Summary:
The Endako deposit is a porphyry Molybdenum deposit. The geometry and geologic occurrence are such that it is sometimes used as an example to define a style of mineralization known as a Endako Style mineralization.
Property Geology
The Endako molybdenite deposit is hosted within the Endako Quartz Monzonite, which is intruded by younger Casey Alaskite toward the north and François Granite toward the south. In the Endako Mine area, Endako Quartz Monzonite has been intruded by aplite, andesite, quartz-feldspar porphyry and porphyritic granite dykes and post-ore basaltic dyke.
The deposit is elongated in a northwest-southeast orientation with a maximum length of 4,800 m and a width of 750 m. The orebody is a series of major en-echelon moly-sulfide veins that strike from north through east across the deposit and dip west to south.
Structural studies that were initiated in 1973 resulted in the creation of the "Endako Vein System" concept. This model is based on the fact that a complex array of structural elements present in the Endako orebody are actually distributed along certain natural axes. When these axes are properly identified, it allows the grouping of the structural elements into natural workable units or systems. Each system, therefore, possesses a definite structural style, and because hydrothermal mineralogy is a function of structure, each system also possesses a characteristic mineralogical style.
Mineralisation And Alteration
At Endako, the mineralisation consists of molybdenite with a gangue of pyrite, magnetite, minor chalcopyrite and rare bornite, bismuthinite, scheelite and specularite. The orebody consists of a series of sub-parallel or en-echelon quartz-molybdenite-pyrite veins and stockwork veins, veinlets and mineralised fractures. The increase in frequency of these veins along a preferred axis form part of the vein system concept.
Mineralisation occurs in milky white to banded or ribboned quartz veins that are often brecciated and healed by quartz and late-stage calcite and minor chalcedony. Molybdenite varies in grain size from very coarse and greasy to microscopic blue-black grains in quartz referred to as "black quartz ore". A pyrite zone lies to the south of, and adjacent to, the orebody with a transitional boundary in the immediate hanging wall of the South Basalt Fault.
Extensive hydrothermal alteration occurs within the Endako ore zone. K-feldspar bearing envelopes develop around quartz-molybdenite veins and barren quartz veins in the footwall of the deposit. Sericite envelopes, consisting of quartz, sericite and pyrite, are developed around quartz-molybdenite and quartz-magnetite veinlets in the orebody, and quartz-pyrite veins in the pyrite zone. Argillic alteration (kaolinization) is pervasive throughout the orebody, ranging from weak to intense.