wider array of analytical approaches including determining the
alloy compositions and morphological studies.
Keywords: Gold; Exploration; Epithermal; Placer; Microchemical
characterization』
1. Introduction
2. Considerations for the study of placer grains derived from
epithermal gold mineralization
2.1. Gold alloy composition
2.2. The opaque inclusion assemblage
2.3. The transparent inclusion assemblage
2.4. Other characteristics of placer gold grains
2.5. Recognition of an epithermal gold signature
2.5.1. Discrimination of epithermal and mesothermal gold
2.5.2. Discrimination between epithermal and authigenic gold
3. Experimental
4. Case studies
4.1. Case 1: Eureka Dome and Eureka Creek
4.1.2. Gold alloy composition
4.1.3. Interpretation of the inclusion assemblages
4.1.4. Comparison with ore deposits of related mineralogy
4.1.5. Summary of information gained from Eureka Creek gold
4.2. Case 2: the Borland Glen prospect, Ochil Hills Perthshire,
Scotland
4.2.1. Characterization of the style of mineralization in Borland
Glen
4.2.2. Relationship of placer gold morphology to microchemical
signature
4.2.3. Identification of phases of Au mineralization in proximal
gold using alloy composition and inclusion assemblage
4.2.4. Interpretation of inclusion assemblages
4.2.5. Use of diagnostic gold grain textures to establish the
geographical range of Discovery Bench mineralization
4.2.6. Summary of information grained from microchemical characterization
of Discovery Bench Gold
5. General discussion
6. Conclusions
Acknowledgements
Appendix
A.1. Microchemical data for gold grains
A.2. Microchemical data for grains from the Ochil Hills
A.3. Microchemical data for grains from localities in the Dalradian
References