High- and intermediate-sulfidation mineralization in the Carmen Quartz-Pyrite-Gold (QPG) deposit, Mankayan District, Northern Luzon, Philippines

The Mankayan district is famous for cotemporaneous porphyry and epithermal mineralization. It hosts a diverse array of ore deposits, including the Lepanto high-sulfidation Cu-Au deposit, the Victoria-Teresa intermediate Au- Ag deposit, Far South East (FSE) porphyry Cu-Au deposit, and several small subeconomic porphyry prospects. The recent exploration activities in this district have discovered a group of gold-rich quartz-pyrite (QPG) veins and breccias, which are located beside and underneath the Lepanto high-sulfidation deposit. Four major QPG orebodies have been defined; these are known as Northwest, Florence East, Florence West, and Carmen (Subang, 2017). Detailed review and documentation of the paragenesis of the Carmen QPG has illustrated that an intermediate-sulfidation phase of mineralization occurs after high-sulfidation mineralization.

The Carmen QPG is located about 800 m to the south of the Lepanto fault, which controlled the Lepanto highsulfidation enargite-Au mineralization, and 200 m to the northwest of Victoria-Teresa intermediate-sulfidation deposit. The host rocks of Carmen QPG include the Late Cretaceous to mid-Miocene Lepanto basaltic-andesitic metavolcanics basement, Plio-Pleistocene intermediate porphyritic intrusions, and overlying Pleistocene Imbanguila dacitic breccias. Weak precursor porphyry mineralization at Carmen is related to diorite and tonalite porphyritic intrusions that are characterized by quartz-pyrite ± sericite ± chalcopyrite veins and stockworks overprinted by later epithermal (QPG) mineralization.

The main epithermal Au-Cu mineralization at Carmen is controlled by a series of subparallel, NNE-striking, steeply dipping fractures, within an area 300 m long by 200 m wide. Seven main fracture zones have been defined (V501 to V507 from SE to NW). The width of each facture zone varies from 1 to 20 m, while the vertical extension can reach over 350 m in the central veins (V505, V506) but only 60–100 m in the side veins (V501, V507).

Each fracture zone is centered by a massive quartz + alunite ± diaspore ± pyrophyllite-altered core. The width of this alteration zone is typically 3-10 m for the central veins and <5 m for the side veins in basaltic metavolcanic basement. In the overlying dacitic breccias, this central alteration zone may be much wider, leading to individual alteration zones joining together to form a 100-m-wide zone. These zones are surrounded by 2- to 8-m-wide domain of quartz-dickite-kaolinite alteration that transitions outward to muscovite and then illite in both the metavolcanic basement and the overlying dacitic breccias.

Three main stages of mineralization are observed in Carmen. Stage 1 is the main-stage high-sulfidation mineralization, which is characterized by an assemblages of massive pyrite + enargite + minor massive dark quartz ± alunite, occurring as veins or cement of hydrothermal breccias along the steeply dipping fractures. Stage 1 mineralization is more intense in the central veins (V505 and V506), but not well developed in the side veins (V501, V502, and V507), which only occur as <1-cm-wide massive sulfide veins or discontinuous breccias. Stage 1 mineralization is related to moderate grades of copper (1-3%) and gold (1-3 g/t).

Stage 2 is an intermediate-sulfidation stage, associated with quartz veins or breccias in massive quartz-alunite ± pyrophyllite ± diaspore alteration zone, occurring along the same fracture as stage 1. Stage 2 is characterized by banded quartz with muscovite, or chalcedony with illite, associated with banded layers of intermediate sulfidation mineral assemblages, including pyrite ± tennantite/tetrahedrite ± sphalerite ± chalcopyrite, and locally native gold. This stage of veins contains high grade gold (3 to >100 g/t) and contributes the main mineable reserve for Carmen. In the central veins (V505 and V506), the stage 1 massive sulfide is normally rebrecciated and cemented by stage 2 minerals. While in the side veins (V501, V502, and V507), stage 2 mineralization is hosted in isolated vein or hydrothermal breccias, which contain minor stage 1 massive sulfides as clasts.

Stage 3 is characterized by widely distributed but not intensive massive luzonite and enargite with minor pyrite, associated with dickite filling in the vugs and cavities of the host rock. Locally, stage 3 luzonite can be observed interbedded with stage 2 chalcopyrite and tennantite.

The characteristics of alteration and mineralization in Carmen QPG initiated with a high-sulfidation epithermal stage (stage 1). The following stage (stage 2) has the features consistent with intermediate-sulfidation mineralization, whereas the final stage (stage 3) again shows the typical high-sulfidation luzonite and enargite mineralization. The paragenesis of alteration and mineralization at Carmen indicates that the physiochemical conditions of fluid drastically and episodically fluctuated during the evolution of the Carmen mineralization.