I don't think anyone can say "the gold is there" until it's there.
This high grade vein is likely discontinuous and spotty. Why? Not
just because it is high grade but because most gold veins
are discontinous and spotty. What you need for gold is structure. what is that? Well when rock folds it may
produce tension fractures at an angle to the beds and
shears at a lower angle to the beds.
These dilational structures formed when rock cracks under
pressure and deformation are the locus or focus of
injection of the silica-carbonate vein systems.
The other thing you need is the heat source system for the vein
fluid circulation and source of the silica and gold. This is
classically thought of as the granite pluton or felsic porphyry.
You may have impressive gold vein systems with no easily discernible
heat source rock but signs of subaqeous volcanism are frequently
there. This heat and hot hydrothermal silica-carbonate fluid from the
adjacent volcanoes substantially alter the rocks and veins within the
rock to a temperature every near the melting point of silica. This
creates a fingerprint temperature that leaves behind distinctive
minerals that are fortmational at a particular temperature and
pressure. This is called greenschist facies metamorphism for
its distinctive colour and association with linearly sheared rocks.
Thus the metamorphic grade of the host rock and particularly
the vein, determined by the contained vein contact minerals tells us
its probable abundance of gold.
So what to look for?
1. deformation and structure (regional mapping, detail mapping,
magnetics, mine correlation)
2. alteration minerals and colouration, especially increasing as
vein is approached (mapping, whole rock analysis, microscopy)
3. Subaqueous volcanism (underwater volcanoes)( mapping,)
4. Mafic volcanics contacting meta-sediments or tuffaceus rocks
(mapping)
5. Silica flooding (lots of quartz in veins)
6. Pervasive carbonitization of rocks increasing as vein structures
are approached. (rocks fizz in shear planes when weak hydrochloric
acid is applied)
7. "Felsic" quartz-felspar porphyries, highly bleached or altered
especially where they are conformable with the other rocks i.e.
laid down at the same time.
8. Gold contained in soils of high grade and in vegetation too.
9. Gold in association with iron sulphides and chalcopyrite
10. Gold in shears and veins of high grade.
11. Minerals that associate or combine with gold in quartz veins,
i.e. tellurium, galena, pyrite, silver, ankerite, micas of all sorts,
chalcopyrite, grunerite, tourmaline.
12. Clay minerals at surface of veins that form when other minerals
break down.. smectite, kaolinite.
13. Formations of rock that are formed at the same pressure and
temperature of gold's melting such as talc and argillite.
14. free grains of gold in the residual non glacial soil close to
quartz veins.
15. cross cutting dykes of aplite that are brick red or other light
colours of felsic composition and extremely fine grained. (related
to the felsic porphyries.)
16. cross cutting diabase or lamprphyre dykes..part of the mafic
volcanic system.
17. distinctive highly coloured and large grained porphyries that
have non euhedral crystals... colours may be light beige to bright
red to white to dark blue.. porphyries may also be evenly and fine
grained..
18. large nearby headframes where workers mine gold
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