Deposit-scale structural architecture of the Sigma-Lamaque gold deposit, Canada—insights from a newly proposed 3D method for assessing structural controls from drill hole data

Abstract

Deposit-scale structural analysis is a practical method of structurally analyzing 3D drilling data. This interpretation method uses readily available distributed 3D data at the most relevant scale for mining and exploration. This method does not depend on collecting traditional structural data from core, such as orientation of planar and linear data, but relies on the premise that grade distributions sampled from drilling provide insight into the structural control of the deposit, with the interpretation of structural geometry done using a computer 3D viewer. This method is applied to the well-studied Archean Sigma-Lamaque gold deposit, which features late-orogenic vein mineralization, and which is controlled by three orders of structural features. The first-order control is the easterly plunging pipe-like conduit parallel to F2 fold plunge defined by the intersection of bedding (S0) and foliation (S2) (~ 1.5 km radius × 3 km down-plunge). The second-order controls, nested within the first-order, are the younger subvertical Late Diorite plugs that intruded vertically into the host rocks along the established S2 foliation anisotropy (1250:400:150 m dimension for Main Plug). The third-order control, laterally limited by first- and second-order structural controls, is the well-documented gold-bearing vein sets (1–10 m thick and up to 1.5 km in extent) that developed in the Late Diorites and in the older folded volcanogenic stratigraphy and diorites. It is proposed that the first-order control defined by the moderately plunging F2 fold is the main fluid pathway resulting from structural permeability formed during earlier folding. The second-order Late Diorite plugs and other competent lithological units, which intersected and fractured during late-stage D2 compression, served as chemically reactive sites causing gold to precipitate along this first-order pathway. It is predicted that intrusions that do not intersect the first-order structural conduits are less likely to be mineralized. Greenfield orogenic-gold exploration requires that the first-order controls are identified, and the geometry of hydrothermal alteration is best understood in the context of the structural architecture of the host rocks.