Download Provocative observations from the Carlin trend: A case for

Provocative observations from the Carlin trend: A case for “unconventional
thinking”
Poul Emsbo, USGS, Denver, United States
Carlin-type deposits in northern Nevada were first recognized in the early 1960’s. Since,
the province has become the third largest gold-producing province in the world with
historic production and reserves of 6,000 t. The genesis of these incredible deposits has
been intensely controversial largely due to their uncommon physical and geochemical
characteristics. Moreover, the Carlin province formed in a distal foreland setting and is
underlain by a sequence of miogeoclinal sedimentary rocks well inboard of accreted
terranes – not a tectonic setting characteristic of arc-related porphyry-epithermal gold
deposits nor orogenic gold deposits. Thus, viewed in the context of deposit-scale
characteristics and gold endowment, Carlin-type deposits in Nevada are unique.
Nonetheless, three conventional models established for other gold deposit types
underpin the genetic hypotheses for Carlin-type deposits. These well established
processes invoke metamorphic, circulated meteoric, or magmatic waters for the
generative fluids. In both metamorphic and circulated meteoric models, adherents
suggest that gold was scavenged from sedimentary or metasedimentary rocks below
the deposits, transported to shallower levels, and deposited along dilatant faults during
mid-Tertiary extension. Currently favored magmatic models, propose that magmas
provided the fluids, ore components, and thermal energy to drive the fluid flow that
transported and deposited gold beyond the source intrusion. Despite strongly held
opinion, the fact remains that the isotopic geochemical, and geological evidence for
these models is equivocal and as is evident from the ongoing debate, no single model
neatly explains features of Carlin-type deposits in northern Nevada.
In light of discrepancies in conventional models, it is time to reexamine and interpreted
anew, and evaluate other processes that may explain the unique gold endowment of
this province. Previously we have proposed a variant of the meteoric circulation model
in which meteoric water evolves to an ore fluid by shallow circulation through sedex Auand S- enriched rocks that formed during mid-Devonian basin evolution. Here extensive
petrographic, geochemical, stratigraphic and biostratigraphic evidence suggests a
massive introduction of mid-Devonian gold into the host rocks of the Carlin trend by a
previously unrecognized variant of the sedex gold system. Specifically, it can now be
shown that Au and associated metals were transported by petroleum and introduced
onto the Devonian seafloor along with the hydrothermal brine of the sedex system,
enriching the host stratigraphy for 100’s km2 across this Au province. Together, these
observations suggest that petroleum system concepts may be important in evaluating
the genesis of this extraordinary gold province.
Volcanogenic and exhalative: A symposium dedicated to Richard W. Hutchinson