<?php
  if(is_category()){
		global $wp_query;

		if(isset($wp_query->posts) && !empty($wp_query->posts)) {
			$tags = array();
			foreach ($wp_query->posts as $post) {
				$posttags = get_the_tags($post->ID);
				if($posttags) {
					foreach ($posttags as $posttag) {
						$tags[] = $posttag->name;
					}
				}
			}

			if(!empty($tags)) {
				$tags =	array_unique($tags);
				$tags = implode(',',$tags);
				echo '<meta name="keywords" content="' . $tags .  '" />';
			}

			wp_reset_postdata();
		}
	}
?>