Petrogenesis and magma fertility of the Heishishan skarn deposit, East Kunlun, NW China: Insights from geochronology, mineralogy, geochemistry, and Sr-Nd-Hf isotopes

The Heishishan skarn Cu-Pb-Zn deposit is located in the Wulonggou area, in the middle part of the East Kunlun Orogenic Belt. The ore body occurs at the contact between the Heishishan monzogranite and marble of the Paleoproterozoic Jinshuikou Group. U-Pb analysis of zircon grains shows that the monzogranite and granodiorite were formed at 241.7 & PLUSMN;1.0 Ma and 243.1 & PLUSMN;0.9 Ma, respectively. Bulk-rock analyses show that the monzogranite has similar chemistry characteristics to those of adakitic rocks, being rich in silicon, aluminum, potassium, sodium, and poor in iron and magnesium, with high Mg# values of 33.95-54.81. The monzogranite samples are enriched in large-ion lithophile elements such as Rb, Ba, U, K, and Sr, and depleted in high-field-strength elements such as Nb, Ta, Ti, and P, with a Sr to Y ratio of 41-63. The granodiorite samples are rich in silicon, potassium and sodium, poor in magnesium, and have a relatively low Mg# values of 30.88-32.61. The granodiorite is enriched in Rb, Ba, and U, and depleted in Th, Nb, Ta, Ti, and P. The whole-rock (87Sr/86Sr)t ratios of the monzogranite and granodiorite range from 0.709607 to 0.709738 and from 0.707084 to 0.707102, respectively; the whole-rock & epsilon;Nd(t) values range from -5.0 to -4.9 and from -3.1 to -3.0, respectively, and the zircon & epsilon;Hf(t) values range from -2.8 to +2.2 and from -1.8 to +1.2, respectively. The geochemistry and Sr-Nd-Hf isotope results imply that the monzogranite originated from crust-mantle magma mixing, whereas the granodiorite originated from partial melting of the lower crust, and both types were formed in a subduction tectonic setting. Estimates of magma fertility using mineral composition show that the monzogranite magma has a high oxygen fugacity, along with high water, S and Cl contents, thus showing strong metallogenic ability. By contrast, the granodiorite magma has a low oxygen fugacity and S content, thus showing weak metallogenic ability. The SO3 contents of the apatite from the monzogranite are much higher than those of the granodiorite, while the Cl contents show the opposite trend. This suggests that this is related to the exsolution of monzogranite magmatic fluid in the shallow crust. Thus, while the SO3 and Cl contents of the apatite are good indicators of the Cu fertility of the magma, the influence of fluid exsolution should also be considered. Determination of monzogranite as the ore-forming rock is achieved by using the spatial contact relationship between the orebody and the intrusive rock, along with the geochemistry and magma fertility. The formation time of the monzogranite (241.7 Ma) approximately represents the metallogenic epoch, which is under the subduction tectonic setting of the PaleoTethys Ocean. The metallogenic materials and fluids are mainly derived from the monzogranite magma. Additionally, the partial melting of the enriched mantle generated high fertility magma, which is critical to the formation of Heishishan skarn deposit.

Automated summary

The Heishishan skarn Cu-Pb-Zn deposit is located in the East Kunlun Orogenic Belt and is associated with the Heishishan monzogranite and marble of the Jinshuikou Group. Geochemical analysis indicates that the monzogranite represents a crust-mantle magma mixing origin, while the granodiorite originated from partial melting of the lower crust. The ore-forming rock is determined based on the spatial contact relationship, geochemistry, and magma fertility. The metallogenic epoch of 241.7 Ma is under the subduction tectonic setting of the PaleoTethys Ocean.