Short-wave infrared (SWIR) spectral and geochemical characteristics of hydrothermal alteration minerals in the Laowangou Au deposit: Implications for ore genesis and vectoring

Abstract

The Laowangou gold deposit is a newly discovered Au deposit in the North Qinling orogenic belt. Based on megascopic and microscopic textural relationships and mineral assemblages, the hydrothermal alteration and mineralization of the Laowangou Au deposit have been divided into porphyry-type (including potassic, chlorite-epidote and mica-quartz alteration stages) and overprinting mineralization periods. Hypogene Au mineralization mainly occurred as muscovite + quartz + Au-bearing pyrite veins during the overprinting mineralization period, and developed in the fault and fracture zone. SWIR analyses indicate that alteration minerals in the Laowangou Au deposit are chlorite (including Mg chlorite, Fe chlorite, and Fe-Mg chlorite), epidote and white micas with less dolomite, brucite and calcite. Two types of white mica are recognized at Laowangou. Type A white mica partially replaces plagioclase and K-feldspar in the Tangzang quartz diorite, representing the mica-quartz stage, whereas Type B white mica coexists with Au-bearing pyrite, quartz, calcite, and mainly developed in the fracture zone, representing the overprinting mineralization period. Compared with metamorphic minerals, Laowangou porphyry-related chlorite shows lower Al, Ti, and As, but higher Sr concentrations; porphyry-related epidote has higher Ti and Bi, but lower As concentrations. Type B white mica has lower Fe and Mg contents, but larger particle sizes and illite crystallinity (IC) values than Type A white mica, similar to geochemical characteristics of metamorphic white mica. These evidences indicate the Laowangou Au mineralization is different from typical magmatic-hydrothermal Au deposits, and the ore-forming fluid related to Au mineralization may have derived from the depth along faults. Higher wavelength of the Al-OH absorption (>2210 nm) and IC values (>1.2) of white mica can be used as effective vector toward high grade Au mineralization (>1 g/t) in the Laowangou Au deposit.