Summary:
Pebble is a porphyry-style copper-gold-molybdenum-silver-rhenium deposit that comprises the Pebble East and Pebble West zones of approximately equal size, with slightly lower-grade mineralization in the center of the deposit where the two zones merge. The Pebble deposit is located at the intersection of crustal-scale structures that are oriented both parallel and obliquely to a magmatic arc which was active in the mid-Cretaceous and which developed in response to the northward subduction of the Pacific Plate beneath the Wrangellia Superterrane.
The oldest rock within the Pebble district is the Jurassic-Cretaceous age Kahiltna flysch, composed of turbiditic clastic sedimentary rocks, interbedded basalt flows and associated gabbro intrusions.
The Pebble East and Pebble West zones are coeval hydrothermal centers within a single magmatic-hydrothermal system. The movement of mineralizing fluids was constrained by a broadly vertical fracture system acting in conjunction with a hornfels aquitard that induced extensive lateral fluid migration.
Mineralization in the Pebble West zone extends from surface to approximately 3,000 ft deep and is centered on four small granodiorite plutons. Mineralization is hosted by flysch, diorite and granodiorite sills, and alkalic intrusions and breccias. The Pebble East zone is of higher grade and extends to a depth of at least 5,810 ft; mineralization on the eastern side of the zone was later dropped 1,970 to 2,950 ft by normal faults which bound the northeast-trending East Graben. The Pebble East zone mineralization is hosted by granodiorite plutons and dykes, and by adjacent granodiorite sills and flysch. The Pebble East and West zone granodiorite plutons merge at depth.
Mineralization at Pebble is predominantly hypogene, although the Pebble West zone contains a thin zone of variably developed supergene mineralization overlain by a thin leached cap. Disseminated and vein-hosted copper-goldmolybdenum-silver-rhenium mineralization, dominated by chalcopyrite and locally accompanied by bornite, is associated with early potassic alteration in the shallow part of the Pebble East zone and with early sodic-potassic alteration in the Pebble West zone and deeper portions of the Pebble East zone. Rhenium occurs in molybdenite and high rhenium concentrations are present in molybdenite concentrates. Elevated palladium concentrations occur in many parts of the deposit but are highest in rocks affected by advanced argillic alteration. High-grade copper-gold mineralization also is associated with younger advanced argillic alteration that overprinted potassic and sodic-potassic alteration and was controlled by a syn-hydrothermal, brittle-ductile fault zone located near the eastern margin of the Pebble East zone. Late quartz veins introduced additional molybdenum into several parts of the deposit.
Within the deposit, the Kahiltna flysch is a well-bedded siltstone with less than 10% coarser-grained wacke interbeds; basalt and gabbro are absent. Bedding within the flysch typically dips less than 25ยบ to the east. The diorite sills are found only in the western half of the deposit, whereas some granodiorite sills extend across the entire deposit. Intrusions and breccias of the alkalic suite occupy the southwest quadrant of the deposit.
The deposit is centered on a group of Kaskanak suite intrusions. Olson (2015) describes the sequence and composition of the intrusions within the Pebble deposit as: 1) earliest, voluminous equigranular granodiorite equivalent to the Kaskanak batholith; 2) transitionally porphyritic granodiorite stocks; 3) early-mineral granodiorite porphyry; 4) inter-mineral quartz granite porphyry; and 5) minor late-mineral high-silica quartz granite porphyry. The north contact of the Pebble East zone pluton is close to vertical, and its upper contact dips shallowly to the west; it remains undelineated to the south and has been dropped into the East Graben by the ZG1 normal fault to the east. Contacts of stocks in the Pebble West zone dip steeply to moderately outward.
The Pebble East zone is entirely concealed by the east-thickening cover sequence. The contact between the flysch and the cover sequence ranges from sharp and undisturbed to structurally disrupted with slippage along the contact. The lower half of the sequence comprises a thick basal conglomerate with well-rounded cobbles and boulders of intrusive and volcanic rock types of unknown provenance, overlain by complex, interlayered, discontinuous lenses of pebble conglomerate, wacke, siltstone, and mudstone. The upper half of the sequence comprises volcanic and volcaniclastic rocks dominated by basalt or andesite and intruded by minor felsic to intermediate sills and/or dykes.
The East Graben is filled by basalt flows and lesser sedimentary rocks that have an uncertain relationship to the cover sequence. The graben fill ranges from approximately 4,265 ft thick north of the ZE fault to a thickness of up to at least 6,400 ft to the south. Basalts in the lower half of the graben are cut by two ~65 Ma monzonite porphyry intrusions, which makes them older than the rocks that cover the Pebble East zone.
Eocene rocks are rare within and proximal to the Pebble deposit. Where thus far encountered, they comprise narrow felsic dykes, a pink hornblende monzonite intrusion intersected at depth in the central part of the East Graben, and a rhyolite flow breccia at the top of the East Graben, south of the ZE fault.
Structure
Within the western part of the Pebble deposit, the Kahiltna flysch occurs as an open, M-shaped anticline with axes that plunge shallowly to the east-southeast (Rebagliati and Payne, 2006). The folding predates intrusive activity at Pebble and diorite sills are commonly thicker where they exploited the hinges of the folds. Folding did not affect the cover sequence.
A Brittle-ductile fault (BDF) zone was identified on the east side of the Pebble deposit where it manifests a zone of deformation defined by distributed cataclastic seams and healed breccias. It strikes north-northeast, extends at least 1.86 mi along strike, is up to 650 ft wide and is vertical to steeply west-dipping. The BDF is truncated on the east by the ZG1 fault and does not affect the cover sequence. Displacement was dextral-oblique/reverse (S. Goodman, pers. comm., 2008), and correlation of alteration domains across the fault limits post-hydrothermal lateral displacement to less than 1,310 ft.
The ZB, ZC and ZD faults occur in the Pebble West zone and exhibit normal offset of diorite and granodiorite sills of between 50 ft and 300 ft. Normal displacement on the ZJ and ZI faults is not well constrained. The ZA fault has about 100 ft of apparent reverse movement. A minimum of 820 ft of normal displacement occurred across the steeply west-dipping ZF fault, juxtaposing mineralized sodic-potassic alteration in the east against poorly mineralized, propylitic and quartz-sericite-pyrite alteration to the west. Scissors-style, south-side-down normal displacement on the ZE fault increases from around 100 ft on its western end to about 980 ft on the east side of the deposit. The ZG1 fault forms the western boundary of the East Graben and has a well-defined normal displacement of approximately 2,100 ft in the north and 2,900 ft in the south, based on offset of the contact between the deposit and the cover sequence. The ZG2 fault, which is parallel to the ZG1 fault, has between 880 ft and 1,800 ft of normal displacement. The ZH fault and possible parallel structures farther east mark the eastern margin of the East Graben but remain undelineated.