Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 1700
Bạn đang xem bản rút gọn của tài liệu. Xem và tải ngay bản đầy đủ của tài liệu tại đây (51.31 KB, 1 trang )
518 MINERALS/Borates
rock-forming minerals; hence, when magma crystallizes, the boron is thought to leave with the released
water. The combination of high temperatures (300–
400 C) and fluids under high pressure at an intrusive
contact could also extract some boron from the adjacent country rocks. These borate skarn deposits, some
of which are associated with iron ores and magnesium deposits of commercial grade, are mined in both
eastern Russia and China.
Exploration
Borate exploration consists of detailed prospecting of
favourable areas followed by drilling, and uses all the
tools available to the exploration geologist. The recognition of trends of favourable host rocks and structures is an important guide to areas that are of possible
interest. Satellite imagery, both real and false-colour,
and standard photographic interpretation can be
successfully used under certain conditions.
In most parts of the world, the identification of a
Cenozoic suite of non-marine fine-grained sediments
and tuffs is the usual starting point for the field geologist, because most commercial borates are associated
with these rocks. Any accumulation of salts in these
sediments is examined closely. Because many borates
are associated with volcanic rocks, volcanic centres,
flows, ash deposits, and tuffs, particularly if they are
zeolite-bearing, may also be favourable guides to
borate prospecting.
Borate exploration begins with visits to Neogene
basins that appear to be favourable for borate deposition. It should be remembered that the mobility of
boron in the Earth’s crust is very high, like chlorine,
bromium, sulphur, etc. Because of its rapid alteration
by water and carbon dioxide, borate can be altered to
calcite and boric acid washed away as follows:
2CaO Á 3B2 O3 5H2 Ocolemaniteị ỵ
2H2 CO3 carbonic acidị ỵ 2H2 O !
2CaCO3 calciteị ỵ 6H3 BO3 boric acidị
The following criteria, based on the criteria outlined
above in the section on the formation of borate deposits, are used during exploration in Turkey, the
USA, and Argentina:
. contribution of Miocene volcanics and volcanoclastics to Neogene playa-lake basins;
. formation of capping limestone;
. claystone–limestone–marl intercalations;
. evidence of hot springs or hydrothermal solutions
carrying boron into the playa lake; and
. evaporitic horizons or an indication of leaching of
them.
The skarn borates of Eastern Europe and Asia were
found by careful prospecting in geologically wellpreserved fold belts where limy sediments are in contact
with potassic to alkaline volcanics. Datolite and danburite occur in skarns where the limestone was originally rich in calcium and silica; axinite, kotoite, and
ludwigite skarns are hosted by dolomitic limestones.
The magnesium borates and borates associated with
iron ores are generally of low grade. Marine borates
are sought in tectonically stable areas with shallow to
outcropping salt structures capped by gypsum, where
near-surface borate deposits are identified by whitish
soil cover and scanty vegetation.
In arid regions, ulexite often accumulates at, or just
beneath, the current surface of salt flats and playas,
indicating that boron is moving within the system.
These recent crusts may also indicate brine deposits
containing boron concentrations of interest. In either
case, additional prospecting is usually justified; playas
are usually prospected by pitting on a 100–500 m
grid. Springs and recent spring deposits containing
anomalous borates may also be used as a guide to
the presence of ore in certain areas.
Geochemical surveys are useful for narrowing
down prospect areas to a drill target. Both soil and
rock-chip sampling techniques are used in exploration programmes, with boron, strontium, arsenic,
and lithium as a common suite of elements. Beryllium
is also used in the search for skarn borates in Russia,
as are complex boron-magnesium-calcium-chlorine
ratios. Sampling of both surface and well water may
be useful. Certain plants are boron sensitive, and
vegetation surveys may prove interesting.
Geophysical surveys, particularly gravity and magnetic, are used to outline target basins or structures
beneath sedimentary basin fill. Resistivity and seismic
surveys have been used to define basin structures and
formations that may be associated with the borates
in that area. Various down-hole well-logging techniques, including natural gamma and neutron probes,
can indicate the approximate percentage of borates
and clay in zones of special interest.
Geological mapping followed by drilling is still the
definitive test in most areas of the world. While rotary
drill methods may be used, cores are generally taken
of the most prospective zones. Assays of B2O3 and
other associated elements (arsenic, lithium, and strontium) are then run on the horizons that appear favourable for borates. Because the saline borates are water
soluble, short core runs are used, but the common
borates generally core well, with recoveries of more
than 90%.
In areas of doubt, two easy field tests can determine
the presence of borates. The original flame test, where
the mineral is soaked with sulphuric acid and alcohol
