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Quartz vein emplacement mechanisms at the E26 porphyry Cu-Au deposit, New South Wales

Anthony C. Harris and Rodney J. Holcombe
Quartz vein emplacement mechanisms at the E26 porphyry Cu-Au deposit, New South Wales (in Alkalic porphyry Cu-Au deposits, BC, Canada, Thomas Bissig (prefacer) and David R. Cooke)
Economic Geology and the Bulletin of the Society of Economic Geologists (June 2014) 109 (4): 1035-1050

Abstract

Porphyry Cu-Au-Mo deposits are characterized by numerous fracturing events. Brittle fracture of mineralized intrusions and the surrounding host rocks is a consequence of hydraulic breakage caused by the interplay of the changing physiochemical properties of magmatic fluids and/or the local tectonic regime. We have mapped more than 1,400 quartz-sulfide vein sets at the Ordovician Endeavour 26 North (E26) deposit, central New South Wales, in order to constrain the processes influencing fracture development around small (approximately 200 m) porphyritic intrusions. Veins from intermediate levels in the E26 deposit define complicated patterns within 10 m of the porphyritic intrusions, whereas more systematic patterns occur up to 50 m from the intrusive contact. The outer parts of the ore deposit are characterized by quartz veins with orientations of approximately 340 degrees and subordinate orthogonal sets of bisecting veins at approximately 240 degrees and approximately 290 degrees . The data show a broad range of vein orientations that define a conical distribution around a vertical axis. There is a rare subset of near-horizontal veins that formed early relative to the main stage of ore-bearing vein emplacement. Many quartz veins exhibit fault displacements (at both micro- and macroscopic scales) and fabrics (including serrate fractures, slickenlines, and asymmetric fiber trails). The distribution of veins suggests that the dominant fracture array developed perpendicular to the direction of bulk finite extension, but significant vein dilation also occurred in orientations with a component of shear strain. We propose a physical model that combines intrusion-driven hydraulic fracturing into a preexisting fracture mesh to help explain the observed vein patterns and to better link fracture development and vein emplacement. Hydraulically driven deformation was important locally at E26, contributing to the formation of the irregular fracture stockwork immediately adjacent to the small intrusive bodies. The more predictable vein patterns that occur away from the porphyritic intrusions require part of the hydraulically driven extension to have reactivated the existing fracture mesh. Parts of this more systematic geometry, including the occurrence of flat veins, are probably related to the emplacement of larger (10-km diameter) magma bodies adjacent to E26, with doming causing the subordinate fracture arrays, including the gross conical distribution. Fluctuating fluid pressures, combined with hydraulically driven extension, caused dilation of preexisting and synmineralization fractures and superposition of veins with similar orientations. Vein patterns suggest that the regional stress state that existed prior to or synchronously with porphyry ore deposit formation strongly influenced vein orientation to within tens of meters away from the mineralizing intrusions. Fractures that localized ore in the E26 porphyry deposit developed in response to the infiltration of fluid batches, including magma and magmatic-hydrothermal fluids, into a regional deformation environment.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 109
Serial Issue: 4
Title: Quartz vein emplacement mechanisms at the E26 porphyry Cu-Au deposit, New South Wales
Title: Alkalic porphyry Cu-Au deposits, BC, Canada
Author(s): Harris, Anthony C.Holcombe, Rodney J.
Author(s): Bissig, Thomasprefacer
Author(s): Cooke, David R.
Affiliation: University of Tasmania, ARC Centre of Excellence in Ore Deposits, Hobart, Tasmania, Australia
Affiliation: University of British Columbia, Mineral Deposit Research Unit, Vancouver, BC, Canada
Pages: 1035-1050
Published: 201406
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 69
Accession Number: 2014-058275
Categories: Economic geology, geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sect., sketch maps
S37°30'00" - S28°15'00", E141°00'00" - E153°30'00"
Secondary Affiliation: University of Tasmania, AUS, AustraliaUniversity of Queensland, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201431
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