Typomorphic Characteristics of Germanium-Enriched Sphalerite Minerals and the Substitution Mechanism of Germanium from Wusihe in Southwestern Margin of the Yangtze Block

Significance: Germanium (Ge) is widely used in important fields such as semiconductors，infrared optics，optical fibers，polymerization catalysts，and medicine，due to its excellent thermal conductivity，electrical conductivity，high refractive index，and low dispersion properties^[1-2]. Notably，nearly three-quarters of industrial Ge worldwide is sourced from sphalerite in lead-zinc deposits^[3-4]. The mineralogical typomorphic characteristics of sphalerite can often effectively reflect the formation environment and genetic type of the deposit^[5-6]. Therefore，conducting deep research on the enrichment mechanism of Ge in lead-zinc deposits and the typomorphic features of Ge-enriched sphalerite is not only significant for further understanding the mineralization patterns of Ge-enriched lead-zinc deposits but also holds practical value for the exploration and development of Ge resources. The Wusihe lead-zinc deposit，located on the southwestern edge of the Yangtze block，is an important lead-zinc polymetallic deposit in the Sichuan—Yunnan—Guizhou metallogenic belt. Experts and scholars have gained a deep understanding of its metallogenic geological background^[13]，sources of metallogenic materials^[14-15]，characteristics of metallogenic fluids^[14，16]，petrogeochemical features^[14-16]，and deposit genesis^[13-18]. Luo Kai et al.^[22] also discovered differential enrichment of Ge in sphalerite from different stages within this deposit. Whether the typomorphic characteristics of sphalerite affect the enrichment of Ge remains worthy of further investigation. Additionally，there are still two differing views on the genesis of the Wusihe lead-zinc deposit: one as a sedimentary exhalative (SEDEX) deposit^[13] and the other as a Mississippi Valley-type (MVT) deposit^[14，16-18]. Therefore，focusing on the research on the typomorphic characteristics of Ge-enriched sphalerite and the genesis of the deposit in this area，this paper primarily adopts two testing methods: microscopic spectrophotometry and LA-ICP-MS. Microscopic spectrophotometry can obtain mineral reflectance，visual reflectance，dominant reflectance wavelength，reflectance saturation，and other characteristics within millimeters，featuring high efficiency and convenience. In geological research，the application of microscopic spectrophotometry is mainly for determining the reflectance of organic matter vitrinite in strata，with less frequent use in measuring the reflectance of metallic minerals. Furthermore，the microscopic spectrophotometer used in this test has a beam spot size reaching the micrometer level，which can match the beam spot size of in-situ composition tests such as LA-ICP-MS. By using microscopic spectrophotometry to test the color (body color) typomorphic features of Ge-enriched sphalerite，we obtain the reflectance color index characteristics of Ge-enriched sphalerite，including visual reflectance (R_vis)，dominant reflectance wavelength (λ_d)，and mean reflectance saturation (P_e). LA-ICP-MS testing technology is a commonly used method for testing trace elements in mineral micro-zones，characterized by high spatial resolution，low detection limits，and low operating costs. By utilizing LA-ICP-MS to test the trace element typomorphic features of Ge-enriched sphalerite，we then quantitatively analyze the elemental content characteristics of Mn，Cu，Ga，Ge，Ag，Cd，In，Pb，Fe，etc. Finally，a comparative analysis is conducted on the two types of data.

Methods: The experimental samples were primarily sourced from the Wusihe lead-zinc deposit located on the north bank of the Dadu River. Based on field geological investigations，systematic sampling was conducted in mining areas 1，2，5，and 12 (as shown in Fig.1c). The preparation of thin sections and probe slices was completed at the Rock and Mineral Testing Center of the Hebei Geological Survey and Mapping Institute，with the thin sections polished to dimensions of 3.7cm×2.6cm×0.6cm and the probe slices to a thickness of 150μm. Microscopic observations of the thin sections and probe slices were performed at the Comprehensive Rock and Mineral Identification Laboratory of the College of Earth and Planetary Sciences，Chengdu University of Technology，using a NiKon LV100POL polarized light microscope equipped with a NiKon DS-Ri2 imaging system. In-situ spectrophotometric analysis of sphalerite was conducted at the National Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering，Chengdu University of Technology. The spectrophotometric testing system used was the Axio Scope.A1 high-resolution microscope equipped with polarized light analysis (A Pol) functionality，jointly produced by Carl Zeiss GmbH and J&M，and paired with the &MSP 400 spectral measurement system. The light source model was HBO 100，and the light-receiving element was a photomultiplier tube containing multiple curved-surface shaped dynodes，an anode，and a cathode，capable of measuring mineral particles as small as 0.5μm under high magnification. The test beam spot was set to 20μm×20μm. The laboratory used double-layered light-blocking curtains，with an outer layer of light blue and an inner layer of red-black，and the light source model was HBO 100，to subtract spectral interference. Before testing，all polished sample surfaces were re-polished using diamond spray (grain size W=1μm). The spectrophotometer was turned on，and after the instrument stabilized (30 minutes)，standard sample calibration and sensitivity adjustments were performed. During testing，the standard sample was recalibrated every 15 minutes，and the standard sample parameters are listed in Table 1. Quantitative calculation of reflectance was performed using the “Selective Wavelength Coordinate Method for Average Daylight” as described by Hardy et al^[24]. LA-ICP-MS in-situ trace element content testing was completed at Wuhan Shangpu Analysis Technology Co.，Ltd. The GeolasPro laser ablation system consisted of a COMPexPro 102 ArF 193nm excimer laser and MicroLas optical system，manufactured by Teledyne Cetac Technologies，model Analyte Excite. The ICP-MS analyzer used was the Agilent 7700e Inductively Coupled Plasma Mass Spectrometer (Agilent Corporation，USA). During laser ablation，helium was used as the carrier gas (370mL/min)，and argon as the compensation gas to adjust sensitivity. Both gases were mixed through a T-connector before entering the ICP. The laser ablation system was equipped with a signal smoothing device to maintain the stability of the ablation signal. The laser beam spot and frequency used in this analysis were 32μm and 6Hz，respectively，with an energy density of 3.0J/cm². The ablation sample signal duration was 40s，the gas background signal duration was 20s，and the washout gas signal duration was 30s. After every 10 test points，1 NIST610 (synthetic silicate glass standard material)，and 2 MASS-1 (sulfide standard samples) were added for correction. Quantitative calculation of element mass fractions was performed using the “Internal Standard-Free Matrix Normalization Method” as described by Liu et al ^[23]. Photographs taken with the polarized light microscope were processed using NIS-Elements BR software; LA-ICP-MS trace element test data were processed using ICPMS DataCal software; and sphalerite reflectance color index test data were processed using Spectra Forensic software. During this testing process，if the relative error of the standard sample minerals was less than 1%，the measured relative error could be controlled within 2%.

Data and Results: The test results of the microspectrophotometer are shown in Table 2，and the test results of LA-ICP-MS are shown in Table 3. In the test results of the microspectrophotometer，the error of visual reflectance is less than 0.3%，the error of the main band of reflection color is less than 1nm，and the error of color saturation is less than 0.001. In the test results of the microspectrophotometer，there is little difference in visual reflectance and the main band of reflection color among different stages of sphalerite，but there is a relatively large difference in color saturation. Compared with Lai et al^[6] test on four lead-zinc deposits (Table 4)，it also shows the characteristics of little difference in visual reflectance and the main band of reflection color，but a relatively large difference in color saturation，which may be due to the characteristics of sphalerite itself. The detection limit of LA-ICP-MS can reach the ng/g level，with significant differences. Therefore，34S and 57Fe with low natural abundance are used for detection. When measuring the element Ge，the mass spectral peaks of elements such as Zn，Zr，Sm，Nd，Ce，and Sn overlap and cause interference. Specifically，⁷⁰Zn and ¹⁴⁰Ge²⁺ interfere with ⁷⁰Ge，¹⁴⁴Sm²⁺ and ¹⁴⁴Nd²⁺ interfere with ⁷²Ge，and ¹⁴⁸Sm²⁺ and ¹⁴⁸Nd²⁺ interfere with ⁷⁴Ge. Because the content of Sm and Nd in sphalerite samples is generally less than 10μg/g，the interference on ⁷²Ge is relatively small. Moreover，⁷²Ge also has high sensitivity，so ⁷²Ge is selected as the measurement isotope. Other detected elements include ⁵⁵Mn，⁶³Cu，⁶⁶Zn，⁷¹Ga，¹⁰⁷Ag，¹¹¹Cd，²⁰⁴Pb，²⁰⁶Pb，²⁰⁷Pb，²⁰⁸Pb，etc. The spot size selected for LA-ICP-MS testing is 32μm，which is closer to the spot size (20μm) of the microspectrophotometer，ensuring the accuracy of trace element test results and a higher degree of matching with the test results of the microspectrophotometer.