No abstract

High-grade ore zones in low-sulfidation epithermal deposits are commonly associated with the occurrence of banded quartz veins. The ore minerals in these veins are heterogeneously distributed and are mostly confined to ginguro bands, which can be identified in hand specimen based on their distinct dark gray to black color. Micro-X-ray fluorescence element maps obtained on representative samples of banded quartz veins show that Au occurs together with Ag minerals in some of the ginguro bands, but Au can also be present in quartz bands that are light gray to white and cannot be macroscopically distinguished from barren bands. The occurrence of compositionally distinct ginguro and gankin bands, the latter being a new term coined here for colloform quartz bands containing primarily electrum or native gold, can be explained by temporal changes in the composition of the ore-forming thermal waters or variations in the conditions of ore deposition. Textural relationships, including the dendritic shape of ore minerals that appear to have grown in a matrix of silica microspheres, suggest that the ginguro and gankin bands have formed as a result of rapid deposition associated with vigorous boiling or flashing of the thermal waters.Minerals 2019, 9, 740 2 of 23 in Japan [15]; Ivanhoe in Nevada [14]; Koryu in Japan [7,12]; Kushikino in Japan [5]; and Midas in Nevada [9]. Some of the best examples of ginguro bands occur at the high-grade Hishikari deposit of Kyushu in Japan [6,8,10,16,17]. At all of these deposits, ginguro bands are characterized by extremely high precious metal grades and can contain thousands of ppm Au and Ag. Many low-sulfidation epithermal deposits would not be economically viable without the occurrence of these high-grade ore bands [11].This contribution reports on the distribution of precious metals in banded low-sulfidation epithermal veins as constrained by micro-X-ray fluorescence (µXRF) mapping of hand specimens. Combined with optical petrography, this study shows that native gold or electrum do not always occur together with Ag sulfides or selenides in ginguro bands, but can also be present in significant quantities in other quartz bands. These Au-bearing quartz bands cannot be visually distinguished from barren quartz bands. The findings highlight that comprehensive petrographic studies are required to fully characterize low-sulfidation epithermal deposits as macroscopic vein characteristics only provide limited information on the location of ore minerals within the veins. Implications for the current understanding of the ore-forming processes are discussed. Materials and Methods Sample LocationsThis study focuses on three representative hand specimens of epithermal ores collected from the Hokuryu deposit of Hokkaido in Japan, the Midas deposit in Nevada, and the Sado Kinzan deposit in Japan.The Hokuryu deposit in the Omu mining camp of Hokkaido in Japan represents a small low-sulfidation epithermal deposit that produced a total of~300,000 metric tons of ore for an estimated 68,000 oz Au and~370,00...

High-grade ores in low-sulfidation epithermal precious metal deposits include banded quartz veins that contain gold dendrites. The processes by which dendrite growth takes place have been subject to debate for decades, especially given that these deposits are known to form from dilute thermal liquids that contain only trace amounts of gold. It is shown here that growth of gold dendrites in epithermal veins at the McLaughlin deposit in California (western USA) originally took place within bands of gel-like noncrystalline silica. The gel provided a framework for the delicate dendrites to form. The high permeability of the gel allowed the diffusion and advection of gold from the thermal liquids flowing across the top of the silica layers to the sites of crystal growth within the gel. Over time, the gel hardened to form opal-AG. This silica phase is thermodynamically unstable and recrystallized to quartz that has a distinct mosaic texture.

The Miocene low-sulfidation epithermal deposits of the Omu camp in northeastern Hokkaido, Japan, are small past-producers of precious metals and represent significant exploration targets for high-grade Au and Ag ores. The quartz textures of ore samples and the distribution of ore minerals within quartz veins were studied to identify the processes that resulted in the bonanza-grade precious metal enrichment in these deposits. In the high-grade vein samples, which are crustiform or brecciated in hand specimen, ore minerals exclusively occur within colloform quartz bands. High-magnification microscopy reveals that ore-bearing colloform bands consist of fine-grained quartz exhibiting relic microsphere textures and quartz having a mosaic texture that formed through recrystallization of the microspheres. The presence of relic microspheres is evidence that the microcrystalline quartz hosting the ore minerals formed through recrystallization of a noncrystalline silica precursor phase. The ore-hosting colloform bands composed of agglomerated microspheres alternate with barren colloform quartz bands that are composed of fibrous chalcedonic quartz and mosaic quartz formed through recrystallization of the chalcedony. The findings of this study are consistent with previous models linking bonanza-grade precious metal enrichment and the formation of bands of noncrystalline silica in low-sulfidation epithermal veins to episodic vigorous boiling or flashing of the hydrothermal system in the near-surface environment.

No abstract