Phoenix AMT Survey Maps Deep VMS Ore Body
Phoenix carried out an AMT test survey last March at the Lalor deposit owned by Hudbay Hudbay Minerals Inc.) near Flin Flon, Manitoba, Canada.
Rich in gold, zinc and copper, the Lalor deposit was discovered in 2007 by drilling a large-loop TDEM anomaly. (See The Phoenix 49, March 2010). Subsequently, dozens of deep drill holes outlined the deposits.
2D Inversion Profile L184 with Drill Section (approx.)
Lalor is a Volcanogenic Massive Sulfide (VMS) deposit. Such deposits are formed in groups on the ocean floor near hot, mineral-rich fluid outflows called “black smokers”. The smokers are associated with the spreading of tectonic plates; see page 4 to learn more about them.
There are more than 800 known VMS deposits worldwide, and over 300 in Canada. The numerous VMS deposits in the Flin Flon area – now several hundred meters below surface and in the center of a continent or “craton” – were once on the sea floor.
HudBay recently established a project for multiple test surveys over the Lalor deposit, with a goal of publicly presenting all the findings. The resulting Lalor Symposium was held in Vancouver, Oct.16–17, 2014, under the auspices of the British Columbia Geophysical Society. Phoenix geophysicist Caroline Finateu presented the results of the Phoenix survey.
The unconstrained 2-D inversion shown above, as is typical, has greater spatial extent than the actual 3-D orebody. But the core of the 2-D inversion (inside the 2.7 ohm-m contour) shows reasonable agreement with the known boundary (left hand side), plunge, depth, and resistivity of the orebody. Other surveys and 3-D inversions reported at the symposium also indicated possible deeper conductive zones, which according to Hudbay cannot be ruled out.
The Lalor deposit, which is now in production, generates considerable 24/7 EM noise. However, Phoenix MT processing techniques were able to mitigate the high levels of ambient noise and provide a satisfactory data set for inversions. The 2D MT inversion along one of the lines (line 184) is shown above.
Coincident Loop TDEM Function for V8 Receiver
MT static shift correction with TDEM typically uses the central-loop configuration. In this configuration, the TX loop is typically about 100m x 100m to 200m x 200m. Here, the V8 receiver measures a single station at the geometric center of the square TX loop.
However, the central-loop configuration is difficult to implement in areas with significant vegetation, such as fields with tall crops, jungle, or heavy bush. In such areas it is still possible for the operator to install the TX loop by paying out the loop wire behind him while he walks round the loop plan following a compass heading. However, it is often difficult to install the receiver magnetic sensor at the center of such a loop, because there is poor visibility or lack of access.
The solution is to combine the wire for the receiver loop with the wire for the transmitter loop in a single multi-conductor cable. This is called the coincident-loop configuration, since two loops are coincident in space. The loop cable is provided in portable segments
typically about 100 to 200m long and weighing about 10kg. The segments are joined by rugged
The new configuration is being used successfully in Japan for a different objective: volcano studies, where the coincident loop is deployed around the rim of a small crater.
The advantages of the coincident loop include insensitivity to minor topographic variations; ease of use, especially in areas with heavy vegetation; greater volume-averaging for the static-shift correction application; and strong RX signal due to the large RX loop area typically at least 100 times greater than the equivalent area of a small portable receiver loop.