Arc Minerals in Zambia
As a disclaimer, I should state that what is posted here is purely my own interpretation of the data we have available to us. It is not to be taken as THE interpretation but it is one that fits the pieces of the puzzle together in a way that makes sense to me.
Some Background
First, I think it's worth having a look at the topography of our licenses and the surrounding areas:
The colour scale is white through to blue indicating high to low altitude.
A couple of things jump out immediately;
- Large scale faulting around the dome which is one of the main factors affecting the location of sulphide deposits, creating pathways for the migration of mineral rich brines. The nearby Sentinel and Enterprise deposits are on fault zones, as are most of Arcs major targets.
- The boundary of Arcs license areas is quite clearly defined in the topography, particularly in the south and east. This corresponds to a change in geology, which is another key factor in understanding where deposits might form.
- The red band which surrounds much of our area is Lower Roan rock. This is the rock from which metals were dissolved by briny fluids and later deposited higher in the geological sequence. Although we only see a band of Lower Roan at surface, in fact it is continuous across most of our licenses below surface. This is key to understanding where mineralisation may have come from.
- The area within the red band is Upper Roan, which overlays the Lower Roan. It is within these rocks that the fluids migrated, travelling via fractures and natural pathways in the rock. Chemical reaction between the fluid and certain rock types caused the metals to drop out of solution.
I'm not a geologist and I suspect that most of you aren't either so I won't
attempt to go into more detail on the geology, but if you do want to know
more, I would recommend viewing this video by David Wood, a senior geologist
with Anglo American and previously involved in the discovery of the nearby
Sentinel deposit when with First Quantum Minerals:
It is very much a talk by a geologist for geologists but there is some very
relevant information, particularly toward the end.
A comprehensive campaign of soil sampling was carried out across the
licenses over several seasons. Here is a map of the copper in soil
concentrations:
You can see several large areas of anomaly highs (in pink) as well as
anomaly lows (in blue). The blue corresponds to the Lower Roan rock and is
the first indication that metals have been stripped from these rocks.
An airborne electromagnetic survey was also carried out during this
time:
Again you can see in blue the Lower Roan rocks which indicate low level
electromagnetic response. This map is more complex to interpret and I'll
come back to this shortly with my own interpretation.
During the Exclusivity Agreement with Anglo American they requested that
soil samples from the original sampling campaign were re-assayed using a
technique called 4 acid digest. This uses the fact that some elements are
not taken into solution by the brines when others are. In this case scandium
is chemically stable whereas copper is not, so by testing the ratio of
copper to scandium in the soil samples it is possible to map where copper
has been leeched and where it has been enriched relative to the background
level.
There is a research paper written by David Wood and others where they
describe this method and how it was used in the mapping of Sentinel.
Here is the Copper Scandium ratio map for Arcs licenses:
Normal background level is indicated by light blue to green. Dark blue
indicates copper depletion with respect to scandium and enrichment is shown
in yellow-to-pink. Again you can see depletion of copper in the Lower Roan
around the whole of the license area.
Initial shallow drilling (150m max) has been guided by the surface copper
anomalies revealed by the soil sampling. As Remy has said (during his
2020 Indaba presentation), they were going for the most obvious low hanging fruit. This drilling
has been successful in finding high grade oxides at Fwiji, Cheyeza East and
also evidence of deeper sulphides at those locations as well as Muswema. A
deeper drilling campaign last year has shown more evidence of deep sulphides
at both Fwiji and Cheyeza East although the drilling campaign was impacted
by external events which limited the amount of drilling which could be
done.
Here is a depth plot of the shallow drill results at Cheyeza East which
include an area of roughly 200mx200m with very high oxide grades, some of
the best ever seen in the Zambian Copperbelt. You can see there is also
evidence of sulphide mineralisation below the oxide cap.
The limited drilling at Fwiji showed some even higher grades particularly
in one drill hole which showed 1m of sulphides at more than 7% Cu eq.
My Interpretation (take with a pinch of salt or two)
First I want to go back to the airborne electromagnetic (EM) map which I
think has some interesting things to tell us. Imagine that you were able to
remove all of the overlying rock from our licences, just leaving the Lower
Roan. If you did another airborne survey what you would expect to see is a
map that is for the most part blue in colour, because metals have been
stripped from those rocks across the entire area. David Wood mentioned in
his video presentation that this isn't a localised process but one which has
been mapped for hundreds of square kms across the Domes region.
So what can we understand by looking at the EM map:
- Any areas which are blue in colour are showing that no mineral deposition has occurred in the rocks above the Lower Roan in that area.
- Any areas which are NOT blue are indicating that mineral deposition HAS occurred in the rocks above that area.
- There is one proviso to the above in that shallow / surface mineralisation does not show up on the EM map. I presume this is by design in order to see what is happening at depth although I don't know that for sure.
Lets first take a look at the area around Lumbeta and Muswema:
What I think we are seeing here is the path of the brines (indicated with
the arrows) as they followed the major fault line down the side of the dome,
then spreading out as they got to the foothills. I believe that this is
likely to be the main source of the mineralisation that we see at Muswema
and Lumbeta.
One interesting comparison is the deposit at Barrick Golds Lumwana mine,
which has formed up against an L-shaped outcrop of Lower Roan in much the
same way as the Muswema anomaly does:
Now lets take a look at the area from Cheyeza / Fwiji to Nyambwezu in the
North East:
My interpretation of what is happening here is that the area around
Nyambwezu is an area of fluid flow from the Lower Roan (in blue) to the
Upper Roan. The large red anomaly may be a fluid trap which has become
mineralised. The whole area from Nyambwezu to Cheyeza is showing a strong EM
response, indicating that this whole area MAY be mineralised as fluids have
flowed down towards Cheyeza.
I noticed that in the RNS on 29th April, Nick made reference to
"targeting the contact between this unusual basal magnetic unit and the
overlying rock formation"
I believe this may be what he was referring to.
I've always thought that Nyambwezu has the potential to be another major
find although it hasn't been the focus of attention to date. In the RNS
from 12th April 2019 notes were made on each of the targets and Nyambwezu was noted to
have an intense EM anomaly and intense copper-cobalt anomaly. The only
target where intense was used in the description.
I've marked a line which follows surface fault lines. It's noticeable that
above Cheyeza this faulting seems to act as a barrier to fluid flow or
creates a pathway to the surface. It is along this line that the high grade
oxides were located. Further North, between Cheyeza and Fwiji, the fluid
flow seems unaffected by the faulting, continuing further West. It's
possible that the mineralisation at Cheyeza West was via this route:
Here is the topographic map and the EM anomaly map for the area between
Fwiji and Cheyeza. When you see it like this, it's pretty clear that the
anomalies (in red) are closely related to the fault line running between
Fwiji and Cheyeza. I think that all of these anomalies are potentially targets for drilling.
And here is a closer view of the area around Cheyeza, showing the previous and planned drilling and the shallow oxides, located on the boundary between the mineralised and non-mineralised areas (blue).
A slideshow view is a bit easier to see how features line up:
And here is the area around Muswema and Lumbeta. The fractures at Muswema again seem to fit well with the EM anomalies.
Thanks.
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