EXTENDED ABSTRACT

Geologic setting and age

The Victoria gold deposit of the Lepanto Consolidated Mining Corporation in Mankayan, Benguet (Figure 1), is the most recently discovered Philippine epithermal deposit which has developed into an economic mine during the last decade. It lies above and ca. 500 m southeast of the Far Southeast porphyry copper-gold deposit and less than 1 km south of the main body of the Lepanto high-sulfidation enargite deposit. Other porphyry copper (e.g., Buaki, Palidan and Guinaoang) and epithermal gold (e.g., Nayak, Suyoc) prospects surround Victoria to the west, south and southeast. The important mineralization occurs within easterly to NE-trending veins hosted mainly in dacitic volcanics and volcaniclastics (Figure 1 and Figure 2). There is ample evidence that the east-trending veins are being cut and/or truncated by the NE-trending ones. The latter is found to have a history of right-lateral strike-slip movement that is probably still active.

Illite separates for ⁴⁰K-⁴⁰Ar dating come from a composite vein + wallrock sample, giving an age of 1.5 ± 0.7 Ma. As illite is found absent in the vein portion of the sample, this age likely corresponds to wallrock alteration.

Mineralization and alteration

Mineralization and alteration occur in four general stages:

1. Early silica-illite-pyrite and chlorite stage - silicification, illitization and pyritization is most evident along fractures and immediate wallrocks of veins, and wane into chloritized rocks away from the veins and fractures.

2. Enargite (high sulfidation) stage> - enargite mineralization corresponding to that in the Lepanto Enargite orebody extends in the Victoria area but very sparsely and in economically insignificant quantities, occupying narrow easterly trending veins. Enargite is being replaced by tetrahedrite/tennantite. A later episode deposited banded pyrite-chalcopyrite-tetrahedrite. At the northern periphery of the Victoria orebody, pyrophylite-kaolinite type advanced argillic alteration is present.

3. Base metal-carbonate-gold (low-sulfidation) stage - several types of veining formed during this stage:

1. Hydrothermal breccia vein - wall rock or early formed quartz veins are shattered, forming jigsaw-like patterns cemented either by massive white quartz or rhodochrosite. The fragments are sometimes rimmed by sulfides before being cemented by quartz.

2. Quartz-base metal vein - earlier quartz veins or younger faults are dilated depositing alternating bands of sulfides (sphalerite ± chalcopyrite ± galena) and quartz. Sphalerite in these veins are dark grey in hand specimen and appear to be more Fe-rich compared to those directly associated with Mn-carbonate. Stockworking of quartz in either argillized or pyritized wallrock has also been observed.

   Tetrahedrite is sometimes present in the vuggy quartz. Bladed quartz, when it occurs, is associated with early sphalerite.

3. Basemetal-carbonate-quartz vein - veins in the northern portion of the deposit form bands of yellow Fe-poor sphalerite ± galena ± quartz, followed by rhodochrosite, then, occasionally, quartz with sparse sulfides and, finally, late stage comb quartz. Bladed or platy Mn-carbonate and quartz occur sporadically. Electrum occurs as inclusions in galena and in microfractures affecting base metal sulfides, and sometimes disseminated in quartz. Silver minerals (acanthite, proustite, pyrargyrite, tetrahedrite and tennantite as identified by EPMA) also occur as microveinlets in sulfides. Bladed quartz and carbonates are also recognized in these veins. Carbonate appears to be absent at deeper levels.

4. Massive to crustiform and botryoidal pyrite/chalcopyrite veins - seem to belong to a late base metal stage and may overlap with stage 4. They either occur as discrete veins or occupy the central portion of earlier dilatant quartz veins, sometimes cementing brecciated quartz.

4. Late quartz and sulfate stage - anhydrite/gypsum cement partially shattered sulfides including the enargite veins or occur as overgrowths on late stage pyrite-chalcopyrite veins (Stage 3d above). Barren quartz is present either as white to amethystic crystalline quartz that occupy late tensional fractures and dilatant openings in older mineralized veins, or as massive bands of alternating white and grey varieties that also recement mineralized breccias. In the enargite vein, EPMA analysis reveals the presence of accessory apatite and a Cu-As-Sb-Sn-V-sulfide mineral (colusite?) similar to that found in the Stage II mineral assemblage of the Lepanto Enargite orebody (Claveria and Hedenquist, 1994; Hedenquist et al., 1998).

A horizontal and vertical zonation of mineral assemblage can be recognized. The distribution of the Mn-carbonates is restricted to the north and western portion of the deposit at level 1000. This carbonate-bearing zone tapers to the southwest at level 850, and seems to be absent at deeper levels. The other parts of the deposit are characterized mainly by quartz-sulfide assemblage. Translucent yellow (Fe-poor) sphalerite is abundant at the northern and central portions of the deposit, while opaque (Fe-rich) sphalerite is more common at the southern area. Although pyrite and chalcopyrite are omnipresent, they are more massive and abundant at the southern portion.

Bonanza veins containing averages of >100 g/t Au are found mostly in the central to northern portion, in the upper levels, within the carbonate-base metal zone.

Fluid Inclusion thermometric studies

Most fluid inclusions in quartz, sphalerite and rhodochrosite are two-phased (i.e., liquid and vapor), although strictly liquid and wholly vapor inclusions are also present. Primary inclusions in quartz gave homogenization temperatures (Th) with a bimodal distribution having ranges of 150-160ºC and 200-300ºC with frequency peak at 240-260ºC. Secondary inclusions have a range of 140-320ºC, with frequency peaks at 230-240ºC, 210-220ºC and 140-150ºC in decreasing importance.

Primary inclusions in sphalerite range from 200 to 240ºC, with peaks at 200-220ºC. Secondary inclusions range from 180 to 240ºC, with a distribution peak at 190-200ºC.

Limited measurements on rhodochrosite fluid inclusions reveal Th of 230-250ºC while secondary inclusions range from 210 to 250ºC.

Freezing runs indicate that in general, primary fluid inclusions have lower melting temperatures (Tm), reflecting higher salinity, than secondary ones. Maximum salinity of primary fluid inclusions is sphalerite is ca. 5-6 wt% and 3-4 wt% for the secondary type. Salinity of fluid in rhodochrosite range from 4wt% in primary, to 3 wt% in secondary inclusions. For quartz, salinity is more erratic, ranging from 2 to 4 wt% in primary inclusions and 2-3% in secondary ones.

Fluid inclusions were also found in transparent flaky anhydrite/gypsum. Th is higher than 145ºC, but exact measurement is made impossible by the blurring of the mineral at higher temperature.

Sulfur isotopes

The?³⁴ for sulfides is quite homogenous, ranging from -2.4 to +1.7‰, while the limited data on gypsum (the only sulfate species found) is from +16.5 to +17.9‰. The sulfide values are within the range of those analyzed from the enargite orebody, -2 to +3‰ (Hedenquist et al., 1998), and the upper portion of the FSE deposit, -3 to +6.1‰ (Imai, in press). On the other hand, ?³⁴S for gypsum in Victoria is lower than those of alunites (20-25‰) of the enargite deposit and anhydrite (17-25‰) from peripheral portions of the FSE deposit. The lower fractionation between the sulfide-sulfate pair in Victoria, if used in thermometric calculations, would indicate a temperature of 370ºC, much higher than the measured values in fluid inclusions. This can be attributed to the fact that gypsum is related to a later stage of mineral deposition, and is, therefore, most probably not in equilibrium with the spatially related sulfides. Unreasonably high temperatures were also calculated for mineral pairs using pyrite-galena and pyrite-chalcopyrite. Pyrite-sphalerite pairs in two samples, on the other hand, gave a temperature range of 209-251ºC, in agreement with observed fluid inclusion measurements.

Implications

1. The geologic setting and mineral assemblage of the Victoria deposit allow for its classification as a porphry-related carbonate-base metal low sulfidation gold deposit.

2. The structural relationship between the sparsely distributed east-trending HS enargite-bearing veins and the NE-trending LS base metal-carbonate-quartz veins suggests that the latter is younger. The 1.5 ± 0.7 m.y.-old ⁴⁰K-⁴⁰Ar age derived from a sample from Victoria can be interpreted as an early alteration (i.e., illitization) age (Stage 1). This would correspond to the early alunite alteration in the Enargite orebody (Arribas et al., 1995; Hedenquiust et al., 1998)

3. ?³⁴S data on sulfides indicate a magmatic origin for sulfur.
    

4. The presence of two phased and vapor-rich primary inclusions in sphalerite, rhodochrosite and quartz, plus the presence of bladed quartz and carbonates suggests boiling. This is deemed as the major mechanism which effected sulfide and gold deposition in Victoria.

5. The relatively high salinity of fluid inclusions in base metals is commonly found in carbonate-base metal LS deposits within the southwest Pacific rim (Corbett and Leach, 1998). This would corroborate the idea of boiling during metal deposition in Victoria. However, in the Victoria deposit, it is also possible that the high salinity is also partly due to the lowering of the Tm depression caused by small amounts of dissolved gas in the liquid (Hedenquist and Henley, 1985). This is supported by Tm vs Th relationship in sphalerite (Figure 3), in which the trend suggests a rise in Tm depression as a result of gas dissociation from the fluids during boiling.

6. The presence of significant amount of carbonates suggest that the mineralizing fluids in Victoria are CO₂-rich. In the general order of deposition, sphalerite (+ other base metals and electrum) generally precedes rhodochrosite or quartz. Boiling with subsequent decrease in temperature could have triggered sulfide deposition. Boiling would also promote degassing and increase of fluid pH, causing carbonate precipitation.

7. The presence of relatively Fe-rich sphalerite and abundance of Fe-Cu base metals in the southern portion of Victoria can be interpreted as due to a greater magmatic contribution it has received compared to the northern portion. This, plus the tapering of the carbonate-bearing zone to the southwest, suggest a fluidflow model in which the mineralizing fluids come mainly from the south (Fig. 2). The FSE porphry intrusive, therefore, appears not to be the source of the Victoria mineralization. The presence of other porphyry bodies south of Mankayan provide additional candidates for the origin of the mineralizing fluids.

8. Considering the fluid flow models in the Enargite (Hedenquist et al., 1998) and Victoria orebodies, it appears that a northward-flowing regional hydrothermal system was established in the Mankayan district at least ca. 1.5 m.y. ago.

9. With regards regional tectonics, the east-trending faults hosting the Enargite veins seem to be restricted within a narrow corridor along the Lepanto Fault, bounded by the West Fault and the North Fault. Left-lateral movement of these faults engendered ENE to E-W tensional fractures. While the Lepanto Fault served as the main avenue for mineralizing fluids, the easterly faults afforded the lateral migration of these fluids, resulting in the widening of the ore zone. The NE-trending faults hosting Victoria are younger and appear to have developed simultaneous with ore deposition. These northeasterlies may be linked to regional NE-trending dextral faulting that Ringenbach (1992) postulated to be due to Riedel-type block rotation during the Pleistocene.

10. As the NE-trending faults appear to be more important in magnitude and distribution than east-trending ones, fault systems similar to that hosting Victoria, especially those occurring north of identified intrusives or porphyry bodies, can be considered very prospective for other Victoria-like deposits in the Mankayan mineral district