The Healy quadrangle Is underlain by a wide variety of sedimentary, volcanic, and plutonic rocks, ranging In age from Precambrlan and (or) early Paleozoic to Recent. There are fifty five map units on the geologic map. All the pre-Cenozolc rocks are Intensely deformed, mainly by overthrusting and folding, and most of them underwent at least one period of low-to medium-grade regional metamorphlsm. This deformation 1s the result of the middle Cretaceous collision and subsequent obductlon of the northward-moving Talkeetna superterrane with and onto the Yukon-Tanana and Nixon Fork terranes of the ancient North American continent. Late Cenozoic deformation, the result of continued northward plate motions, has modified but not substantially altered the geology of the quadrangle.
The juxtaposition of disparate geologic terranes in southern Alaska has been previously interpreted to be mainly the result of several hundred kilometers of right lateral offset along the Denali fault system in Cenozoic time. Recent geologic investigations in the Healy quadrangle strongly suggest that Cenozoic horizontal displacements of such magnitude along the Denali fault system do not exist. In the Healy quadrangle, isograds and metamorphic facies boundaries of an early Late Cretaceous metamorphic belt trend across the Cenozoic McKinley strand of the Denali system without significant horizontal offsets. The present geologic makeup of most of southern Alaska is primarily the result of the Talkeetna superterrane, consisting of the previously assembled Peninsular terrane and Wrangellia, colliding with and subsequently being thrust upon the Yukon‐Tanana and Nixon Fork terranes of the ancient North American continent in about middle Cretaceous time. The leading edge of the Talkeetna superterrane faces a wide, complexly deformed zone that contains numerous northwestward thrust miniterranes tectonically intermixed with Jurassic and Cretaceous flysch. The flysch is interpreted to have been deposited mostly in the narrowing and subsequently collapsed oceanic basin between the converging continental blocks. The postcollisional Denali fault system developed in Cenozoic time across the already accreted continental margin, in eastern Alaska along an older, Cretaceous suture.
STRUCTURE and ZONING: Ore minerals sparsely disseminated or in stockwork of thin quartz-sulfide veins. ORE CONTROLS: Deposits commonly occur in skarn and polymetallic vein and replacement districts outboard of all other types of mineralization. Fracture permeability is the most important ore control, Primary rock permeability may be imprtant locally STRUCTURAL SETTING: Shear zones, axial plane fractures in folded rocks ORE DEPOSIT GEOMETRY: Irregular bodies, locally conformable to bedding ALTERATION: Silicification (Taylor, Star Pointer, Cove), and decalcification (Star Pointer) of carbonate rocks; sericite-clay in clastic rocks (Candelaria). EFFECT OF WEATHERING:Leaching and redeposition of Ag as cerargyrite forms bonanza deposits (White Pine district, Nevada; Vekol, Arizona).
This compendium of ore deposits models was assembled for the Colombia Mineral Resource Assessment Project. The objectives of the compendium are: (1) to define mineral deposit types so that all project members have a common vocabulary or deposit classification scheme to which mineral occurrences, favorable geologic environments, favorable geochemical anomalies, and tonnage-grade models may be related; (2) to provide data on the environments of ore deposition so that favorable rocks, structures, and tectonic settings can be easily recognized by project geologists; and (3) to relate possibly the associations of elements within geochemical anomalies to specific deposit types so that geochemical data may be more easily interpreted. A key-word index and an element association index are included to help in achieving the second and third objectives.The compendium is not complete at this stage and many models useful in the Colombia assessment still need to be added. Appropriate models for the Proterozoic shield environments of eastern Colombia are needed.Alteration; Minerals produced by reaction of ore-forming fluids with rocks.List in assemblages. Show zonal or temporal relation between assemblages. Use terms such as potassic (potassium-feldspar+biotite), phyllic (white mica+pyrite), argillic (clay4white mica), advanced argillic (clay+pyrophyllite+alunite+A^O^ minerals) as they apply. Include skarn mineral assemblages where appropriate.Ore controls; List special stratigraphic, structural, or geochemical features that are believed to have influenced ore-mineral deposition.Weathering; Optional. List any special weathering characteristics or secondary minerals that might serve as prospecting guides.Geochemical signature; Elements expected to be anomalous (enriched or depleted) in and near the deposit. List element assemblages and show zonal arrangement where possible.Examples; A Colombian or Andean example should be included where possible.References; One reference for each example. Give name and year. Include complete reference on a separate sheet. Additional information neededSketch; Where possible, include a well-labeled map or section of a deposit, or a cartoon of an ideal deposit, showing ore controls, zoning, and approximate dimensions.
Sandstone copper, shale-hosted copper, redbed Cu, continental redbed, Kupferschiefer type, marine paralic type, reduced facies Cu, Revett Cu. DESCRIPTION Sediment-hosted copper deposits are stratabound, that is, they are restricted to a narrow range of layers within a sedimentary sequence but do not necessarily follow sedimentary bedding. They are epigenetic and diagenetic, that is, they are formed after the host sediment is deposited, but in most cases, prior to lithification of the host. They form independently of igneous processes.
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