June 27th 2017Epithermal and Orogenic gold-deposits in Slovakia - by J. Majzlan
The sand- and claystones of the external west carpathians is unsuitable for ore deposits. The highly metamorphic gneissic rocks and granites – also called ‚Variscan Basement‘ – of the internal west carpathians on the other hand is highly suited for ore deposits.
In postvariscan rocks no Ore-deposits can be found.
In the Dúbrava Au-Sb-deposit stibnite-veins as well as monazite, in which the gold is concentrated, can be found with a mineral concentration sometimes so high granite had to be mixed in to lower the concentration in order to properly process the ore. These minerals were formed due to fluid circulation following the subduction of the Tethys about 150-130 Ma ago.
There is no active mining at the moment, but still reserves are available.
In Lúbietová malachite, manganese- and iron-oxide and phosphate were propably formed between two mylonite layers in an oxidation zone 21 Ma ago. These minerals later were alterated by chalcopyrite and precipitated due to hydrothermal fluids circulating following a small lateral subduction.
Text by M. Hofner
June 9th 2017
Mining in Bavaria – Exploration, exploitation and usage by Bernhard Ratzke
Bergbau in Bayern – Exploration, Gewinnung und Anwendung
Bergbau in Bayern – Exploration, Gewinnung und Anwendung
In the introduction Mr. Ratzke stated that bentonite - named after Fort Benton - is a wrongly labled, industrial mineral which composition is mainly of montmorillonite, kaolinite, feldspar, mica, calcite, quartz, illite and cristobalite and can be found in the regions between Ulm, Augsburg and Passau and in the triangle made up by Moosburg, Mainburg and Landshut, in which the Clariant Produkte GmbH is mining their bentonite. Following the introduction it was explained, that there are two types of bentonite: the Na-rich and the Ca-rich ‚bavarian type‘, which is less reactive than the previously mentioned Na-bentonite and therefore usually chemically activated. This activation is either done acetous by hydrochloric acid and used for the food industry afterwards, or alkaline by sodium bicarbonate and then used for the casting or drilling industry. The speaker then talked in detail about the steps necessary to open a new mine. At first the Bergamt Süd-Bayern (Mining authority of southern Bavaria) has to issue a drilling license, after which the landowner has to give his permission for drilling on his property before exploration itself can start. For exploration sampling-holes are drilled by a vehicle-mounted drilling rig to the upper boundary of the bentonite, out of which samples are taken by a twist drill. These sampling-holes are drilled in a distance of approximately 80-100 m and when Bentonit is found the distance is lowered to create a more accurate representation of the deposit, the so called „Lagerstättenplan“, which is the next step on the way to a new mine. According to this „Lagerstättenplan“ the area of mining is determined, followed by the aquisition of the needed land. Before mining can begin, an operating-plan has to be established, in which size and position of the mining area, the thickness of the bentonitelayer, overburden, recultivation, biology, archaeology and many more topics have to be detailed. After mining has ended the area is recultivated and given back to the previous owner. At the end Mr. Ratzke stated that on average there are 11 m of overburden and only 1.5 m of
bentonite in the six active mines the Werk Bergbau Clariant Produkte GmbH currently opertes, which produce 340‘000 tonnes of bentonite each year. Also there are currently two mines in preperation and six in recultivation. On our field trip to the Werk Bergbau Clariant Produkte GmbH on the 9th of June 2017 we were able to attend the drilling process during the last few meters of drilling and the taking of the samples, of which we were allowed to take some with us. During drilling it was stated that the well trained drilling rig operator is able to notice the upper boundary of the bentonite due to a slight rise in pressure needed to pierce the bentonite, in comparison to the pressure needed to drill through sand and gravel. Right after that we visited one of the six active mines, where Mr. Ratzke gave a short overview over the different qualities of the bentonite and the process of its mining. After that we had some time to take a look around the mine, which luckily was not operating this day.
You can also see some pictures of our fieldtrip when you click here.
Text by M. Hofner
January 27th in Geneva
Porphyry Copper Deposits: From Bottoms to Tops - by R. Sillitoe
Richard Sillitoe graduated from London University where he went on to earn a Ph.D. degree in 1968. After three years with the Geological Survey of Chile and a Shell postdoctoral research fellowship at Imperial College London, he has operated for more than 40 years as an independent consultant to mining companies, international agencies and foreign governments. He has worked on a wide variety of mineral deposits and prospects in nearly 100 countries worldwide, but focuses on the epithermal gold and porphyry copper environments. Published research has earned him awards in Europe, Australia and North and South America, including the Silver and Penrose Gold Medals of the Society of Economic Geologists, of which he was President in 1999 – 2000.
Basically, porphyries are generated in about 1.5 km depth at convergent plates (subduction zones) within a time lap of 10 – 30 Ma between fertilizing the mantle and generation of the porphyries. Therefore it usually contains footprints of up to a dozen different stages, leading to so-called deposit-clustering or –alignment and confronting explorationists on brownfields with different types of basement features they have to analyze. Stocks and dykes are created by uprising fluids whereas it has to be pointed out that size and quality of a deposit are not linked to each other. Even if every magmatic arc is different, we have to distinguish three major styles:
Constructed accretion-reprisms (carbonatous, ferrous) -> poor
Huge calderas by arc-linked extensions (eruptive) -> poor and largely explored
Extensional arc (inter-arc-extension, development of basins [marianna type]) -> rich
In summary: contraction leads to accumulation of magma, a better magmatic fluid and finally to higher-degree-deposits. The mineralization itself is subdivided in early minerals (before mineralization of surrounding rocks), intermineral (implaces whilst mineralization) and late minerals which take place widely later. Contact zones of different stages allow to distinguish them by means of chemism, structure, mathematics et cetera:
Magmatic veins (trunkated)
Floating veins: Zeolith in margin, intrusive contact
Chilled margin (smaller minerals -> darker)
Alignment of phenocrysts very close to contact
Disregarding El Teniente (Chile) and Resolution (Arizona), which are hosted by paleo- or proterozoic dolomite, three of the most bearing deposits are based in basaltic stone (overall-percentage: 80 – 90 %). The best example is the Giant Chuquicamata, which consists of clasts and fine matrix or hydrothermal/igneous cement at different compositions. We differentiate between the magmatic-hydrothermal tribe, which develops out of magmatic fluid at intermineral build, forming a bubble until rock pressure overlines the hydrostatic pressure. This comparatively small bubble lies near surface and is mostly two-times more concentrated in ores than it’s original source (e.g. Maria Mine or Rio Blanco – Los Broncos; double name due to legal boundaries). The second one, the phreatic tribe, has a muddy matrix created at the last stage of mineral intrusions with groundwater coming down through fractures. The magmatic chamber only provides heat via conduction to evaporate the water. The third and last is the phreatomagmatic tribe, a juvenile magmatic component in breccia, consisting of broken phenocrysts in tuff-matrix. In contrast to the second tribe, the hot magma interacts directly with cool groundwater. These three types are leading to either an aphanitic, volcanic or phaneritic, intrusive appearance. An usual paragenesis is tourmaline, quartz, sulfides and sometimes oxides.
The thermodynamically linked deposit-types can be displayed as phase-fields between sulfidation and red/ox-reaction, creating the differentiation in advanced argillic (silica, pyrite), sericitic (quartz, sericite, pyrite, chalcopyrite, bornite), chlorite-sericite (mafic, feldspar, py, cp), potassic/propylitic (epidote, chlorite, py, cp, bn) and sodic-calcic (calcic amphibole + sodic component + diopside, magnesium). When the sericitic begins to destroy the potassic phase of an magmatic intrusion, copper is freed and can be precipitated elsewhere. If the intrusion lies next to a thinly bedded carbonate with high permeability due to alteration, it leads to three different deposit types: The scarn environment between source and scarn front bearing copper and zinc, the CRD’s (carbonate replacement deposits) bearing silver, gold and lead in mean distance and at least the disthal fringes (Hg). Resulting veins can highly differ in appearance due to several stages of genesis and consist of various types (e.g. B-type: linear, sutured vein – EDM-type: finely dispersed within a halo).
In order to strike a magmatic intrusion and therefore a potential profitable deposit, a so-called ‚lithic cap‘ is an important factor. It is usually sedimentary formed after intrusion, acts as an important rock replacement and finally leads to a ledge in topography. Most interesting about a lithic cap is the area around it (initial focus) as there could be erosion remnants of a steam heated horizon (e.g. intermineral sulfidation), indicating a deposit underneath and used for telescoping them. Another hint is the discovery of tourmaline, pyrite and bornite in sericitic rocks pointing to copper or higher py/bn-percentage in larger depths as well as a decreasing pH-value the closer you get to the intrusion.
Even if we know much about generation of ore-deposits today, we still have no clue, why we can get potassic, calcic and sericitic phases out of one magma. One question out of hundreds…
Text by C. Faist
December 23rd 2016
The Discovery and Geology of the Sakatti Cu-Ni-PGE Deposit – by Dr. Christian Ihlenfeld
The 2.05 Ga old nickel-sulfide deposits of Sakatti – formed in large flowthrough magma intrusions – are located in Lapland (northern Finland) roughly 150 km north of the artic circle in the Greenstone Belt.
For the exploration of the Sakatti Deposit first a theoretical geological model was created, followed by airborne measurements of the deviation of the earths magnetic field and an examination of the till geochemistry in the area, which has proven as the most effective sample technique on this exploration.
Following this geochemical analysis it was possible to correlate the surface located tills to sub surface rocks and by this locating the underground ore deposits, which were probed by taking drilling cores.
Text by M. Hofner
December 13th 2016
"Fluid-mediated metal transport in subduction zones and its link to arc-related giant ore deposits: Constraints from a sulfide-bearing HP vein in lawsonite eclogite (Tianshan, China)“ - by Dr. Reiner Klemd
In the research area he proved a continous slab watering and fluid flow through and out the slab, so there was a clear transition from the H2O-rich, ‘wet’ blueschist to the ‚dry‘ eclogite formed by devolatilization. This process leads to a reduction of porosity and permeability. Subsequently he distinguished two different forms of fluid-transport: first the pervasive form within a dihedral angle of at most 60o, which always needs a fluid for dissolving, transport and precipitation. If the angle is bigger than 60o – as in this example – , no fluid flow is possible. Secondly the channelized fluid flow along fractions oft he slab (dehydration embrittlements), which is considerably stronger.
In the mesozoic Tianshan-area the biggest accessible open blueschist can be found in the so called eclogitic belt, which is generally dominated by HP pelitic-felsic schists. After the disappearance oft he glaciers, glaucophane and omphazite are located next to each other, differentiated in pillows and venes. Furthermore Klemd recognized, that the p-T-pathes were strongly different in the same zone, probably arised out of the location at a subduction zone where the rock was altered. He reasoned that the eclogite-front moves forward and replaces the glaucophane, whilst the freed H2O escaped via veins, the ‚fluid-highways‘. This and the metasomatized upper mantle are leading to a strong enrichment of Sr, Pb (300%; circa 110% in the eclogitic selvage) and metals like Cu, Au and Ga.
Text by M. Hofner
November 14th 2016
"Ore formation and hydrothermal processes in Schwarzwald" by Prof. Gregor Markl
In the introduction Prof. Markl gave a short overview on the geographic and geological situation of the Schwarzwald, which is located in the south-west of Germany, and therefore mentioned that about 1000 hydrothermal veins - formed during the Jurassic and Cretaceous - can be found there. Unfortunately it is only visible in one active mine and one that will open in the upcoming years in that region.
He then continued with the explanation of the general forming of ore deposits by mixing of fluids from the sediment and basement aquifer in the area around the active Klara-mine and stated for this purpose that fluid-mobilisation and previous forming of salt-deposits is necesarry because the Cl-ions in the fluid increase the dissolubility of e.g. copper and silver.
To prove this concept Prof. Markl showed that the possible fluid-mobilisation by heating, salt-enrichment or diagenese only occur in an area between 6 and 12 cubic kilometers, then mentioned that the fluid-volume in Schwarzwald was somewhere around 11 cubic kilometers and concluded that the process of fluid-mixing is therefore possible.
The average composition of this hydrothermal veins with a volume of about 10,000,000 m³ had a percentage of 46% fluorite, 35% quarz and 19% barite and depict a perfect record of palaeo-fluid-flows.
With 26 visitors the talk was well attended.
Text by M. Hofner