High-Grade Copper and Gold Deposited During Postpotassic Chlorite-White Mica-Albite Stage in the Far Southeast Porphyry Deposit, Philippines

Abstract

Ninety-eight underground diamond holes (~102 km) drilled by Far Southeast Gold Resources Inc. at the Far Southeast porphyry Cu-Au deposit, Philippines, from 2011 to mid-2013, provide a three-dimensional exposure of the deposit between 700- and –750-m elevation, with surface at ~1,400-m elevation. Far Southeast contains an inferred resource of 891.7 million tonnes (Mt) averaging 0.7 g/t Au and 0.5 wt % Cu, equivalent to 19.8 Moz Au and 4.5 Mt Cu. This contribution reports the spatial and temporal distribution of alteration and mineralization at Far Southeast, notably a white-mica–chlorite-albite assemblage that formed after early secondary biotite and before late quartz–white-mica–pyrite alteration and that is associated with the highest copper and gold grades. Alteration assemblages were determined by drill core logging, short-wavelength infrared (SWIR) spectral analysis, petrographic examination, and a quantitative evaluation of materials by scanning electron microscopy (QEMSCAN) study. Alteration is limited around sparse veins or pervasive where vein density is high and the alteration halos coalesce. The alteration and mineralization zones with increasing depth are as follows: (1) the lithocap of quartz-alunite–dominated advanced argillic-silicic alteration that hosts part of the Lepanto high-sulfidation Cu-Au epithermal deposit (mostly above ~700-m elevation), (2) an aluminosilicate-dominated zone with coexisting pyrophyllite-diaspore ± kandite ± alunite and white mica (~700- to ~100-m elevation), (3) porphyry-style assemblages characterized by stockwork veins (below ~500-m elevation), (4) the 1 wt % Cu equivalent ore shell (~400- to –300-m elevation), and (5) an underlying subeconomic zone (about –300- to –750-m elevation, the base of drilling). The ore shells have a typical bell shape centered on a dioritic intrusive complex. The paragenetic sequence of the porphyry deposit includes stage 1 granular gray to white quartz-rich (± anhydrite ± magnetite ± biotite) veins with biotite-magnetite alteration. These were cut by stage 2 lavender-colored euhedral quartz-rich (± anhydrite ± sulfides) veins, with halos of greenish white-mica–chlorite-albite alteration. The white mica is largely illite, with an average 2,203-nm Al-OH wavelength position. The albite may reflect the mafic nature of the diorite magmatism. The quartz veins of this stage are associated with the bulk of copper deposited as chalcopyrite and bornite, as well as gold. Thin Cu sulfide (chalcopyrite, minor bornite) veins with minor quartz and/or anhydrite (paint veins), with or without a white-mica halo, also occur. These veins were followed by stage 3 anhydrite-rich pyrite-quartz veins with white-mica (avg 2,197 nm, illite)–pyrite alteration halos. Combined with previous studies, we conclude that this porphyry system, including the Far Southeast porphyry and Lepanto high-sulfidation Cu-Au deposits, evolved over a period of 0.1–0.2 m.y. Three diorite porphyry stocks were emplaced, and by ~1.4 Ma biotite-magnetite–style alteration formed with quartz-anhydrite veins and deposition of ≤0.5% Cu and ≤0.5 g/t Au (stage 1); coupled with this alteration style, a barren lithocap of residual quartz with quartz-alunite halo plus kandite ± pyrophyllite and/or diaspore formed at shallower depth (>700-m elevation). Subsequently, lavender quartz and anhydrite veins with bornite and chalcopyrite (high-grade stage, avg ~1 wt % Cu and ~1 g/t Au) and white-mica–chlorite-albite halos formed below ~400-m elevation (stage 2). They were accompanied by local pyrite replacement, the formation of hydrothermal breccias and Cu sulfide (paint) veins. Stage 2 was followed at ~1.3 Ma by the formation of igneous breccias largely along the margins of the high-grade zones and stage 3 pyrite-quartz-anhydrite ± chalcopyrite veins with white-mica (mostly illitic) halos. At shallower depths in the transition to the base of the lithocap, cooling led to the formation of aluminosilicate minerals (mainly pyrophyllite ± diaspore ± dickite) with anhydrite plus high-sulfidation-state sulfides and pyrite veinlets. Consistent with previous studies, it is likely that the lithocap-hosted enargite-Au mineralization formed during this later period.