Figure 1: Location of principal shallow conductors at Alpala (anomalies C0-C10; 5 Ohm/m surfaces, red) and shallow zones of electrical chargeability (85 msecs, yellow-green) shown on iso-surfaces of MVI magnetics (blue). The main concentration of shallow conductive anomalies (caused by sulphides, fracturing and clays) and chargeable rocks (caused by disseminated sulphide and/or clays) occurs under the Northwest Alpala lithocap. Conductive bodies form more discreet responses than the chargeable responses and tightly constrain areas of interest that lie in magnetic rocks beneath these stronger conductive responses. The enveloping chargeability anomalies (yellow-green) are typical of the pyrite shell that forms around porphyry systems.
Figure 2: A northwest-trending belt of deep magnetic anomalies (brown) are overlain by chargeability anomalies which are concentrated above the northwest lithocap, consistent with an extensive pyrite shell within the lithocap above and around the Northwest Alpala porphyry target (T1).
Figure 3: Image looking northeast, showing an MVI model iso-surface (0.0054 SI units; turquoise surface) defining the Central Alpala and Northwest Alpala MVI anomaly, the location of drill holes 1-9 and two horizontal slices through the Alpala Orion Deep Conductivity model. The rocks within and surrounding the magnetic anomaly at Central and Northwest Alpala are conductive, caused by the likely presence of sulphides and fracturing (veining) associated with magnetic rocks that contain hydrothermal magnetite associated with potassic alteration. Importantly Hole 5 (532 m @ 1.05% Cu and 1.08 g/t Au) tested the southeast margin of the highly prospective T1 target.
Figure 4: Location of the high priority target T1 at Alpala beneath the northwest lobe of the surface alteration lithocap (white outline), while the dashed white outlines show areas of potential target extension beyond the core target area. Targets T2, T3 and T4 are additional targets. The base map is a plan view at 800m RL (approximately 800m depth) of a DC conductivity model, with conductive rocks in purple and resistive rocks in white. Target T1 and T2 occur within strong magnetic anomalies that exhibit electrical conductivity responses around their upper parts (due to sulphides, fracturing and clay alteration), and are overlain by zones of acid alteration and geochemical anomalism.
Figure 5: The spatial coincidence between deep Orion MT conductivity (contoured horizontal slice through the MT conductivity model), deep magnetic anomalism (turquoise MVI anomaly), acid alteration defined by pyrophyllite and kaolinite (Figure 6), and geochemical anomalism at surface (Cu and Mo outlines in red and blue), provide strong evidence that the copper and gold mineralisation encountered by drilling at Central Alpala extends a significant distance to the northwest through the T1 area, and may also extend to the south east (T1-Ext).
Figure 6: Location of high priority Northwest Alpala target (T1) over the northwest end of the deep MVI magnetic (turquoise) anomaly and against the Alpala Footwall Structure. The strongest part of the Deep Conductivity model at Alpala coincides with the top of the magnetic anomaly, consistent with a concentration of sulphide and fracturing in this region (yellow ovoid shape).
Cornerstone Glossary Alteration Changes in the chemical or mineralogical composition of the rock, usually produced by hydrothermal solutions or weathering A-Vein An early generation of vein in porphyry copper deposits that form whilst the rock is extremely hot and partly ductile. Ductile deformation of the solidifying magma produces contorted and segmented (worm-like) vein textures. These are high temperature veins and can host substantial copper sulphide minerals in association with quartz. Bornite Copper-iron sulphide (Cu 5 FeS 4 ). An ore of copper occasionally associated with chalcopyrite or chalcocite. Oxidize rapidly at surface (within minutes) to a purple-blue iridescence. B-Vein A particular generation of abundant quartz stockwork veins in porphyry copper deposits characterized by quartz fill and crack-seal textures. B-veins are often lined with copper-sulphide minerals along the centreline of the vein, and also locally along the margins of the vein. Chalcocite Black or dark grey copper sulphide mineral (Cu 2 S). Chalcocite is an important ore mineral of copper, often occurring as a result of secondary enrichment (oxidation and weathering). Chalcopyrite Brassy or golden-yellow ore mineral of copper (CuFeS 2 ). The most common ore mineral of copper. Chargeability The ability of earth materials to hold a charge for an extended period of time. Conductivity The ability of a material to conduct electrical current. In isotropic material, it is the reciprocal of resistivity. Units are Siemens/m. C-Vein A particular (abundant) generation of stockwork veins in porphyry copper deposits characterized by monomineralic sulphides. E.g. a vein comprising of only chalcopyrite, or of only bornite. C-veins generally form later than B-veins and typically cross-cut or re-open earlier B-veins. Hydrothermal Of or pertaining to heated water, to its actions, or its products. Induced Polarization (IP) Induced Polarization (IP) is a technique of measuring an induced potential field in the ground in order to map the geological subsurface. From measurements of the induced potential field the chargeability and resistivity of the subsurface can be calculated. These measurements are made in either the time domain or frequency domain using various configurations of transmission electrodes and multiple potentiometer receivers. Intrusive A body of magma emplaced into a rock and then solidified upon cooling. Isosurface A 3 dimensional surface linking all points of equal value. Lithocap A shallow region of intense silica and clay hydrothermal alteration that commonly occurs over porphyry Cu-Au deposits Magnetite A strongly magnetic shiny black iron-oxide mineral - (Fe,Mg)Fe 2 O 4. Magnetic Vector Inversion A recently developed geophysical computational technique for modelling magnetic field data. Magnetotelluric An electromagnetic geophysical method for inferring the earth's subsurface electrical conductivity from measurements of natural geomagnetic and geoelectric field variation at the Earth's surface. Investigation depth ranges from 300m below ground by recording higher frequencies down to 10,000m or deeper with long-period soundings. Developed in the USSR and France during the 1950s, MT is now an international academic discipline and is used in exploration surveys around the world Metallurgy The science that deals with procedures used in extracting metals from their ores and purifying metals. Porphyry Cu Copper orebodies that are formed from hydrothermal fluids that originate from a voluminous magma chamber several kilometres below the deposit itself. Predating or associated with those fluids are vertical dikes of porphyritic intrusive rocks from which this deposit type derives its name. Resistivity Resistivity is an active geophysical technique that uses probes to introduce an electrical current into the ground, measuring the resistance of the rocks to the passage of an electric current. Secondary Magnetite Magnetite formed by hydrothermal processes as opposed to primary magnetite that crystallize directly from a magma. Sheeted Veins Vein sets where most of the veins are broadly parallel and close-spaced. These often occur in or near to fault zones. Stockwork Veins Vein sets which have multiple orientations.