In New Mexico, there are at least 100,000 abandoned mine waste
rock piles with widely ranging mineralogical and geochemical compositions.
To better understand the environmental implications of metal mobility
in regions of minimal precipitation (< 250 mm/year) a mineralogical
and geochemical study was implemented for five mine waste rock
piles, some of which contained sulfides, as well as the drainage
systems from these areas in the Hillsboro mining district. Furthermore,
this study will give insight into the mineralogical mechanisms
governing metal mobility in arid environments.
The average metal content of a Laramide vein waste rock pile is
1,200 ppm copper, 230 ppm lead, 550 ppm zinc, and 26 ppm arsenic,
but a stream sediment sample directly below the area contains
190 ppm copper, 52 ppm lead, 150 ppm zinc, and 8 ppm arsenic.
The mineralogical assemblage of the same Laramide vein waste rock
pile consists of (in order of abundance) quartz, albite, microcline,
chlorite, illite, hornblende, muscovite, pyrite, chalcopyrite,
hematite, minor bornite, chalcanthite, and actinolite. The stream
sediment contains igneous rock fragments, quartz, albite, chlorite,
hematite, minor pyrite, calcite, gypsum, and cuprite.
Mineralogical characteristics of mineral grains and their weathering
rind products were examined with the electron microprobe to examine
the chemical breakdown of minerals that release metals to the
environment. Several minerals have weathering rinds of different
mineralogical and chemical compositions than their cores. Pyrite
and chalcopyrite appear to be the most reactive to surficial weathering
and tend to show the thickest weathering rinds. Chemical composition
of a pyrite core/rind transect showed iron concentration increased
from 46% to 66% while sulfur decreased from 54% to 2%. Chemical
composition of a chalcopyrite core/rind transect showed copper
concentration decreased from 33% to 3%, iron increased from 30%
to 58%, and sulfur decreased from 34% to 0.3%. Sulfur lost from
the system will be removed by the formation of sulfuric acid after
oxidation occurs. Clays, iron oxide/hydroxide, iron sulfate, and
primary texture rinds exist in the waste rock piles analyzed.
Metal ratios of mine waste rock pile/stream sediment sample are
copper 6.3, lead 4.4, zinc 3.6, and arsenic 3.2. Metal mobility,
therefore, can be described in the following manner: As > Zn
> Pb > Cu. Supergene oxidation of galena created a cerrusite
rind, which effectively shields the galena grain from further
weathering and release of lead and sulfur. This may decrease the
level of lead mobility depending on the overall acid producing
capability of the waste rock pile. Metals present in sulfide mineral
weathering rinds are in higher concentrations than those in rinds
surrounding oxides or silicates. Oxides and silicates need more
time and water to break down and showed at most a minimal clay
rind. This indicates metal mobility is higher in the sulfide minerals,
which therefore enhances metal availability to the environment.