Micaceous picrites of the Sikhote-Alin ridge

Petukhova L.L., Prikhodko V.S.

Institute of Tectonics and Geophysics n.a. Yu.A. Kosygin,

Far Eastern Branch of the Russian Academy of Sciences, Khabarovsk, Russia

šššššš Regarding the issue of diamond potential, the Sikhote-Alin ridge comprises rare and specific igneous formations of high interest, such as micaceous picrites, which were discovered in the alluvial sediments of the Kedrovka river and as part of dikes and volcanic pipes near the settlement of Ariadnoe (Primorsky Krai). Micaceous picrites were studied by us along the left bank of the Malinovka river, in between the Pad Malaya, Pozhiga and Pad Bolshaya creeks, where picrites comprise a thin subvertical dike of north-west strike and a small volcanic pipe within host terrigenous sediments of Jurassic period.

š ššššššNear the contact with host rocks the dike consists of brecciated variations of picrite, where rock debris are represented mainly by sandstone, and the central part of the dike is structured by rocks with big-grained (sometimes – large-grained) impregnations of amphibole, mica, and carbonaceous globules. Picrites are characterized by high concentration of biotitite, quartz and carbonate rock. They are penetrated by thin veinlets of barite.š The volcanic pipe is formed by intensively altered and withered picrites, thus it is difficult to judge about its inner structure.š

šššššš Micaceous picrites are grey-greenish rocks of porphyry structure and globular texture. They are impregnated by kaersutite, phlogopite (amber mica, over 10%), clinopyroxene and rarely chromspinel. Their primary mass is comprised of felted aggregate of kaersutite, phlogopite, ilmenite and igneous carbonate. Olivine has not been indicated in those rocks. š

Table 1. Representative assay results of micaceous picrites

šš

Note: 5,7 – dike, 14,16 – volcanic pipe; oxides-mass. %, the rest-g/t. Assaying was done by the Central laboratory “Dalgeology”, REE and rare elements were analyzed by Khabarovsk Innovation Analytical Center (Institute of Tectonics and Geophysics) with ISP-MS.

ššššššš According to the concentrations of silica, sum alkali and sum magnesium and calcium (see Table 1), the rocks can be classified as alkaline picrites. Their original chemical composition has been probably distorted by strong hydrothermal alternations, which enabled further active drift of many components. This assumption is indirectly proven by high concentration of CO₂ and high impurity level in the rocks.

ššššššš Geochemical peculiarity of micaceous picrites lies in their enrichment with big ionic lithophilous elements (Ti, Ta, Nb, Zr), light lanthanide elements and phosphorus, which may be indicative of specific composition of palliac source of micaceous picrites. šSum concentration of REE is very high and reaches 600 ppm. Besides they sharply prevail over heavy elements; the ratio (La/Yb) is up to 60.š

šššššš Judging by grades of iron group elements (๓r, Co, Ni, Ti, V) alkaline picrites are in the intermediate position between ultrabasic and basic rocks. It has been indicated that processes of intensive alternation of rocks have not resulted in substantial redistribution and migration of iron group elements.š Nickel/copper ratio varies within 4.7 – 6.5; chrome/vanadium proportion fluctuates from 2.4 to 1.0. Such ratio rates are characteristic of rocks of picrite family.

Table 2.š Chemical composition of minerals of micaceous picrites

Note: 1 – 6 – dike of alkaline picrites; 7 – volcanic pipe; 1,2,3 – kaersutites; 4 – clinopyroxene; 6- chromspinel; 5,7 – mica. Analyses were done with CAMEBAX-micro analyzer in the Institute of Volcanology and Seismology, Far Eastern Branch of the Russian Academy of Sciences (Petropavlovsk-Kamchatsky).

šššššš Assays (see Table 2) of the main minerals of alkaline picrites illustrate peculiar features of their composition. Low aluminiferous monocline pyroxene is related to diopside-sahlite with a small part of aegirine. Kaersutite and phlogopite are characterized by high grades of titanium and aluminum. Analytical results of amphiboles of the rocks indicate that kaersutite with high silica proportion and sum alkali contains minimal concentrations of titanium, aluminum and calcium. Points of chromspinellid compositions bearing titanium coincide with a trend of chromspinellid meimechites within the Sikhote-Alin ridge. šIlmenite of the major mass of picrites contains small amounts of magnesium, manganese, aluminum and chrome. Ore minerals are represented by pyrite and chalcopyrite; accessory minerals – by apatite, monazite, orthite and sphene. Comparative analysis of phlogopite’s composition in the dike rocks and in the volcanic pipe (assays 5&7 Table 2) enables to conclude that picrite minerals of the volcanic pipe are more ferrous. š

ššššššš Petro-geochemical data represented above show that micaceous picrites under study are intensively enriched with K, Ti, Nb, Zr & REE. Thus they can be viewed as products of potassium-bearing ultramafic melts formed out of metasomatically altered pallial source.š