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<h1><a href="#">Diamond Volcanology</a></h1>
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<br>
<h1>Diamond Volcanology</h1>
<br>
<hr>
<br>
<p style="text-align:center";>
<a href="#Introduction">Introduction</a><br>
<a href="#Argyle">Evidence for the Phreatomagmatic Origin of the Argyle Pipe</a><br>
<a href="#ArgyleSum">Summary of Observations at Argyle</a><br>
<a href="2_Ellendale">Ellendale Lamproite Field</a><br>
<a href="3_WesternDistrict.html">Volcanic Features of the Western District Plains of Victoria</a></p>
<h2><a name="Introduction">Introduction</a></h2>
<br>
<p>The subject of diamond geology and volcanology is really challenging - on
the one side there is the classic De Beers approach of fluidisation and on the
other is the phreato-magmatists camp initially led by Dr Volker Lorenz. Both
have their applications and I think in reality both styles exist in nature and
may occur in the same pipe but at differing times during intrusive events. </p>
<p>Most of my experience relates to the olivine and leucite lamproites of the
West and East Kimberley of Western Australia (see also my pages on
<a href="argyle.htm">Argyle</a> and
<a href="ellendale.htm">Ellendale</a>). The Argyle pipe is an olivine lamproite diatreme with an age of
about 1178 Ma and I will present information below to support the
phreatomagmatic origin of the pipe. The West Kimberley lamproites are all
relatively new (in Australian geology terms!), with ages of about 20 - 25 Ma,
and also show many features illustrating the phreatomagmatic nature of these
pipes (I will add a section on Ellendale in the future).</p>
<p>I spent a number of years while working at the Argyle mine looking at the
geology and petrography of the pipe. What struck me was the similarity with the
phreatomagmatic deposits of basaltic volcanics that had been brought to our
attention by Dr Volker Lorenz. Dr Volker Lorenz was consulting for Argyle at the
time and brought a wealth of information on basaltic volcanics and the
phreatomagmatic process. I visited the Tertiary volcanic deposits of Western
Victoria in south eastern Australia, where exposures were examined and
photographed. (I hope to be able to put some of these photos on this page in the
future – the exposures are excellent).</p>
<p>These deposits in Western Victoria were all terrestrial volcanics that had
been erupted on to dry land (in general). Argyle and these deposits show
similarities and differences. Phreatomagmatic eruptions occur when hot magma
(molten rock) comes into contact with water, either as groundwater, lakes or
oceans. The energy released when the water flashes into steam is enormous and
very deadly if you happen to be in the immediate area! The results on land are
usually big holes in the ground!</p>
<p>After the volcanic activity has subsided the craters tend to be filled with
reworked pyroclastics and crater lake sediments, these features are called maars.
For more info on these types of features see papers by Lorenz and others, for
example see Lorenz
1985 (<a href="Bibliography.html">(Bibliography)</a>) plus numerous other papers since then.</p>
<p>For a discussion of the Argyle pipe see a my paper by Boxer, Lorenz and
Smith entitled "The Geology and Volcanology of the Argyle (AK1) Lamproite
Diatreme, Western Australia" which was published by the Geological Society of
Australia as Special Publication No.14 in 1989. The Special Publication is
called Kimberlites and Related Rocks and Volume 1 is concerned with "Their
Composition, Occurrence, Origin and Emplacement" of diamondiferous rocks. These
are the proceedings of the Fourth International Kimberlite Conference (4thIKC)
that was held in Perth, WA in 1986.</p>
<p><a href="#top">Back to top</a></p>
<p><i>Anyway, how about some evidence?</i></p>
<h2><a name="Argyle">THE ARGYLE PIPE</a></h2>
<h3>Accretionary Lapilli</h3>
<img src="images/Argyle/Full_AccLapillii_Fig7.jpg"></p>
<p>The photograph above was taken
in the northern part of the Argyle diatreme and illustrates accretionary lapilli
in bedded sandy tuffs (quartzose lapilli ash tuffs). The accretionary lapilli
are classic in structure and are very similar to those found in other wet
volcanic environments.</p>
<img src="images/Argyle/Full_AccLapilli1.jpg">
<p>Two issues are of importance here, 1. the eruptions have
been wet, and 2. the eruptions have to have been subaerial at this time. The
photo below shows a close up of the accretionary lapilli seen in drill core.</p>
<h3>Drop Stones</h3>
<img src="images/Argyle/Full_AK1_Dropstone.jpg">
<p>The other feature, which proves the subaerial nature of the eruptions at
the time of formation of the tuffs in the northern part of the diatreme, is the
occurrence of drop stones. The photo above nicely shows a drop stone in sandy
tuffs from the northern part of the pipe. This photograph is courtesy of Chris
Smith (Rio Tinto). </p>
<h3>Internal Pipe Geology</h3>
<p>Meanwhile in the central and southern areas of the pipe, a different
picture is evident. I logged many kilometres of diamond drill core in an attempt
to unravel the internal structure of the pipe. A great deal of work was done but
correlation of the internal geology was very limited and only then with gross
textural groupings; bedded, ragged or blocky types of textures.</p>
<img src="images/Argyle/Full_Argyle_Bedded_STuffs_Shales.jpg">
<p>At the northern end of the pipe, there were exposures of sandy tuffs
overlain by fine grained sediments (see photo to left), which were interpreted to
be crater lake sediments. This would indicate that the northern end of the pipe
has exposed the highest levels in the diatreme.</p>
<h3>Water Escape Features</h3>
<p>In the northern, an upper parts of the pipe, sandstone dykes cross cut
the sandy tuffs. These are interpreted to be clastic dykes formed as water
escape features. They are well documented in sandstone sequences (see
<a href="Bibliography.html">Lowe 1975</a>). I do not know of any other descriptions of clastic dykes in a
volcanic diamond deposit. Other features that indicate water escape features
from water saturated volcanoclastics at Argyle include disturbance to bedding
and entrainment of fine grained ash. The photo on the left indicated the disturbance
of bedding in tuffaceous sandstone that occurs interbedded with the sandy tuffs
in the southern part of the Argyle pipe.</p>
<img src="images/Argyle/Full_Argyle_TuffSst.jpg">
<p>The chaotic sandy tuffs at Argyle, which form the majority of the pipe,
lack any coherent internal structure that can be traced for any significant
distance. The have the appearance of being mixed and may represent lahar type
material. They may simply be the result of intense mixing caused by the
eruptions in the vent and the abundance of water producing a tuffaceous soup.</p>
<img src="images/Argyle/Full_AK1_ChaoticTuff.jpg">
<h3>Juvenile Clasts</h3>
<p>The juvenile clasts in the sandy tuffs of the Argyle pipe comprise two main
types; blocky” clasts and ragged clasts. Blocky clasts are typically fine
grained, non-vesicular (in general) and have a predominance of straight faces.</p>
<br>
<img src="images/Argyle/Full_BlockyClast.jpg">
These clasts appear similar to the type 1 clasts of
<a href="Bibliography.html">Wohletz (1983)</a> who states that these types of clasts are the most commonly
observed shapes in coarse (>63 micron) hydrovolcanic ash.</p>
<br>
<img src="images/Argyle/Full_AK1-RaggedTuff.jpg">
<p>The ragged clasts are generally light coloured, deformed to wispy in shape
and commonly highly vesicular. The photo below shows a section of core
comprising ragged tuff.</p>
<br>
<img src="images/Argyle/Full_AK1_RaggedTS.jpg">
<br>
<p>In thin section, the ragged clasts show clear evidence of deformation, as
shown by these two images. The top image is a polished slab of drill core
and the other, below, is a thin section showing a ragged clast.</p>
<p><a href="#top">Back to top</a></p>
<h2 name="ArgyleSum">Summary of Observations at the Argyle Pipe</h2>
<p>The overall appearance was one of a chaotic mess. A number of
petrographic features were relevant to my final interpretation of what had
happened in the southern area of the pipe.</p>
<p>1. the sandy tuff, which comprises the economically mineralised material,
is a quartzose lapilli ash tuff. The lapilli are olivine lamproite clasts, which
are either a massive, fine grained and blocky, type or a flattened vesicular
type (fiamme – “ragged tuff”) that occur in a matrix of quartz sand grains and
fine grained ash,</p>
<p>2. the occurrence of accretionary lapilli,</p>
<p>3. tuffaceous sandstone dykes cross cutting the tuffs at various angles
that appear to be water escape features and due to the dewatering of water
saturated volcanoclastic sediments,</p>
<p>4. the quartz sand grains in the matrix of the tuffs are largely single
grains or broken single grains. This contrasts with the sand grains in the host
rocks, which have been silicified by quartz over-growths.</p>
<p>5. the presence of low-angle cross bedding was suggestive of deposition
from high energy base surge volcanic eruptions.</p>
<p>Additional photographs of water escape and other features are in the paper
by Boxer et al mentioned above.</p>
<p>All the evidence points to the fact that the Argyle pipe was intruded into
a shallow marine environment where there was an abundance of quartz sand and
water. It is envisaged that the pipe was emplaced on a shallow shelf and that
sand continually poured into the vent during its formation, allowing deep
penetration of the sand to great depths in the diatreme.</p>
<p>The water escape features seen in the pipe are represented by sandstone
dykes and occur in exactly the same way as sandstone dykes in sandy sediments
and are dewatering features. The occurrence of dropstones and accretionary
lapilli in the northern part of the diatreme indicated that at later stages in
the pipe formation, water flow into the vent was restricted, probably by the
formation of a tuff ring around the vent, and that the non-quartzose lapilli ash
tuffs (non-sandy tuffs) where probably formed at this time.</p>
<p>At the far northern part of the Argyle pipe, there was an exposure of
shales (prior to mining) overlying the quartzose lapilli ash tuffs and probably
represents the deposition of crater lake sediments over the top of the vent. </p>
<img src="images/Volcanology/Full_Sutur.jpg">
<p>The photo left is how I would have imagined the later stages of the Argyle
eruption to look like. This photo was taken by Dr Volker Lorenz of the Surtur
eruption near Surtsey in the Atlantic Ocean in 1966.</p>
<p>Note the steam filled eruption cloud and low vent. At Argyle one could
image many carats of microdiamonds being scattered by the will of the wind!</p>
<p><a href="#top">Back to top</a></p>
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