• Abstract and Figures
• Public Full-text

CONTACT INFORMATION Mining Records Curator Arizona Geological Survey 416 W. Congress St., Suite 100 Tucson, Arizona 85701 520-770-3500 http://www.azgs.az.gov [email protected] The following file is part of the James Doyle Sell Mining Collection ACCESS STATEMENT These digitized collections are accessible for purposes of education and research. We have indicated what we know about copyright and rights of privacy, publicity, or trademark. Due to the nature of archival collections, we are not always able to identify this information. We are eager to hear from any rights owners, so that we may obtain accurate information. Upon request, we will remove material from public view while we address a rights issue. CONSTRAINTS STATEMENT The Arizona Geological Survey does not claim to control all rights for all materials in its collection. These rights include, but are not limited to: copyright, privacy rights, and cultural protection rights. The User hereby assumes all responsibility for obtaining any rights to use the material in excess of “fair use.” The Survey makes no intellectual property claims to the products created by individual authors in the manuscript collections, except when the author deeded those rights to the Survey or when those authors were employed by the State of Arizona and created intellectual products as a function of their official duties. The Survey does maintain property rights to the physical and digital representations of the works. QUALITY STATEMENT The Arizona Geological Survey is not responsible for the accuracy of the records, information, or opinions that may be contained in the files. The Survey collects, catalogs, and archives data on mineral properties regardless of its views of the veracity or accuracy of those data. r r b 1148 L! } 4 . ~ 1 A61, - -- ----------------------- --- ~> ABSTRACT Gold is found intimately associated with the sulphide minerals in several Precambrian iron formations . Arsenopyrite and pyrrhotite .generally make up the bulk of the sulphide, while lesser amounts of pyrite, chalcopyrite, and/or loellingite may be present . Quartz, chlorite and carbonate are the common` gangue minerals, however iron rich garnets and amphiboles may be present where metamorphism has- reached the amphibolite facies . Sulphide minerals at the Homestake and Contwoyto Lake deposits are the most obvious examples of sedimentary exhalitive origin, even though they have been highly deformed and metamorphosed . Sulphides at the Cullaton Lake deposit and Central Patricia Mine are related to brecciated and deformed zones in quartz magnetite iron formation . They are characterized by remobilization at lower (greenschist facies) metamorphism and excellent replacement textures present . These deposits are more i nti miately associated ;; i th volcanic rcc1;s and represent a more proximal facies of exhalitive iron formation . ' Gold deposits in the Little Long Lac area are of a similar type . All of these deposits show significant similarities and differences, even within a single area . A model emphasizing an exhalitive origin and including modification by later events and contributions by other processes best explains the origin of these iron formations and their associated gold deposits .. INTRODUCTION Spectacular increases in the price of gold recently have been accompanied by an equivalent increase in interest and activity in many known gold deposits . Gold deposits of particular interest in this regard are those related to Pre- cambrian iron formation . The Homestake Mine is the largest gold mine in North America . It is a classic mineral deposit and is well documented in the liter- ature of economic geology . Two other deposits, namely the Contwoyto Lake property of the International Nickel Company . of Canada and the Cullation Lake "B Zone" of O'Brien Gold Mines Limited were discovered in the course conven- tional prospecting in the early nineteen sixties . Unlike many former producers that were high-graded or proved uneconomical even with the relatively low mining costs of the thirties, these deposits contain significant tonnages of ore grade material averaging 0 .75 oz/ Au/ton . Their main disadvantages have been and are remoteness of location coupled with expensive capital costs re- . lative to proven ore reserves . Several gold deposits in Northern Ontario' are also related to sulfides in Precambrian iron formation, particularly in the Crow River, Little Long Lac and Kirkland-Larder Lake areas . These areas con- tain several long term profitable mining operations and several of which°ilave also been the subject of recent feasibility studies and diamond drilling . The locations of known gold deposits of this type in North America are shown in Figure 1 . Finally, the reader's attention is drawn to the reports of gold found in iron formations from Australia, Brazil, Central and Southern Africa, and India (Sawkins~and Rye, 1974), however, these are beyond the scope of the present study . f The Homestake Mine The Homestake Mine is located at Lead, South Dakota near the northern end of the Black Hills . It was discovered in 1978 .during the working of rich` placer deposits in Deadwood Gulch . During the 9G years it has been in operation, over 2 30 million ounces of gold and 7 million ounces of silver . have been produced . The geology of the deposit .-(Figure 2) has been described by several company geologists, including Staughter (1968), Noble (1950), Noble et al .(1949), and Noble and Harder (1948) . All of the ore is confined to the Homestake formation, a fine-grained quartz-sideroplesite schist csideroplesito + (Fed5 Mg25) COD which is com- monly further metamorphosed to a coarser grained cummingtonite-grunerite schist above the garnet isograd . This unit is approximately 200 feet thick but true stratigraphic thicknesses are obscured by thickening in the noses and thinning in the limbs of folds . Conformable with the Homestake formation are the under- lying Poorman formation - 2,000 feet plus of grey laminated phyllite or mud- stone and the younger Ellison formation - 3,000 feet of grey to black quart- mica phyllite and interbedded quartzites . Tertiary pplift due to intrusion of .quartz monzonite stocks and porphyry =" sills created a large dome structure with Paleozoic sedimentary rocks dipping outward from a core of Eocet`e sfocks and Precambrian schists . Within the Pre- I1 cambrian schist area, rocks are tightly and complexly folded (Figure 2) . Early isoclinal folds in which the rocks have deformed plastically during flexural slip folding plunge 10° - 45° to the southeast, with axial planes dipping to the northwest . In numerous instances the axial planes are themselves folded . These folds vary from the dominant Poorman anticline (several miles in length) to much smaller folds that may become so tightly closed that some : formations are separated into pods-or-lenses . A later phase of folding in- volves superposed "cross folds" caused by penetrative shear folding along planes at a small angle to the axial planes of the earlier folds (Chinn, 1970) . These folds are often best developed in the noses and occasionally on the limbs of the early isoclinal folds, where they cause a pronounced thickening of'the Homestake formation and form favorable structures for the localization of mineralization . • 3 In three dimensions these mineralized structures form pencil or rod like ore shoots . These shoots may coalesce with other shoots and continue at depth until their fold structure dies out . Ore forms only a very small amount of the known Homestake formation (Norton, 1974) . The ore bodies are more or less completely chloritized portions of the cummingtonite or sideroplesite schist-of the Homestake formation . The ch'lorit- ized areas no longer contain the sedimentary quartz bands prevelant in other parts of the formation but rather abundant vein quartz which appears-to be derived from the sedimentary quartz and may be localized in the cross folds (Plate V, Noble, 1950) . The chlorite wall rock immediately adjacent to these veins generally carry good values of gold . The chlorite schist usually con- tains slightly less than ten percent sulphide minerals, mainly pyrrhotite''or pyrite with a smaller amount of arsenopyrite . Pyrrhotite and pyrite may contain small inclusions of gold and occur as tiny grains and elongated lenses of grains often aligned along the bedding of the chlorite, cummingtonite, or sideroplesite schist . Medium to coarse grained euhedral arsenopyrite crystals occur concentrated in zones parallel to bedding . Arsenopyrite is most abundant in the chlorite zones and containsmost of the high grade gold in the form .of tiny inclusions and veins . The average grade of ore from the Homestake Mine has been about 0 .33 ounces of gold per ton of rock for several years . In addition to the Homestake deposit and the recent placer deposits, gold has also been found in the basal conglomerate of the-Cambrian Deadwood form- ation (believed to be a fossil placer) and in vertical fractures of-Tertiary age found in Paleozoic rocks of the Lead area . In the early 1900's some Oro- duction was obtained from gold-arsenopyrite mineralization related to plunging fold structures found in cherty iron rich meta-sediments similar to but st ra- tigraphically higher than the Homestake formation in the Roubaix-Galena district southeast of Lead (Bayley, 1970) and in the'Rochford area south of Lead (Bayley, 1972) . 4 Tertiary geologic activity and mineralization have been used in the past to argue for a Tertiary hydrothermal origin far the Homestake ores (eg . Noble, 1950) versus a Precambrian hydrothermal or syngenetic origin . The only strong evidence for Noble's Tertiary origin of the Homestake min- eralization is the areal distribution of tertiary ore bodies . Chinn (1970) has shown that the "cross folds" to which the ore bodies are restricted, were probably not formed by tertiary intrusives `(Rye and Rye, 1974, p . 297) . On the other hand considerable geologic and isotopic evidence (Table I) indicates a Precambrian origin in a sedimentary-exhalite environment followed by later 1,600 tl .Y .

Load more

  184

Copy Link