斑岩铜矿
地质学
地球化学
方位(导航)
铜
铜矿石
采矿工程
地球科学
古生物学
冶金
地图学
地理
热液循环
流体包裹体
材料科学
作者
Carlos Carrasco Godoy,I. H. Campbell,NULL AUTHOR_ID
出处
期刊:Economic geology and the bulletin of the Society of Economic Geologists
[Society of Economic Geologists]
日期:2024-07-04
摘要
Abstract Zircon is a common mineral in igneous rocks, which is resistant to both chemical weathering and physical abrasion. Its chemistry can potentially be used to distinguish ore-forming porphyry magmas from barren magma systems. This study compiles >23,000 zircon analyses from >30 porphyry deposits, barren intrusions, and rivers to determine the principal geochemical characteristics of fertile zircons using predictive modeling, and compares them with traditional geochemical thresholds. The results show that the Eu/Eu* and Dy/Yb ratios, P content, and the curvature at the end of rare earth element (REE) patterns (λ3) are the most diagnostic characteristics of fertile zircons. The use of geochemical thresholds, as Boolean conditions, reach their maximum performance for Eu/Eu* and Dy/Yb (sensitivity [sens] = 0.73, specificity [spec] = 0.90), but it is outperformed by the random forest model (sens = 0.91, spec = 0.93) in the testing set. Explanatory analysis of the models shows that the fertility signal in zircons becomes stronger as the porphyry system evolves and is accompanied by an overall decrease in the middle to light REE and P content, characteristics that are absent in barren zircons. We attribute the observed difference in λ3 to the co-crystallization of other accessory phases, suggesting that the changes in the zircon Ce anomaly is controlled by the depletion of light and middle REE. The low P content in fertile zircons is caused by extensive crystallization of apatite. Fertile zircons have an excess of (REE + Y)3+, which we attribute to charge-balance by H+ in hydrous magmas. Simple machine learning algorithms outperform the traditional geochemical discriminators in their predictions and provide insights into characteristics that have not previously been considered for evaluating porphyry copper fertility using zircon geochemistry. We propose simplified methods that can be easily incorporated into exploration workflows.
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