G. Simon et al., Geochemistry and textures of gold-bearing arsenian pyrite, Twin Creeks, Nevada: Implications for deposition of gold in Carlin-type deposits, ECON GEOL B, 94(3), 1999, pp. 405-421
Citations number
53
Categorie Soggetti
Earth Sciences
Journal title
ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS
This study relates morphology, size, texture, and chemistry of arsenian pyr
ite in the Twin Creeks Carlin-type gold deposits to the chemical position a
nd amount of gold that it contains. Arsenian pyrite is confirmed as the mos
t important gold-hosting mineral and four types of gold-bearing arsenian py
rite are recognized. Among these four types, the lowest amount of gold and
arsenic (<1 wt % As; 17-60 ppm Au) is contained in arsenian pyrite with rel
atively large grain sizes (10-30 mu m) It formed at relatively high tempera
tures (250 degrees C) and K+/H+ ratios (adularia stable), probably relative
ly slowly The largest amount of gold and arsenic (1.05-2.43 wt % As; 595-1,
465 ppm Au) is contained in arsenian pyrite with small grain sizes (up to 2
mu m) that formed at low temperatures (120 degrees-200 degrees C) and mode
rate K+/H+ ratios (illite stable), probably by relatively rapid deposition.
XANES, EXAFS, and SIMS analyses show that gold in arsenian pyrite is both s
tructurally bound (Au-I) and in submicroscopic inclusions of free gold (Au-
0). Highest amounts of structurally bound gold were observed in the relativ
ely coarse-grained arsenian pyrite noted above, whereas most inclusions of
free gold were observed in fine-grained arsenian pyrite. In general, there
is a good correlation between gold and arsenic in arsenian pyrite, although
Au/As ratios vary from one pyrite to another. It is suggested that gold in
arsenian pyrite was deposited by direct deposition of native gold and by a
dsorption of Au-I from hydrothermal solutions. Adsorption of gold by arseni
an pyrite can take place from solutions that are not saturated with respect
to native gold. Relative volumes of gold-bearing pyrite from different par
agenetic stages at Twin Creeks suggest that as much as 50 percent of the to
tal gold in the deposit could have formed from solutions that were not satu
rated with respect to native gold.
Because both Au-0 and Au-I are found together in arsenian pyrite, it is lik
ely that they were deposited by the same process. Consideration of possible
depositional reactions suggests that a decrease in the activity of reduced
sulfur is most likely to cause gold deposition.