I would hate to get in an argument as all my knowledge is purely bookish due to a long severe addiction to reading ;=]
The definition I gave was written using several sources that seemed to basically agree. The results may well be unclear.
And as I mentioned, the usage of the term is often unclear or abusive in some of the news releases.
I have always seen "mantos" classified under magmatic hydrothermal systems with skarns in academic literature, that doesn't mean that the phenomenon doesn't occur in sedimentary rocks, but the mantos are often sandwiched between other magmatic rocks within the host.
"manto" mean layer, coating and reminds me of the French word "manteau" although they have derived in two very different geological meanings.
Ex: Causa il traffico, la quantit… maggiore di inquinamento resta evidentemente sul manto d'asfalto.
Most of the pollutants caused by traffic apparently remain on the road surface.
Over a year ago, there was a very interesting text on the subject in the Hard Rock Analyst about one of the properties of Regeena Resources Inc.
"La Cucaracha, northern Chile
This 1,200 Ha project consist of a series options from separate owners, and is located about 100 Km north of the regional commercial and port city of La Serena. Regeena is earning 75% to 100% of the various portions. The largest portion is being farmed in from one the regions larger copper producers. The project is located beside the
Pan-American Highway, which is both the main road and power corridor for the region.
The property contains a large skarn alteration system hosting high grade copper with gold mineralization. The near surface portion of the mineralization has been oxidised to direct leaching copper minerals and free gold. There has been past exploration in the form of test pitting and adit development to examine below a leached cap, but no
trenching to test long exposures or drilling to test depth.
Skarn deposits
DEPOSIT TYPE and EXPLORATION PARAMETERS
Skarns are metasomatic replacement deposits which develop when host rocks are altered by both heat and fluid transfer along the edge of intruding magma. The best host for these deposits is an impure limestone, or other limy (calcium carbonate bearing) sedimentary or volcanic rock. More porous rocks aid and channel the alteration. Skarn may also develop within the intrusive rock which provides the heat and fluids for the replacement. Strictly speaking a skarn is located at the contact between intrusive and host. Deposits of essentially the same character which develop away from contacts are called mantos. Skarn is the more generic term and may indicate either.
Individual skarn zones tend to have variable grades and thickness, and deposits within a region may have variable economic metal components. They usually form relatively small and irregularly shaped deposits in the 2 to 4 million tonne range. There are a number of deposits in the 10-20 million tonne range and a very few skarn systems which total over 100 million tonnes. There is no "typical" skarn, but the main producers host copper, tungsten, zinc-lead and gold-silver in varied combinations. The great virtue of skarns is high grade and a relatively short development period from initial discovery. Mantos deposits tend to be of more tabular from, and are defined as such in some areas; mantos tend to be of more consistent or predictable grade, once they are understood. In South America mantos may indicate an oxidised body.
Stronger zones result where rock layering (depositional bedding) or fractures in the host are dipping towards or pointing at the intruding maga, and where fractures developed in the cooling intrusive point at the host rock. Zones may develop where the flow from the intrusive is impeded by a impermeable rock. Mantos are often formed in porous beds between two more impervious rock units. The other major influence is bowl shaped embayments or other features which create large contact surfaces between the intrusive and host rocks. This aids rapid and complete alteration of the host, just as spreading batter thin on a griddle speeds cooking and prevents lumps. Recognising these various relationships is the key to full evaluation of a skarn systems potential. La Cucaracha's grade potential is evident in the surface exposures. Regeena is now sorting out these various relationships to determine form and size potential of the deposits.
La Cucaracha is a series of skarns and silicified limestone interbedded with volcanic units. The system has been traced for at least 2 Km of strike and appears to be half this distance across strike. The individual beds appear to be up to 75 m (250 ft) thick. Earlier this year RGA sampled various pits and adit exposing oxide mineralization below the leached capping. A series of 8 samples from the 70 m long adit averaged 1.8% copper and 1.6 g/t (0.05) gold. Limited testing indicated that direct leaching of the material recovered 90% of the copper. The rough average of all the sampling was 2% copper and 2 g/t gold. Neither the leached capping nor the silicified limestone was sampled for gold content.
Regeena expects to complete up to 14 drill holes and a series of bulldozer trenches this year. The information from this work will be used to re-evaluate the system and various targets to determine priorities for further testing after the new year. This is a large system, and the grades of the near surface exposures is quite good. These systems can be frustrating in the early going until the details of the geometry is worked out. Given the size of the system and its location near amenities needed for production, we feel patience will be rewarded with a sizeable deposit."
My pleasure, Leo. |
