The Dizon porphyry copper-gold deposit is located within the municipality of San Marcelino in southern Zambales province, western Central Luzon in the Philippines, about 80 km northwest of Manila.
The oldest rocks in the Dizon district comprise troctolite gabbro and pyroxenite believed to be exhumed remnants of the breached ophiolite basement related to the nearby Eocene Zambales ophiolite which are underlain by a horneblende diorite stock and in turn, partly overlain by a dacite porphyry volcanic flow. The basal volcanic flows and pyroclastics of the latter are interpretted to be part of an inlier of the Mt. Pinatubo stratovolcano complex of probable Late Miocene age and consist of predominantly andesitic and dacitic flows with locally intercalated lithic and lapilli tuff and lahar. These rocks most likely underlie a younger capping below the slopes of the current stratovolcano. In the Dizon Mine area, this lithologic unit is intruded by a number of intermediate stocks and exhibits varying degrees of alteration and mineralisation.
A series of small intrusive stocks are recognised, clustered in the vicinity of the mine, intruding the basal volcanics. These intrusives range in composition from from microdiorite to hornblende diorite, and exhibit varying degrees of hydrothermal alteration which extends into the intruded volcanics. The most highly mineralised of these quartz diorite porphyry stocks is exposed on the eastern flank of Mount Pau. Both the mineralised stocks and surrounding volcanics have been eroded, and much of the ore lost. A marginally mineralised intra-mineral dyke of biotite dacite porphyry occurs on the southern margin of the Pua stock which probably represents late magmatic differentiates that formed during the waning stage of the hydrothermal activity of the intrusives. Sericite associated with the mineralised Pau stock has been aged at 2.7±0.3 Ma.
The post-mineral Pua diatreme is an upward-flaring structure of probable Pleistocene age has been exposed by mining on the southeast flank of Mt. Pua. This diatreme partially destroyed the orebody, accounting for its tapered southern boundary and removal of the southern part of the alteration halo. It consists of a chaotic assemblage of dacite and andesitic boulders, cobbles and pebbles embedded in a matrix of tuffaceous material. Based from its northern section exposed in the pit, it was about 1000 m in diameter and appears to taper downward.
A number of breccia pipes and pebble dykes have been exposed along the southern and southwestern wall of the pit along the contact of the diatreme, believed to represent hydrothermal breccias formed during the solphataric stage of the diatreme. Breccia fragments generally comprise well-rounded boulders and cobbles of various rock types embedded in a matrix of clay or rock flour. Mineralised fragments are common within the breccia fill. Pebble dykes, some of which cross-cut the orebody, are related to the hydrothermal breccia plpes. They occur as narrow, irregular, tabular bodies closely similar in composition to the hydrothermal breccia, with erratic strikes and dips.
In plan, the Dizon orebody is heart-shaped, with a diameter of about 600 m, while in section, it resembles a molar tooth, forming a cap to the main quartz diorite intrusion, straddling the contact with the intruded volcanic rocks. It has an incomplete set of "molar roots" following the steeply dipping flanks of the stock.
Distinct zoning of alteration and mineralisation is evident, as follows:
(1) a core of K-silicate alteration developed largely within the intrusive, characterised by well-developed quartz veining, a preponderance of biotite with subordinate chlorite, some magnetite and approximately equal amounts of chalcopyrite, bornite and pyrite;
(2) a phyllic-argillic assemblage of quartz-sericite-clay-chlorite overprinting the K-silicate zone, characterised by the pervasive replacement of feldspars by clay minerals and sericite, the comp1ete replacement of the mafic minerals by ch1orite and a high pyrite-chalcopyrite ratio and only minor amount of bornite;
(3) an advanced argillic zone that probably formed a pre-erosion capping to the phyllic-argillic zone, to now occupy the peripheral zone characterised by pervasive quartz or clay replacement, an abundance of diaspore, and pyrophyllite and the widespread occurrence of pyrite and hematite;
(4) an outer propylitic zone characterised by strong chlorite and a sprinkling of epidote and calcite, and a variable amount of disseminated pyrite; and
(5) sericitic alteration associated with WNW-trending fault veins and gash fractures cutting into the phyllic-argillic alteration zones, in which sericite and pyrite are predominant intermixed with the clays and gouge.
Ore-grade copper-gold mineralisation is largely restricted to the K-silicate and the phyllic zones and to some extent in the advanced argillic alteration, occurring as stockworks, fracture fillings and disseminated sulphides and totalling around:
140 Mt @ 0.43% Cu, 0.93 g/t Au and 2.5 g/t Ag.
There is a generally close positive correlation between the tenor of Au and Cu within the ore. Gold occurs in a free form, generally < 37 µm with a high percentage around 12 µm. The remainder of the gold appears to be either completely occluded in quartz, calcite or in the sulphides, or attached to these minerals but partly exposed. The occurrence of silver, is less well known and is inferred to be associated in solid solution with the other minerals. A number of minor, potentially deleterious, base metal minerals are present, including cinnabar, enargite, stibnite, tetrahedrite, galena and sphalerite. These minerals appear to have been mostly introduced by WNW-trending late-stage fault veins that cut across the orebody are and possibly related to the Pua diatreme.
(Source: Porter Geoconsultancy, http://www.portergeo.com.au/, 1997)