Geology And Gold Mineralization Of The Rattlesnake Hills Granite Mountains Wyoming

Author : Kathleen Durk Autenrieth
Publisher :
ISBN 13 :
Total Pages : 518 pages
Book Rating : 4.:/5 (922 download)

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Download or read book Geologic Setting of Gold Mineralization in the Rattlesnake Hills, Wyoming written by Kathleen Durk Autenrieth and published by . This book was released on 2012 with total page 518 pages. Available in PDF, EPUB and Kindle. Book excerpt: New detailed geologic and alteration mapping, 40Ar/39Ar geochronology, and geochemistry has been performed on an 18 km2 area in the Rattlesnake Hills of central Wyoming to establish the geologic context for gold mineralization. The study area is underlain by an Archean greenstone belt whose units include: tonalite gneiss, quartzite, banded iron formation, pelitic schist, amphibolite gneiss, metabasalt, and metagraywacke later cut by amphibolites and granite. These units are folded into a tight, northwest- trending, map-scale syncline that is cross-cut by a - 1494 Ma diabase dike. The only significant fault is the south-dipping North Granite Mountains fault, which experienced minor reverse slip during Cretaceous Laramide deformation and then minor normal slip during the Miocene. The greenstone belt is intruded and overlain by a middle Eocene volcanic complex. The complex within the study area consists of a 45.45 Ma trachydacite dome, 45.40 trachyte and 45.08 to 45.8 Ma trachyandesite domes, 44.65 to 44.72 Ma hypabyssal phonolite diatremes, and younger carbonatite dikes and veins. The domes and diatremes are 100-300 m in diameter and appear evenly distributed throughout the study area. Significant hypogene alteration accompanied the volcanic event, producing propylitic, argillic, potassic, and carbonate alteration assemblages that alter Archean, Mesoproterozoic and Eocene rocks in a north-northeast trending swath. The alteration patterns form a semi-concentric pattern, where lower grade alteration assemblages appear on the outer portion of the alteration path and increase in grade inwards. Whole rock 0́−eochemistry and x-ray diffraction indicates argillic alteration assemblages are commonly overprinted by potassium metasomatism. Supergene alteration of the Archean rocks is also present, producing a zone of gossan and iron-oxide staining parallel to strike. Brecciation is also common, produced by explosions related to diatreme formation and carbonatite emplacement. Gold concentrations recognized to date show a spatial correlation to areas of potassic alteration and areas of carbonatite and carbonate veins. Furthermore, gold mineralization occurs as veins and veinlets associated with adularia and dolomite gangue. These relations support the interpretation that the process of gold mineralization was closely associated with potassium metasomatism and carbonate alteration. Finally, gold concentrations are restricted to a metagraywacke within the Archean UT Creek Formation, evidence that host rock lithology also influenced gold precipitation. A conceptual model for Eocene volcanism and mineralization in the Rattlesnake Hills is as follows: (1) 46-45 Ma emplacement of subalkaline trachydacite domes, alkaline trachydacite, trachyandesite and trachyte domes and plugs; circulation of acidic to nearly neutral hydrothermal fluids, and development of propylitic and argillic alteration assemblages. (2) 45-44.5 Ma emplacement of explosive diatremes and intrusion of phonolite plugs, accompanied by pulses of brecciation. (3)