The people, science and technology behind discovery

Best targets yet to be drilled at Promesa’s Alumbre project in Peru

by Virginia Heffernan on July 31, 2014 applied

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Figure 1:  The image on the left shows the historical ground based magnetic data (i.e. Total Maganetic Intensity) and the image on the right illustrates 3D MVI inversion in grey underneath with historical ground based magnetics in the background and drillhole traces Stage 1 program.

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Figure 2: Plan view of the magnetic susceptibility 3D MVI inversion with Stage 1 drillhole traces (Magnetics (pink) – Isosurfaces of susceptibility, +10 x 10-3 SI* in pink and dark pink 30 x 10-3 SI isosurface).

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Figure 3:  Cross Section View at 9,065,750 N of drillholes 3, 4 and 5 with andesitic volcanics (green colour) overlying porphyritic tonalites and diorite intrusives (beige colour) at Alumbre.

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Figure 4:  Plan view - Magnetic susceptibility 3D MVI inversion with copper field sample (Magnetics (pink) – Isosurfaces of susceptibility, +10 x 10-3 SI* in pink and dark pink 30 x 10-3 SI isosurface).

With drilling costs averaging more than $200 per metre, explorers are looking for ways to improve confidence in the drill targets they choose.  Australian junior Promesa Ltd (ASX Code: PRA) believes it may have found an answer in geophysical inversion or, more specifically, Magnetization Vector Inversion (MVI).

“Detailed surface and subsurface geology, geochemistry, geophysics and the recent 3D modelling using the MVI process has increased confidence in our exploration model,” says Ananda Kathiravelu, executive director of Promesa, which is exploring the Alumbre copper-molybdenum-gold porphyry system in northern Peru. “There is now a substantial body of information in support of the district-scale potential of the project.”

Magnetization Vector Inversion takes into account geological processes and geophysical effects that alter magnetization direction such as deformation, anisotropy, remanent magnetization and high susceptibilities – all obstacles for traditional susceptibility modelling. MVI is particularly effective at low latitudes, where the low inclination of the Earth’s magnetic field can make magnetic interpretation challenging.

The MVI technique is part of Geosoft VOXI Earth Modelling, a cloud-based geophysical inversion software that generates 3D voxel models from airborne or ground gravity and magnetic data.  By harnessing the processing power of the cloud, VOXI has made large, multi-parameter inversion modelling faster, more responsive and effective as a tool for target delineation.

At Alumbre, Geophysicist Barry Bourne, ‎Principal Consultant at Terra Resources PTY Ltd, reinterpreted historical ground magnetic data (Fig 1) using the MVI methodology to estimate magnetic susceptibility. The resulting model suggests a significant link between surface Cu-Mo-Au mineralization and a large subsurface area with a strong magnetic signature (Fig 2). There is potential for multiple porphyry intrusive centres.

Recent drilling has provided further validation of the model, intercepting seven metres grading 0.72% copper coincident with increased magnetite (Fig 3). The combination of magnetic susceptibility, MVI and downhole and surface geochemistry indicates mineralization associated with strongly potassic felsic to intermediate intrusives, hydrothermal breccias and andesitic tuffs may continue at depth, west and north of the discovery hole.

By incorporating the drilling data, Promesa will further refine the MVI model and use it to target boreholes for a second phase of drilling in August. Ground magnetic anomalies extend throughout the project area and the established association between copper and magnetite (Fig 4) increases their prospectivity.

Alumbre has never been systematically explored for porphyry style deposits, but magnetic modelling using methods such as MVI is catching on as an effective means of generating targets within similar porphyry systems, particularly at low latitudes.

“Magnetic inversion has developed into an important tool for targeting potential porphyry systems at Alumbre,” Promesa noted in a recent release. “This evaluation has resulted in a significant new interpretation of the magnetic susceptibility model of the project area.”

Source: Promesa Ltd

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