Petrology And Alteration-mineralization Evolution Rhythm Of Hornfels In Jiama Copper Polymetallic Deposit, Tibet

The Jiama Copper polymetallic deposit is located in the central part of the Gangdese-Nyainqentanglha terrane. Metal resources of EQCu exceeds14,000,000t. This study has discussed on the petrology and alteration-mineralization evolution rhythm of hornfels based on the basic geological mapping, core logging, thin section description and microanalyses and also combined with electron microprobe analysis, major elements analysis, trace element LA-ICP-MS in situ analysis of biotite, infrared spectrography analysis of biotite and contouring of hornfels thickness and Cu, Mo, Au, Ag, Pb, Zn mineralization element content formed in them by212drilling holes.Research shows that the hornfels thermal metamorphism is well developed in Jiama polymetallic copper deposit. Rock types of hornfels can be divided into the pre-oreforming hornfels and the syn-oreforming ones, belonging to AEH Face with the main mineral paragenetic association—albite+biotite+quartz. The pre-oreforming hornfels consist of protogenic biotite hornfels with typical mottled and banded structure, and felsic hornfels, characterized by refining-grain, most phenocryst mineral without independent morphology. The syn-oreforming hornfels include silicified hornfels and hydrothermal biotite-chlorite hornfels, characte-rized by phenocryst mineral formed in good crystal habit. Based on the stratum contact relationship and the rock fabric, protoliths of hornfels are inferred as sandy slates and carbonaceous slates by comparision for features of petrochemistry and geochemistry.On condition that hornfels are natural trace material for thermal metamorphism, the author contours the thickness of hornfels and mineralization formed in them, and contours the concentration center of abundance of Cu and Mo, which helps to indicate the location of the concealed porphyry with the latest exploration results.The biotite from hornfels can be distinguished into protogenic and hydrothermal biotite.. Compared with protogenic biotite, there is more Cu enriched in hydrothermal biotite. But Mo spread in two sorts of biotite diffusely, and don’t have characteristic of selective occurrence. Hydrothermal biotite from hornfels can be divided into A, B, C and D type according paragenetic association of alteration minerals and alteration attitude. A type hornfels hydrothermal biotite occurs in quartz vein with the main alteration mineral paragenetic association quartz+biotite (Q+Bi), B type biotite in quartz vein with main alteration mineral paragenetic association quartz+biotite+chlorite (Q+Bi+Chl), C type biotite in quartz vein with main alteration mineral paragenetic association quartz+biotite+kaolinite (Q+Bi+Kin), and D type biotite grows in surface hydrothermal biotite-chlorite alteration with alteration mineral accompanying combination biotite-hlorite (Bi-Chl) indicating potassic alteration or retrometamorphism-potassic alteration. A, C and D type hornfels hydrothermal biotite can indicate Cu mineralization in hornfels well. The Cu, Pb and Zn contents from rocks have positive correlation with that in hornfels hydrothermal biotite.Alteration-mineralization evolution rhythm of hornfels shows surface potassic alteration with chalcopyrite mineralizationâ†'retrometamorphism potassic alteration with chalcopyrite mineralizationâ†'surface chloritization+chlorite in quartz vein with chalcopyrite mineralizationâ†'nervation silicification (quartz+chalcopyriteâ†'quartz+chalcopyrite+molybdeniteâ†'quartz+molybdenite) from the bottom up in space; and early stage large-scale rich acid siliceous hydrothermal solution total-rock metasomatic metamorphism (giving rise to good self-organization construction) with pyriteâ†'teeming of potassium-rich silicate mineral biotite with chalcopyrite and pyrite formingâ†'hydrolyzation retrometamorphism of potassium-rich silicate mineral with chalcopyrite and pyrite formingâ†'teeming of hydrothermal chloritization with chalcopyrite and pyriteâ†'partial crumb silication with chalcopyriteâ†'nervation silication (quartz+chalcopyriteâ†'quartz+chalcopyrite+molybdeniteâ†'quartz+molybdenite) from early to late. The early molybdenite grows along vein wall of quartz vein, and the later one occurs in the centre of quartz vein, and molybdenite in the end appears in the miarolitic structure of quartz vein. In any stage, molybdenite always displays dissemination formation in the early, then becomes successive dense dissemination, and changes into schistose texture self-structured crystal in the end. The meticulous core logging shows single core hand specimen indicates information from surface alteration to nervation alteration, from alteration mineral paragenetic association or accompanying combination to occurence state, by which can help to distinguish intercalated relationship and formation order of all kinds of veins, and further indicate alteration-mineralization evolution rhythm of hornfels.The study on Petrology and alteration-mineralization evolution rhythm of hornfels in Jiama deposit has solved problems as rock types and genesis significance of hornfels, ore-bearing potential of biotites and their mineralization signatures, and alteration-mineralization evolution rhythm of hornfels, which help to indicate the location of the concealed porphyry, and to explain genesis of accumulation of massive amount mineralization element in hornfels-type orebody.