eCite Digital Repository

A Carbonaceous sedimentary source-rock model for carlin-type and orogenic gold deposits

Citation

Large, RR and Bull, SW and Maslennikov, VV, A Carbonaceous sedimentary source-rock model for carlin-type and orogenic gold deposits, Economic Geology and The Bulletin of The Society of Economic Geologists, 106, (3) pp. 331-358. ISSN 0361-0128 (2011) [Refereed Article]

Copyright Statement

© 2011 Society of Economic Geologists, Inc. Society of Economic Geologists, 7811 Shaffer Parkway, Littleton, CO 80127, USA

DOI: doi:10.2113/econgeo.106.3.331

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 Details

Item Type:Refereed Article
Keywords:carbonaceous sedimentary rocks, orogenic, Carlin-type, gold deposits
Research Division:Earth Sciences
Research Group:Geology
Research Field:Ore Deposit Petrology
Objective Division:Mineral Resources (excl. Energy Resources)
Objective Group:Mineral Exploration
Objective Field:Precious (Noble) Metal Ore Exploration
Author:Large, RR (Professor Ross Large)
Author:Bull, SW (Dr Stuart Bull)
ID Code:74063
Year Published:2011
Web of Science® Times Cited:127
Deposited By:Centre for Ore Deposit Research - CODES CoE
Deposited On:2011-11-10
Last Modified:2017-10-30
Downloads:3 View Download Statistics

Repository Staff Only: item control page