Moran nickel sulphide orebody discovery using DHTEM

The Moran nickel orebody (June 2009 resource est. 456,000t @ 7.1% Ni for 32,400t contained Ni metal) was discovered in 2008 when underground drill-hole LSU-152 intersected a 12.3 m thick interval of mineralisation with an average grade of 6.0% Ni. The discovery drill-hole had been designed to test a strong conductor detected by transient electromagnetic surveys read in drill-holes that had passed above and below Moran.

Moran is located approximately 1.1 km south of the Long orebody on the eastern flank of the Kambalda Dome. Both orebodies are interpreted to be hosted within the same komatiite lava channel. The exploration program was premised on the observation that fertile komatiite lava channels are often mineralised over significant distances.  Independence Group theorized in their first quarterly report after purchasing the Long-Victor operation from WMC that an undiscovered orebody may lie close to the intersection of 0.76 m @ 8.12% Ni in historic surface diamond drill hole KD6067B.

This conceptual target was the justification for developing an exploration decline south from the Long underground workings.  An extensive program of underground drilling produced some encouraging sub-economic intersections near the face of the decline, aided by down-hole electromagnetic surveys read with an underground transmitter loop.  But the geophysical target leading to the eventual discovery was generated from wildcat holes drilled several hundred meters further to the south, a program motivated by the observation of a thick section of hanging wall disseminated sulphide mineralization in historic surface hole KD6068.  DHTEM surveys in LSU-140 and LSU-141 generated strong anomalies, and it was the follow-up of this target that resulted in the Moran discovery.

The discovery DHTEM anomaly in LSU-141 was identified through a diligent review by Mine Geophysicist Jacob Paggi of 100 m spaced reconnaissance measurements taken up-hole from a detailed survey of the bottom 100 m of the hole (down-dip from a remobilized sulphide intersection in LSU-140 above).

The decay curve of the axial component DHTEM response at 200 m down-hole displays a late-time exponential transient with a time-constant of 247 milliseconds – consistent with a very strong conductor.

This result in the reconnaissance survey prompted immediate follow-up with a detailed survey centered on 200 m down-hole.  Hole LSU-140 was also surveyed close to this interval.

Modeling of these responses using the current ribbon approximation to a plate-like conductor implemented in the Maxwell software by ElectroMagnetic Imaging Technology (EMIT) formed the basis for designing the discovery drill hole.  However, it proved to be a long drawn-out process leading to the discovery intersection in LSU-152, owing to all available collar positions producing down-dip hole trajectories and near-misses.  The mining engineers eventually solved the problem with a $1 million hanging-wall drill drive, which provided the perfect collar location for LSU-152.

The schematic cross-section

shows the frustrating succession of holes leading up to the drilling of LSU-152 from the 525 drill drive. The presence of a prospective embayment structure in the footwall basalt was recognized early in the program by Chief Geologist Somealy Sheppard, and this was a key ingredient in the success of the program.

The video above shows the series of ineffective holes drilled after target identification.  The discovery process took months and would not have reached a successful conclusion without the support of Managing Director Chris Bonwick and Mine Manager Brett Hartman.

The first cut developed into Moran ore shows why this orebody was such a spectacular conductor.  The massive and heavy matrix ore together produce a plate-like body with a conductance of tens of thousands of Siemens.  Other elements of the success story were innovative technology (proprietary high-powered transmitter, underground EM transmitter loop, Atlantis fluxgate magnetometer DHTEM system, SmartEM instrument), highly experienced geologists/geophysicists, diligent field geophysicists/technicians, supportive management and dogged persistence.

Reference:  David M. Johnson, Somealy Sheppard, Jacob Paggi, and John Coggon (2010) Discovery of the Moran massive nickel sulphide deposit using down-hole transient electromagnetic surveying. ASEG Extended Abstracts 2010: 21st Geophysical Conference: pp. 1-4. [Download PDF]