Isotopic Composition and Alteration Characteristics of Dacite Porphyry, and Their Prospecting Significance in the Dabaoshan Copper Deposit of Guangdong Province

Abstract

BACKGROUND The main types of copper deposits are porphyry and skarn types. Dacite porphyry is a rock type commonly developed in the porphyry Cu-Mo-Au system. The rock can be used as a marker clue for ore prospecting and ore-forming rock when there is well-developed hydrothermal alteration. The Dabaoshan deposit is currently the largest copper polymetallic deposit in the South China Block. Previous studies have shown that the dacite porphyry of the mining area is not only the ore-forming parent rock, but also a favorable ore-bearing wall rock. However, there are some debates about the relationship between dacite porphyry and mineralization in this mining area. With the breakthrough of prospecting for medium and large dacite porphyry ore bodies in recent years, the relationship between dacite porphyry and mineralization has once again become a concern. OBJECTIVES To understand the relationship between dacite porphyry alteration and mineralization of the Dabaoshan deposit. METHODS Based on field work and indoor analysis, X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS) were used to determine copper, tungsten, silicon, aluminum and calcium in Dabaoshan dacite porphyry samples. The isotope composition, metal element contents and alteration characteristics of the dacite porphyry in the mining area were used to discuss the relationship between alteration and prospecting direction. RESULTS The copper content ranged from 107 to 6909μg/g, which was consistent with the copper ore grade of 0.3%-0.5% in the mining area. The LOI and other geochemical indicators that can represent the degree of alteration were positively correlated with copper. Hydrothermal alteration was widely developed in dacite porphyry, including potassium, prophyliticition and quartz-sericitization from early to late stage. Among different stages of alterations, the copper content of quartz-sericitization was 2 to 5 times higher than other alteration types, indicating that quartz-sericitization was closely related to copper mineralization and was an important prospecting marker clue. CONCLUSIONS The deep alteration intensity of the dacite porphyry (below the 270m elevation) has not been weakened, suggesting a great prospecting potential. In addition, the contact zone between dacite porphyry and the Jurassic strata, and the xenoliths in the dacite porphyry are also important prospecting directions, which may host skarn-type copper-rich deposits.

Keywords

• south china block
• dabaoshan
• porphyry copper deposit
• dacite porphyry
• x-ray fluorescence spectrometry
• inductively coupled plasma-mass spectrometry
• major, minor and trace elements
• metallogenic epoch