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Вскрытие невидимого золота: использование наноразмеров для оценки содержания золота, микроэлементов и изотопов серы в пирите из золоторудных месторождений типа Карлин
One-hundred-nm-resolution secondary ionizing mass spectrometry (nanoSIMS) was used to determine the distribution of Au, Cu, As, Sb, and S, and stable isotopes of S (34S/32S) in gold-bearing pyrite from two refractory Carlin-type gold deposits: West Banshee, northern Carlin Trend, and Turquoise Ridge, Getchell Trend, located in northern Nevada. NanoSIMS maps reveal that gold occurs in two discrete episodes in each deposit. Elevated gold concentrations correlate with elevated concentrations of As, Sb, Cu ± Te, and lower 34S/32S ratios, compared to periods when gold was not deposited. Precipitation of elevated gold and trace elements at West Banshee was followed by precipitation of pyrite with lower gold concentrations, whereas at Turquoise Ridge precipitation of trace element-rich pyrite was followed by precipitation of late ore-stage minerals. These NanoSIMS results are consistent with formation of the deposits by the episodic incursion of gold-rich fluids into hydrothermal systems otherwise dominated by gold-poor fluids. Such gold-rich fluid pulses may be related to incursions of magmatic fluid, which have been shown to have high concentrations of gold, arsenic, and copper in porphyry and high-sulfidation epithermal systems.
Приводятся новые термобарометрические и изотопно-геохимического данные исследования руд и пород нижней известняковой части разреза месторождения. Изучение газово-жидких включений в минералах указывает на принадлежность рудообразующего флюида к солевой системе CaCl2-KCl-NaCl-H2O с концентрацией солей 6.4-9.2 мас. % экв. NaCl. Изотопный состав C, O и Sr карбонатов вмещающих известняков и руд варьирует в пределах δ13C от 2 до -4.0‰ PDB, δ18O от 12 до 23.5‰ SMOW. (87Sr/86Sr)nom = 0.7048-0.7079 и указывает на наличие двух изотопных резервуаров - метаморфогенного флюида, уравновешенного с известняками и магматогенного флюида. Суммарное содержание РЗЭ варьирует от 3.7 до 54.8 г/т, обнаруживая тенденцию увеличения в ряду: неизмененный известняк вмещающей толщи (3.7-5.6 г/т)-перекристаллизованный и мраморизованный известняк (8.7-15.5 г/т)-карбонат всех рудоносных образований (21.8-54.8 г/т). Изотопное датирование циркона (SHRIMP) из цемента рудоносной брекчии дало конкордантный возраст 518.5 ± 3.7 млн. лет. Эта датировка существенно древнее девонского возраста формирования толщ известняка и вулканогенных пород. Предполагается, что циркон и возможно часть тонкого золота попадает в глинистую фракцию известняка, а затем в цемент брекчий при размыве более древних отложений.
A variety of techniques have been used in attempts to date the mineralization in Carlin-type deposits in the Great Basin, with highly variable results. These techniques and results are reviewed in this paper, along with presentation of two new dates, for the Rodeo deposit on the Carlin trend and for the Barneys Canyon deposit in Utah. Complete resetting of sericite by hydrothermal fluids of the temperature and duration of hydrother-mal activity that form Carlin-type deposits is considered highly unlikely. Therefore, dates from sericite are generally of questionable value unless that sericite can be shown to have formed during the same hydrothermal event during which Au was deposited. For the deposits that have been investigated to date, sericite dates rarely, if ever, record the age of Au mineralization. However, sericite ages do appear to record pre-Au events in some districts. Such events may have contributed to ground preparation and, to a much lesser extent, to the tenor of the ore. Fission-track and U/Th-He techniques provide important age constraints on mineralization in some districts but also suffer from a less than clear association with Au. Rb-Sr dating of galkhaite, an Hg sulfosalt, provides the only direct age of mineralization, but galkhaite has been recognized in only a few locations (and dated in only two deposits). In a similar manner, adularia is contemporaneous with Au at Twin Creeks, but Twin Creeks is the only Carlin-type deposit where adularia has been reported. Several other techniques (U-Pb, Re-Os, Sm-Nd) have been used in attempts to date the deposits but with limited success.
Preface to SEG Compilation on Carlin-type Gold Deposits
John Muntean
Nevada Bureau of Mines and Geology
University of Nevada Reno
Since the discovery of the original Carlin gold deposit in 1961, approximately 200 million ounces of gold in Carlin-type deposits have been mined or identified in northern Nevada, making the United States one of the world’s largest gold producers for the past several decades. Despite their importance, Carlin-type deposits remain enigmatic. Controversy over their origin mainly stems from the very fine grained character of the ore and gold. Carlin-type gold deposits are largely hydrothermal replacement bodies with visually subtle alteration dominated by decarbonatization and silicification of carbonate-bearing host rocks. Gold occurs in solid solution or as submicron particles in very fine grained disseminated arsenian pyrite or marcasite. To date, over 90% of the gold mined from what most geologists would call Carlin-type deposits has come from northern Nevada, and debate exists whether similar-looking deposits elsewhere share a comparable origin or whether the deposits in Nevada represent a unique occurrence.
