Observed and Experimental Pit Lake Water Quality in Low Sulfidation (Quartz-Adularia) Gold Deposits, Nevada

ABSTRACT
Within the next 10 to 15 years, over 35 mines in Nevada will
have lakes in their open pits after dewatering and mining ceases. The
large bulk-mineable deposits in Nevada mined below the water table
are of several types, including low sulfidation quartz-adularia Au
deposits, Carlin-type Au, quartz-alunite precious metal, and porphyry-
Cu(-Mo) deposits. The quartz-adularia type systems have
been evaluated by sampling from four Nevada open-pit mines that
currently contain water, and by conducting static leaching experiments
using rock and water samples from one example deposit.
Decreases with time in both alkalinity and Ca were observed in the
experimental waters, as well as the sampled pit lakes, suggesting
that co-precipitation of metals with calcite and/or clays as groundwaters
interact with wall rocks may be an important process in the
pit-water evolution.
Each of the four pit lakes studied developed in similar geologic
settings related to the general characteristics of the ore deposit
group, yet the water quality in the lakes exhibits some distinct differences,
particularly in one of the deposits (Boss). Principal factors
contributing to differences in water chemistry among the four pit
lakes include variable dilution with surface waters and varying
degrees of evaporative concentration. In general, pH is near neutral
to slightly alkaline in all of the pit-lake waters, and metals concentrations
are low. Most metals are present in concentrations well
below the Nevada drinking water standards. Concentrations of As
are low in three of the four pit lakes evaluated. Water from the Boss
pit lake, in southern Nevada, is much more concentrated than that of
the other three lakes, and some components of concern are also considerably
elevated over predictions based on experimental leaching
tests using the Boss deposit wall rocks and local groundwaters. The
differences between the Boss lake samples and the experimental
results, and between the other three pit lakes evaluated, can be partly
attributed to the location of the Boss pit lake at the edge of a
Quaternary pluvial lake. The addition of pluvial sediments, rich in
evaporitic components, and of surface water that has dissolved
these, has the potential to contribute high concentrations of Na, Ca,
and SO4. None of the other lakes evaluated have been exposed to
this influence. The contribution of these elements from a surficial
source to the Boss pit lake was not simulated in the experimental
tests, which explains the large discrepancy between observed and
experimental results.