DOI: 10.6084/m9.figshare.29421362
title: Perkins Discontinuities: structurally controlled grade patterns diagnostic of epigenetic gold mineralisation at the deposit-scale

description: This study establishes a robust methodology using raw, desurveyed. uninterpolated drill-sampled 3D-grade data to identify superposed structural symmetries and determine mineralisation timing at the deposit-scale. This methodology has transformative implications for exploration and resource estimation of gold and other structurally controlled commodities. We introduce the concept of Perkins Discontinuities—planar boundaries that abruptly terminate mineralisation continuity, creating asymmetric grade distributions. These features , including fractures, low-displacement faults, veins, dykes or intrusions—define asymmetric mineralisation patterns unrelated to fault displacement. Such asymmetry arises from minor permeability contrasts across fractures intersecting the primary tectonic grains defined by fold axes. Our ‘outside-in’, deposit-scale structural analysis of drill-sampled assay-grade data from five well-established epigenetic gold deposits—Ballarat East (Victorian goldfields), Exodus (Carlin Trend), Bronzewing, Aphrodite from the Bardoc Tectonic Zone and Coolgardie (Yilgarn Craton)—reveals two key structural symmetry axes. A quasi-cylindrical axial symmetry (Axis 1) that parallels early shear zones and folds, while oblique Perkins Discontinuities define a later superposed axial symmetry (Axis 2). If Axis 1 and Axis 2 are parallel, gold mineralisation timing coincides at earliest with the first ductile deformation (syn-folding), although a later timing remains possible. Conversely, if Axis 2 is oblique to Axis 1, mineralisation must post-date folding and fracturing, ruling out syngenetic or syn-fold origins proposed by orogenic shear-zone gold models. Applying this approach to three deposits classified as syngenetic—Haile (South Carolina), Chelopech (Bulgaria) and Vaal Reef (Witwatersrand Basin, South Africa)—demonstrates their epigenetic nature. Using Fast Lagrangian Analysis of Continua finite difference modelling of Darcy fluid flow, we show upward-flowing fluids converge towards antiform apices and diverge from synforms, explaining the prevalence of orogenic gold in antiformal saddle reefs and scarcity in synformal trough reefs. A similar process may occur where fracture networks converge in an A-shaped geometry, as at Chelopech. Recognising Perkins Discontinuities and integrating their geometry with deposit-scale grade data redefines gold emplacement timing as late-orogenic in many goldfields, challenging both crustal-scale shear-zone models of orogenic gold emplacement and syngenetic classifications. This refined structural and fluid-flow framework impacts exploration targeting and resource estimation by highlighting post-fold and post-fracture controls on gold localisation, reshaping our understanding of major gold deposit formation. We establish a robust methodology using raw, desurveyed, uninterpolated 3D drill-sampled data to identify superposed structural symmetries, precisely determining mineralisation timing at deposit-scale. <i>Perkins Discontinuities</i> are planar features such as fractures, faults, veins, dykes or intrusions that abruptly terminate mineralisation continuity, creating asymmetric grade distributions caused by minor permeability contrasts between adjacent blocks across the discontinuity. Quantitative deposit-scale structural analysis reveals two rotational symmetry axes: an axis parallel to early folds and shear zones, and a later, oblique axis defined by Perkins Discontinuities, challenging traditional interpretations that suggest ore emplacement occurs before deformation (syngenetic) or simultaneously with folding and ductile shearing. Finite difference modelling of Darcy fluid flow using <i>Fast Lagrangian Analysis of Continua</i> (FLAC) explains why gold accumulates in antiformal fold hinges, showing how folded layers of contrasting permeabilities—along with slight permeability contrasts across fractures that form Perkins Discontinuities—localise mineralisation.