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Metasedimentary rock-hosted Au-(Sb-Hg) Deposits within the Stibnite Roof pendant, Yellow Pine Mining District, Idaho
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AuthorKonyshev, Sergey A
AdvisorMuntean, John L.
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The Yellow Pine Mining District, located in central Idaho, has produced nearly900,000 ounces of gold and significant amounts of silver (>2.5 million ounces), antimony(50,000 tons) and tungsten (845K units). Midas Gold Inc., who is currently exploring thedistrict, has defined a substantial gold resource and the largest defined antimony resourcedeposit in the conterminous United States. Their goal is to resume production, providedthat ongoing economic, engineering, environmental, and permitting studies havefavorable outcomes, which, thus far, have been moving along successfully. Goldmineralization in the district is located mainly within Late Cretaceous granitic rocks ofthe Atlanta lobe of the Idaho Batholith. Significant mineralization also occurs withinlower Paleozoic metasedimentary rocks in the Stibnite roof pendant. Locally, marblehostedgold mineralization shares some similarities to Carlin-type deposits in Nevada.The metasedimentary rock-hosted mineralization in the Yellow Pine district has bothstructural and stratigraphic controls and is characterized by a Au+Ag+As±Sb±Hggeochemical signature. Native gold and electrum are rare. Gold primarily occurs inarsenian pyrite and arsenopyrite, likely either as nanoparticles or in the crystal structureof these minerals. The Hangar Flats, West End, and Yellow Pine deposits are situated lessthan 6 km from the western edge of the Thunder Mountain volcanic complex, which ispart of the Eocene Challis magmatic complex. Dikes related to the Eocene magmatismoccur within the district.Midas Gold is developing resources that remain in the previously mined YellowPine and West End deposits, as well as in the covered Hangar Flats prospect. Goldiimineralization at the Hangar Flats and Yellow Pine deposits occurs in intrusive rocksassociated with the Atlanta Lobe of the Idaho Batholith, within which mineralization islocalized along the north-to-northeast striking Meadow Creek fault zone. At the WestEnd deposit, mineralization occurs predominantly in metasedimentary rocks and, to alesser degree, in intrusive rocks. Mineralization is strongly controlled by the West Endfault zone, a northeast-striking splay of the Meadow Creek fault zone. Carbonate-hostedgold mineralization is widespread in northwest-striking, steeply-dipping, block-faultedNeoproterozoic to Ordovician metasedimentary rocks that are part of the Stibnite roofpendant.Compared to the Hangar Flats and Yellow Pine deposits, which are intrusivehosted,relatively little was known about the ore controls, ore and alteration mineralparagenesis, and deportment of gold in the metasedimentary rock-hosted occurrences inthe Stibnite roof pendant. Gold mineralization is typically disseminated, but is alsopresent in several temporally distinct generations of veins, which overprint each other,thereby complicating the study. Determining which mineralization event deposited thebulk of the gold was an important objective of this study.The work involved in this study began with field mapping and sampling of keymetasedimentary-hosted gold occurrences, along with core logging, mapping ofaccessible benches in open pits and construction of compilation maps and cross-sectionsof the deposits. This was followed by petrography, including transmitted and reflectedlight microscopy, scanning electron microscopy, and cathodoluminescence; and analysesof pyrite and arsenopyrite with an electron microprobe and laser ablation ICP-MS, asiiiwell as geochronology and thermochronology. Particular emphasis was placed onunderstanding the specific lithologic, stratigraphic, and structural controls on carbonatehostedgold mineralization and identifying cross-cutting relationships between thevarious styles of mineralization, structures, and intrusions. To better understand thechanges caused by hydrothermal fluids, paragenetic studies of ore mineralogy andhydrothermal alteration were completed by sampling transects from fresh rock intoincreasing intensity of hydrothermal alteration and mineralization within the samelithology. Emphasis was placed on understanding the deportment of gold and othermetals in each style of mineralization. In addition, an apatite fission track study wasconducted to try to establish when the roof pendant was exhumed relative to the differentstages of mineralization.Comparisons were made to intrusion-hosted mineralization in the district andCarlin-type deposits in Nevada, as well as similar metasedimentary rock-hosted golddeposits from around the world including deposits in Alaska, China, and Australia bycomparing: 1) ore mineralogy, 2) gold deportment, 3) controls on mineralization(structural and lithologic controls), 4) alteration, 5) host rocks, and 6) trace elementgeochemistry.The overall dissertation is composed of three stand-alone chapters for publicationpurposes as individual papers. Chapter One covers the West End deposit, the largestmetasedimentary-hosted prospect in the Stibnite roof pendant. West End has the bestexposure and most data from previous mining operations and a recent drill programcarried out by Midas Gold Inc., so it has the most available data available for the study.ivChapter Two focuses on the variation of lower Paleozoic metasedimentary rock-hostedhydrothermal mineralization throughout the Stibnite roof pendant, including a briefsummary of West End, and descriptions of the Stibnite and Garnet pits, the trend betweenFern and Saddle prospects, and the Hermes mercury deposit. Chapter Three is dedicatedto the apatite fission track study, which examines the exhumation history of the Stibniteroof pendant. We use our own work to compare it to other regional studies from centralIdaho and then tie it to the exhumation history of the Stibnite roof pendant and themineralization history previously described in Chapters One and Two.Beyond the Yellow Pine Mining District, this research helps further theunderstanding of other metasedimentary-hosted gold deposits, including intrusionrelated,orogenic gold deposits, and Carlin- type gold deposits