Tramaine James, PhD
T. James1, S. Perrouty1, D. Kontak1
1Harquail School of Earth Science, Laurentian University, Sudbury, ON, Canada
The two dominant Au systems within Precambrian greenstone belts are orogenic (ORG) and intrusion related (IRG; i.e., magmatic-hydrothermal). Classification of gold deposits into one of these two types is still much disputed due to their similar features, and in some cases the overprinting of IRG by ORG systems, adding further complexity. This study differentiates these systems using the geochemical signature of pyrite grains, analysed through laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) traverses and maps. Samples were collected from gold deposits within the Mazaruni greenstone belt of the Guiana Shield, Guyana, South America. This include two IRG systems (Toroparu Au-Cu(-Ag) and Eagle Mountain Audeposits) and several ORG systems (Aurora Gold Mine:Rorys Knoll, Mad Kiss and Alec Hilldeposits; Omai Gold Mines: Wenot deposit; Oko West gold deposit; and Sona Hill deposit).These deposits are hosted within Rhyacian-age mafic to felsic volcanic, volcaniclastic and intrusive host rocks metamorphosed to greenschist facies assemblages.
Time slice domain (TSD) data obtained from LA-ICP-MS maps and traverses, plotted on Au/Ag binary plots, revealed distinct patterns: IRG deposits exhibit randomly dispersed Au/Ag ratios, whereas ORG systems cluster at Au/Ag = 0.1, 1 or 100. Principal component analysis (PCA) onthe TSD (standardised using a centered log-ratio transformation) identified four significant components (PC1 PC4) with eigenvalues greater than 1.29. K-means clustering analysis of these components subdivided the dataset into four groups. The analysis showed that IRG deposits are enriched in Cu, As, Se, Ag, Sb, Au, Pb and Bi, whereas ORG deposits are depleted in those elements and enriched in Co, Ni and Te. Notably, both IRG deposits have As-rich pyrite (As values between 1000 to 10000 ppm), with As causing lattice distortions that facilitate element substitution due to ionic radius, captured by PC1. This methodology determined that As, Se, Ag, Sb, Au, Pb and Bi are the discriminator elements for differentiating IRG and ORG deposits in the Mazaruni greenstone belt and these findings are broadly applicable to similarly aged greenstone belts worldwide.