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Calo Prospect: An Unusually Large and Intense IP Geophysical Anomaly
Calo is one of Mindoro's highest priority porphyry copper-gold targets. It covers an unusually large and intense IP chargeability anomaly, seven kilometers south of Freeport McMoRan's Taysan porphyry copper-gold deposit, and has abundant copper-gold and gold showings along its margins, as well as extensive porphyry-related alteration within a very favorable geological setting. It is bounded in the southeast by the Pica porphyry copper-gold system.
Main features are shown on the Calo-Pica Compilation map. Porphyry copper-gold mineralization was discovered by Mindoro on the Pica Prospect in 2005. Pica occurs in a high range of hills on the Lobo Project and is associated with intense advanced argillic (silica cap) and intermediate argillic alteration at surface. A geophysical survey in late 2004 defined chargeability anomalies extending over an area of approximately 1.5 kilometers by 2.5 kilometers, which was tested by four drill holes. The second drill hole intersected 213 meters at a grade of 0.18% copper, 0.30 g/t gold and 1.91 g/t silver from 22 to 235 meters, associated with phyllic alteration. It seems apparent that Pica is just the southeastern edge of a much larger system.
In the meantime, a cluster of low- and high-sulphidation epithermal copper-gold and gold showings were located in the far northwest of the Calo Prospect. This led to a major induced polarization (IP) and ground magnetic survey of 141 line kilometers. Background chargeability values are in the range of five to eight msecs. An area of at least seven kilometers by four kilometers of greater than 20 msecs has been defined, which encompasses zones of greater than 50 msecs, and includes values to as much as 100 msecs, and more. The Calo anomaly links with the Pica chargeability anomaly in the far southeast.
The chargeability anomalies, which occur at the intersection of strong regional northwest to southeast, and northeast to southwest structural features, suggest large concentrations of metal sulphides below the younger cover volcanics, and which geological evidence suggests are related to a cluster of porphyry copper-gold centers.
The area is extensively covered by young Quaternary volcanics and tuff which obscure large parts of the prospective underlying geology. However, in the northwest of the Calo prospect, erosion has exposed windows of altered andesite volcanics and small diorite intrusive stocks with areas of advanced argillic alteration covering several copper showings, and areas of phyllic and SCC (sericite-clay-chlorite) alteration. High-sulphidation epithermal copper-gold-silver mineralization associated with quartz veins and stockworks, and skarn-related mineralization have been located in these altered rocks, and are interpreted to indicate proximity to a porphyry system.
Evidence that the sulphides defined by the IP survey relate to porphyry copper-gold mineralization include: the Pica porphyry copper-gold mineralization already drill-intersected by Mindoro on the far southeastern margin, the erosional windows of advanced argillic, phyllic and SCC (sericite-clay-chlorite), copper-stained, massive silica boulders, interpreted as porphyry system lithocap located in the southern part of Calo; abundant epithermal copper and gold showings occur along the western and eastern margins of Calo, and include high-sulphidation epithermal copper-gold-silver veins and skarn mineralization; the occurrence of a diatreme breccia. All are features characteristic of Philippine porphyry copper-gold systems.
In addition, in the southwest part of Calo, outcrops of young volcanic agglomerate cover, with clasts cemented by supergene copper minerals and silica have been mapped over an area of 100 meters by 300 meters. Two trench channel samples gave 2.62% copper over 30 meters, and 2.17% copper over 20 meters. This exotic copper occurrence, typical of those associated with South American porphyry copper deposits, also implies a significant copper source in the underlying rocks.
Drill-testing with a large diamond drill rig commenced in late 2006. Given the very large size of the target area, initial planning is for six drill holes in 3,500 meters as a preliminary test of the anomaly cluster. These will be widely-spaced holes, up to two kilometers apart, with the objective of vectoring into mineralization centers, and will likely be the start of a major ongoing program. As an illustration of the large target size to be evaluated at Calo, the Anglo American/Philex Gold Boyongan porphyry copper-gold deposit is shown on the Calo-Pica Compilation map to scale.
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