Orogenic gold and geologic time: a global synthesis

The “chessboard” classification scheme of mineral deposits: Mineralogy and geology from aluminum to zirconium

Lode gold deposits commonly consist of pyrite and lesser amounts of lead and zinc sulfides accompanied by quartz and calcite. Their exact origin remains controversial mainly because of the scarcity of reliable geochronological data. Here we present direct Rb-Sr dating of pyrite from a lode gold deposit and propose the method as a useful geochronological technique for gold mineralization for which age data are poor. A positive correlation between present-day 87 Sr/ 86 Sr and 87 Rb/ 86 Sr ratios of pyrite (FeS 2 ) subsamples from the Linglong gold mine of China corresponds to an age of 122–123 Ma, which dates the age of gold mineralization. The Sr, Nd, and Pb isotopic compositions of pyrites and the associated rocks suggest that the hydrothermal fluids responsible for the pyrite and gold mineralization were probably derived from a mixed source (i.e., degassing of mafic magmas and meteoric water that had leached the country rocks).

Rb-Sr, Sm-Nd, and Pb isotope systematics of pyrite: Implications for the age and genesis of lode gold deposits

The growth in technological innovation that has occurred over the past decades has, in part, been possible because an increasing number of metals of the periodic table are used to perform specialized functions. However, there have been increasing concerns regarding the reliability of supply of some of these metals. A main contributor to these concerns is the fact that many of these metals are recovered only as by-products from a limited number of geopolitically concentrated ore deposits, rendering their supplies unable to respond to rapid changes in demand. Companionality is the degree to which a metal is obtained largely or entirely as a by-product of one or more host metals from geologic ores. The dependence of companion metal availability on the production of the host metals introduces a new facet of supply risk to modern technology. We evaluated companionality for 62 different metals and metalloids, and show that 61% (38 of 62) have companionality greater than 50%. Eighteen of the 38—including such technologically essential elements as germanium, terbium, and dysprosium—are further characterized as having geopolitically concentrated production and extremely low rates of end-of-life recycling. It is this subset of companion metals—vital in current technologies such as electronics, solar energy, medical imaging, energy-efficient lighting, and other state-of-the-art products—that may be at the greatest risk of supply constraints in the coming decades.

https://advances.sciencemag.org/content/advances/1/3/e1400180.full.pdf

By-product metals are technologically essential but have problematic supply

Thallium pollution in China: A geo-environmental perspective.

Omai is a high tonnage, low-grade, world-class gold deposit located in the Paleoproterozoic Guiana Shield. It is the second most important gold deposit in the Guiana Shield (after Las Cristinas, Venezuela), and one of the largest in South America (4.0 million oz.). Sm-Nd and Sr isotope data are presented for host rocks and for scheelite from auriferous quartz-carbonate-scheelite-sulfide-telluride veins from the Omai deposit. Gold-bearing veins are hosted by the Paleoproterozoic Barama-Mazaruni Supergroup, a greenstone belt sequence consisting of mafic volcanic rocks interbedded with sedimentary rocks that are intruded by quartz-feldspar porphyry and rhyolite dikes. This lithologic sequence was folded and metamorphosed to lower greenschist facies during the Paleoproterozoic Trans-Amazonian orogeny. The volcano-sedimentary unit was intruded by a post-tectonic quartz monzodiorite-diorite-hornblendite stock. Initial Nd isotope ratios for the Omai volcanic rocks range from ɛNd=+2.1 to +4.2. These values suggest that this part of the Guiana Shield was a site of new crust formation during the Paleoproterozoic and was not contaminated by older (Archean), reworked continental crust. Initial Nd isotope ratios for the Omai stock range between +0.5 and +2.3, which suggest limited contamination with previously formed continental crust. Although the Nd isotopic ratios of gold-related scheelites overlap with those of the host rocks, particularly the tholeiitic basalts at the interpreted time of vein emplacement, the lack of both isotopic mixing and significant Nd movement during the hydrothermal process suggest that the Nd isotope composition can be used to determine the isotopic characteristics of the ore fluid source area. At Omai, the ore fluid is largely derived from a radiogenic Nd source, represented by mantle or lower crustal reservoirs. Strontium isotope ratios for the scheelites cluster tightly between 0.7019 and 0.7021. The Sr isotope data suggest that unlike Nd, Sr was significantly mobile during the hydrothermal process. The fluids responsible for the Omai deposit may have picked up Sr along the flow path. The constant low Sr isotope values of scheelites probably reflect the key role that the local tholeiitic basalts played as the main source of Sr in the fluids. Whereas Nd isotopes identify the fluid source area, the Sr isotopes map the fluid flow paths.

Nd and Sr isotope study of hydrothermal scheelite and host rocks at Omai, Guiana Shield: implications for ore fluid source and flow path during the formation of orogenic gold deposits

Lithostratigraphy, geochronology and gold metallogeny in the northern Guiana Shield, South America: a review

Geochemical behavior under tropical weathering of the Barama-Mazaruni greenstone belt at Omai gold mine, Guiana Shield

Here we report the first example of a thallium-only deposit, which was discovered in the northeast margin of the Yangtze block, eastern China, in the transition zone between the Yangtze Block, North China Block and Dabie orogenic belt. Ore from the Xiangquan deposit will be mined, ore-dressed and smelted independently for thallium. The ore deposit is hosted in Lower Ordovician micrite, calcilutite and marl within the Dalongwang Mountain Xiao Mountain anticline. The ore zones are fold and fault controlled. Thallium occurs mainly in pyrite but minor amounts form as lorandite (TlAsS2) and hutchinsonite (TlFeS2). Fluid inclusion studies of two stages of fluorite intimately associated with ore-forming pyrite yielded homogenization temperatures of 120 to 220 °C with a salinity of 1.5 to 6.0 equivalent wt.% NaCl. Thallium was originally derived by hydrothermal emanations onto the sea floor and deposited in calcareous sediments. Subsequently in the Early Cretaceous (Yanshanian period) these source beds were reworked to form thallium enriched minerals in hydrothermal veins.

G. Voicu

Papers

Nd and Sr isotope study of hydrothermal scheelite and host rocks at Omai, Guiana Shield: implications for ore fluid source and flow path during the formation of orogenic gold deposits

Lithostratigraphy, geochronology and gold metallogeny in the northern Guiana Shield, South America: a review

Geochemical behavior under tropical weathering of the Barama-Mazaruni greenstone belt at Omai gold mine, Guiana Shield

A preliminary geological and geochemical study of the Xiangquan thallium deposit, eastern China: the world's first thallium-only mine

Geological setting of the Paleoproterozoic Rosebel gold district, Guiana Shield, Suriname