Improved process control through real-time measurement of mineral content Page: 2 of 9
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Commercially available XRF spectrometers gene;
consist of a source of excitation, a radiation detector
a display of the spectral response of the sample. Exc
tion sources are either X-ray tubes, or sealed sources
radioactive material. For field use, X-ray sources are
probably not practical because of their size and pow
requirements. However, compared to radioactive soi
X-ray tubes have the advantage of operating on dem
Devices that use radioactive sources are better suites
field applications because they are lightweight (typi<
1-2 pounds), they can run off batteries, and they can
easily housed in a small portable unit. However, rad
tive sources must be replaced periodically, on the or
once per year. Radiation detectors are made from a
ety of semiconductor materials, which differ with re
to resolution, maintenance requirements, cost, opera
conditions, and range of detection. The best radiatio
source and detector for the test site are under investi
MINERALOGY OF THE MINE
The results presented below are based on analysis
field samples collected at the Phelps Dodge Morenc
Mine in southeastern Arizona. The Morenci mining
described below, as paraphrased from R. T. Moolick and
J. J. Durek (1971):
Diorite Porphyry: The southwestern part of the in-
trusive complex is a gray mottled diorite porphyry
containing large phenocrysts of hornblende and labra-
Quartz monzonite porphyry: The monzonite por-
phyry intrusive has the greatest exposed area and is
the principal ore bearing rock. It consists of small,
losely-packed phenocrysts-of orthoclase, albite, and
igoclase in a microcrystalline groundmass of quartz
and feldspar. Small quartz phenocrysts are present
only locally, and quartz is generally confined to the
groundmass. Biotite appears to have been abundant
but rarely preserved. When only weakly altered, the
rock is gray, brownish gray or greenish gray; it is gen-
e aty-strongly altered and light gray or white.
Granite porphyry: Much of the central park'f li 'T-'
trusive complex consists of granite porphyry-otA-
ing medium to large we spaced phenocrysts .f 01
thoclase, albite and quartz. Several ages of granite
porphry occur and have intrusive contacts and ed
textural differences. The youngest granite porpih
contains euhedral qua enocryst ch as n
in diameter and is weakly e older
strict is the largest producing porphyry-copper deposit i1 grardc pzrpnyry usually cntains airallcr guarz
North America. The porphyry-copper deposits in this dis- phenocrysts and more closely spaced feldspar pheno-
trict are associated with granitic rodWnose dbooj n the Morenci f ietPct #1 a Jcrjgeqij RfrMjejiBjj developed
crystals in a fine-grained matrix; fo9lw 0l less e rock types g #sp zggto these stages are diorite porphyry, quartz-
of the geology in this region see thd R f t r and granite porphyry. The rocks pictured indicate the tremendous variety in
R. Titley and C. L. Hicks (1971). TV1W g 8* r jrs at a single *ia main ore body in the Morenci district encompasses
to that of other large copper deposits in volcanic arcs as- about two-thirds of the quartz monzonite porphyry intru-
sociated with subduction zones. Economically important sion Moolick and Durek renort that intense fracturing in
minerals in these deposits generally occur as small grains the
that are either dispersed throughout the rock, or concen- into
trated along closely spaced fractures; these deposits are am y
typically found in and around porphyrytic felsic plutons gen
that are within a few kilometers of the earth's surface.
The Precambrian rocks consist of schist, quartzite, gran-
ite, and granodiorite (see R. T. Moolick and J. J. Durek,
1971). The basement rock is overlain by about 300 meters
(1000 ft) of Paleozoic sedimentary deposits consisting of
quartzite, limestone, and shale. These sedimentary rocks
are, in turn, overlain by remnants of Cretaceous shale and
sandstone that are as much as 250 meters (840 ft) thick.
Tertiary volcanic flows and intrusive pipes of basalt, an-
desite and rhyollite encircle the district.
It does not appear that there was any igneous activity in
the region between the Precambrian period and the Lara-
mide intrusion that occurred during the Cretaceous-
Tertiary period. The stocks or laccolithes and associated
dikes and sills created at that time are almost entirely por-
phyritic in texture, and developed during three distinct
stages. The rock types corresponding to these stages are
As explained by S. C. Creasey (1971), porphyry intru-
sion of a mixed sedimentary sequence was followed by
extensive alteration of the wall rocks and porphyry.
Creasey writes that it is difficult to distinguish between
the metamorphic contact and the hydrothermal alteration.
There are three primary zones where alteration silicates
and ::ded: ;
" Propylitic Zgne ,with chlorite, epidotecalcite, talc,
green b i cu zn
" Amrillic Zone wi- n11z seicite kaolimte mon
morillo ,1 5480 8360 145
* Phyllic Zone, with quartz-sericite and pyrite.
Ore-bearing minerals are found in all of these alteration
zones, as well as in the host rock. The primary ore-
Figure 3. A rock sample from the older-granite-porphyry zone. Alteration is quartz-sericite. Copper
oxide is present in the sample as chrysocolla. The copper concentration measured using XRF analy-
sis is 8360 ppm or 0.83% for this sample.
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Turler, Daniel; Karaca, Murat; Davis, William B.; Giauque, Robert D. & Hopkins, Deborah. Improved process control through real-time measurement of mineral content, article, November 2, 2001; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc717590/m1/2/: accessed November 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.