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Pyrite and Pyrrhotite textures and composition in sediments, laminated quartz veins, and reefs at Bendigo Gold Mine, Australia: Insights for ore genesis

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Thomas, HV and Large, RR and Bull, SW and Maslennikov, VV and Berry, RF and Fraser, R and Froud, S and Moye, R (2011) Pyrite and Pyrrhotite textures and composition in sediments, laminated quartz veins, and reefs at Bendigo Gold Mine, Australia: Insights for ore genesis. Economic Geology, 106 (1). pp. 1-31. ISSN 978-1-887483-01-8

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Abstract

The various types, textures, and compositional zoning of pyrite in the gold-bearing saddle reefs, quartz veins,
and surrounding sedimentary rocks provide new information on the potential source and timing of gold and
arsenic and related fluid processes responsible for mineralization at Bendigo. Nodular diagenetic pyrite in the
black shale tops to sandstone turbidites is enriched in invisible gold and arsenic with mean values of 0.61 ppm
Au, 1,300 ppm As, and Au/Ag <1, based on LA-ICPMS analyses. Other elements enriched in the diagenetic
pyrite within the organic-rich shales are Mn, Zn, Mo, Cu, V, Ba, Ag, Cd, Tl, Co, Ni, Bi, Pb, and Te. In contrast,
euhedral- and growth-zoned hydrothermal pyrite in the turbidites and bedding-parallel laminated quartz veins
contains lower contents of most trace elements but has higher contents of invisible Au and As, especially on
the outermost rim of the pyrite. The gold-rich pyrite rims generally become thicker (a few to hundreds of
microns) in proximity to the gold-bearing saddle reefs. Pyrite in the reef commonly has the highest levels of
invisible Au and As and the lowest levels of other trace elements. It is characterized by Au/Ag >1 and Au/Pb
>0.01. In the deepest stratigraphic levels, below known productive gold reefs, diagenetic pyrite in the most carbonaceous
shales has been replaced by pyrrhotite during metamorphism. LA-ICPMS analyses reveal that the
disseminated pyrrhotite contains similar levels of Ni and Co to the diagenetic pyrite but is strongly depleted in
As and Au. The spatial relationships between organic-rich shales, folded bedding-parallel laminated quartz
veins, and gold-arsenic−bearing saddle reefs, combined with the consistent trends in the trace element composition
of pyrite hosted by these three geological elements, is interpreted to indicate that the black shales were
an initial source of Au and As, and the laminated quartz veins acted as the initial pathways for hydrothermal
fluid flow carrying Au and As from the source shales to the saddle reefs. Maximum gold and arsenic input into
the reefs, principally as free gold plus arsenopyrite, occurred late during deformation toward the end of the
hydrothermal cycle and is expressed by the Au-As−rich rims to hydrothermal pyrite in the sedimentary host
rocks, laminated quartz veins, and reefs. This corresponds with final fold lockup and the development of
through-going fault arrays linking adjacent anticlines. The source of Au and As for this final, and most economically
important, fluid-flow event is considered to be from carbonaceous shales deeper in the basin, where
original gold-bearing diagenetic arsenian pyrite reacts with organic matter and is converted to pyrrhotite, with
release of Au, As, and S to the metamorphic fluid.

Item Type: Article
Keywords: orogenic gold, syngenetic gold, diagenetic pyrite, turbidite-hosted gold, Bendigo
Journal or Publication Title: Economic Geology
Page Range: pp. 1-31
ISSN: 978-1-887483-01-8
Identification Number - DOI: 10.2113/econgeo.106.1.1
Additional Information:

Copyright © 2011 Society of Economic Geologists

Date Deposited: 21 Apr 2011 05:32
Last Modified: 18 Nov 2014 04:16
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