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A Carbonaceous Sedimentary Source-Rock Model for Carlin-Type and Orogenic Gold Deposits

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Large, RR and Bull, SW and Maslennikov, VV (2011) A Carbonaceous Sedimentary Source-Rock Model for Carlin-Type and Orogenic Gold Deposits. Economic Geology, 106 (05/201). pp. 331-358. ISSN 0361-0128

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Abstract

This paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for Au and
As in sediment-hosted orogenic and Carlin-type gold deposits and develops a corresponding genetic model. In
this two-stage basin-scale model, gold and arsenic are introduced early into black shale and turbidite basins
during sedimentation and diagenesis (stage 1) and concentrated to ore grades by later hydrothermal, structural,
or magmatic processes (stage 2). In reduced continental margin basin settings, organic matter, sedimented
under anoxic to euxinic conditions, immobilizes and concentrates gold, arsenic, and a range of trace elements
(particularly V, Ni, Se, Ag, Zn, Mo, Cu, U) present in marine bottom waters, into fine-grained black mudstone
and siltstone of slope and basin facies. During early diagenesis, gold and certain other trace elements (Ni, Se,
Te, Ag, Mo, Cu, ±PGE) are preferentially partitioned into arsenian pyrite that grows in the muds. These
processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and
arsenic, commonly from 5 to 100 ppb Au and 10 to 200 ppm As. Rare organic- and sulfide-rich metalliferous
black shales may contain up to 1 to 2 ppm Au and over 1,000 ppm As, present as refractory gold in arsenian
pyrite and nanoparticles of free gold.
During late diagenesis and early metamorphism (stage 2) the diagenetic arsenian pyrite is recrystallized to
form coarser grained pyrite generations, and the organic matter is cooked to bitumen. Under higher grade
metamorphism (lower greenschist facies and above) arsenian pyrite in carbonaceous shales is converted to
pyrrhotite. These processes release gold, arsenic, sulfur and other elements (Sb, Te, Cu, Zn, Mo, Bi, Tl, and
Pb) from the source rocks to become concentrated by hydrothermal processes, locally to produce gold ores, in
structural sites such as fold hinge zones, shear or breccia zones within or above the black shale sequence.
LA-ICP-MS analyses of diagenetic pyrite in carbonaceous sediments, both associated and not associated
with gold deposits, suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite, are
indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits. Application
of this sedimentary source-rock model enables a systematic exploration approach for sediment-hosted gold
deposits, based on the distribution, composition and structure of carbonaceous shale sequences and their contained
diagenetic pyrite.

Item Type: Article
Keywords: Carlin-type Orogenic gold deposits Carbonaceous Sedimentary
Journal or Publication Title: Economic Geology
Page Range: pp. 331-358
ISSN: 0361-0128
Additional Information:

Copyright © 2011 Society of Economic Geologists

Date Deposited: 11 Aug 2011 04:57
Last Modified: 18 Nov 2014 04:21
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