Greens Creek mine is owned by Hecla Mining through wholly owned subsidiaries. Greens Creek Mine is owned and operated pursuant to the Greens Creek Joint Venture Agreement by Hecla Greens Creek Mining Company (57,75%), Hecla Alaska LLC (29.73%) and Hecla Juneau Mining Company (12.52%).
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Summary:
The Greens Creek deposit is a volcanogenic massive sulfide deposit with a relatively high precious metal content compared to other deposits of its type. The host rock consists predominantly of marine sedimentary and mafic to ultramafic volcanic and plutonic rocks, which have been subjected to multiple periods of deformation. These deformational episodes imposed intense tectonic fabrics and folds within the rock. The deposits occur at the contact between Mississippian-age mafic meta-volcanic footwall and a hanging wall of Triassic-age argillite and basalt. Extensive hydrothermal alteration occurred within the meta-volcanic footwall prior to and during ore deposition, converting the basalts to sericite-rich, phyllitic schist. At ore deposition, thick and extensive lenses of base and precious metals, with pyrite and barite, formed at the footwall-hanging wall contact. Major sulfide minerals include pyrite, sphalerite, galena, and tetrahedrite/tennantite.
Work by Taylor and Johnson (2010) in the USGS Professional Paper 1763 indicated that the Greens Creek deposit displays a range of syngenetic, diagenetic, and epigenetic features that are typical of volcanic massive sulfide deposits (VMS), sedimentary exhalative (SEDEX), and Mississippi Valley-type (MVT) genetic models. Based on those observations the investigators indicated that the Greens Creek mineral deposit was a ‘hybrid’ type with elements of several deposit models.
Due to variations in mineralization, structural complexity, and spatial location, the Greens Creek mineralization is segregated into nine separate mineralized zones. In order from easternmost and highest elevations to westernmost, the zones are:
• East;
• West;
• 9A;
• Northwest West;
• Upper Plate;
• 5250;
• Southwest;
• 200 South;
• Gallagher.
The mineralization is stratigraphically controlled and typically found at the contact between the phyllites (stratigraphic footwall) and the argillites (stratigraphic hanging wall). Due to the intense structural deformation, mineralization may be tightly folded into the phyllite or argillite packages such that the original stratigraphic relationships are unclear. In rare cases there may be areas where the mineralized materials are stratigraphically above the phyllite/argillite contact but still proximal to it.
On a gross deposit scale the mineralization trends N 30° W and plunges to the south at approximately 20°. The East Zone outcrops at the eastern edge of the mineral deposit, dips to the west, and transitions into the West Zone near a tight F2 fold where the mineral horizon transitions from a nearly flat orientation to a nearly vertical wall dipping steeply to the west. The East and West zones are bounded on the west by the Maki Fault system which offset the mineral horizon to the north in a dextral sense. The western deformation boundary of the Maki Fault tends to be a continuous fault splay which is called the Kahuna Fault. The mineralization hosted inside the fault zone are called the 9A Zone.
West of the Kahuna Fault, the Northwest West Zone represents the offset portion of the West Zone. Above and to the south of the Northwest West Zone is the main trend of mineralization which includes the Southwest Zone followed by the 200S Zone further down plunge. The 5250 Zone is the along the upper mineralized trend evidenced in the East Zone, just offset across the Maki-Kahuna Fault system.
The Gallagher Zone lies to the west of the 200 South Zone and is west of a second major dextral fault zone known as the Gallagher Fault. Offset on a post-mineralized dike swarm, the trend of the 200S Zone into the Gallagher Fault and the similar structural and chemical styles between the southern 200S and Gallagher mineral zones all indicate that the Gallagher Zone is simply the fault offset of the 200S Zone.
East Zone
The East Zone outcrops at the discovery “Big Sore” gossan and extends down-dip to the west until it is deformed and offset by the D2.5 Klaus Shear at depth or by the Maki Fault at its southern extent. The mineralization occurs along the phyllite/argillite contact and varies from 1 to 30 feet (0.3-9 m) in thickness.
At the surface the mineralization dips at 60–80° to the west with the argillite on the bottom or eastern side. The dip shallows with depth to near-horizontal as a result of F2 folding. Where the mineral body terminates into the Maki Fault drag folding has rotated the mineralization nearly 900 to 850 feet along strike. This geometry indicates that the entire Greens Creek deposit is on an overturned major antiform with stratigraphic up being down.
West Zone
The West Zone is the down-dip extension of the East Zone below the Klaus Shear, and from 75 to 1100 ft in elevation. While quite variable, the overall trend strikes N 30°W for over 2,500 ft (762 m) of strike length and 1,025 ft of vertical extent (75 to 1,110 ft). The thickness is also highly variable from less than 10 ft (3 m) to over 300 ft (91 m) in the central portions.
9A Zone
The 9A Zone is the most structurally dismembered zone at Greens Creek as it lies within the Maki- Kahuna Fault Zone. The general orientation of the mineral body is striking to the northwest and dipping steeply to the west but many internal fault splays cut mineralization at differing orientations. In plan, mineralized widths range between less than 5 ft (1.5 m) up to 100 ft (30 m).
Northwest West Zone
The Northwest West Zone is an extension of the West and East zones with the 9A Zone tying the three together through the Maki-Kahuna Fault system. The structural setting is dominated by a pair of recumbent F2 folds. The upper fold is an argillitecored syncline while the lower fold is a phyllite-cored anticline. Mineral types and mineralization are similar to what has previously been described for the West Zone, with MFB and MFP dominate with some WSI and WCA intermixed.
In the Northwest Zone some mineralization is located up to 100 ft off the mine contact into the hanging wall argillite. Mineral types are a mixture of mostly massive and white-siliceous material types with lesser carbonate, baritic material and mineralized argillites. This zone is particularly rich in zinc, iron and copper with lower silver relative to most of the Greens Creek deposit, again because this is part of the main core of the deposit encompassing the lower East, West, 9A, and Northwest West zones.
Upper Plate Zone
The Upper Plate Zone is located at the far northern end of the Greens Creek deposit and above the northwest West Zone. It is a smaller body representing the top of the Northwest West Zone which was caught up in a very large F2 fold located on the bottom of a major D2.5 shear known locally as the Upper Shear Zone. The recumbent fold has an amplitude of over 3,000 ft with an argillite core no more than 200 ft thick. Mineralization is found mostly on the upper and lower contacts of the fold but does in places cross into the argillite core.
5250 Zone
Immediately west of the Kahuna-Maki Fault system is a lower temperature lens of barite-rich mineralization which links to the north with the Northwest-West Zone mineral body. This lens, known as the 5250 Zone is continuous for up to 1,200 ft (366 m) along a N300W trend and represents the uppermost mineralization trend at Greens Creek.
The mineral types are dominated by white baritic material (WBA) with lesser massive mineral and minor amounts of carbonate and siliceous mineral types. The silver grades are typically higher than average for the Greens Creek mineral bodies while zinc, lead and gold are below average. The mineralized material occurs along the phyllite/argillite mine contact and trends approximately N 35° W. The interpretation shows two limbs of a fold; the western limb dips generally 30º to the west/southwest and the eastern limb dips more steeply at approximately 80º.