00:00:00
today's outcrop of the day is this
00:00:02
rhyolite porphyry with unusually large
00:00:05
fenucorists of quartz plagio Clays
00:00:08
cafels bar and biotite why is that so
00:00:12
good because that mineral set is the
00:00:15
perfect set of indicators for the level
00:00:17
of intensity of hydrothermal alteration
00:00:21
because each one of them goes to a
00:00:24
different mineral at a different stage
00:00:26
in that alteration sequence
00:00:29
biotites the first to go it gets a
00:00:32
little bit wrinkly cleavage first then
00:00:34
it gets converted to chlorite and
00:00:36
eventually the entire crystals will go
00:00:38
to muscovite plagioclase is next to go
00:00:42
and it usually converts to a mixture of
00:00:44
serousite quartz and carbonate
00:00:48
and then finally K felsbar converts to a
00:00:51
similar mixture and it usually goes
00:00:53
along with the Matrix as well
00:00:56
quartz crystals survive the entire
00:00:58
process because quartz is part of the
00:01:01
alteration assemblage so that mineral is
00:01:04
stable with respect to the hydrothermal
00:01:06
fluid in this case there's some extra
00:01:08
Clues because the surface has this
00:01:11
really pitted texture that's because
00:01:14
there used to be some big feldspar Fanta
00:01:16
crisps in here they've been altered to
00:01:18
sarasite and then weathered out to leave
00:01:21
these oblong-shaped holes
00:01:23
but the quartz and a crests are
00:01:26
unaffected by the alteration so they
00:01:29
stand proud of the surface and if I
00:01:31
break this open
00:01:37
and if I tilt this around in the sun you
00:01:41
should be able to see the sparkling
00:01:43
serousite grains here in what used to be
00:01:46
a large phenicrest of feldspar
00:01:49
and over here if I tilt it around there
00:01:53
you should be able to see the brilliant
00:01:55
reflection of a hexagonal shape that's
00:01:58
used to be a biotite finacrest and is
00:02:01
now muscovite
00:02:03
and there's some quartz panicrests here
00:02:05
and here
00:02:07
that are completely unaltered because
00:02:09
quartz is part of the philic alteration
00:02:11
assemblage so the primary phenicrests
00:02:14
are unaltered this is the same rhyolite
00:02:17
pore free with silic alteration at level
00:02:19
two on my scale of naught to five
00:02:22
the pledge of glaze finish have been
00:02:24
altered to a mix of green sarasite
00:02:27
quartz and carbonate
00:02:29
the kfeld spars which you can see from
00:02:31
that bright cleavage reflection have
00:02:33
survived almost unharmed with a little
00:02:36
water on the sample you can see the
00:02:38
altered plagio Clay is a little more
00:02:39
distinctly and the black patches are
00:02:42
chloride after biotite partially
00:02:44
converted to muscovite
00:02:46
one useful trick when you're mapping
00:02:48
phyllicorderation in the field is the
00:02:51
sound it makes when you hit it with a
00:02:52
geopick
00:02:54
this piece here has level four filic
00:02:56
alteration and when I hit it
00:03:05
that dull knocking sound is quite
00:03:08
distinct from the typical pin you get
00:03:11
when you hit an unaltered Rock
00:03:14
and here you can see the philic
00:03:16
alteration all of the big feldspar
00:03:18
phenicrysts have gone to Sarah site and
00:03:21
the quartz finacrysts have survived the
00:03:24
alteration this is the same porphyry
00:03:26
with weak propolitic alteration and you
00:03:29
can see the surface is much smoother
00:03:31
without those pits from the weathered
00:03:33
out feldspars and if I hit it
00:03:40
hear that more of a ping from the hammer
00:03:43
that's because the feldspars are still
00:03:45
pretty much intact and on this surface
00:03:48
here you can see the feldspars and the
00:03:50
quartz grains both sticking proud of the
00:03:53
surface while the Matrix is now the
00:03:56
softest thing and in fact only the mafic
00:03:59
minerals the biotites are altered to
00:04:02
chlorite so they are a little bit
00:04:04
recessive but the feldspar as you can
00:04:07
see still almost perfect and in fact
00:04:10
still showing good cleavage and on this
00:04:13
broken piece here you can see this
00:04:16
feldspar here that's a big k-fels Bar
00:04:18
Crystal and you can see it's twinned and
00:04:21
half of it there is reflecting really
00:04:23
good sunlight and that's because it's
00:04:27
almost unaltered kfeld's bar is a little
00:04:30
more resistant to alteration than
00:04:32
plagioclase and it tends to be the last
00:04:34
feldspar to survive so in propolitical
00:04:38
duration it's almost unaffected
00:04:41
that you can see a biotite there now
00:04:43
it's a gray green color that's because
00:04:46
it's all altered to chlorite this is a
00:04:48
finer grain version of the Paw free but
00:04:50
it has the same set of fantacrest
00:04:52
minerals and it's a perfect example of
00:04:55
propolitical oration at level two just
00:04:59
about here
00:05:01
if I tilt it around you can see the
00:05:04
reflections off a nice hexagonal
00:05:06
euhedral Crystal of what used to be
00:05:09
biotite and it's now gone to chloride
00:05:12
and you can tell it's chlorite because
00:05:14
it's a gray green color and the surface
00:05:17
is slightly fuzzy it's not a nice shiny
00:05:20
clean reflection like biotite it's kind
00:05:23
of fuzzy and that's what happens when
00:05:25
you convert a biotite crystal to
00:05:27
chlorite
00:05:29
and just next to it
00:05:32
over here if I tilt that around
00:05:35
you can see there's a feldspar Fender
00:05:38
Crest it still has a really good clean
00:05:41
bright reflection
00:05:42
and that's what happens in propolitical
00:05:44
Alteration the mafic minerals
00:05:46
particularly biotite are the first to
00:05:49
get altered to chlorite and the
00:05:52
feldspars generally survive until things
00:05:54
get a bit more intense
00:05:56
now if you see that feldspar finna crisp
00:05:59
there seems to have a bit of green core
00:06:01
in it and so do a lot of the other ones
00:06:04
like this one over here and this one
00:06:05
here and particularly that one there
00:06:08
although it's a bit small
00:06:10
that green core is epidote and what
00:06:14
happens is that feldspar Fanta crisps
00:06:16
tend to grow with a calcic core and a
00:06:19
sodic rim and the available calcium in
00:06:21
the core
00:06:22
tends to let epidote grow there first
00:06:25
because epidote is a calcium mineral so
00:06:28
you find these green cores in the middle
00:06:31
of the feldspar finnicris so that
00:06:34
combination of epidote in the Fels bars
00:06:36
and chloride in the biotites is a
00:06:40
classic propolitic assemblage
00:06:43
another good thing about this porphry is
00:06:45
that it illustrates a good way to tell
00:06:47
the difference between a crystal tooth a
00:06:50
lava or an intrusive paw free
00:06:53
my general rule of thumb for making that
00:06:55
distinction is to look at the proportion
00:06:57
of the finacres that are broken
00:07:00
if lots of them are broken up into
00:07:01
fragments then it's most likely a tooth
00:07:04
if most of them are still good Crystal
00:07:06
shapes then it's most likely a lava or
00:07:09
an intrusive pore free
00:07:12
in this case you can see the feldspar
00:07:14
crystals were absolutely perfect because
00:07:16
they've been altered and plucked out by
00:07:19
weathering and you can see the nice
00:07:21
smooth Crystal faces in there
00:07:24
and this one you can even see the
00:07:25
twinning shape of a k-feldspar
00:07:28
another really nice smooth Crystal face
00:07:31
there so they're not broken and the
00:07:34
quartz crystals also a nice equine
00:07:37
shapes not shards or broken fragments so
00:07:41
this is almost certainly an intrusive
00:07:43
porphyry not a tough
00:07:46
when I'm mapping mineralization systems
00:07:49
I always try to map the intensity of
00:07:51
hydrothermal alteration
00:07:53
because hydrothermal alteration is the
00:07:56
footprints of the hydrothermal fluid
00:07:59
the more alteration the more fluid
00:08:01
you've had through there and that means
00:08:04
the more likely you are to have had
00:08:06
mineralization deposited
00:08:09
so it's really good to find rocks that
00:08:11
have an indicator of how intense that
00:08:14
alteration is
00:08:16
so if you're looking for that there's no
00:08:18
better indicator than a nice coarse
00:08:20
grain porphry with a mixture of
00:08:22
kefelsbar plagioclase and biotite
00:08:25
fenucorists
