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The Calo Porphyry Copper-Gold Prospect
A cluster of unusually large Induced Polarization (IP) anomalies has been defined at Calo, with extremely high chargeability readings of up to 100 msecs against a background of 6-8 msecs. 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. 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) alteration; 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; and the occurrence of a diatreme breccia. All are features characteristic of Philippine porphyry copper-gold systems.
Much of the anomaly cluster, which is at least four kilometers by six kilometers in extent, is covered by younger rocks; with the source, therefore, being obscured. The anomalies are interpreted as related to porphyry-copper-gold mineralization, possibly with multiple mineralization centers. High-sulphidation epithermal copper-gold showings, along with advanced argillic alteration - which are usually indicative of proximity to porphyry mineralization in the Philippines - as well as diatreme breccias and porphyry-type alteration, occur around the margins of the chargeability anomalies, and in erosional windows.
Porphyry copper-gold mineralization has already been drill-intersected by Mindoro on the far-eastern margin of Calo, where Pica drill hole two intersected 213 meters of 0.18 percent copper and 0.30 grams per tonne gold below a cap of advanced argillic alteration. In the southwest part, young volcanic cover above the chargeability anomaly contains exotic copper mineralization, which is typically associated with porphyry copper deposits. The volcanic breccia contains clasts cemented by supergene copper minerals and have been mapped over an area of 100 meters by 300 meters. Two trench channel samples gave 2.62 percent copper over 30 meters, and 2.17 percent 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.
Calo-Pica Compilation
Main features are shown on the Calo-Pica Compilation map above. Reconnaissance drill-testing with a large diamond drill rig commenced in late 2006. Seven wide-spaced holes, up to two kilometers apart, were completed for a total of 3,323 meters. Objective was to intersect indications of porphyry copper-gold alteration/mineralization and then to vector into mineralization centers with closer spaced drilling. Location of the drill holes is shown on the Calo-Pica Compilation map:
- Hole 1 encountered young cover rocks down to 162 meters, then interbedded tuffs, volcanic breccias and andesitic lavas hole bottom of 502 meters. Opaline silica and illitic clay alteration were confirmed by petrography near hole bottom. Fracture controlled and disseminated fine pyrite increase to 15 percent with depth. The increased pyrite is associated with moderately anomalous moderate copper and zinc, and weakly anomalous gold and arsenic. The significant pyrite content may suggest a porphyry source. The hole was prematurely terminated due to continuous ground collapse.
- Hole 2 intersected intercalated volcanic breccias, porphyritic andesite, basaltic andesite and tuffs, was drilled to 620 meters. Significant hydrothermal opaline, chalcedonic silica and barite occur, possibly indicating pulses of epithermal alteration. Structurally controlled sericite alteration is also present. This alteration is weak and probably distal to a porphyry system.
- Hole 3 intersected intercalated sediments and andesitic volcanics. Zones of moderate argillic alteration were encountered near surface, with propylitic alteration to end of hole at 226 meters. Within the altered andesitic unit, galena, chalcopyrite and sphalerite occur in veinlets and fracture fills. A 10.5 meter wide fault breccia occurs at 94 meter depth with stringers of chalcopyrite-pyrite. Throughout the propylitized unit, pyrite disseminations and trace chalcopyrite occur. Only weak porphyry-related alteration/mineralization zoning is evident in this hole.
- Hole 4 to 60 meters intersected argillic to intermediate argillicaltered andesitic flows and lapilli tuffs, with gypsum veins plus minor pyrite (to 10 percent) and chalcopyrite. To hole bottom at 360 meters, weakly propylitized porphyritic andesite with localized sericite and epidote alteration was encountered. Only weak porphyry-related alteration/ mineralization was encountered in this hole.
- Hole 5 intersected weakly argillic-altered andesitic tuff down to 46 meters, followed by argillic to intermediate argillic-altered, gypsum-anhydrite veined lithic and andesitic lapilli tuff down to 250 meters, with a 4 meter massive vuggy silica intercepted from 61 to 65 meters from which a sample assayed 1.18 g/t gold. To end of hole at 456 meters, propylitized porphyritic andesite occurs, intruded by narrow propylitic and phyllicaltered diorite dykes, which are likely distal to a porphyry system.
- Hole 6 intersected cover volcanics to 296 meters, and below these, the Talahib Volcanics to end of holes at 358 meters; generally with weak alteration, but containing structurally-controlled zones of argillic alteration and pyritization (to 5 percent pyrite). No significant mineralization was encountered in this hole.
- Hole 7 intersected 580 meters of cover volcanics, volcaniclastics and tuff above the Talahib Volcanics. Clasts of diorite intruding volcanics were found in the tuff (which is a lahar, or locally transported flow debris). Petrology by Mindoro's independent petrology consultant describes strong potassic (secondary biotite to 20 percent), clay (to 10 percent) and chlorite (to 10 percent) alteration and silicification (to 25 percent), with from trace to one percent chalcopyrite (copper-iron sulphide) and up to three percent hydrothermal magnetite in clasts of the diorite and volcanics.
These clasts are interpreted to be derived from the core of a porphyry copper-gold system some hundreds of meters to the west, where there is a diatreme about 700 meters distant. This diatreme contains argillic-altered clasts set in fine pyritic (up to 15 percent) matrix. Diatremes are commonly associated with porphyry copper-gold deposits in SE Asian island arcs. Anomalous soil values of up to 150 parts per million copper are noted in proximity to the diatreme. Cover thickness near the diatreme is thin. Hole 7 passed from the lithic tuff into propyliticaltered andesitic volcanics at 710 meters.
All results to date are being carefully evaluated to assist in vectoring in to the mineralization center in a second, more focused, round of drilling. Clearly the diatreme area will be one of the priority areas for that more detailed drilling.
ZZ Calo Exotic Copper, San Miguel
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