Abstract
This study uses Re-Os isotopes from sulfide minerals (molybdenite, pyrite, chalcopyrite,
bornite, sphalerite, arsenopyrite, and galena) and magnetite from high (400–500°C)
and low and low (<300°C) temperature mineralization assemblages associated with the
intrusion of igneous rocks to trace the source of Os and by inference the other metals.
The source of Os is used as a proxy for the source of copper and gold in these systems.
Concentrations of Re-Os for all sulfides except molybdenite in various deposit types vary
from 0.005–2 ppb Os and 0.120–500 ppb Re. Initial
187Os/188Os ratios derived from isochron plots ranges from
0.15–50. These data show interesting relationships among deposits of different
styles within the district, region, and worldwide scales. On the district-deposit scale,
Re-Os isotopes illustrate a complex behavior for the source of Os and by inference other
transition and noble metals. The source of Os can be linked to one intrusive event,
multiple intrusive events, and the sedimentary rocks into which the magmas intersect.
On a regional scale, in the Andean Cordillera, Re-Os isotopes reveal a correspondence
between the initial 187Os/188Os ratios from sulfides of the
ore deposit and amount of copper present in porphyry copper deposits. Eleven deposits
sampled from different crustal blocks and different mineralization ages form a trend in
which deposits that contain large amounts of copper have less radiogenic
187Os/ 188Os initial ratios, suggesting a greater mantle
component. Smaller deposits have more radiogenic Os suggesting greater crustal components.
On a global scale, the significant observation for the deposits that form in arc
environments is that all of the calculated initial 187 Os/188Os
ratios are greater then the chondritic mantle. This indicates that there must be a crustal
source (recycled in a metasomatized mantle, lower/upper continental crust, or subducting
slab) that contributes Os and by inference other metals present in porphyry coppers. Another
feature of the data set is that gold-bearing systems in arc environments associated with
porphyry intrusions have crustal signatures (0.5–2.5) regardless of the style of
the related ore body. For instance, the low/high sulfidation epithermal gold systems in
Bucaramanga Columbia (187Os/ 188Os initial 1.2), the massive
sulfide ores of the Grasberg, Irian Jaya (187Os/188Os initial
0.7–1.2), and Cu-Au skarns in the Hedley District, British Columbia
(187Os/ 188Os initial 1.7–2.5) all have large crustal
components.
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