Gold quartz passage
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Gold quartz veins are hydrothermal ore and mineral veins that are widespread worldwide and consist largely of quartz (as a gangue) with sprinkles of sulfides and native gold . Until well into the 19th century, they were the most important source of " mountain gold " from the rock and were only surpassed in their importance by sedimentary " wash gold ". Accordingly, gold quartz veins were one of the classic targets for gold seekers and prospectors at the time of the gold rush . However, quartz is one of the most widespread minerals in the earth's crust. Quartz veins with sulfides are among the most common mineral veins. Due to the weathering resistance of quartz, they are also easy to identify on the earth's surface, but as a rule they are free of gold. It took and still needs great patience to locate the few gold-bearing passages among all these "deaf" passages. While the soap deposits are largely exhausted and only of interest to hobby gold seekers, gold quartz veins can still be important vein deposits today.
Plutonic sequence
These gold quartz veins can be found in already deeply eroded orogenous zones of various ages and in the exposed metamorphic basement . As a rule, they are bound to igneous bodies ( plutons ) that have penetrated the adjacent rock at a greater depth (mostly in marine sediments and volcanic rocks that are tectonically deformed by the folding of the mountains ). Since there were not only high pressures but also high temperatures at the time the glowing liquid intrusions took place , the minerals were excreted in narrow crevices opened by tectonic movements under catomal to mesothermal conditions (400–200 ° C). In the depths they sometimes even merge into tourmaline- bearing tunnels, which are considered pneumatolytic formations (over 400 ° C), very close to the magma chamber.
The origin of the dissolved minerals in the fluids is by no means always clear. They can come from the rock melt as well as from the host rock ( lateral secretion ).
The mineral association ( paragenesis ) of these gold quartz veins is very simple: 97–98% is quartz, the rest is sulfide and solid gold. The sulphides are mostly pebbles , as well as arsenic , copper pebbles and occasionally a little antimony luster . The gold contains approx. 10-20% silver and has grown into the other minerals in the form of mostly small droplets and leaves.
The tunnels are usually not very thick (0.5–3 m), but they often extend very deeply and form extensive tunnels of densely packed tunnels staggered one behind the other. Well-known examples are the Mother Lode in the Sierra Nevada (California), the gold districts of Fairbanks in the Yukon Basin (Alaska), Porcupine , Yellowknife and Kirkland Lake in the Canadian Shield (Canada), the gold fields of Bendigo and Ballarat (in the archaic greenstone belts Western Australia), the Kolar district in Mysore (India), as well as numerous occurrences in the Siberian table .
In Europe, the gold quartz veins in the Hohe Tauern were of great importance from ancient times to the Middle Ages. At the end of the 18th century, the Goldkronach mines in the Fichtel Mountains were managed by Alexander von Humboldt .
Sub-volcanic sequence
Gold quartz veins that are tied to sub-volcanic intrusions closer to the surface of the earth, as well as to volcanic chimneys and crevices , are found in younger mountain formation zones that have not yet been eroded as deeply . The mineralization is not only found in tunnels, but also in finely distributed impregnations in tectonic rustling zones , volcanic debris ( breccias ) and porous tuffs . Excretions in open cavities ( drusen ) with diverse mineral paragenesis are common. In addition to the rising meso- to epithermal solutions (300–100 ° C), meteoric groundwater now also plays a role, which seeps into the subsoil and is heated.
In contrast to the plutonic gold quartz veins, the mineralogical composition of the gangue and ore is very complex here. In addition to quartz (often in the amethyst variety or as microcrystalline chalcedony ), calcite is also found as gangue, as well as rhodochrosite and various zeolites . The gold is very rich in silver ( electrum ) and is accompanied by solid silver, as well as by various silver ores such as acanthite , prostite , pyrargyrite , Freibergite etc., which are often worth building themselves . The occurrence of gold tellurides , such as sylvanite and calaverite , and gold selenides is also typical .
The minerals of the adjacent rock (mostly andesite , dacite , rhyolite or trachyte ) were decomposed by the aggressive hydrothermal solutions and converted into (typically greenish) secondary rocks ( propylitization ).
Important examples of sub-volcanic epithermal gold quartz veins include Cripple Creek, Colorado, Comstock Lode, Nevada, and neighboring Gold Hill . As with the numerous deposits in Mexico, the gold in Comstock Lode is already well behind the silver. Worldwide they are particularly tied to the rocky magmatism and volcanism in connection with subduction zones , so in New Zealand, Papua New Guinea, Peru etc.
The most important occurrences in Europe are found in connection with the andesitic-Dazitic volcanic rocks of the Carpathian inner arc: in the Slovak Ore Mountains , Vihorlot Mountains (Slovakia) and the Transylvania Ore Mountains (Romania).
literature
- Siegfried Matthes: Mineralogy. An introduction to the special mineralogy of petrology and reservoir science . Springer, Berlin / Heidelberg / New York / Tokyo 1983, section I. The gold and gold-silver formations , p. 241f.